Contents

16.1 Digital “Modes” 16.5 Networking Modes 16.1.1 Symbols, Baud, Bits and Bandwidth 16.5.1 OSI Networking Model 16.1.2 Error Detection and Correction 16.5.2 Connected and Connectionless 16.1.3 Data Representations Protocols 16.1.4 Compression Techniques 16.5.3 The Terminal Node Controller (TNC) 16.1.5 Compression vs. Encryption 16.5.4 PACTOR-I 16.2 Unstructured Digital Modes 16.5.5 PACTOR-II 16.2.1 (RTTY) 16.5.6 PACTOR-III 16.2.2 PSK31 16.5.7 -TOR 16.2.3 MFSK16 16.5.8 CLOVER-II 16.2.4 DominoEX 16.5.9 CLOVER-2000 16.2.5 THROB 16.5.10 WINMOR 16.2.6 MT63 16.5.11 16.2.7 Olivia 16.5.12 APRS 16.3 Fuzzy Modes 16.5.13 Winlink 2000 16.3.1 Facsimile () 16.5.14 -STAR 16.3.2 Slow-Scan TV (SSTV) 16.5.15 P25 16.3.3 , Feld-Hell or Hell 16.6 Digital Mode Table 16.4 Structured Digital Modes 16.7 Glossary 16.4.1 FSK441 16.8 References and Bibliography 16.4.2 JT6M 16.4.3 JT65 16.4.4 WSPR 16.4.5 HF Digital Voice 16.4.6 ALE

Chapter 16 — CD-ROM Content Supplemental Files • Table of digital mode characteristics (section 16.6) • ASCII and ITA2 tables • tables for PSK31, MFSK16 and DominoEX • Tips for using FreeDV HF digital voice software by Mel Whitten, KØPFX Chapter 16 Digital Modes

There is a broad array of digital modes to service various needs with more coming. The most basic modes simply encode text data for transmission in a keyboard-to-keyboard chat- The first type environment. These modes may or may not include any mechanism for error detection transmissions were digital using or correction. The second class of modes are generally more robust and support more so- . Computers have now phisticated data types by structuring the data sent and including additional error-correction become so common, households information to properly reconstruct the data at the receiving end. The third class of modes often have several and there are a large number of other digital discussed will be networking modes with protocols often the same or similar to versions information appliances such used on the Internet and computer networks. as tablets, netbooks and smart phones. -mail and texting are displacing traditional telephone communications and Amateur Radio 16.1 Digital “Modes” operators are creating new digital The ITU uses Emission Designators to define a “mode” as demonstrated in the Modula- modes to carry this traffic over the tion chapter. These designators include the bandwidth, type and information air or to solve a particular need. being sent. This system works well to describe the physical characteristics of the modula- Many new modes can be entirely tion, but digital modes create some ambiguity because the type of information sent could be implemented in software for use text, image or even the audio of a CW session. As an example, an FM data transmission of with a computer , making 20K0F3D could transmit spoken audio (like FM 20K0F3E or 2K5J3E) or a CW signal (like tools for experimentation and implementation available worldwide 150H0A1A). These designators don’ help identify the type of data supported by a particular via the Internet. mode, the speed that data can be sent, if it’ error-corrected, or how well it might perform in This chapter will focus on the hostile band conditions. Digital modes have more characteristics that define them and there protocols for transferring various are often many variations on a single mode that are optimized for different conditions. We’ll data types. The material was written need to look at the specifics of these unique characteristics to be able to determine which or updated by Scott Honaker, N7SS, digital modes offer the best combination of features for any given application. with support from Alan Bloom, N1AL, and Pete Loveall, AE5PL. Kok Chen W7AY contributed the 16.1.1 Symbols, Baud, Bits and Bandwidth sections on RTTY, PSK, and MFSK. The basic performance measure of a digital mode is the data rate. This can be measured a Mel Whitten, KØPFX, and David number of ways and is often confused. Each change of state on a transmission medium defines Rowe, VK5DGR, contributed a new a symbol and the symbol rate is also known as baud. (While commonly used, “baud rate” is section on FreeDV digital voice. redundant because “baud” is already defined as the rate of symbols/second.) Modulating a Modulation methods are covered carrier increases the frequency range, or bandwidth, it occupies. The FCC currently limits in the Modulation chapter and digital modes by symbol rate on the various bands as an indirect (but easily measurable) the Digital Communications means of controlling bandwidth. supplement on the Handbook The is the product of the symbol rate and the number of bits encoded in each CD discusses the practical symbol. In a simple two-state system like an RS-232 interface, the bit rate will be the same considerations of operating using these modes. This chapter as baud. More complex waveforms can represent more than two states with a single symbol addresses the process by which so the bit rate will be higher than the baud. For each additional bit encoded in a symbol, the data is encoded, compressed number of states of the carrier doubles. This makes each state less distinct from the others, and error checked, packaged and which in turn makes it more difficult for the receiver to detect each symbol correctly in the exchanged. presence of noise. A .34 may transmit symbols at a baud rate of 3420 baud and each symbol can represent up to 10 discrete states or bits, resulting in a gross (or raw) bit rate of 3420 baud × 10 or 34,200 bits per second (bit/s). Framing bits and other overhead reduce the net bit rate to 33,800 bit/s. Bits per second is abbreviated here as bit/s for clarity but is also often seen as bps. Bits per second is useful when looking at the protocol but is less helpful determining how long it

Digital Modes 16.1 and a retransmission requested. The com- Table 16.1 bination is called hybrid automatic repeat Data Rate Symbols and Multipliers request (hybrid ARQ). Name Symbol Multiplier There are many error correcting code kilobit per second kbit/s or kbps 1000 or 103 Megabit per second Mbit/s or Mbps 1,000,000 or 106 (ECC) available. Extended Go- Gigabit per second Gbit/s or Gbps 1,000,000,000 or 109 lay coding is used on blocks of ALE data, for example, as described in the section below on G-TOR. In addition to the ability to detect and correct errors in the data packets, the takes to transmit a specific size file because bit inversions. If the last two bits were flipped modulation scheme allows sending multiple the number of bits consumed by overhead to 01011001 (the ASCII letter ), it would data streams and interleaving the data in such is often unknown. A more useful measure still pass the parity check because it still has a way that a noise burst will disrupt the data for calculating transmission times is bytes an even number of bits. Parity is also rarely at different points. per second or Bps (note the capitalization). used on 8-bit data so it cannot be used when Although there are only eight bits per byte, transferring binary data files. with the addition of start and stop bits, the Checksum is a method similar to the 16.1.3 Data Representations difference between bps and Bps is often ten- “check” value in an NTS message. It is gen- When comparing digital modes, it is im- fold. Since the net bit rate takes the fixed erally a single byte (8-bit) value appended portant to understand how the data is con- overhead into account, Bytes per second can to the end of a packet or frame of data. It veyed. There are inherent limitations in any be calculated as bpsnet/8. Higher data rates is calculated by adding all the values in the method chosen. PSK31 might seem a good can be expressed with their metric multipliers packet and taking the least significant (most choice for sending data over HF links because as shown in Table 16.1. unique) byte. This is a simple operation for it performs well, but it was only designed for Digital modes constantly balance the even basic processors to perform quickly but text (not 8-bit data) and has no inherent error relationship between symbol rate, bit rate, can also be easily mislead. If two errors occur correction. It is certainly possible to use this bandwidth and the effect of noise. The in the packet of equal amounts in the opposite modulation scheme to send 8-bit data and add Shannon-Hartley theorem demonstrates the direction (A becomes and becomes Y), error correction to create a new mode. This maximum channel capacity in the presence the checksum value will still be accurate and would maintain the weak signal performance of Gaussian white noise and was discussed the packet will be accepted as error-free. but the speed will suffer from the increased in the Modulation chapter in the Channel Cyclic redundancy check (CRC) is similar overhead. Similarly, a digital photo sent via Capacity section. This theorem describes to checksum but uses a more sophisticated analog SSTV software may only take two how an increased symbol rate will require formula for calculating the check value of a minutes to send, but over VHF packet it could an increase in bandwidth and how a reduced packet. The formula most closely resembles take 10 minutes, despite the higher speed of signal-to-noise ratio (SNR) will reduce the long division, where the quotient is thrown a packet system. This doesn’t mean SSTV is potential throughput of the channel. away and the remainder is used. It is also more efficient. Analog SSTV systems gener- common for CRC values to be more than a ally transmit lower resolution images with no single byte, making the value more unique error correction. Over good local links, the 16.1.2 Error Detection and likely to identify an error. Although other VHF packet system will be able to deliver and Correction error detection systems are currently in use, perfect images faster or of higher quality. Voice modes require the operator to man- CRC is the most common. ually request a repeat of any information TEXT REPRESENTATIONS required but not understood. Using proper ERROR CORRECTION Morse code is well known as an early code phonetics makes the information more easily There are two basic ways to design a pro- used to send text data over a wire, then over understood but takes longer to transmit. If tocol for an error correcting system: auto- the air. Each letter/number or symbol is rep- 100% accuracy is required, it may be nec- matic repeat request (ARQ) and forward error resented with a varying length code with the essary for the receiver to repeat the entire correction (FEC). With ARQ the transmitter more common letters having shorter . message back to the sender for verification. sends the data packet with an error detection This early varicode system is very efficient Computers can’t necessarily distinguish be- code, which the receiver uses to check for and minimizes the number of state changes tween valuable and unnecessary data or iden- errors. The receiver will request retransmis- required to send a message. tify likely errors but they offer other options sion of packets with errors or sends an ac- The Baudot code (pronounced “bawd- to detect and correct errors. knowledgement (ACK) of correctly received OH”) was invented by Émile Baudot and is data, and the transmitter re-sends anything the predecessor to the character set currently ERROR DETECTION not acknowledged within a reasonable period known more accurately as International Tele- The first requirement of any accurate sys- of time. graph Alphabet No 2 (ITA2). This code is tem is to be able to detect when an error With forward error correction (FEC), the used for radioteletype communications and has occurred. The simplest method is parity. transmitter encodes the data with an error- contains five bits with start and stop pulses. With 7-bit ASCII data, it was common to correcting code (ECC) and sends the encoded This only allows for 25 or 32 possible char- transmit an additional 8th parity bit to each message. The receiver is not required to send acters to be sent, which is not enough for character. The parity bit was added to make any messages back to the transmitter. The re- all 26 letters plus numbers and characters. the total number of 1 bits odd or even. The ceiver decodes what it receives into the “most To resolve this, ITA2 uses a LTRS code to binary representation for an ASCII letter Z is likely” data. The codes are designed so that select a table of upper case (only) letters and 1011010. Sent as seven bits with even parity, it would take an “unreasonable” amount of a FIGS code to select a table of numbers, the parity bit would be 0 because there are noise to trick the receiver into misinterpreting punctuation and special symbols. The code already an even number of 1 bits and the result the data. It is possible to combine the two, is defined in the ITA2 codes table on the CD would be 01011010. The limitation of parity so that minor errors are corrected without included with this book. is that it only works with an odd number of retransmission, and major errors are detected Early computers used a wide variety of al-

16.2 Chapter 16 phabetic codes until the early 1960s until the advent of the American Standard Code for In- Table 16.2 formation Interchange or ASCII (pronounced Typical Audio Formats “ESS-key”). At that point many computers Audio Format Bits per Dynamic Maximum kbytes per Sample Range Frequency Minute standardized on this character set to allow 44.1 kHz stereo 16 98 22.050 kHz 10,584 simple transfer of data between machines. 22 kHz mono 16 98 11 kHz 2,640 ASCII is a 7-bit code which allows for 27 8 kHz mono 8 50 4 kHz 960 or 128 characters. It was designed without the control characters used by Baudot for more reliable transmissions and the letters appear in English alphabetical order for easy million also called true color). True color is AUDIO DATA REPRESENTATIONS sorting. The code can be reduced to only six most commonly used with digital cameras Like images, audio can be stored as a sam- bits and still carry numbers and uppercase and conveniently provides eight bits of reso- pled waveform or in some type of primitive letters. Current FCC regulations provide that lution each for the red, green and blue colors. format. Storing a sampled waveform is the amateur use of ASCII conform to ASCII as Newer scanners and other systems will often most versatile but can also require substantial 30 36 defined in ANSI standard X3.4-1977. The generate 30-bit (2 colors) and 36-bit (2 storage capacity. MIDI (musical instrument international counterparts are ISO 646-1983 colors). Images with 4- and 8-bit color can digital interface, pronounced “MID-ee”) is a and International Alphabet No. 5 (IA5) as store more accurate images than might be common music format that stores instrument, published in ITU-T Recommendation V.3. obvious because they include a palette where note, tempo and intensity information as a A table of ASCII characters is presented as each of their 16 or 256 colors respectively musical score. There are also voice coding “ASCII Character Set” on the CD included are chosen from a palette of 16 million. This techniques that store speech as allophones with this book. palette is stored in the file which works well (basic human speech sounds). ASCII has been modified and initially ex- for simple images. The GIF format only sup- As with images, storage of primitives can panded to eight bits, allowing the addition of ports 256 colors (8-bit) with a palette and save storage (and transmission times) foreign characters or line segments. The dif- . but they are not as rich as a high quality sam- ferent extended versions were often referred Digital photographs are raster images at a pled waveform. Unfortunately, the 44,100 Hz to as code pages. The IBM PC supported code specific resolution and color depth. A typical 16-bit sample rate of an audio compact disc page 437 which offers line segments, and low resolution digital camera image would (CD) requires 176,400 bytes to store each be 640×480 pixels with 24-bit color. The 24- English Windows natively supports code page second and 10,584,000 bytes for each minute bit color indicated for each pixel requires 1252 with additional foreign characters and of stereo audio. symbols. All of these extended code pages in- three bytes to store (24 bits / 8 bits per byte) The Nyquist-Shannon sampling theorem clude the same first 128 ASCII characters for and can represent one of a possible 224 or states that perfect reconstruction of a signal backward compatibility. In the early 1990s 16,777,216 colors with eight bits each for red, is possible when the sampling frequency is efforts were made to support more languages green and blue intensities. The raw (uncom- at least twice the maximum frequency of the directly and Unicode was created. Unicode pressed) storage requirement for this image signal being sampled. With its 44,100 Hz generally requires 16 bits per character and would be 640×480x3 or 921,600 bytes. This sample rate, CD audio is limited to a maxi- can represent nearly any language. relatively low resolution image would require mum frequency response of 22,050 Hz. If More recent schemes use varicode, where the significant time to transmit over a slow link. only voice-quality is desired, the sample most common characters are given shorter codes Vector images are generally created with rate can easily be dropped to 8000 Hz pro- (see http://en.wikipedia.org/wiki/Prefix_ drawing or CAD packages and can offer sig- viding a maximum 4000 Hz frequency re- code). Varicode is used in PSK31and MFSK nificant detail while remaining quite small. sponse. (See the DSP and Software Radio to reduce the number of bits in a message. Because vector files are simply drawn at the Design chapter for more information on Although the PSK31 varicode contains all desired resolution, there is no degradation sampling.) 128 ASCII characters, lower-case letters con- of the image if the size is changed and the bit depth tain fewer bits and can be sent more quickly. storage requirements remain the same at The (number of bits used to rep- Tables of PSK and MFSK varicode characters any resolution. Typical drawing primitives resent each sample) of an audio signal will are included on the CD included with this include lines and polylines, polygons, circles determine the theoretical dynamic range or book. and ellipses, Bézier curves and text. Even signal-to-noise ratio (SNR). This is expressed computer font technologies such as TrueType with the formula SNR = (1.761 + 6.0206 × IMAGE DATA REPRESENTATIONS create each letter from Bézier curves allowing bits) dB. A dynamic range of 40 dB is adequate Images are generally broken into two basic for flawless scaling to any size and resolution for the perception of human speech. Table types, raster and vector. Raster or im- on a screen or printer. 16.2 compares the audio quality of various ages are simply rows of colored points known Raster images can be resized by dropping formats with the storage (or transmission) as pixels (picture elements). Vector images or adding pixels or changing the color depth. requirements. are a set of drawing primitives or instructions. The 640×480×24-bit color image mentioned DATA REPRESENTATIONS These instructions define shapes, placement, above could be reduced to 320×240×16-bit size and color. Similar coding is used with color with a raw size of 153,600 bytes — The most basic video format simply stores plotters to command the pens to create the a significant saving over the 921,600 byte a series of images for playback. Video can desired image. original. If the image is intended for screen place huge demand on storage and bandwidth Bitmap images can be stored at various display and doesn’t require significant detail, because 30 frames per second is a common color depths or bits per pixel indicating how that size may be appropriate. If the image is rate for smooth-appearing video. Rates as low many colors can be represented. Common printed full sized on a typical 300 dpi (dots as 15 frames per second can still be consid- color depths are 1-bit (2 colors), 4-bit (16 per inch) printer, each pixel in the photo will ered “full-motion” but will appear jerky and colors), 8-bit (256 colors), 16-bit (65,536 explode to nearly 100 dots on the printer and even this rate requires substantial storage. colors also called high color) and 24-bit (16 appear very blocky or pixilated. Primitive formats are less common for video

Digital Modes 16.3 but animation systems like Adobe Flash are made on the HF bands. More information put over the wire. Winlink uses a compres- often found on the web. about the project can be found at http:// sion scheme called B2F and sees an average codec2.org. Digital voice is discussed 44% improvement in performance since most elsewhere in this chapter. Winlink data is uncompressed previously. 16.1.4 Compression Many online services offer “Web Accelera- Techniques LOSSLESS COMPRESSION tors” that also compress data going over the There are a large variety of compression TECHNIQUES wire to achieve better performance. There algorithms available to reduce the size of data One of the earliest lossless compression is a slight delay (latency) as a result of this stored or sent over a transmission medium. schemes is known as . Huff- compression but over a slow link, the ad- Many techniques are targeted at specific types man coding creates a of commonly used ditional latency is minimal compared to the of data (text, audio, images or video). These data values and gives the most common val- performance gain. compression techniques can be broken into ues a lower bit count. Varicode is based on this two major categories: lossless compression mechanism. In 1984, Terry Welch released and . Lossless algorithms code with improvements to a scheme from LOSSY COMPRESSION SCHEMES are important for compressing data that must Abraham Lempel and Jacob Ziv, commonly Lossy compression schemes depend on the arrive perfectly intact but offer a smaller com- referred to as LZW (Lempel-Ziv-Welch). The human brain to “recover” or simply ignore the pression ratio than lossy techniques. Pro- Lempel-Ziv and variants are the missing data. Since audio, image and video grams, documents, databases, spreadsheets basis for most current compression programs data have unique characteristics as perceived would all be corrupted and made worthless and is used in the GIF and optionally TIFF by the brain, each of these data types have if a lossy compression technique were used. graphic formats. LZW operates similarly to unique compression algorithms. New com- Images, audio and video are good candi- Huffman coding but with greater efficiency. pression schemes are developed constantly to dates for lossy compression schemes with The actual amount of compression achieved achieve high compression rates while main- their substantially higher compression rates. will depend on how redundant the data is and taining the highest quality. Often a particular As the name implies, a lossy compression the size of the data being compressed. Large file format actually supports multiple com- scheme deliberately omits or simplifies that files will achieve greater compression rates pression schemes or is available in different data to be able to represent it efficiently. The because the common data combinations will versions as better methods are developed. In- human eye and ear can easily interpolate be seen more frequently. Simple text and doc- depth discussion of each of these algorithms missing information and it simply appears uments can often see 25% compression rates. is beyond the scope of this book but there are to be of lower quality. Spreadsheets and databases generally consist some important issues to consider when look- Compression of a real-time stream of of many empty cells and can often achieve ing at these compression methods. data such as audio or video is performed nearly 50% compression. Graphic and video by software or firmware (from compression will vary greatly depending on Lossy Audio Compression coder-decoder). Codecs provide the real- the complexity of the image. Simple images Audio compression is based on the psy- time encoding or decoding of the audio or with solid backgrounds will compress well choacoustic model that describes which parts video stream. Many codecs are proprietary where complex images with little recurring of a digital audio signal can be removed or and have licensing requirements. Codecs data will see little benefit. Similarly, music substantially reduced without affecting the can be implemented as or will compress poorly but spoken audio with perceived quality of the sound. Lossy audio application plug-ins or even as digital ICs, little background noise may see reasonable compression schemes can typically reduce as with the P25 IMBE and D-STAR AMBE compression rates. the size of the file 10- or 12-fold with little codecs. Run length encoding (RLE) is a very simple loss in quality. The most common formats There are also specifically designed scheme supported on bitmap graphics (Win- currently are MP3, WMA, AAC, low bit-rate codecs for voice that are more dows BMP files). Each value in the file is a and ATRAC. accurately called vocoders. Vocoders are color value and “run length” specifying how optimized for voice characteristics and can many of the next pixels will be that color. It Lossy encode it extremely efficiently. Conversely, works well on simple files but can make a file The Joint Photographic Experts Group they cannot effectively process non-voice larger if the image is too complex. developed the JPEG format (pronounced signals. This is easily demonstrated with a It is important to note that compressing a “JAY-peg”) in 1992 and it has become the mobile phone by listening to music through previously compressed file will often yield primary format used for lossy compression a voice connection, such as when a call is a larger file because it simply creates more of digital images. It is a scalable compression placed on hold. Mobile phones use highly overhead in the file. There is occasionally scheme allowing the user to determine the efficient vocoders to minimize the bandwidth some minor benefit if two different compres- level of compression/quality of the image. of each voice channel which allows more sion algorithms are used. It is not possible to Compression rates of 10-fold are common channels per tower. This also means that non- compress the same file repeatedly and expect with good quality and can be over 100-fold at voice sounds such as music or mechanical any significant benefit. Modern compression substantially reduced quality. The JPEG for- sounds are not well reproduced. software does offer the additional benefit of mat tends to enhance edges and substantially Good vocoders are extremely valuable and being able to compress groups of files or even compress fields of similar color. It is not well vigorously guarded by their patent holders. whole directory structures into a single file suited when multiple edits will be required This has made it difficult for amateurs to for transmission. because each copy will have generational loss experiment with digital voice techniques. The compression mechanisms mentioned and therefore reduced quality. David Rowe, VK5DGR, worked for several above allow files to be compressed prior to years on a project called CODEC2. This is transmission but there are also mechanisms Lossy Video Compression a very effective open source vocoder made that allow near real-time compression of the Because of the massive amount of data available at the end of 2012. It has been transmitted data. V.92 implement a required for video compression it is almost incorporated into the FDMDV software and LZJH adaptive compression scheme called always distributed with a lossy compression there has been extensive testing and tuning V.44 that can average 15% better through- scheme. Lossless compression is only used

16.4 Chapter 16 when editing to eliminate generational loss. file is opened, this format information is read .264, Sorenson 3 (used by QuickTime), Video support on a computer is generally and the appropriate is activated and fed WMV (Microsoft), VC-1, RealVideo (Re- implemented with a codec that allows encod- into the data stream to be decoded. alNetworks) and . ing and decoding the video stream. Video files The most common current codecs are The basic mechanism of video compres- are containers that can often support more H.261 (video conferencing), MPEG-1 (used sion is to encode a high quality “key-frame” than one video format and the specific format in video CDs), MPEG-2 (DVD Video), H.263 that could be a JPEG image as a starting information is contained in the file. When the (video conferencing), MPEG-4, DivX, , image. Successive video frames or “inter-

FLDIGI By Ken Humbertson, WØKAH, and Fig 16.A, an ICOM IC-7000 is controlled modulating tones of the signal to that Jeff Coval, ACØSC by and thus shows the current offset within your receive bandwidth. You may have heard of the free operating frequency and mode of Click the CQ action button on the fldigi digital mode software fldigi by David H. 21.070 MHz USB, the QSO frequency of display. The transmitted text is displayed Freese Jr., W1HKJ. It is very popular 21071.511 (kHz) (in the windows at the in red above the waterfall display. When for use in emergency communications, upper left corner), and the current mode the text is finished,fldigi will return to for example, because of its multimode of BPSK31 (lower left corner). receive mode. The tutorial “Beginner’s capabilities and its ability to work When you see multiple signals in the Guide to Fldigi” at www.w1hkj.com/ with a companion program flmsg that waterfall, fldigi will decode whichever beginners.html is recommended and generates standard message forms to one you choose when you place the the program has an extensive Help file. be transmitted using fldigi. The software mouse cursor over a signal of interest The program can be used with an supports more than 34 modes (as of and line up the two vertical red lines on internal sound card or external sound early 2013) as well as variations of your screen with the sides of the signal, card adapter such as the Tigertronics tones, bandwidth, baud rates, number as shown near 1500 Hz. When you Signalink USB (www.tigertronics.com) of bits, and other variations for many change modes or bandwidths within a or West Mountain Radio Rigblaster modes. Versions of fldigi are available mode, the spacing of the red lines will series (www.westmountainradio. for /Free-BSD, Windows XP/W2K/ adjust to the new bandwidth. Fig 16.B com). Setting up a sound card to use NT/Vista/Win7, and from W1HKJ’s (right) shows that by selecting RTTY-45 fldigi may require manipulation of the download page at www.w1hkj.com/ under the OP MODE tab, you will notice audio device configuration for your download.html. that the red line spacing increases to computer’s operating system. Follow the If you have a computer with a sound match the familiar RTTY tone shift of instructions in the fldigi manuals and card and , you can begin 170 Hz. Simply place the two red lines the manufacturer’s manuals if you are using fldigi to receive with no additional over the signal you wish to decode and using an external adapter. A digital data hardware. A quick example would be to the program does the rest. interface that allows you to connect the tune to 14.070 MHz or 21.070 MHz USB Fig 16.B shows a decoded ARRL sound card to your rig’s microphone during the day. Set the radio speaker digital bulletin from January 2, 2013. input is recommended. (See the volume to a normal listening level. Start While the examples show BPSK31 and Assembling a Station chapter and the fldigi on the computer with sound card RTTY, there are many operating modes Digital Communications supplement and microphone connected and you to choose from, including CW. on the CD-ROM.) should see a waterfall display similar to Actions in fldigiare performed by Fldigi is frequently updated to include Fig 16.A (left). action buttons that invoke macros — new modes that are being developed. If your rig has computer control text scripts that control the program. The program is a user-friendly way to capability, fldigi can likely interface with it For example, to call CQ use the become active on the digital modes, a to display frequency and mode, as well mouse to click on an unoccupied spot rapidly expanding aspect of Amateur as control the radio from the program. In in the waterfall display. This shifts the Radio.

Fig 16.A Fig 16.B

Digital Modes 16.5 frames” contain only changes to the previous frame. After some number of frames, it is Table 16.3 necessary to use another key-frame and start Audio and Video Bit Rates over with inter-frames. The resolution of the Audio images, and compression quality 8 kbit/s Telephone quality audio (using speech codecs) 32 kbit/s AM broadcast quality (using MP3) determine the size of the video file. 96 kbit/s FM broadcast quality (using MP3) 224-320 kbit/s Near-CD quality, indistinguishable to the average listener (using MP3) BIT RATE COMPARISON Video Table 16.3 provides an indication of mini- 16 kbit/s Videophone quality (minimum for “talking head”) mum bit rates required to transmit audio and 128-384 kbit/s Business videoconference system quality video such that the average listener would 1.25 Mbit/s VCD (video compact disk) quality not perceive them significantly worse than 5 Mbit/s DVD quality the standard shown. 10.5 Mbit/s Actual DVD video bit rate

16.1.5 Compression vs. sages encoded for the purpose of obscuring because many of these services utilize en- Encryption their meaning.” Compressing a file with ZIP cryption of various types. Banks and other There is some confusion about compres- or RAR (common file compression methods) retailers may encrypt their entire transactions sion being a form of encryption. It is true that and transmitting it over the air is simply an to insure confidentiality of personal data. a after compression can no longer be efficient use of spectrum and time and is not Other systems as benign as e-mail may simply read unless uncompressed with the appropri- intended to “obscure its meaning.” encrypt passwords to properly authenticate ate algorithm. In the United States the FCC As amateur digital modes interact more users. The FCC has offered no additional defines encryption in part 97.113 as “mes- with Internet-based services, the issue arises guidance on these issues.

16.2 Unstructured Digital Modes The first group of modes we’ll examine mark signal. Fig 16.1 shows the character D ters. The two keys behave much like the caps are generally considered “sound card modes” sent by RTTY. lock key on a modern typewriter keyboard. for keyboard-to-keyboard communications. Instead of locking the keyboard of a type- Because each of these modes is optimized for BAUDOT CODE writer to upper case shift or lower case shift, a specific purpose by blending multiple fea- The Baudot code (see the Handbook CD) the two teletypewriter keys lock the state of tures, they often defy simple categorization. is a 5-bit code; thus, it is capable of encoding the teletypewriter into the LTRS table or the only 32 unique characters. Since the combi- FIGS table. LTRS and FIGS, among some nation of alphabets, decimal numbers and other characters such as the space character, 16.2.1 Radioteletype (RTTY) common punctuations exceeds 32, the Bau- appear in both LTRS and FIGS tables so that Radioteletype (RTTY) consists of a fre- dot code is created as two sets of tables. One you can send LTRS, FIGS and shift no matter quency shift keyed (FSK) signal that is modu- table is called the LTRS Shift, and consists which table the encoder is using. lated between two carrier frequencies, called mainly of alphabetic characters. The second To send the letter , you need to first make the mark frequency and the space frequency. table is called the FIGS Shift, and consists sure that the decoder is currently using the The protocol for amateur RTTY calls for the mainly of decimal numerals and punctua- LTRS table, and then send the Baudot code- mark carrier frequency to be the higher of tion marks. Two unique Baudot characters word for Q, 17 ( or “hex” value). the two in the RF spectrum. The difference called LTRS and FIGS are used by the sender If the same hex 17 is received when the de- between the mark and space frequencies is to command the decoder to switch between coder is in the FIGS shift, the number 1 will called the FSK shift, usually 170 Hz for an these two tables. be decoded instead of the Q. Modern software amateur RTTY signal. Mechanical teletypewriter keyboards have does away with the need for the operator to At the conventional data speed of 60 WPM, two keys to send the LTRS and FIGS charac- manually send the LTRS and FIGS codes. binary information modulates the FSK signal at 22 ms per bit, or equivalent to 45.45 baud. Characters are encoded into binary 5-bit Bau- dot coded data. Each character is individually synchronized by adding a start bit before the 5-bit code and by appending the code with a stop bit. The start bit has the same duration as the data bits, but the stop bit can be anywhere between 22 to 44 ms in duration. The stop bit is transmitted as a mark carrier, and the RTTY signal “rests” at this state until a new character comes along. If the number of stop bits is set to two, the RTTY signal will send a minimum of 44 ms of mark carrier before the next start bit is sent. A start bit is sent as a space carrier. A zero in the Baudot code is sent as a space signal and a one is sent as a Fig 16.1 — The character “D” sent via RTTY.

16.6 Chapter 16 When the operator sends a character from Hz) is lower in frequency, the receiver will nal. The USB modulator will then place the the LTRS table, the software first checks to be set to LSB. In general, modem tone pairs corresponding mark carrier at the higher of make sure that the previous character had also can be “reversed” (see the Inverted Signals the two FSK carrier frequencies. When LSB used a character from the LTRS table; if not, section), so either an LSB or a USB receiver transmission is used, the mark tone must be the software will first send a LTRS character. can be used. Moreover, the tone pairs of many the lower of the two audio tones in the audio Noise can often cause the LTRS or FIGS modems, especially software modems, can be FSK signal. The LSB modulator will then character to be incorrectly received. This will moved to place them where narrowband filters place the corresponding mark carrier at the cause subsequent characters to decode into are available on the receiver. higher of the two FSK carrier frequencies. As the wrong Baudot character, until a correct In the past, audio FSK demodulators were when receiving, the actual audio tones are of LTRS or FIGS is received (see also USOS built using high-Q audio filters followed by no importance. The important part of AFSK below). Instead of asking that the message a “slicer” to determine if the signal from the is to have the two audio tones separated by be repeated by the sender, a trick that many mark filter is stronger or weaker than the 170 Hz, and have the pair properly flipped to RTTY operators use is to observe that on signal that comes out of the space filter. To match the choice of USB or LSB transmis- a standard QWERTY keyboard, the Q key counter selective fading, where ionospheric sion. (See the Modulation chapter for more is below the 1 key in the row above it, the propagation can cause the mark carrier to information on USB and LSB modulation key is below the 2 key, and so on. Q and become stronger or weaker than the space and the relationship between modulating 1 happen to share the same Baudot code; carrier, the slicer’s threshold can be designed frequency and transmitted signal frequency.) W and 2 share the same Baudot code, and to adapt to the imbalance. Once “sliced” into When using AFSK with older transceiv- so forth. Given this visual aid, a printed UE a bi-level signal, the binary stream is passed ers, it is wise to choose a high tone pair so can easily be interpreted by context to 73 and to the decoder where start bit detection cir- that harmonic by-products fall outside the TOO interpreted as 599. cuitry determines where to extract the 5-bit passband of the transmitter. Because of this, If the sender uses one stop bit, an RTTY character data. That data is then passed to a popular tone pair is 2125 Hz/2295 Hz. Most character consists of a total of seven bits after the Baudot decoder, which uses the current transceivers will pass both tones and also have adding the start and stop bits. If the sender LTRS or FIGS state to determine the decoded good suppression of the harmonics of the two uses 1.5 stop bits, each RTTY character has character. The mark and space transmitted tones. Not all transceivers that have an FSK a total length of 7.5 bits. The least significant carriers do not overlap, although this can oc- input are FSK transmitters. Some transceivers bit of the Baudot code follows the start bit cur after they pass through certain HF propa- will take the FSK keying input and modulate of a character. gation conditions. Sophisticated demodula- an internal AFSK generator, which is then tors can account for this distortion. used to modulate an SSB transmitter. In this INVERTED SIGNALS A software modem performs the same case, the transmitter is really operating as an There are times when the sender does not functions as a hardware terminal unit, ex- F2B emitter. This mode of operation is often comply with the RTTY standard and reverses cept that the software modems can apply called “keyed AFSK.” the mark and space order in the spectrum. more sophisticated mathematics that would This is often called an “inverted” signal or be too expensive to implement in hardware. “SPOTTING” AN RTTY SIGNAL “reverse shift.” Most RTTY modulators and Modern desktop computers have more than By convention, RTTY signals are identi- demodulators have a provision to reverse the enough processing speed to implement an fied by the frequency of the mark carrier on shift of an inverted signal. RTTY demodulator. Software modems first the RF spectrum. “Spotting” the suppressed convert the audio signal into a sequence of DEMODULATION AND DECODING carrier frequency dial of an SSB receiver is binary numbers using an analog-to-digital useless for someone else unless they also The most common way to decode an RTTY converter which is usually part of an audio know whether the spotter is using upper or signal is to use a single sideband (SSB) re- chip set either on the motherboard or sound lower sideband and what tone pair the spot- ceiver to first translate the two FSK carri- card. Everything from that point on uses ter’s demodulator is using. The mark and ers into two audio tones. If the carriers are numerical algorithms to implement the de- space carriers are the only two constants, 170 Hz apart, the two audio tones (called modulation processes. so the amateur RTTY standard is to spot the the tone pair) will also be 170 Hz apart. The frequency of the mark carrier. RTTY demodulator, in the form of a terminal FSK VS AFSK MODULATION unit (TU) or a software modem (modulator- An RTTY signal is usually generated as an DIDDLE CHARACTERS demodulator), then works to discriminate In between the stop bit of a preceding char- between the two audio tones. Some packet F1B or F2B emission. F1B is implemented acter and the start bit of the next character, TNCs (terminal node controllers) can be made by directly shifting an RF carrier between the RTTY signal stays at the mark frequency. to function as RTTY demodulators, but often the two (mark and space) frequencies. This When the RTTY decoder is in this “rest” state, they do not work as well under poor signal- method of generating FSK is often called a mark-to-space transition tells a decoder that to-noise conditions because their filters are direct FSK, or true FSK, or simply FSK. the start of a new character has arrived. Noise matched to packet radio FSK shifts and baud F2B is implemented by shifting between two that causes a start bit to be misidentified can rate instead of to RTTY shift and baud rate. audio tones, instead of two RF carriers. cause the RTTY decoder to fall out of sync. As long as the tone pair separation is The resultant audio is then sent to an SSB 170 Hz, the frequencies of the two audio tones transmitter to become two RF carriers. This After losing sync, the decoder will use subse- can be quite arbitrary. Many TUs and modems method of first generating an audio FSK sig- quent data bits to help it identify the location are constructed to handle a range of tone pairs. nal and then modulating an SSB transmitter of the next potential start bit. If reception uses a lower sideband (LSB) re- to achieve the same FSK spectrum is usually Since all mark-to-space transitions are ceiver, the higher mark carrier will become the called AFSK (audio frequency shift keying). potential locations of the leading edge of a lower of the two audio tones. If upper sideband AFSK can be generated by using either an start bit, this can cause multiple characters to (USB) is used, the mark carrier will remain upper sideband (USB) transmitter or a low- be incorrectly decoded until proper synchro- the higher of the tone pair. It is common to er sideband (LSB) transmitter. With a USB nization is again achieved. This “character use 2125 Hz as the mark tone and 2295 Hz transmitter, the mark tone must be the higher slippage” can be minimized somewhat by not as a space tone. Since the mark tone (2125 of the two audio tones in the audio FSK sig- allowing the RTTY signal to rest for longer

Digital Modes 16.7 than its stop bit duration. An idle or diddle ror hits than a demodulator that is designed varicode bits directly modulate the PSK31 character (so called because of the sound of for 170 Hz shift. Likewise, a signal that is modulator, causing a 180° phase change if the demodulated audio from an idle RTTY transmitted using 170 Hz shift will not be the varicode bit is a 0 and keeping a constant signal sending the idle characters) is inserted optimally copied by a demodulator that is phase if the varicode bit is a 1. With QPSK31, immediately after a stop bit when the operator designed for a 200 Hz shift. the varicode bits first pass through two con- is not actively typing. The idle character is a To conserve spectrum space, amateurs volution encoders to create a sequence of bit non-printing character from the Baudot set have experimented with narrower FSK pairs (dibits). Each dibit is then used to shift and most often the LTRS character is used. shifts, down to 22.5 Hz. At 22.5 Hz, optimal the QPSK31 modulator into one of four dif- Baudot encodes a LTRS as five bits of all ones demodulators are designed as minimal shift ferent phase changes. making it particularly useful when the decoder keyed (MSK) instead of frequency shift keyed PSK63 is a double-clock-rate version of is recovering from a misidentified start bit. (FSK) demodulators. a PSK31 signal, operating at the rate of one An RTTY diddle is also useful when there symbol every 16 ms (62.5 symbols per sec- is selective fading. Good RTTY demodula- SOME PRACTICAL ond). PSK125 is a PSK31 signal clocked at tors counter selective fading by measuring CHARACTERISTICS four times the rate, with one symbol every the amplitudes of the mark and space sig- When the demodulator is properly imple- 8 ms (125 symbols per second). Although nals and automatically adjusting the decod- mented, RTTY can be very resilient against PSK63 and PSK125 are both in use, includ- ing threshold when making the decision of certain HF fading conditions, namely when ing the binary and quadrature forms, the most whether a mark or a space is being received. selective fading causes only one of the two popular PSK31 variant remains BPSK31. If a station does not transmit diddles and has FSK carriers to fade while the other carrier Most implementations of PSK31 first gen- been idle for a period of time, the receiver remains strong. However, RTTY is still sus- erate an audio PSK31 signal. The audio signal will have no idea if selective fading has af- ceptible to “flat fading” (where the mark and is then used to modulate an SSB transmitter. fected the space frequency. By transmitting space channels both fade at the same instant). Since BPSK31 is based upon phase reversals, a diddle, the RTTY demodulator is ensured There is neither an error correction scheme it can be used with either upper sideband of a measurement of the strength of the space nor an interleaver (a method of rearranging (USB) or lower sideband (LSB) systems. carrier during each character period. — interleaving — the distribution of bits to With QPSK31 however, the 90° and 270° make errors easier to correct) that can make an phase shifts have to be swapped when using UNSHIFT-ON-SPACE (USOS) RTTY decoder print through a flat and deep LSB transmitters or receivers. Since the Baudot code aliases characters fade. The lack of a data interleaver, however, (for example, Q is encoded to the same 5-bit also makes RTTY a very interactive mode. PSK31 VARICODE code as 1) using the LTRS and FIGS Baudot There is practically no latency when com- Characters that are sent in PSK31 are en- shift to steer the decoder, decoding could turn pared to a mode such as MFSK16, where coded as varicode, a variable length prefix into gibberish if the Baudot shift characters are the interleaver causes a latency of over 120 code as varicode. As described earlier, char- altered by noise. For this reason, many ama- bit durations before incoming data can even acters that occur more frequently in English teurs use a protocol called unshift-on-space be decoded. This makes RTTY attractive to text, such as spaces and the lower-case e, (USOS). Under this protocol, both the sender operating styles that have short exchanges are encoded into a fewer number of bits than and the receiver agree that the Baudot charac- with rapid turnarounds, such as in contests. characters that are less frequent in English, ter set is always shifted to the LTRS state after Although RTTY is not as “sensitive” as such as the character q and the upper case E. a space character is received. In a stream of text PSK31 (when there is no multipath, PSK31 PSK31 varicode characters always end that includes space characters, this provides has a lower error rate than RTTY when the with two bits of zeros. A space character is additional, implicit, Baudot shifts. same amount of power is used) it is not af- sent as a one followed by the two zeros (100); Not everyone uses USOS. When used fected by phase distortion that can render even a lower case e is sent as two ones, again ter- with messages that have mostly numbers and a strong PSK31 signal from being copied. minated by the two bits of zero (1100). None spaces, the use of USOS causes extra FIGS When HF propagation conditions deteriorate, of the varicode code words contain two con- characters to be sent. A decoder that com- RTTY can often function as long as suffi- secutive zero bits. Because of this, the PSK31 plies with USOS will not properly decode an cient power is used. Tuning is also moderately decoder can uniquely identify the boundary RTTY stream that does not have USOS set. uncritical with RTTY. When the signal-to- between characters. A special “character” Likewise, a decoder that has USOS turned noise ratio is good, RTTY tuning can be off in PSK31 is the idle code, which consists of off will not properly decode an RTTY stream by 50 Hz and still print well. nothing but the two prefix bits. A long pause that has USOS turned on. at the keyboard is encoded into a string of even numbers of zeros. The start of a new OTHER FSK SHIFTS AND RTTY 16.2.2 PSK31 character is signaled by the first non-zero bit BAUD RATE PSK31 is a family of modes that uses dif- received after at least two consecutive zeros. The most commonly used FSK shift in ferentially encoded varicode (see the next amateur RTTY is 170 Hz. However, on rare section), envelope-shaped phase shift key- CONVOLUTION CODE occasions stations can be found using 425 ing. BPSK31, or binary PSK31, operates at As described earlier, QPSK31 encodes the and 850 Hz shifts. The wider FSK shifts are 31.25 bit/s (one bit every 32 ms). QPSK31, varicode stream with two convolution encod- especially useful in the presence of selective or quadrature PSK31, operates at 31.25 baud. ers to form the dibits that are used to modulate fading since they provide better frequency Each symbol consists of four possible quadra- the QPSK31 generator. Both convolution en- diversity than 170 Hz. ture phase change (or dibits) at the signaling coders are fourth-order polynomials, shown Because HF packet radio uses 200 Hz rate of one dibit every 32 ms. QPSK31 sends in Fig 16.2. shifts, some TNCs use 200 Hz as the FSK a phase change symbol of 0°, 90°, 180° or The varicode data is first inverted before the shift for RTTY. Although they are mostly 270° every 32 ms. bits are given to the convolution encoders. The compatible with the 170 Hz shift protocol, Characters that are typed from the key- first polynomial generates the most significant under poor signal to noise ratio conditions board are first encoded into variable-length bit and the second polynomial generates the these demodulators will produce more er- varicode binary digits. With BPSK31, the least significant bit of the dibit. Since we are

16.8 Chapter 16 Table 16.4 QPSK31 Modulation Dibit Phase Change 00 0˚ 01 90˚ 10 180˚ 11 270˚

of one symbol and the previous one. With BPSK31, the output can be compared to a threshold to determine if a phase reversal or a non-reversal is more likely. The decoder then looks for two phase reversals in a row, followed by a non-reversal, to determine the beginning Fig 16.2 — QPSK31 convolution encoders. of a new character. The bits are gathered until two phase reversals are again seen and the accumulated bits are decoded into one of the characters in the varicode table. working with binary numbers, the GF(2) sums symbols of QPSK31, the amplitude of the QPSK31 decoding is more involved. As shown in the above figure are exclusive-OR signal does not reach zero. It dips to only half in the BPSK31 case, the phase difference functions and the delay elements x1, x2, x3 and the peak power in between the two symbols. demodulator estimates the phase change from x4 are stages of binary shift registers. As each To provide a changing envelope when the one bit to another. However, one cannot sim- inverted varicode bit is available, it is clocked operator is idle at the keyboard, a zero in ply invert the convolution function to derive into the shift register and the two bits of the the varicode (remember that an idle varicode the data dibits. Various techniques exist to dibit are computed from the shift register taps. consists of two 0 bits) is encoded as a 180° decode the measured phase angles into dibits. phase change between two BPSK31 symbols. MODULATION The Viterbi algorithm is a relatively simple A 1 in the varicode is encoded as no phase algorithm for the convolution polynomials PSK31 uses both differential phase shift change from one BPSK31 symbol to the next used in QPSK31. The estimated phase angles modulation and envelope modulation to symbol. This changing envelope allows the can first be fixed to one of the four quadra- maintain its narrow-band characteristics. The receiver to extract bit timing information even ture angles before the angles are submitted to most common way to generate an envelope- when the sender is idle. Bit clock recovery is the Viterbi algorithm. This is called a hard- shaped PSK31 signal is to start with baseband implemented by using a comb filter on the decision Viterbi decoder. in-phase (I) and quadrature (Q) signals. Both envelope of the PSK31 signal. A soft-decision Viterbi decoder (that is I and Q signals settle at either a value of The convolution code that is used by not that much more complex to construct) +1 or a value of –1. When no phase transi- QPSK31 converts a constant idle stream of usually gives better results . The soft-decision tion is needed between symbols, the I and Q zeros into a stream of repeated 10 dibits. To decoder uses arbitrary phase angles and some signals remain constant (at their original +1 produce the same constant phase change measure of how “far” an angle is away from or –1 values). See the Modulation chapter during idle periods as BPSK31, the QPSK31 one of the four quadrature angles. Error cor- for information on creating I and Q signals. 10 dibit is chosen to represent the 180° phase rection occurs within the trellis that imple- To encode a 180° transition between sym- shift modulating term. Table 16.4 shows the ments the Viterbi algorithm. References to bols, both I and Q signals are slewed with a QPSK31 modulation. convolution code, trellis and the Viterbi al- cosinusoidal envelope. If the in-phase sig- To produce bit clocks during idle, combined gorithm can be found at http://en.wikipedia. nal had a value of +1 during the previous with the particular convolution code that was org/wiki/Convolutional_code. symbol, it is slewed to –1 cosinusoidally. chosen for QPSK31, results in a slightly sub- If the in-phase signal had a value of –1 in optimal (non-Gray code) encoding of the four the previous symbols, it is slewed cosinusoi- dibits. At the end of a transmission, PSK31 16.2.3 MFSK16 dally to +1. The quadrature signal behaves stops all modulation for a short period and MFSK16 uses -ary FSK modulation with in a likewise manner. To encode a 90° phase transmits a short unmodulated carrier. The 16 “tones” (also known as 16-FSK modula- shift in QPSK31, only the in-phase signal is intended function is as a squelch mechanism. tion), where only a single tone is present at any slewed between the two symbols, the quadra- instant. MFSK16 has a crest factor of 1, with ture signal remains constant between the two DEMODULATION AND DECODING no wave shaping performed on the data bits. symbols. To encode a 270° phase shift in Many techniques are available to decode The tone centers of MFSK16 are separated by QPSK31, only the quadrature signal is slewed differential PSK, where a reference phase 15.625 Hz. Data switches at a rate of 15.625 between the two symbols; the in-phase signal is not present. Okunev has a good presenta- baud (one symbol change every 64 ms). remains constant between the two symbols. tion of the methods (reference: Yuri Okunev, Characters are first encoded into the The envelope of the real signal remains Phase and Phase-difference Modulation MFSK16 varicode, creating a constant bit constant if there is no phase change. When in Digital Communications (1997, Artech stream whose rate is one bit every 32 ms. the signal makes a 180° phase transition, the House), ISBN 0-89006-937-9). This results in a similar, although not identi- amplitude of a PSK31 signal will drop to zero As mentioned earlier, there is sufficient am- cal, character rate as PSK31. The difference in between the two symbols. The actual phase plitude information to extract the bit clock is due to the different varicode tables that are reversal occurs when the signal amplitude is from a PSK31 signal. The output of a differ- used by the two modes. zero, when there is no signal energy. Dur- ential-phase demodulator is an estimate of the This bit stream is clocked into a pair of ing the 90° and 270° phase shifts between phase angle difference between the centers 6th-order convolution encoders. The result

Digital Modes 16.9 can contain two or more consecutive zero bits, as long as the consecutive zeros are at the tail of a code word. Character boundaries are determined when two or more consecutive zeros are followed by a one. CONVOLUTION CODE As described earlier, MFSK16 encodes the varicode stream with two convolution encoders to form the dibits that are passed on to the interleaver. Both convolution encoders are sixth order polynomial, shown in Fig 16.3. The first polynomial generates the most significant bit and the second polynomial generates the least significant bit of the dibit. Since we are working with binary numbers, the GF(2) sums shown in the above figure Fig 16.3 — MFSK16 convolution encoders. are exclusive OR functions and the delay elements x1, x2, x3, x4, x5 and x6 are stages of binary shift registers. As each varicode bit is a pair of bits (dibit) for each varicode bit, is available, it is clocked into the shift register at a rate of one dibit every 32 ms. Consecu- and the two bits of the dibit are computed tive pairs of dibits are next combined into from the shift register taps. sets of four bits (quadbit or nibble). Each INTERLEAVER nibble, at the rate of one per 64 ms, is then passed through an interleaver. The interleaver HF fading channels tend to generate produces one nibble for each nibble that is “burst” errors that are clumped together. sent to it. Each nibble from the interleaver is An interleaver is used to permute the errors then Gray-coded and the resultant 4-bit Gray temporally so that they appear as uncorrelated code is the ordered index of each 16 tones that errors to the convolution decoder. MFSK16 are separated by 15.625 Hz. The result is a uses 10 concatenated stages of an IZ8BLY 16-FSK signal with a rate of one symbol per Diagonal Interleaver. More information on 64 ms. Since the symbol time is the reciprocal the interleaver can be found at www.qsl.net/ of the tone separation, a 16-point fast Fourier zl1bpu/MFSK/Interleaver.htm. Fig 16.4 transform (FFT) can be conveniently used as ­illustrates how a single IZ8BLY interleaver an MFSK16 modulator. spreads a sequence of bits. The bits enter the IZ8BLY interleaver in MFSK16 VARICODE the order 0, 1, 2, 3, 4,... and are passed to Although the varicode table that is used the output in the order 0, 5, 10, 15, 8, 13, ... by MFSK16 (see the Handbook CD) is not (shown in the diagonal boxes). In MFSK16, the same as the one used by PSK31, they the output of one interleaver is sent to the share similar characteristics. Please refer input of a second interleaver, for a total of to the PSK31 section of this chapter for a 10 such stages. Each stage spreads the bits more detailed description of varicode. Unlike out over a longer time frame. Fig 16.4 — IZ8BLY interleaver. PSK31 varicode, MSK16 varicode encodings This concatenated 10-stage interpolator

Fig 16.5 — Bit spreading through interpolation.

16.10 Chapter 16 is equivalent to a single interpolator that is of tuning accuracy and frequency stability be uniquely identified until the lowest and 123 bits long. Fig 16.5 shows the structure of and can be susceptible to multipath distor- highest of the 16 tones have passed through the single interpolator and demonstrates how tion. DominoEX specifically addresses these the receiver. This contributes to the decoding four consecutive input bits are spread evenly issues. To avoid tuning issues, IFK (incre- latency of MFSK16. There is no such latency over 123 time periods. mental frequency keying) is used. With IFK, with IFK. An error burst can be seen to be spread over the data is represented not by the frequency Since IFK depends upon frequency dif- a duration of almost two seconds. This gives of each tone, but by the frequency difference ferences and not absolute tone frequencies, MFSK16 the capability to correct errors over between one tone and the next. It also uses off- DominoEX tolerates tuning errors and drift- deep and long fades. While it is good for cor- set incremental keying to reduce inter-symbol ing signals without requiring any additional recting errors, the delay through the long in- interference caused by multipath reception. automatic frequency tracking algorithms. terleaver also causes a long decoding latency. These techniques provide a tuning tolerance of Like MFSK16, the DominoEX signal is 200 Hz and a drift of 200 Hz/minute. Domi- not wave-shaped and has constant output GRAY CODE noEX also features an optional FEC mode that power. The baud rates for DominoEX are The Gray code creates a condition where tones increases latency but provides communica- shown in Table 16.5. The tone spacings for that are next to one another are also different by tions over even more difficult channels. More DominoEX 11, DominoEX 16 and Domi- a smaller Hamming distance. This optimizes information can be found online at www.qsl. noEX 16 have the same values as their baud the error correction process at the receiver. net/zl1bpu/MFSK/DEX.htm. rates. The tone spacings for DominoEX 8, References to Gray code and Hamming dis- DominoEX uses M-ary FSK modulation DominoEX 5 and DominoEX 4 are twice the tance can be found at http://en.wikipedia.org/ with 18 tones in which only a single tone is value of their baud rates. wiki/Gray_code and http://en.wikipedia. present at any instant. Information is sent as a Unlike PSK31 and MFSK16, characters in org/wiki/Hamming_distance and http:// sequence of separate 4-bit (“nibble”) symbols. DominoEX are encoded into varicode nibbles en.wikipedia.org/wiki/Hamming_distance. The value of each nibble is represented during instead of encoding into varicode bits. The transmission as the position of the single tone. DominoEX varicode table can be found in the DEMODULATION AND DECODING The position of the tone is computed as the CD-ROM included with this book. A 16-point FFT can be used to implement difference of the current nibble value from a set of matched filters for demodulating an the nibble value of the previously transmitted BEACON MESSAGE MFSK16 signal once the input waveform is symbol. In addition, a constant offset of 2 is Instead of transmitting an idle varicode properly time aligned so that each transform applied to this difference. Because there are symbol when there is no keyboard activity, is performed on an integral symbol and the 18 tones, any possible 4-bit value between DominoEX transmits a “beacon” message signal is tuned so that the MFSK16 tones are 0 and 15 can be represented, including the from an alternate set of varicode (SECVAR perfectly centered in the FFT bins. A refer- offset of 2. columns in the DominoEX varicode table). ence to a matched filter can be found at http:// The additional offset of 2 tone positions en- This user-supplied repeating beacon message en.wikipedia.org/wiki/Matched_filter. sures that a transmitted tone is separated from is displayed at the receiving station when the The 16 output bins from an FFT demodula- the previously transmitted tone by at least sending station is not actively sending the tor can first be converted to the “best” 4-bit two tone positions. It is thus impossible for primary message. On average, the character index of an FFT frequency bin, or they can two sequential symbols to result in the same rate of the beacon channel is about half of the be converted to a vector of four numerical tone being transmitted for two sequential tone character rate of the primary channel. values representing four “soft” bits. The four periods. This means sequential tones will al- bits are then passed through an MFSK16 de- ways be different by at least two positions, an FORWARD ERROR CORRECTION interleaver. In the case of “soft decoding,” important consideration in maintaining sync. (FEC) the de-interleaver would contain numerical This minimum separation of successive When FEC is turned off, DominoEX has values rather than a 0 or 1 bit. tones of incremental frequency keying (IFK) very low decoding latency, providing an The output of the de-interleaver is passed in DominoEX reduces the inter-symbol dis- interactive quality that approaches RTTY into a convolution decoder. The Gray code tortion that results from a pulse being tem- and PSK31. The first character is decoded makes sure that adjacent FFT bins also have porally smeared when passing through an virtually instantly by the receiver after it is the lowest Hamming distance; i.e., the most HF channel. The double-tone spacing of transmitted. Because of that, FEC is not usu- likely error is also associated with the clos- DominoEX modes (see Table 16.5) further ally used even though it is available in most est FFT bin. The hard or the soft Viterbi reduces inter-symbol distortion caused by software that implements DominoEX. Algorithm can be used to decode and cor- frequency smearing. DominoEX FEC is similar to the FEC that rect errors. Incremental frequency keying allows the is used in MFSK16. When FEC is on, each DominoEX nibbles to immediately decode 4-bit IFK symbol that is decoded by the re- without having to wait for the absolute tone ceiver is split into two di-bits. The dibits enter 16.2.4 DominoEX to be identified. With MFSK16, a tone cannot an identical convolution coder to that used DominoEX is a digital mode with MFSK (multi-frequency shift keying) designed for simplex chats on HF by Murray Greenman, ZL1BPU. It was designed to be easier to Table 16.5 use and tune than other similar modes, of- Comparison of DominoEX Modes fer low latency for contesting or other quick Mode Baud/ BW Tones Speed FEC Tone Spacing exchange situations, offer reliable copy down (sec) (Hz) (WPM) (WPM) DominoEX 4 3.90625 173 18 ~25 ~12 Baud rate ×2 into the noise floor, and work well as an DominoEX 5 5.3833 244 18 ~31 ~16 Baud rate ×2 NVIS (near-vertical incidence skywave, see DominoEX 8 7.8125 346 18 ~50 ~25 Baud rate ×2 the Propagation of Radio Signals chapter) DominoEX 11 10.766 262 18 ~70 ~35 Baud rate ×1 mode for emergency communications. DominoEX 16 15.625 355 18 ~100 ~50 Baud rate ×1 Generally MFSK requires a high degree DominoEX 22 21.533 524 18 ~140 ~70 Baud rate ×1

Digital Modes 16.11 by MFSK16. However, instead of a 10-stage DSSS signal looks like nothing else than sults in superb impulse noise rejection. At IZ8BLY interleaver (see Fig 16.4), only wideband BPSK. the same time, in the frequency domain, sig- 4 cascaded stages of the basic 4-bit interleaver “The system proved to be efficient and nificant portions of the signal can be masked are present in DominoEX. In the presence we found it comparable to the other mod- by unwanted noise or other transmissions of long duration fading, the performance ern digital modes. Being totally different in without any noticeable effect on successful of the shortened interleaver is moderately its architecture, it shows better performance reception. poor when used with DominoEX 16 and during certain circumstances, while in oth- On each of the 64 tones, the transmission DominoEX 22. However, the interleaver is ers it shows no actual gain. In particular, it data rate is fairly slow, which suits the nature quite efficient in countering fading when performs better under multipath where nor- of ionospheric disturbances. Despite the low FEC is used with DominoEX 4, DominoEX mal BPSK can’t track arriving symbols, but data rate, good text speed is maintained be- 5 and DominoEX 8, with their longer symbol in quiet environments it doesn’t show any cause the text is sent on many tones at once. periods. improvement over plain BPSK. This is ex- The system runs at several different speeds, Since DominoEX works in dibit units pected because of the losses that occur due to which can be chosen to suit conditions but rather than nibble units when FEC is turned the imperfect autocorrelation of the codes.” 100 WPM is typical of the MT63-1K mode. on, it also switches to using the same binary Chip64 has a total data rate of 37.5 bit/s Although the 1 kHz bandwidth mode is typi- varicode used by MFSK16 instead of us- and the more robust Chip128 is 21.09 bit/s. cal, MT63 can also run at 500 Hz and 2 kHz ing the nibble-based varicode. DominoEX Both use the same varicode used by MFSK16. bandwidth where the tone spacing and baud does not implement Gray code as is used by The software can be downloaded from http:// rate are halved or doubled and the throughput MFSK16 FEC. xoomer.virgilio.it/aporcino/Chip64/index. is halved or doubled, respectively. Even without FEC, DominoEX works well htm and more information is available at Tuning of MT63 modes is not critical. This under many HF propagation conditions, in- www.arrl.org/technical-characteristics. is because the mode can use FEC techniques cluding the ITU NVIS (“Mid-latitude Dis- Spread spectrum is discussed in the Modu- to examine different combinations of the 64 turbed”) propagation profile. However, there lation chapter, as well. tones that calculate the correct location with- are conditions where DominoEX is not usable in the spectrum. As an example, MT63-1K unless FEC is switched on, specifically the 16.2.5 THROB will still work if the decoder is off-frequency CCIR Flutter and ITU High Latitude Moder- by as much as 100 Hz. MT63-2K requires ate Conditions profiles. DominoEX modes, Throb is an experimental mode written even less precision and can tolerate an error especially those with tone spacings that are by Lionel Sear, G3PPT, and gets the name of 250 Hz. twice the baud rate, are very robust even un- from the “throbbing” sound it makes on the The incredible noise immunity comes at a der these extreme conditions once FEC is air. It uses either single tones or pairs from a price beyond the large bandwidth required. switched on. possible nine tones spaced 8 or 16 Hz apart, There is a large latency caused by the error DominoEX performance charts (character resulting in a bandwidth of 72 or 144 Hz, correction and interleaving process. Quick- error rates versus signal-to-noise ratios) are respectively. It has three transmission speeds turnaround QSOs are not possible because included as the HTML document “Domi- — 1, 2 and 4 throbs/s — resulting in data rates there is a several second delay between typing noEX Performance” on the CD-ROM ac- of 10, 20 and 40 WPM, respectively. The the last character and it being transmitted. companying this book and online at http:// 1 and 2 throb/s speeds use a tone spacing of Without confirming each transmission homepage.mac.com/chen/Technical/Domi- 8 Hz for a 72 Hz bandwidth and the 4-throb/s with some type of ARQ mode, there is no noEX/Measurements/index.html. speed uses a spacing of 16 Hz for a 144 Hz more robust digital mode than MT63. The bandwidth. It is implemented as a stand- mode was evaluated and recommended for CHIP64/128 alone application or included in a multimode Navy MARS message handling. The evalu- Chip64 and Chip128 modes were released ­package such as MixW (www.mixw.net). ation is published on the Navy MARS web- in 2004 by Antonino Porcino, IZ8BLY. The site (www.navymars.org), along with other modes were tested on the air by IZ8BLY, 16.2.6 MT63 information on this mode. Murray Greenman, ZL1BPU, (who also con- MT63 is a mode developed by Pawel Jalo- tributed in the design of the system), Chris cha, SP9VRC. MT63 is very complex with 16.2.7 Olivia Gerber, HB9BDM, and Manfred Salzwedel, wide bandwidth, low speed and very high OH/DK4ZC. According to IZ8BLY, “The Olivia is an MFSK-based protocol de- noise immunity. By using 64 different modu- design of this new digital mode served to signed to work in difficult (low signal-to- lated tones, MT63 includes a large amount of introduce the spread spectrum technology noise ratio plus multipath propagation) extra data in the transmission of each charac- among radio amateurs by providing a com- conditions on the HF bands. The signal can ter, so that the receiving equipment can work munication tool to experiment with. Its pur- still be copied accurately at 10 dB below the out, with no ambiguity, which character was pose was to prove that it’s possible to take noise floor. Olivia was developed in 2003 sent, even if 25% of the character is obliter- advantage of the spread spectrum techniques by Pawel Jalocha, SP9VRC, and performs ated. MT63 also features a secondary channel even on the HF channels, making the com- well for digital data transfer with white noise, munication possible under conditions where that operates simultaneously with the main fading and multipath, polar path flutter and traditional narrowband modes fail. channel that can be used for an ID or beacon. auroral conditions. “Among the different possible implemen- MT63 likely has the most extensive error Olivia transmits a stream of 7-bit ASCII tations of spread spectrum, Chip64 uses the correction and can be quite processor inten- characters. The characters are sent in blocks so-called Direct Sequence Spread Spectrum sive. It uses a Walsh function that spreads the of five with each block requiring two seconds (DSSS). In a DSSS transmission, the low data bits of each 7-bit ASCII character across to transmit. This results in an effective data speed signal containing the data bits to be all 64 of the tones of the signal spectrum rate of 2.5 characters/second or 150 char- transmitted is mixed (multiplied) with a and simultaneously repeats the information acters/minute. A transmission bandwidth of greatly higher speed signal called code. The over a period of 64 symbols within any one 1000 Hz and the baud rate of 31.25 MFSK result of this mixing operation, called spread- tone. This coding takes several seconds. The tones/second, also known as Olivia 1000/32, ing, is a high-speed bit stream which is then combination of time domain (temporal) and is the most common. To adapt to different transmitted as a normal DBPSK. Indeed, a frequency domain (spectral) interleaving re- propagation conditions, the number of tones

16.12 Chapter 16 and the bandwidth can be changed and the in the first layer the orthogonal functions are a constant tone) and to minimize the chance time and frequency parameters are propor- cosine functions, with 32 different tones. Since for a false lock at the synchronizer, the char- tionally scaled. The number of tones can be 2, only one of those 32 tones is being sent at a acters encoded into the Walsh function pass 4, 8, 16, 32, 64, 128 or 256 and the bandwidth time, the demodulator measures the ampli- through a scrambler and interleaver. The re- can be 125, 250, 500, 1000 or 2000 Hz. tudes of all the 32 possible tones and identifies ceiver synchronizes automatically by search- The Olivia is constructed of two layers: the the tone with the highest amplitude. In the ing through time and frequency offsets for a lower, modulation and FEC code layer is a second layer every ASCII character is encoded matching pattern. classical MFSK while the higher layer is an as one of 64 possible Walsh functions. The More information can be found online at FEC code based on Walsh functions. More receiver again measures the amplitudes for http://n1su.com/olivia and Olivia is sup- detail on Walsh functions is available online all 64 vectors and selects the greatest as the ported in a number of digital multimode at en.wikipedia.org/wiki/Walsh_function. true value. packages such as MixW, MultiPSK and Ham Assuming Olivia 1000/32 is being used, To avoid simple transmitted patterns (like Radio Deluxe.

16.3 Fuzzy Modes There is a group of modes referred to as satellites. It features a start tone that triggers content is sent as digital data and not directly “fuzzy modes” because although they are the receiving system, originally used to allow represented in the modulation scheme.) machine generated and decoded, they are the receiving drum to come up to speed. It There are a number of different SSTV designed to be human-read. These include also includes a phasing signal with a periodic “modes” that define image resolution and color facsimile (fax), slow-scan TV (SSTV) and pulse that synchronizes the receiver so the scheme. A color image takes about 2 minutes to Hellschreiber. image appears centered on the page. A stop transmit, depending on mode. Some black and tone, optionally followed by black, indicates white modes can transmit an image in under the end of the transmission. The APT format 10 seconds. More information about SSTV 16.3.1 Facsimile (fax) is shown in Table 16.6. may be found in the Image Communications Facsimile was developed as a mechanical- Stations with Russian equipment some- supplement on the Handbook CD. ly transmitted technology where the source times use RPM 60 or 90 and sometimes an material was placed on a spinning drum and IOC of 288. Photofax transmissions such as scanned line by line into an electrical signal those from North Korea use RPM 60 and 16.3.3 Hellschreiber, which would be transmitted by wire or over an IOC 352 with gray tones, and satellite Feld-Hell or Hell the air. It is important that the receiving sta- rebroadcast use also RPM 120 IOC 576, with Hellschreiber is a facsimile-based mode tion have their drum spinning at the correct gray tones (4 or more bit depth). For software developed by Rudolph Hell in the 1920s. The speed in order to correctly recreate the image. decoding of weather fax images it is best to name is German and means “bright writer” or A value known as the index of cooperation decode with Black and White (2-bit depth). “light writer” and is a pun on the inventor’s (IOC) must also be known to decode a trans- name. In Hellschreiber, text is transmitted by mission. IOC governs the image resolution dividing each column into seven pixels and and is the product of the total line length and 16.3.2 Slow-Scan TV (SSTV) transmitting them sequentially starting at the the number of lines per unit length divided by Slow-Scan TV or SSTV is similar to fac- lowest pixel. Black pixels are transmitted as π. Most fax transmissions are sent with LPM simile where a single image is converted a signal and white as silence at 122.5 bit/s (RPM) at 120 and an IOC of 576. to individual scanned lines and those lines (about a 35 WPM text rate). Originally the Facsimile is generally transmitted in single sent as variable tones between 1500 and text was printed on continuous rolls of paper sideband with a tone of 1500 Hz represent- 2300 Hz. Modern systems use computer soft- so the message could be any length. ing black and 2300 Hz representing white. ware and a sound card to generate and receive Even though each pixel is only transmit- The automatic picture transmission (APT) the required tones. (Some SSTV communi- ted once, they are printed twice, one below format is used by most terrestrial weather cation uses purely digital protocols, such as the other. This compensates for slight timing facsimile stations and geostationary weather WinDRM described later, in which the picture errors in the equipment that causes the text to slant. If properly in sync, the text will appear as two identical rows, one below the other or a line of text in the middle with chopped lines top and bottom. Regardless of the slant, it is always possible to read one copy of the text. Table 16.6 Facsimile Automatic Picture Format Since the text is read visually, it can be sent in nearly any language and tends to look like Signal Duration IOC576 IOC288 Remarks Start Tone 5 s 300 Hz 675 Hz 200Hz for color fax modes an old dot matrix printer. More information Phasing Signal 30 s White line interrupted by can be found online at www.qsl.net/zl1bpu/ black pulse FUZZY/Feld.htm and Randy, K7AGE, has Image Variable 1200 lines 600 lines At 120 LPM a great introduction to Hellschreiber avail- Stop Tone 5 s 450 Hz 450 Hz able on YouTube at www.youtube.com/ Black 10 s watch?v=yR-EmyEBVqA.

Digital Modes 16.13 16.4 Structured Digital Modes This group of digital modes has more software will insert one if necessary. teor trail ionization. It will also account for structured data. This provides more robust Timed message sequences are a must, and Doppler shift of ± 400 Hz. JT6M includes data connections and better weak signal per- according to the procedures used by common some improvements that allow working sig- formance or more sophisticated data. Each consent in North America, the westernmost nals many dB weaker than FSK441. of these modes bundles data into packets station transmits first in each minute for 30 JT6M uses 44-tone FSK with a synchroniz- or blocks that can be transmitted and error seconds, followed by 30 seconds of receiv- ing tone and 43 possible data tones — one for checked at the receive end. ing. WSJT messages are also limited to 28 each character in the supported alphanumeric characters which are plenty to send call sign set, the same set used for FSK441. The sync and signal report information but clearly does 16.4.1 FSK441 tone is at 1076.66 Hz, and the 43 other possible not make WSJT a good ragchew mode. tones are spaced at intervals of 11025/512 = FSK441 was implemented in the WSJT The meteor scatter procedures are also 21.53 Hz up to 2002.59 Hz. Transmitted sym- software by Joe Taylor, K1JT, in 2001. well-defined and supported directly by tem- bols are keyed at a rate of 21.53 baud, so each FSK441 was designed for meteor scatter us- plates in the WSJT software. Signal reports one lasts for 1/21.53 = 0.04644 s. Every 3rd ing four-tone frequency shift keying at a rate are conventionally sent as two-digit num- symbol is the sync tone, and each sync symbol of 441 baud and was given the technical name bers. The first digit characterizes the lengths is followed by two data symbols. The transmis- FSK441. Meteor scatter presents an unusual of pings being received, on a 1-5 scale, and sion rate of user data is therefore (2/3)×21.53 problem because the signals are reflected by the second estimates their strength on a 6-9 = 14.4 characters per second. The transmitted the ionization trails left by meteors that of- scale. The most common signal reports are signal sounds a bit like piccolo music. ten last less than a second. The signals are 26 for weak pings and 27 for stronger ones, Contacts in FSK441 and JT6M are often weak, of short duration with widely varying but under good conditions reports such as 38 scheduled online on Ping Jockey at www. signal strength and include some amount of and higher are sometimes used. Whatever pingjockey.net. Doppler shift. Since these meteors enter the signal report is sent to the QSO partner, it is atmosphere constantly, this mode can be used important that it not be changed. You never any time of day or year to make contacts on know when pings will successfully convey 16.4.3 JT65 VHF between 500 and 1100 miles with a fragments of your message to the other end of JT65 is another WSJT-supported mode de- single large Yagi and 100 W. the path, and you want the received informa- signed to optimize Earth-Moon-Earth (EME) FSK441 uses a 43-character alphabet com- tion to be consistent. contacts on the VHF bands, and conforms patible with the earlier PUA43 mode by Bob More information can be found at the WSJT efficiently to the established standards and Larkin, W7PUA. This alphabet includes let- website in the user manual and other articles procedures for such QSOs. It also performs ters, numbers and six special characters and at http://pulsar.princeton.edu/~joe/K1JT. well for weak signal VHF/UHF, and for HF is shown in the Table 16.7. These characters skywave propagation. JT65 includes error- are encoded by the four tones at 882, 1323, 16.4.2 JT6M correcting features that make it very robust, 1764 and 2205 Hz and are designated 0-3 even with signals much too weak to be heard. in the table below. For example, the letter T JT6M is another WSJT mode optimized for With extended transmission duration and has the code 210 and is transmitted by send- meteor and ionospheric scatter on 6 meters. three decoders, JT65 can reliably decode 24 ing sequentially the tones at 1764, 1323 and Like FSK441, JT6M uses 30 second periods to 30 dB below the noise floor. 882 Hz. Since the modulation rate is speci- for transmission and reception to look for JT65 does not transmit messages character fied as 441 baud, or 441 bit/s, the character enhancements produced by short-lived me- by character, as done in Morse code. Instead, transmission rate is 441/3 = 147 characters whole messages are translated into unique per second. At this speed, a ping lasting 0.1 s strings of 72 bits, and from those into sequenc- can convey 15 characters of text. Four signals of 63 six-bit symbols. These symbols are (000, 111, 222 and 333) generate single solid Table 16.7 transmitted over a radio channel; some of them tones that are easily decoded and are reserved FSK441 Character Codes Character Tones Character Tones may arrive intact, while others are corrupted for the standard meteor scatter messages — 1 001 H 120 by noise. If enough of the symbols are cor- R26, R27, RRR and 73. 2 002 I 121 rect, the full 72-bit compressed message can When receiving FSK441, the receiver time 3 003 122 be recovered exactly. The decoded bits are and frequency need to be synchronized pre- 4 010 123 then translated back into the human-readable cisely. The computer clock must be set as ac- 5 011 130 curately as possible (ideally to WWV or other 6 012 M 131 message that was sent. The coding scheme 7 013 132 and robust FEC assure that messages are never time standard) before attempting to transmit 8 020 133 or receive. When the receiver hears a burst received in fragments. Message components 9 021 P 200 cannot be mistaken for one another, and call of signal the decoder attempts to identify the . 022 Q 201 correct frequency shift and align the mes- , 023 202 signs are never displayed with a few characters sage. For reasons of transmission efficiency, ? 030 S 203 missing or incorrect. There is no chance for no special synchronizing information is em- / 031 T 210 the letter O or R in a call sign to be confused # 032 U 211 bedded in an FSK441 message. Instead, the with a signal report or an acknowledgment. space 033 V 212 JT65 uses 60-second transmit-receive se- proper synchronization is established from $ 100 W 213 the message content itself, making use of the A 101 X 220 quences and carefully structured messages. facts that (a) three-bit sequences starting with B 102 Y 221 Standard messages are compressed so that 3 are never used, and (b) the space character is 103 0 223 two call signs and a grid locator can be trans- coded as 033, as shown in FSK441 character D 110 E 230 mitted with just 71 bits. A 72nd bit serves as 112 Z 231 a flag to indicate that the message consists of code table. Messages sent by WSJT always G 113 contain at least one trailing space and the arbitrary text (up to 13 characters) instead of

16.14 Chapter 16 call signs and a grid locator. Special formats tor, and transmitter power in dBm. The pro- ports the HF digital voice mode. allow other information such as call sign pre- gram can decode signals with S/N as low as The AR9800 uses a protocol developed by fixes (for example, ZA/PA2CHR) or numeri- –28 dB in a 2500 Hz bandwidth. Stations Charles Brain, G4GUO. The protocol uses cal signal reports (in dB) to be substituted for with Internet access can automatically up- the AMBE (Advanced Multi-Band Excita- the grid locator. load their reception reports to a central da- tion) codec from DVSI Inc. to carry voice. The aim of source encoding is to compress tabase called WSPRnet, which includes a It uses 2400 bit/s for voice data with an addi- the common messages used for EME QSOs mapping facility. tional 1200 bit/s for Forward Error Correction into a minimum fixed number of bits. After WSPR implements a protocol similar to for a total 3600 bit/s data stream. The protocol being compressed into 72 bits, a JT65 mes- JT65 called MEPT_JT (Manned Experimen- is detailed below: sage is augmented with 306 uniquely defined tal Propagation Tests, by K1JT). In receive • Bandwidth: 300-2500 Hz, 36 carriers error-correcting bits. The FEC coding rate is mode the program looks for all detectable • Symbol Rate: 20 ms (50 baud) such that each message is transmitted with MEPT_JT signals in a 200-Hz passband, de- • Guard interval: 4 ms a “redundancy ratio” of 5.25. With a good codes them, and displays the results. If noth- • Tone steps: 62.5 Hz error-correcting code, however, the resulting ing is decoded, nothing will be printed. In T/R • Modulation method: 36 carriers: performance and sensitivity are far superior mode the program alternates in a randomized DQPSK (3.6K) to those obtainable with simple five times way between transmit and receive sequences. • AFC: ±125 Hz message repetition. The high level of redun- Like JT65, MEPT_JT includes very efficient • Error correction: Voice: Golay and dancy means that JT65 copes extremely well and strong forward error Hamming with QSB. Signals that are discernible to the correction. Received messages are nearly • Video/Data: Convolution and Reed- software for as little as 10 to 15 seconds in always exactly the same as the transmitted Solomon a transmission can still yield perfect copy. message, or else they are left blank. • Header: 1 s; 3 tones plus BPSK train- JT65 requires tight synchronization of Basic specifications of the MEPT_JT ing pattern for synchronization time and frequency between transmitter and mode are as follows: • Digital voice: DVSI AMBE2020 receiver. Each transmission is divided into • Transmitted message: call sign + coder, decoder 126 contiguous time intervals or symbols, 4-character-locator + dBm Example: • Signal detection: Automatic Digital de- each lasting 0.372 s. Within each interval the “K1JT FN20 30” tect, Automatic switching between analog waveform is a constant-amplitude sinusoid at • Message length after lossless compres- mode and digital mode one of 65 pre-defined frequencies. Half of the sion: 28 bits for call sign, 15 for locator, 7 • Video Compression: AOR original channel symbols are devoted to a pseudo- for power level; 50 bits total. adaptive JPEG random synchronizing vector interleaved • Forward error correction (FEC): Long- AOR has more information online at www. with the encoded information symbols. The constraint convolutional code, K =32, r = 1/2. aorusa.com/ard9800.html and Amateur Ra- sync vector allows calibration of relative time • Number of channel symbols: nsym = dio Video News featured a number of digi- and frequency offsets between transmitter (50 + K – 1) × 2 = 162. tal voice modes including the AOR devices and receiver. A transmission nominally be- • Keying rate: 12000/8192 = 1.46 baud. available on DVD at www.arvideonews. gins at t = 1 s after the start of a UTC minute • Modulation: Continuous phase 4-FSK. com/dv/index.html. and finishes at t = 47.8 s. Tone separation 1.46 Hz. WSJT does its JT65 decoding in three phas- • Synchronization: 162-bit pseudo-ran- WinDRM es: a soft-decision Reed-Solomon decoder, dom sync vector. Digital Radio Mondiale (DRM) — not to the deep search decoder and the decoder • Data structure: Each channel symbol be confused with Digital Rights Manage- for shorthand messages. In circumstances conveys one sync bit and one data bit. ment — is a set of commercial digital audio involving birdies, atmospherics, or other in- • Duration of transmission: (162 × broadcasting technologies designed to work terference, operator interaction is an essential 8192)/12000 = 110.6 s over the bands currently used for AM broad- part of the decoding process. The operator • Occupied bandwidth: About 6 Hz casting, particularly shortwave. DRM can can enable a “Zap” function to excise bird- • Minimum S/N for reception: Around fit more channels than AM, at higher qual- ies, a “Clip” function to suppress broadband –27 dB on the WSJT scale (2500 Hz refer- ity, into a given amount of bandwidth, using noise spikes and a “Freeze” feature to limit ence bandwidth). various MPEG-4 codecs optimized for voice the frequency range searched for a sync tone. The current version and documentation or music or both. As a digital modulation, Using these aids and the program’s graphi- of WSPR can be found at www.physics. DRM supports data transmission in addition cal and numerical displays appropriately, the princeton.edu/pulsar/K1JT/wspr.html to the audio channels. The modulation used operator is well equipped to recognize and and WSPRnet propagation data is available by DRM is COFDM (coded orthogonal fre- discard any spurious output from the decoder. at http://wsprnet.org. quency division multiplex) with each carrier The JT65 procedures are also well-defined coded using QAM (quadrature amplitude and supported directly by templates in the modulation). The use of multiple carriers in WSJT software. More information can be 16.4.5 HF Digital Voice COFDM provides a reasonably robust signal found at the WSJT website in the user manual on HF. and other articles at http://pulsar.princeton. AOR Two members of Darmstadt University of edu/~joe/K1JT. In 2004, AOR Corporation introduced Technology in Germany, Volker Fisher and Al- its HF digital voice and data modem, the exander Kurpiers, developed an open-source AR9800. Digital voice offers a quality simi- software program to decode commercial DRM 16.4.4 WSPR lar to FM with no background noise or fading called Dream. Typically, the DRM commer- WSPR (pronounced “whisper”) imple- as long as the signal can be properly decod- cial shortwave broadcasts use 5 kHz, 10 kHz ments a protocol designed for probing po- ed. The AR9800 can alternatively transmit or 20 kHz of bandwidth (monaural or stereo) tential propagation paths with low-power ­binary files and images. AOR later released and require large bandwidths in receivers. transmissions. Each transmission carries a the AR9000 which is compatible with the Shortly after, Francesco Lanza, HB9TLK, station’s call sign, Maidenhead grid loca- AR9800 but less expensive and only sup- modified the Dream software to use the DRM

Digital Modes 16.15 technology concepts in a program that only • Fast/slow SNR estimation Table 16.8 used 2.5 kHz of bandwidth for ham radio • Microphone and speaker signal audio ALE Tones use. His software, named WinDRM, includes equalizer Frequency Data file transfer functionality and is available for • Control of transmitter PTT via RS-232 750 Hz 000 download at http://n1su.com/windrm/. levels 1000 Hz 001 • Works with one (receive only) or two 1250 Hz 011 FreeDV (transmit and receive) sound cards, for 1500 Hz 010 FreeDV is a Windows and Linux application example a built-in sound card and USB 1750 Hz 110 that allows any SSB radio to be used for low . 2000 Hz 111 2250 Hz 101 bit-rate digital voice. Speech and call sign data More details and the software can be found 2500 Hz 100 is compressed down to 1400 bit/s, which then at http://freedv.org as well as written and modulates an 1125 Hz wide QPSK signal, video setup guides. A coordinating website which is then applied to the microphone input for FreeDV QSOs is available at http://qso. tion. At the conclusion of the QSO, one of of an SSB radio. On receive, the signal is k7ve.org. the stations sends a disconnect signal to the received as SSB, then further demodulated other station, and they each return their ALE stations to the scanning mode. Some military / and decoded by FreeDV. 16.4.6 ALE FreeDV was coded from scratch by David commercial HF transceivers are available with Witten, KDØEAG, (GUI, architecture) and Automatic link establishment (ALE) was cre- ALE available internally. Amateur Radio op- David Rowe, VK5DGR, (Codec2, modem ated as a series of protocols for government erators commonly use the PCALE soundcard implementation, integration). The FreeDV users to simplify HF communications. The pro- software ALE controller, interfaced to a ham design and user interface is based on FDMDV, tocol provides a mechanism to analyze signal transceiver via rig control cable and multi- which was developed by Francesco Lanza, quality on various channels/bands and choose frequency antenna. HB9TLK. Francesco received advice on the best option. The purpose is to provide a The ALE waveform is designed to be com- modem design from Peter Martinez, G3PLX. reliable rapid method of calling and connect- patible with the audio passband of a standard Bruce Perens, K6BP has been a thought leader ing during constantly changing HF ionospheric SSB radio. It has a robust waveform for reli- on open source, patent-free voice codecs for propagation, reception interference and shared ability during poor path conditions. It consists Amateur Radio. He has inspired, promoted spectrum use of busy or congested HF chan- of 8-ary frequency-shift keying (FSK) modu- and encouraged the development of Codec2 nels. It also supports text messages with a very lation with eight orthogonal tones, a single and FreeDV. A team of reviewers and beta robust protocol that can get through even if no tone for a symbol. These tones represent three testers also supported the development of voice-quality channel can be found. bits of data, with least significant bit to the FreeDV as credited on the http://freedv.org Each radio ALE station uses a call sign right, as shown in Table 16.8. home page. or address in the ALE controller. When not The tones are transmitted at a rate of 125 FreeDV is entirely open source — even actively in communication with another sta- tones per second, 8 ms per tone. The resultant the voice codec. This makes it unique among tion, each HF SSB transceiver constantly scans transmitted bit rate is 375 bit/s. The basic ALE Amateur Radio digital voice systems that through a list of frequencies, listening for its word consists of 24 bits of information. De- typically rely on a proprietary voice codec call sign. It decodes calls and soundings sent tails can be found in Federal Standard 1045, that is not available to ham experimentation. by other stations, using the bit error rate to Detailed Requirements at www.its.bldrdoc. store a quality score for that frequency and gov/fs-1054a/ 45-detr.htm. FreeDV Design sender call sign. It would require a lot of time for the radio • Codec2 voice codec and FDMDV modem To reach a specific station, the caller sim- to go through the sequence of calling a station • 14, 50 baud QPSK voice data carriers ply enters the call sign, just like dialing a on every possible frequency to establish a link. • 1 center BPSK carrier with 2× power for phone number. The ALE controller selects Time can be decreased by using a “smarter” fast and robust synchronization the best available frequency and sends out way of predictive or synchronized linking. • 1.125 kHz spectrum bandwidth (half brief digital selective calling signals contain- With Link Quality Analysis (LQA), an ALE SSB) with 75 Hz carrier spacing ing the call signs. When the distant scanning system uses periodic sounding and linking • 1400 bit/s data rate with 1375 bit/s voice station detects the first few characters of its signals between other stations in the network coding and 25 bit/s text for call sign ID call sign, it stops scanning and stays on that to stay in touch and to predict which channel • No interleaving in time or FEC, resulting frequency. The two stations’ ALE controllers is likely to support a connection to the desired in low latency, fast synchronization and quick automatically handshake to confirm that a link station at any given time. Various stations may recovery from fades of sufficient quality is established and they are be operating on different channels, and this • 44.1 or 48 kHz sample rate, sound card ready to communicate. enables the stations to find and use a common compatible When successfully linked, the receiving open channel. station which was muted will typically emit The PCALE software developed by Key Features an audible alarm and visual alert for the re- Charles Brain, G4GUO, is available for • Waterfall, spectrum, scatter and audio ceiving operator of the incoming call. It also download at http://hflink.com/software. oscilloscope displays indicates the call sign of the linked station. The Much more ALE information and real-time • Adjustable squelch operators then can talk in a regular conversa- data is available online at http:/hflink.com.

16.16 Chapter 16 16.5 Networking Modes The modes described in this section operate allows greater flexibility (and complexity) they are the intended recipient. In a packet- using features and functions associated with when mixing features. switched network, connectionless mode computer-to-computer networking. Even As a data packet moves through these lay- transmission is a transmission in which each though communication using these modes ers the header or preamble is removed and packet is prepended with a header containing may not involve the creation of a network, the any required action performed before the data a destination address to allow delivery of the modes are referred to as “networking modes” is passed to the next layer, much like peel- packet without the aid of additional instruc- because of their structure. In cases such as ing away layers of an onion until just the tions. A packet transmitted in a connection- Winlink 2000 and D-STAR, the most com- basic clean data is left. The OSI model does less mode is frequently called a datagram. mon use is to implement a networked system not define any interfaces between layers; it In connection-oriented protocols, the sta- and those features are described along with is just a conceptual model of the functions tions about to exchange data need to first the modes and protocols used to implement required. Real-world protocols rarely imple- declare to each other they want to “establish communications within the network. ment each layer individually and often span a connection”. A connection is sometimes multiple layers. defined as a logical relationship between This description is by no means exhaustive the peers exchanging data. Connected pro- 16.5.1 OSI Networking Model and more information can be found online tocols can use a method called automatic The Open Systems Interconnection Model and in every networking textbook. Table repeat request (ARQ) to insure accurate de- or OSI Model is an abstract description for 16.10 shows the placement of commonly livery of packets using acknowledgements computer network protocol design. It defines recognized protocols within the OSI layered and timeouts. This allows the detection and seven different layers or functions performed structure. correction of corrupted packets, misdelivery, by a protocol stack. In the OSI model, the duplication, or out-of-sequence delivery of highest level is closest to the user and the the packets. lowest is closest to the hardware required 16.5.2 Connected and Connectionless modes can have error cor- to transport the data (network card and wire Connectionless Protocols rection and detection included by a higher or radio). The seven layers are described in The protocols discussed to this point have layer of the protocol but they have no mecha- Table 16.9. The modes examined previously been connectionless meaning they don’t es- nism to request a correction. An advantage implemented the protocols as a monolithic tablish a connection with a specific machine of connectionless mode over connection-ori- stack where all the functions are performed for the purpose of transferring data. Even ented mode is that it has a low data overhead. inside a single piece of code. The modes de- with packetized modes like FSK441 with a It also allows for multicast and broadcast scribed in this section implement network- destination call sign specified, the packet is (net-type) operations, which may save even ing features in a more modular fashion. This transmitted and it’s up to the user to identify more network resources when the same data needs to be transmitted to several recipients. In contrast, a connected mode is always uni- cast (point-to-point). Table 16.9 Another drawback of the connectionless OSI Seven Layer Networking Model mode is that no optimizations are possible 7 — Application Layer End-user program or “application” that uses the network when sending several frames between the 6 — Presentation Layer The format of data after transfer (code conversion, encryption) same two peers. By establishing a connection 5 — Session Layer Manages the transfer process at the beginning of such a data exchange the 4 — Transport Layer Provides reliable data transfer to the upper layers components (routers, bridges) along the net- 3 — Network Layer Controls data routing 2 — Data Link Layer Provides error detection and flow control work path would be able to pre-compute (and 1 — Physical Layer Signal used on the medium—voltage, current, frequency, etc. hence cache) routing-related information, avoiding re-computation for every packet. Many network modes incorporate both types of protocol for different purposes. In the Internet TCP/IP protocol, TCP is a connection-oriented transport protocol where Table 16.10 UDP is connectionless. Networking Protocols in the OSI Model Layer Examples IP Protocol Suite 7 — Application NNTP, DNS, FTP, 16.5.3 The Terminal Node Gopher, HTTP, DHCP, Controller (TNC) SMTP, SNMP, TELNET While a terminal node controller (TNC) 6 — Presentation ASCII, EBCDIC, MIDI, MPEG MIME, SSL is nominally an OSI Physical layer device, 5 — Session Named Pipes, NetBIOS, Half Duplex, Sockets, Session the internal firmware often implements a establishment in TCP protocol such as PACTOR that handles all Full Duplex, Simplex 4 — Transport TCP, UDP the routing, and error correction through the 3 — Network AX.25 IP, IPsec, ICMP, IGMP transport layer. This greatly simplifies the 2 — Data Link 802.3 (Ethernet), 802.11a/b/g/n MAC/LLC, PPP, SLIP, PPTP, L2TP coding of any protocol or application that ATM, FDDI, Frame Relay, HDLC, Token Ring, uses these devices. ARP (maps layer 3 to layer 2 address) A TNC is actually a computer that con- 1 — Physical RS-232, T1, 10BASE-T, 100BASE-TX, POTS, tains the protocols implemented in firmware DSL, 802.11a/b/g/n, Soundcard, TNC, Radio and a modem (modulator/demodulator). The

Digital Modes 16.17 TNC generally connects to a PC as a serial or USB device on one side and to the radio with appropriate audio and PTT cables on the other. Most of the newer rigs have dedicated data connections available that feature audio lines with fixed levels that are unaffected by settings in the radio. These jacks make swapping mike cables unnecessary when switching between voice and digital modes. Bypassing internal audio processing circuitry eliminates a number of issues that can cause problems with digital modes and makes the Fig 16.6 — PACTOR packet structure. use of digital modes more reproducible/reli- able by eliminating a number of variables when configuring equipment. These same Table 16.11 data jacks are recommended when using a PACTOR Timing computer sound card. Object Length (seconds) Although many of the modes discussed can Packet 0.96 (200 baud: 192 bits; 100 baud: 96 bits) use a computer sound card to generate the re- receive time 0.29 quired modulation and a separate mechanism Control signals 0.12 (12 bits at 10 ms each) to support push-to-talk (PTT), a TNC offers Propagation delay 0.17 some advantages: Cycle 1.25 • TNC hardware can be used with any computer platform. • A computer of nearly any vintage/per- formance level can be used. retaining the platform independence and shown in Table 16.12. Each of the signals is • Data transmission/reception is unaf- robustness of a separate TNC. The TNC-X 12-bits long. The characters differ in pairs fected by computer interruptions from virus from Coastal Chipworks at http://tnc-x.com in eight bits (Hamming offset) so that the checkers or other “inits.” is a good example of a KISS-mode TNC. chance of confusion is reduced. If the CS is • Initialization settings are held internal not correctly received, the TX reacts by re- peating the last packet. The request status can to the TNC and can easily be reset as need- 16.5.4 PACTOR-I ed — once working, they stay working. be uniquely recognized by the 2-bit packet • Virtually eliminates the computer as a PACTOR, now often referred to as PAC- number so that wasteful transmissions of pure problem/failure point. TOR-I, is an HF radio transmission system RQ blocks are unnecessary. • Offers features independent of the developed by German amateurs Hans-Peter­ The receiver pause between two blocks computer (digipeat, BBS, APRS beacon, Helfert, DL6MAA, and Ulrich Strate, is 0.29 s. After deducting the CS lengths, telemetry, weather beacon, and so forth). DF4KV. It was designed to overcome the 0.17 s remains for switching and propagation The majority of TNCs are designed for 300 shortcomings of AMTOR and packet radio. delays so that there is adequate reserve for or 1200-bit/s packet and implement the Bell It performs well under both weak-signal and DX operation. high-noise conditions. PACTOR-I has been 103 or Bell 202 modulation respectively. A CONTACT FLOW multimode communications processor (MCP) overtaken by PACTOR-II and PACTOR-III or multi-protocol controller (MPC) may offer but remains in use. In the listen mode, the receiver scans any received packets for a CRC match. This the capability to operate RTTY, CW, AM- TRANSMISSION FORMATS TOR, PACTOR, G-TOR, Clover, fax, SSTV method uses a lot of computer processing and other modes in addition to packet. Some All packets have the basic structure shown resources, but it’s flexible. of these modes are only available in TNC in Fig 16.6, and their timing is as shown in A station seeking contacts transmits CQ hardware because the real-time operating Table 16.11. packets in an FEC mode, without pauses system in the TNC provides a more reliable • Header: Contains a fixed bit pattern to for acknowledgment between packets. The platform to implement the mode and it also simplify repeat requests, synchronization and transmit time length, number of repetitions helps protect proprietary intellectual property. monitoring. The header is also important for and speed are the transmit operator’s choice. KISS-Mode TNCs have become popular. the Memory ARQ function. In each packet (This mode is also suitable for bulletins and These devices simply provide the modem that carries new information, the bit pattern other group traffic.) Once a listening station and filters to implement the baseband signals is inverted. has copied the call, the listener assumes the for a type of digital modulation. They rely • Data: Any binary information. The TX station role and initiates a contact. Thus, on the computer software to generate the format is specified in the status word. Cur- the station sending CQ initially takes the RX appropriate packet protocol and complete rent choices are 8-bit ASCII or 7-bit ASCII station role. The contact begins as shown in the mode. This means the software must be (with Huffman encoding). Characters are Table 16.13. written specifically to support these TNCs not broken across packets. ASCII RS (hex With good conditions, PACTOR’s normal by creating the entire AX.25 packet with the 1E) is used as an IDLE character in both signaling rate is 200 baud, but the system data embedded, rather than simply sending formats. automatically changes from 200 to 100 baud the data to the TNC expecting the TNC to • Status word: See Table 16.11 and back, as conditions demand. In addi- frame the packet. By leaving the TNC to • CRC: The CRC is calculated according tion, Huffman coding can further increase handle only the baseband signal generation to the CCITT standard, for the data, status the throughput by a factor of 1.7. There is no and data recovery, much simpler, smaller and and CRC. loss of synchronization speed changes; only less expensive designs are possible while still The PACTOR acknowledgment signals are one packet is repeated.

16.18 Chapter 16 as received data to reassemble defective data Table 16.12 blocks into good frames. This reduces the PACTOR Control Signals number of transmissions and increases the Code Chars (hex) Function throughput of the data. CS1 4D5 Normal acknowledge CS2 AB2 Normal acknowledge CS3 34B Break-in (forms header of first packet from RX to TX) 16.5.6 PACTOR-III CS4 D2C Speed change request PACTOR-III is a software upgrade for ex- All control signals are sent only from RX to TX isting PACTOR-II modems that provides a data transmission mode for improved speed and robustness. Both the transmitting and receiving stations must support PACTOR-III Table 16.13 for end-to-end communications using this PACTOR Initial Contact mode. Master Initiating Contact PACTOR-III’s maximum uncompressed Size (bytes) 1 8 6 speed is 2722 bit/s. Using online compres- Content /Header /SLAVECAL /SLAVECAL/ sion, up to 5.2 kbit/s is achievable. This re- Speed (baud) 100 100 200 quires an audio passband from 400 Hz to Slave Response 2600 Hz (for PACTOR-III speed level 6). The receiving station detects a call, determines mark/space polarity, and decodes 100 baud On an average channel, PACTOR-III is more and 200-bd call signs. It uses the two call signs to determine if it is being called and the quality than three times faster than PACTOR-II. On of the communication path. The possible responses are: good channels, the effective throughput ra- tio between PACTOR-III and PACTOR-II First call sign does not match slave’s call sign (Master not calling this slave) none can exceed five. PACTOR-III is also slightly Only first call sign matches slave’s call sign more robust than PACTOR-II at their lower (Master calling this slave, poor communications) CS1 SNR edges. First and second call signs both match the slaves The ITU emission designator for PAC- (good circuit, request speed change to 200 baud) CS4 TOR-III is 2K20J2D. Because PACTOR-III builds on PACTOR-II, most specifications like frame length and frame structure are adopted from PACTOR-II. The only sig- When the RX receives a bad 200-baud and Markov compression and powerful Vit- nificant difference is PACTOR III’s multi- packet, it can acknowledge with CS4. TX erbi decoding to increase transfer rate and tone waveform that uses up to 18 carriers immediately assembles the previous packet sensitivity into the noise level. The effective while PACTOR-II uses only two carriers. in 100-baud format and sends it. Thus, one transfer rate of text is over 1200 bit/s. Features PACTOR-III’s carriers are located in a packet is repeated in a change from 200 to of PACTOR II include: 120 Hz grid and modulated with 100 sym- 100 baud. • Frequency agility — it can automati- bols per second DBPSK or DQPSK. Channel The RX can acknowledge a good 100-baud cally adjust or lock two signals together coding is also adopted from PACTOR-II’s packet with CS4. TX immediately switches over a ±100 Hz window. Punctured Convolutional Coding. to 200 baud and sends the next packet. There • Powerful data reconstruction based is no packet repeat in an upward speed change. upon computer power — with over PACTOR-III Link Establishment The RX station can become the TX station 2 Mbyte of available memory. The calling modem uses the PACTOR-I by sending a special change-over packet in • Cross correlation — applies analog FSK connect frame for compatibility. When response to a valid packet. RX sends CS3 as Memory ARQ to acknowledgment frames the called modem answers, the modems ne- the first section of the changeover packet. and headers. gotiate to the highest level of which both This immediately changes the TX station to • Soft decision making — Uses arti- modems are capable. If one modem is only RX mode to read the data in that packet and ficial intelligence (AI), as well as digital capable of PACTOR-II, then the 500 Hz responds with CS1 and CS3 (acknowledge) information received to determine frame PACTOR-II mode is used for the session. or CS2 (reject). validity. With the MYLevel (MYL) command a user PACTOR provides a sure end-of-contact • Extended data block length — when may limit a modem’s highest mode. For ex- procedure. TX initiates the end of contact transferring large files under good condi- ample, a user may set MYL to 1 and only by sending a special packet with the QRT bit tions, the data length is doubled to increase a PACTOR-I connection will be made, set set in the status word and the call of the RX the transfer rate. to 2 and PACTOR-I and II connections are station in byte-reverse order at 100 baud. The • Automatic recognition of PACTOR- available, set to 3 and PACTOR-I through III RX station responds with a final CS. I, PACTOR-II and so on, with automatic connections are enabled. The default MYL is mode switching. set to 2 with the current firmware and with • Intermodulation products are canceled PACTOR-III firmware it will be set to 3. If 16.5.5 PACTOR-II by the coding system. a user is only allowed to occupy a 500 Hz This is a significant improvement over • Two long-path modes extend frame channel, MYL can be set to 2 and the modem PACTOR-I, yet it is fully compatible with timing for long-path terrestrial and satellite will stay in its PACTOR-II mode. the older mode. PACTOR-II uses 16PSK to propagation paths. The PACTOR-III Protocol Specification is transfer up to 800 bit/s at a 100 baud rate. This is a fast, robust mode that has excellent available online at www.scs-ptc.com/pactor. This keeps the bandwidth less than 500 Hz. coding gain as well. PACTOR-II stations ac- html. More information can also be found PACTOR-II uses digital signal processing knowledge each received transmission block. online at www.arrl.org/technical-charac- (DSP) with Nyquist waveforms, Huffman PACTOR-II employs computer logic as well teristics or in ARRL’s HF Digital Handbook

Digital Modes 16.19 by Steve Ford, WB8IMY. dundancy from source data. Therefore, fewer when using AX.25 packet radio on HF radio The protocol specifications and equipment bits are needed to convey any given message. led Ray Petit, W7GHM, to develop a unique for PACTOR-IV have been released, but the This increases data throughput and decreases modulation waveform and data transfer pro- mode is not yet legal for US amateurs. The transmission time — valuable features for tocol that is now called CLOVER-II. Bill symbol rate for PACTOR-IV is 1800 baud, HF. G-TOR uses run-length encoding and Henry, K9GWT, supplied this description of but FCC rules limit US amateurs to 300 baud two types of Huffman coding during normal the CLOVER-II system. below the upper end of 10 meters. PACTOR- text transmissions. Run-length encoding is CLOVER modulation is characterized by IV is being used outside the US by individual used when more than two repetitions of an the following key parameters: amateurs and by Winlink stations that are not 8-bit character are sent. It provides an es- • Very low base symbol rate: 31.25 sym- subject to FCC rules. It is not known if or when pecially large savings in total transmission bols/second (all modes). this restriction will be lifted. time when repeated characters are being • Time-sequence of amplitude-shaped transferred. pulses in a very narrow frequency spectrum. • Occupied bandwidth = 500 Hz at 16.5.7 G-TOR The Huffman code works best when the sta- tistics of the data are known. G-TOR applies 50 dB below peak output level. This brief description has been adapted Huffman A coding with the upper- and lower- • Differential modulation between from “A Hybrid ARQ Protocol for Narrow case character set, and Huffman B coding with pulses. Bandwidth HF Data Communication” by upper-case-only text. Either type of Huffman • Multilevel modulation. Glenn Prescott, WBØSKX, Phil Anderson, code reduces the average number of bits sent The low base symbol rate is very resistant WØXI, Mike Huslig, KBØNYK, and Karl per character. In some situations, however, to multipath distortion because the time be- Medcalf, WK5M (May 1994 QEX). there is no benefit from Huffman coding. The tween modulation transitions is much lon- G-TOR is short for Golay-TOR, an inno- encoding process is then disabled. This deci- ger than even the worst-case time-smearing vation of Kantronics. It was inspired by HF sion is made on a frame-by-frame basis by the caused by summing of multipath signals. By automatic link establishment (ALE) concepts information sending station. using a time-sequence of tone pulses, Dolph- and is structured to be compatible with ALE. The real power of G-TOR resides in the Chebychev “windowing” of the modulating The purpose of the G-TOR protocol is to properties of the (24, 12) extended Golay er- signal and differential modulation, the total provide an improved digital radio communi- ror-correcting code, which permits correction occupied bandwidth of a CLOVER-II signal cation capability for the HF bands. The key of up to three random errors in three received is held to 500 Hz. features of G-TOR are: bytes. The (24, 12) extended Golay code is a Multilevel tone, phase and amplitude mod- • Standard FSK tone pairs (mark and half-rate error-correcting code: Each 12 data ulation gives CLOVER a large selection of space) bits are translated into an additional 12 parity data modes that may be used (see Table 16.14). • Link-quality-based signaling rate: 300, bits (24 bits total). Further, the code can be The adaptive ARQ mode of CLOVER senses 200 or 100 baud implemented to produce separate input-data current ionospheric conditions and automati- • 2.4-s transmission cycle and parity-bit frames. cally adjusts the modulation mode to produce • Low overhead within data frames The extended Golay code is used for maximum data throughput. When using the • Huffman data compression — two G-TOR because the encoder and decoder Fast bias setting, ARQ throughput automati- types, on demand are simple to implement in software. Also, cally varies from 11.6 byte/s to 70 byte/s. • Embedded run-length data compression Golay code has mathematical properties The CLOVER-II waveform uses four • Golay forward-error-correction coding that make it an ideal choice for short-cycle tone pulses that are spaced in frequency by • Full-frame data interleaving synchronous communication. More informa- 125 Hz. The time and frequency domain char- • CRC error detection with hybrid ARQ tion can also be found online at www.arrl. acteristics of CLOVER modulation are shown • Error-tolerant “Fuzzy” acknowledg- org/technical-characteristics or in ARRL’s in Figs 16.7, 16.8 and 16.9. The time-domain ments. HF Digital Handbook by Steve Ford, shape of each tone pulse is intentionally Since one of the objectives of this protocol WB8IMY. shaped to produce a very compact frequency is ease of implementation in existing TNCs, spectrum. The four tone pulses are spaced in the modulation format consists of standard time and then combined to produce the com- tone pairs (FSK), operating at 300, 200 or 16.5.8 CLOVER-II posite output shown. Unlike other modulation 100 baud, depending upon channel conditions. The desire to send data via HF radio at schemes, the CLOVER modulation spectrum G-TOR initiates contacts and sends ACKs high data rates and the problem encountered is the same for all modulation modes. only at 100 baud. The G-TOR waveform con- sists of two phase-continuous tones (BFSK), spaced 200 Hz apart (mark = 1600 Hz, space = 1800 Hz); however, the system can still op- Table 16.14 erate at the familiar 170 Hz shift (mark = 2125 CLOVER-II Modulation Modes Hz, space = 2295 Hz), or with any other con- As presently implemented, CLOVER-II supports a total of seven venient tone pairs. The optimum spacing for different modulation formats: five using PSM and two using a 300-baud transmission is 300 Hz, but you combination of PSM and ASM (Amplitude Shift Modulation). trade some performance for a narrower band- Name Description In-Block Data width. Rate Each transmission consists of a synchro- 16P4A 16 PSM, 4-ASM 750 bps nous ARQ 1.92-s frame and a 0.48-s inter- 16PSM 16 PSM 500 bps val for propagation and ACK transmissions 8P2A 8 PSM, 2-ASM 500 bps (2.4 s cycles). All advanced protocol features 8PSM 8 PSM 375 bps QPSM 4 PSM 250 bps are implemented in the signal-processing soft- BPSM Binary PSM 125 bps ware. 2DPSM 2-Channel Diversity BPSM 62.5 bps Data compression is used to remove re-

16.20 Chapter 16 Table 16.15 Data Bytes Transmitted Per Block Block Reed-Solomon Encoder Efficiency Size 60% 75% 90% 100% 17 8 10 12 14 51 28 36 42 48 85 48 60 74 82 255 150 188 226 252

Table 16.16 Correctable Byte Errors Per Block Block Reed-Solomon Encoder Efficiency Size 60% 75% 90% 100% 17 1 1 0 0 51 9 5 2 0 85 16 10 3 0 255 50 31 12 0

ciency” being the approximate ratio of real data bytes to total bytes sent). 60% efficiency corrects the most errors but has the lowest net data throughput. 100% efficiency turns Fig 16.7 — Amplitude vs time plots for CLOVER-II’s four-tone waveform. the encoder off and has the highest through- put but fixes no errors. There is therefore a tradeoff between raw data throughput versus Data is modulated on a CLOVER-II signal pulse. Further, the phase state is changed only the number of errors that can be corrected by varying the phase and/or amplitude of the while the pulse amplitude is zero. Therefore, without resorting to retransmission of the tone pulses. Further, all data modulation is the wide frequency spectra normally associat- entire data block. differential on the same tone pulse — data is ed with PSK of a continuous carrier is avoided. Note that while the In Block Data Rate represented by the phase (or amplitude) differ- This is true for all CLOVER-II modulation for- numbers listed in the table go as high as 750 ence from one pulse to the next. For example, mats. The term phase-shift modulation (PSM) bit/s, overhead reduces the net throughput when binary phase modulation is used, a data is used when describing CLOVER modes to or overall efficiency of a CLOVER trans- change from 0 to 1 may be represented by a emphasize this distinction. mission. The FEC coder efficiency setting change in the phase of tone pulse one by 180° CLOVER-II has four “coder efficiency” and protocol requirements of FEC and ARQ between the first and second occurrence of that options: 60%, 75%, 90% and 100% (“effi- modes add overhead and reduce the net ef-

Fig 16.8 — A frequency-domain plot of a CLOVER-II waveform. Fig 16.9 — Spectra plots of AMTOR, HF packet-radio and CLOVER-II signals.

Digital Modes 16.21 ficiency. Tables 16.15 and 16.16 detail the apply for CLOVER-2000, except for the higher 16.5.10 WINMOR relationships between block size, coder ef- data rates inherent to CLOVER-2000. Just like While the various PACTOR modes cur- ficiency, data bytes per block and correctable CLOVER, all data sent via CLOVER-2000 rently dominate and generally represent the byte errors per block. is encoded as 8-bit data bytes and the error- best available performance HF ARQ pro- With seven different modulation formats, correction coding and modulation formatting tocols suitable for digital messaging, PC four data-block lengths (17, 51, 85 or 255 processes are transparent to the data stream sound cards with appropriate DSP software bytes) and four Reed-Solomon coder effi- — every bit of source data is delivered to the can now begin to approach PACTOR perfor- ciencies (60%, 75%, 90% and 100%), there receiving terminal without modification. mance. The WINMOR (Winlink Message are 112 (7 × 4 × 4) different waveform modes Control characters and special “escape se- Over Radio) protocol is an outgrowth of the that could be used to send data via CLOVER. quences” are not required or used by CLO- work SCAMP (Sound Card Amateur Mes- Once all of the determining factors are con- VER-2000. Compressed or encrypted data sage Protocol) by Rick Muething, KN6KB. sidered, however, there are eight different may therefore be sent without the need to SCAMP put an ARQ “wrapper” around Barry waveform combinations that are actually insert (and filter) additional control charac- Sanderson’s RDFT (Redundant Digital File used for FEC and/or ARQ modes. ters and without concern for data integrity. Transfer) then integrated SCAMP into a Five different types of modulation may be Client and Server for access to the Winlink used in the ARQ mode — BPSM (Binary 16.5.9 CLOVER-2000 message system. (More on Winlink in a later Phase Shift Modulation), QPSM (Quadrature section.) SCAMP worked well on good chan- CLOVER-2000 is a faster version of PSM), 8PSM (8-level PSM), 8P2A (8PSM nels but suffered from the following issues: CLOVER (about four times faster) that uses + 2-level Amplitude-Shift Modulation) and The RDFT batch-oriented DLLs eight tone pulses, each of which is 250 Hz • 16P4A (16 PSM plus 4 ASM). were slow and required frame pipelining, wide, spaced at 250 Hz centers, contained The same five types of modulation used in increasing complexity and overhead. within the 2 kHz bandwidth between 500 and ARQ mode are also available in Broadcast • RDFT only changed the RS encod- 2500 Hz. The eight tone pulses are sequen- (FEC) mode, with the addition of 2-Chan- ing on its 8PSK multi carrier waveform tial, with only one tone being present at any nel Diversity BPSM (2DPSM). Each CCB is to achieve a 3:1 range in speed/robustness instant and each tone lasting 2 ms. Each frame sent using 2DPSM modulation, 17-byte block which is not enough. consists of eight tone pulses lasting a total of size and 60% bias. The maximum ARQ data • RDFT was inefficient in Partial Frame 16 ms, so the base modulation rate of a CLO- throughput varies from 336 bit/s for BPSM to recovery (no memory ARQ). VER-2000 signal is always 62.5 symbols per 1992 bit/s for 16P4A modulation. BPSM is • RDFT was a 2.4 kHz mode and limited second (regardless of the type of modulation most useful for weak and badly distorted data to narrow HF sub bands. being used). CLOVER-2000’s maximum raw signals, while the highest format (16P4A) • SCAMP’s simple multi-tone ACK/ data rate is 3000 bit/s. needs extremely good channels, with high NAK did not carry session ID info, increas- Allowing for overhead, CLOVER-2000 SNRs and almost no multipath. ing chances of fatal cross session contami- can deliver error-corrected data over a stan- Most ARQ protocols designed for use with nation. dard HF SSB radio channel at up to 1994 bit/s, HF radio systems can send data in only one WINMOR is an ARQ mode generated or 249 characters (8-bit bytes) per second. direction at a time. CLOVER-2000 does not from the ground up to address the limita- These are the uncompressed data rates; the need an OVER command; data may flow in tions of SCAMP/RDFT and leverage what maximum throughput is typically doubled for either direction at any time. The CLOVER was learned. Today, a viable message system plain text if compression is used. The effec- ARQ time frame automatically adjusts to (with the need for compression and binary at- tive data throughput rate of CLOVER-2000 match the data volume sent in either or both tachments) requires true “error-free” delivery can be even higher when binary file transfer directions. When first linked, both sides of of binary data. To achieve this there must be mode is used with data compression. the ARQ link exchange information using some “back channel” or ARQ so the receiv- The binary file transfer protocol used by six bytes of the CCB. When one station has ing station can notify the sender of lost or HAL Communications operates with a ter- damaged data and request retransmission or minal program explained in the HAL E2004 a large volume of data buffered and ready to send, ARQ mode automatically shifts to an repair. Table 16.17 outlines the guidelines engineering document. Data compression al- used in the development of WINMOR. gorithms tend to be context sensitive — com- expanded time frame during which one or more 255 byte data blocks are sent. Perhaps the most challenging of these re- pression that works well for one mode (say, quirements are: text), may not work well for other data forms If the second station also has a large volume of data buffered and ready to send, its half • The ability to quickly tune, lock and (graphics, for example). The HAL terminal acquire the signal which is necessary for program uses the PK-WARE compression of the ARQ frame is also expanded. Either or both stations will shift back to CCB level practical length ARQ cycles in the 2-6 s algorithm, which has proved to be a good range. general-purpose compressor for most com- when all buffered data has been sent. This • The ability to automatically adapt the puter files and programs. Other algorithms feature provides the benefit of full-duplex data transfer but requires use of only simplex modulation scheme to changing channel may be more efficient for some data formats, conditions. An excellent example of this is particularly for compression of graphic im- frequencies and half-duplex radio equipment. This two-way feature of CLOVER can also Pactor III’s extremely wide range of speed/ age files and digitized voice data. The HAL robustness (18:1) and is one reason it is Communications CLOVER-2000 modems provide a back-channel “order-wire” capabil- ity. Communications may be maintained in such an effective mode in both good and can be operated with other data compression poor channel conditions. algorithms in the users’ computers. this chat mode at 55 WPM, which is more than adequate for real-time keyboard-to-keyboard The most recent development effort has CLOVER-2000 is similar to the previous focused on 62.5 baud BPSK, QPSK and version of CLOVER, including the transmis- communications. More information can also be found at 16QAM and 31.25-baud 4FSK using 1 sion protocols and Reed-Solomon error detec- (200 Hz), 3 (500 Hz) and 15 (2000 Hz). With tion and correction algorithm. The original www.arrl.org/technical-characteristics or in ARRL’s HF Digital Handbook by Steve carriers spaced at twice the symbol rate. descriptions of the CLOVER Control Block These appear to offer high throughput and (CCB) and Error Correction Block (ECB) still Ford, WB8IMY.

16.22 Chapter 16 Table 16.17 WINMOR Development Guidelines Absolute Requirements Desirable Requirements Work with standard HF (SSB) radios Modest CPU & OS demands Accommodate Automatic Connections Bandwidth options (200, 500, 3000 Hz) Error-free transmission and confirmation Work with most sound cards/interfaces Fast Lock for practical length ARQ cycles Good bit/s/Hz performance ~ P2 goal Auto adapt to a wide range of changing channel conditions Efficient modulation and demodulation for acceptable ARQ latency Must support true transparent binary to allow attachments Selective ARQ & memory ARQ to maximize throughput and robustness. and compression Must use loosely synchronous ARQ timing to accommodate Near Pactor ARQ efficiency (~70% of raw theoretical throughput) OS and DSP demands

robustness especially when combined with which would render QAM modes poor in Hz is possible in fair to good channels multi-level FEC coding. fading channels. (0.68 - 0.82 bit/s/Hz measured) WINMOR uses several mechanisms for In trying to anticipate how WINMOR • Good ARQ efficiency — 70-75% error recovery and redundancy. might be integrated into applications they • Throughput is currently competitive 1) FEC data encoding currently using: came up with a “Virtual TNC” concept. This with P2 and P3 and significantly better than • 4,8 Extended Hamming Dmin = 4 essentially allows an application to integrate P1 (used in ACK and Frame ID) the WINMOR protocol by simply treating More information about WINMOR will • 16-bit CRC for data verification the WINMOR code as just another TNC and be available as it develops at www.winlink. Two-level Reed-Solomon (RS) FEC for writing a driver for that TNC. Like all TNCs org/WINMOR. data: there are some (<10) parameters to set up: call • First level Weak FEC, for example RS sign, timing info, sound card, keying mecha- 140,116 (corrects 12 errors) nism, etc. A sample image of the virtual TNC 16.5.11 Packet Radio • Second level Strong FEC, for example appears in Fig 16.10. Amateur packet radio began in Canada RS 254,116 (corrects 69 errors) The WINMOR software DLL can even after the Canadian Department of Commu- 2) Selective ARQ. Each carrier’s data con- be made to appear as a physical TNC by nications permitted amateurs to use the mode tains a Packet Sequence Number (PSN). The “wrapping” the DLL with code that accesses in 1978. The FCC permitted amateur packet ACK independently acknowledges each PSN it through a virtual or a TCP/IP radio in the US in 1980. In the first half of so only carriers with failed PSNs get repeated. port. Like a physical TNC WINMOR has a the 1980s, packet radio was the habitat of a The software manages all the PSN accounting “front panel” with flashing lights. But since small group of experimenters who did not and re-sequencing. operation is automatic with no front panel mind communicating with a limited number 3) Memory ARQ. The analog phase and user interaction required the WINMOR TNC of potential fellow packet communicators. amplitude of each demodulated symbol is can be visible or hidden. In the second half of the decade, packet ra- saved for summation (phasor averaging) over WINMOR looks promising and the testing dio took off as the experimenters built a net- multiple frames. Summation is cleared and re- to date confirms: work that increased the potential number of started if max count reached. Reed-Solomon • Sound card ARQ is possible with a mod- packet stations that could intercommunicate FEC error decoding done after summation. ern CPU and OS while making acceptable and thus attracted tens of thousands of com- 4) Multiple Carrier Assignment (MCA). CPU processing demands. (CPU Loading municators who wanted to take advantage of The same PSN can be assigned to multiple of < 20% on a 1.5 GHz Celeron/Win XP) this potential. carriers (allows tradeoff of throughput for • Throughput and robustness can be Packet radio provides error-free data trans- robustness). Provides an automatic mecha- adjusted automatically to cover a wide fer via the AX.25 protocol. The receiving nism for frequency redundancy and protec- range of bandwidth needs and channel station receives information exactly as the tion from interference on some carriers. conditions. (10:1 bandwidth range, 57:1 transmitting station sends it, so you do not 5) Dynamic threshold adjustment (used on throughput range) waste time deciphering communication er- QAM modes) helps compensate for fading • ARQ throughput in excess of 0.5 bit/s/ rors caused by interference or changes in propagation. This simplifies the effort re- quired to build other protocols or applications on top of packet technology since much of the data transfer work is handled seamlessly by the TNC in the lower layers. There are a number of other advantages to the packet radio design: • Packet uses time efficiently, since - et bulletin-board systems (PBBSs) permit packet operators to store information for later retrieval by other amateurs. • Packet uses the radio spectrum efficient- ly, since one radio channel may be used for multiple communications simultaneously, or one radio channel may be used to intercon- nect a number of packet stations to form a Fig 16.10 — WINMOR sound card TNC screen. “cluster” that provides for the distribution

Digital Modes 16.23 protocol with a ham in a distant city and rest assured that you’re not overburdening the sys- tem. Your packets simply join the constantly moving “freeway” of data. They might slow down in heavy traffic, but they will reach their destination eventually. This adaptive system is used for all TCP/IP packets, no matter what they contain. TCP/IP excels when it comes to transfer- ring files from one station to another. By using the TCP/IP File Transfer Protocol (FTP), you can connect to another station and transfer computer files — including software. As might be imagined, transferring large files can take time. With TCP/IP, however, you can still send and receive mail (using the SMTP protocol) or talk to another ham while the transfer is taking place. When you attempt to contact another sta- tion using TCP/IP, all network routing is per- formed automatically according to the TCP/ IP address of the station you’re trying to reach. In fact, TCP/IP networks are transparent to the average user. To operate TCP/IP, all you Fig 16.11 — DX spotting clusters (based on PacketCluster software) are networks need is a computer, a 2 meter FM transceiver comprised of individual nodes and stations with an interest in DXing and contesting. and a TNC with KISS (keep it simple, stupid) In this example, N1BKE is connected to the KC8PE node. If he finds a DX station on capability. As you might guess, the heart of the air, he’ll post a notice — otherwise known as a spot — which the KC8PE node your TCP/IP setup is software. The TCP/IP distributes to all its local stations. In addition, KC8PE passes the information along to the W1RM node. W1RM distributes the information and then passes it to the KR1S node, software set was written by Phil Karn, KA9Q, which does the same. Eventually, WS1O — who is connected to the KR1S node — sees and is called NOSNET or just NOS for short. the spot on his screen. Depending on the size of the network, WS1O will receive the There are dozens of NOS derivatives available information within minutes after it was posted by N1BKE. Many such networks can also today. All are based on the original NOSNET. be found on the web or are accessible by the use of TELNET software. NOS takes care of all TCP/IP functions, using your “KISSable” TNC to communicate with the outside world. The only other item neces- sary is your own IP address in Network 44, of information to all of the clustered stations. in the AX.25 Level 2 protocol firmware. The termed AMPRNet (AMateur Packet Radio The DX PacketCluster nodes are typical ex- digipeater function is handy when you need a Network). Individual IP Address Coordina- amples (see Fig 16.11). Each local channel relay and no node is available, or for on-the- tors assign addresses to new TCP/IP users in may be connected to other local channels to air testing. Digipeaters are used extensively the 44.x.x.x subnet based on physical loca- form a network that affords interstate and with APRS (Automatic Packet Reporting tion worldwide. Your local coordinator can international data communications. This System) covered below. be found on AMPR.org on the list at http:// network can be used by interlinked packet TCP/IP noh.ucsd.edu/~brian/amprnets.txt. bulletin-board systems to transfer informa- More packet information can be found in tion, messages and third party traffic via HF, Despite its name, TCP/IP (Transmission the annual ARRL/TAPR Digital Communi- VHF, UHF, satellite and the Internet. Control Protocol/Internet Protocol) is more cations Conference proceedings and on the • Packet uses other stations efficiently, than two protocols; it’s actually a set of several TAPR (Tucson Amateur Packet Radio) web- since any packet-radio station can use one protocols. TCP/IP provides a standardized set site at www.tapr.org. TAPR also maintains or more other packet-radio stations to relay of protocols familiar to many and compatible the AX.25 protocol specification at www. data to its intended destination. with existing network technologies and appli- tapr.org/pub_ax25.html. • Packet uses current station transmitting cations. TCP/IP has a unique solution for busy and receiving equipment efficiently, since the networks. Rather than transmitting packets same equipment used for voice communica- at randomly determined intervals, TCP/IP 16.5.12 APRS tions may be used for packet communica- stations automatically adapt to network de- APRS (http://aprs.org) was developed by tions. The outlay for the additional equipment lays as they occur. As network throughput Bob Bruninga, WB4APR, for tracking and necessary to add packet capability to a voice slows down, active TCP/IP stations sense digital communications with mobile GPS station may be less than $100. the change and lengthen their transmission equipped stations with two-way radio. APRS delays accordingly. As the network speeds DIGIPEATERS is different from regular packet in four ways: up, the TCP/IP stations their delays • Integration of maps and other data dis- A digipeater is a packet-radio station ca- to match the pace. This kind of intelligent plays to organize and display data pable of recognizing and selectively repeat- network sharing virtually guarantees that all • By using a one-to-many protocol to up- ing packet frames. An equivalent term used in packets will reach their destinations with the date everyone in real time the network industry is bridge. Virtually any greatest efficiency the network can provide. • By using generic digipeating so that prior TNC can be used as a single-port digipeater, With TCP/IP’s adaptive networking knowledge of the network is not required because the digipeater function is included scheme, you can chat using the TELNET • Provides worldwide transparent Internet

16.24 Chapter 16 backbone, linking everyone worldwide will provide some idea how busy the channel ing the D-STAR radio positions to APRS-IS. APRS turns packet radio into a real-time is. In much of the country the APRS network Those positions can be gated to local RF by tactical communications and display system is well developed and a beacon doesn’t need APRS IGates if the APRS IGate sysop en- for emergencies, public service applications more than a couple hops to cover a wide area ables this feature. and global communications. Normal packet and find an IGate. Keith Sproul, WU2Z, operates an e-mail radio has shown usefulness in passing bulk gateway that allows APRS messages to be message traffic (e-mail) from point to point. APRS SOFTWARE converted to short e-mail or text messages. It has been difficult to apply conventional There is APRS software available for most When entering a message, E-MAIL is used for packet to real time events where information any platform with varying levels of support. the destination address. The message body has a very short life time and needs to get to Some of the most common are DOSAPRS must then contain the actual e-mail address everyone. (DOS), WinAPRS / APRS+SA / APRSPoint followed by a space and very short message. The APRS network consists of five types / UI-View (Windows), MacAPRS (Macin- If the message is picked up by an IGate it of APRS stations: tosh), PocketAPRS (Palm devices), APRSce will be sent to the WU2Z mail server and out • Basic — Full transmit and receive ca- (Windows CE devices) and Xastir / X-APRS to the recipient, with a confirmation APRS pability (Linux). Because APRS beacons find their message. For dozens of other messaging op- • Tracker — Transmit-only device (por- way to the Internet via IGates, it is also pos- tions, check online at http://aprs.org/aprs- table or weather station) sible to track station or view the network on- messaging.html. • RELAY — Provides basic digipeating line. There is a good network view available There is connectivity between APRS • WIDE — Dedicated digipeaters with at http://aprs.fi and individual station infor- and the Winlink system via APRSLink. specific coverage areas mation can be found at http://map.findu. APRSLink monitors all APRS traffic gated • IGate — Internet gateways to repeat com/call sign. The Findu.com site also offers to the Internet, worldwide, and watches for APRS packets to servers on the Internet historical weather data, a message display and special commands that allow APRS users APRS stations are generally set to beacon a large list of other queries. to: their position and other information at fre- • Read short e-mail messages sent to their quent intervals. Fixed stations don’t require a APRS INTEGRATION WITH OTHER call [email protected] GPS and should beacon infrequently to avoid TECHNOLOGIES • Send short e-mail messages to any valid crowding the channel with needless reports. APRS is very similar to the AIS (Auto- e-mail address or Winlink 2000 user Moving mobile stations can receive their co- matic Identification System) used to track • Perform e-mail related maintenance (see ordinates from a GPS and should report more ships in coastal waters. The AIS transpon- commands below) frequently. More sophisticated APRS devices der equipment is becoming more common • Be notified of pending Winlink e-mail support “smart beaconing” and “corner pin- in commercial ships and private vessels. The via APRS message ning” where a station will beacon more fre- AIS information can be displayed simultane- • Query APRSLink for information on the quently when it moves faster or automatically ously with APRS data on the aprs.fi website. closest Winlink RMS packet station beacon when changing direction more than More information about AIS can be found at Attention must be paid to the APRS traffic a predefined angle. An APRS beacon will http://en.wikipedia.org/wiki/Automatic_ generated when using these features but they generally contain a call sign with Second- Identification_System. can be very handy. Details on the APRSLink ary Station ID (SSID), position report (lat/ APRS RF networks worldwide are inter­ system are available at www.winlink.org/ long), heading, speed, altitude, display icon connected via APRS-IS. APRS-IS is an aprslink. type, status text and routing information. It ad hoc network of Amateur Radio servers, may also contain antenna/power information, gateways (IGates), and clients (display weather data or short message data. software). APRS-IS facilitates world-wide 16.5.13 Winlink 2000 Call signs generally include an SSID that messaging without having to define special Winlink 2000 or WL2K is a worldwide historically provided information about the paths. Because all clients have access to all radio messaging system that takes advantage type of station. For example, a call sign of gated packets, databases such as http://aprs. of the Internet where possible. It does this N7SS-9 would generally indicate a mobile fi and http://findu.com can store and parse in order to allow the end-user more radio station. Since there are several systems in the packets for later retrieval from browsers spectrum on the crowded spectrum. The sys- use, the SSID information is not definitive. and other clients. APRS-IS messaging tem provides radio interconnection services Recently a number of dedicated APRS radios support allows many interfaces for the RF including: e-mail with attachments, position have appeared that feature an internal TNC ham running an APRS client such as call sign reporting, graphic and text weather bulletins, and APRS software. These radios include a lookups, e-mail, and calling CQ world-wide. emergency / disaster relief communications, method of displaying and entering messages More information on many of these features and message relay. The PACTOR-I, II, and without requiring the use of a computer mean- can be found at www.aprs-is.net. III protocols are used on HF, and AX.25 ing their user can directly respond to a mes- APRS position reports are also avail- Packet, D-STAR and 802.11 are used on sage sent. There is value in knowing someone able from GPS-equipped D-STAR radios VHF/UHF. can respond and it is recommended they use via DPRS gateways (more on D-STAR in Winlink 2000 has been assisting the mari- an SSID of -7. Tactical call signs can also be a later section). The DPRS specification de- time and RV community around the clock for used if the assigned call sign is included as fines how to translate the continuous stream many years. More recently there has been an part of the status text. of GPS position reports in the D-STAR DV increasing interest in emergency communi- The APRS packet includes PATH infor- stream to individual APRS packets. This defi- cations (emcomm), and the Winlink 2000 mation that determines how many times it nition restricts the number of APRS packets development team has responded by adding should be digipeated. A local group should generated to prevent overrunning a local RF features and functions that make the system be able to offer guidance on how the PATH network because the D-STAR radios continu- more reliable, flexible and redundant. The should be set to not overload the local net- ously stream new position reports while the role of Winlink 2000 in emergency commu- work. Listening to squawking signals on the user is talking. Most D-STAR repeaters have nications is to supplement existing method- national APRS frequency of 144.390 MHz DPRS IGates running on the gateway provid- ologies to add another tool in the toolkit of

Digital Modes 16.25 the volunteer services deploying emergency communications in their communities. CMS SanDiego CMS Canada The Winlink 2000 system is a “star” based network containing five mirror-image, redun­ dant Common Message Servers (CMS) loca­ ted in San Diego (USA), Washington DC (USA), Vienna (Austria), Halifax (Canada) and Perth (Australia). These ensure that the system will remain in operation should any piece of the Internet become inoperative. Each Radio Message Server node (RMS) is tied together as would be the ends of a spoke Internet on a wheel with the hub function performed by the Common Message Servers. Traffic goes in and out between the CMS and the Internet e-mail recipient, and between the end users and the Radio Message Server HF (RMS) gateways. Multiple radio-to-radio

123 456 ad­dresses may be mixed with radio-to- 789 0# Internet e-mail addresses, allowing complete * flexibility. VHF/UHF Because each Radio Message Server gate- RMS Pactor 1 way is a mirror image of the next, it does not Telnet matter which station is used. Each can provide over 700 text-based or graphic weather prod- HF

123 ucts, and each can relay the user’s position 456 789 to a web-based view of reporting users and * 0# interoperates with the APRS system. RMS Packet HBK0434 RMS Pactor 2 One of the most important objectives in the eyes of the Winlink development team was to Fig 16.12 — Winlink 2000 system. reduce the use of the HF spectrum to only that required to exchange messages with a user, and to do that at full “machine” speeds. The sage servers on HF were known as PMBOs stations, can immediately and automatically HF spectrum is very crowded, and limiting (Participating Mail Box Offices) and there connect a local amateur station to the Internet the forwarding of messages between WL2K was a packet radio to TELNET bridge compo- for emergency traffic. Using a standard SMTP RMS stations to the Internet, a great deal of nent for VHF/UHF called Telpac. These have e-mail client, the Paclink mini-e-mail server radio air time is eliminated, making the time been replaced with the newer RMS PACTOR can replace a network of computers (behind a and spectrum available to individual users (HF) and RMS Packet (VHF/UHF) software router) as a transparent substitute for normal either for message handling or for other although the older terms are still in use. Over SMTP mail. WL2K uses no external source operations. A number of the RMS PACTOR the air, PACTOR is used on HF, AX.25 packet for sending or receiving Internet e-mail. It is a servers found on HF restrict their protocols to on VHF/UHF and both employ the B2F com- stand-alone function which interacts directly PACTOR II (400 to 800 bit/s) and pressed binary format to maximize transmis- with the Internet rather than through any ex- PACTOR III (1400 to 3600 bit/s.) In doing so, sion efficiency. A diagram of the Winlink ternal Internet service provider. these RMS stations typically have a much 2000 architecture is shown in Fig 16.12. Airmail is independently developed, dis- higher ratio of traffic minutes and message tributed and supported by Jim Corenman, counts to connect times than do the RMS CLIENT ACCESS TO WINLINK KE6RK. It is the oldest and most widely ­stations that also receive the slower The primary purpose of the Winlink 2000 used program for sending and receiving mes- PACTOR I (100 to 200 bps) protocol. In­ network system is to assist the mobile or re- sages using the WL2K system. Airmail may other words, the amount of traffic that is motely located user and to provide emergency be used for HF Pactor, VHF/UHF Packet, passed with an Express station is much great- e-mail capabilities to community agencies. and for TELNET connections over any TCP/ er for an equivalent amount of connect time Because of this, WL2K supports a clean, IP ­medium including the Internet and high- with approximately the same number of con- simple interface to the Internet SMTP e-mail speed radio media, such as D-STAR and nections. On average, this translates to a PAC- system. Any message sent or received may in- HSMM. Once connected to a WL2K station, TOR I station downloading an 80,000 byte clude multiple recipients and multiple binary message transfer is completely automatic. file in approximately 80 minutes while on attachments. The radio user’s e-mail address, On the ham bands, Airmail can transfer mes- PACTOR III, the same download takes ap- however, must be known to the system as a sages automatically with any station sup- proximately six minutes. Note that PACTOR radio user or the message will be rejected. porting the BBS or F6FBB protocols, such IV is not yet legal for US amateurs as discussed This simple Internet interface protocol has an as Winlink 2000, F6FBB, MSYS and other earlier in the PACTOR section. added benefit in case of an emergency where Airmail stations. When used with WL2K, The RMS servers provide endpoint con- local services are interrupted and the system Airmail also contains position reporting nectivity to users via HF or VHF and can be must be used by non-Amateur groups as an capabilities, and a propagation prediction found worldwide. When connecting via the alternative to normal SMTP e-mail. program to determine which of the partici- Internet, users can connect directly to the Connecting to any one of the WL2K pub- pating Winlink stations will work from any- CMS server operational closest to them via licly-used RMS Packet stations via HF or where on Earth. Airmail also contains a lim- TELNET. Formerly, the full-featured mes- the specialized non-public emcomm RMS ited mail server that allows it to host e-mail

16.26 Chapter 16 independently from the Winlink network. then sent to the user. The global catalog cur- its on HF frequencies by RMS sysops. The To obtain a copy of Airmail, including rently includes over 700 available weather, default time per any 24-hour period is 30 the installation and operating instructions, propagation, and information bulletins, in- minutes, however, the user may request more download the program from the Airmail cluding, instructions for using the system, time from the RMS sysop should it be needed. web page at www.siriuscyber.net/ham and World news, and piracy reports. All WL2K The time limit is individual to each RMS support is available from the Airmail user RMS stations support a single global catalog station. Utilization of the PACTOR-II and group at http://groups.yahoo.com/group/ which ensures users can access any bulletin PACTOR-III protocols are a great timesaver, airmail2000. from any RMS. Bulletins can contain basic allowing the user up to 18 times the volume When an RF connection is not available text, graphic fax or satellite images, binary of messages over that of PACTOR-I for the (or necessary) but web access is available, or encoded files like GRIB or WMO weather same period of time. Winlink 2000 messages can be retrieved or reports. Local processing is used to re-process The system has a number of other sec- sent via a web interface. The web browser images to sizes suitable for HF Pactor trans- ondary features to help keep it healthy. Ex- access is limited to text-based messages with- mission. The system prevents bulletin du- tensive traffic reports are collected, the state out the use of bulletins or file attachments. plication and automatically purges obsolete of individual RMS stations are monitored Password-protected access to the Winlink time-sensitive weather bulletins and replaces and reported if it becomes inactive and daily mailbox is available through the Winlink them with the current version. backups are performed automatically at all page at www.winlink.org/webmail. There The system also has the ability to contain RMS stations as well as the Common Mes- is also a terminal mode for interactive key- bulletins with attachment information which sage Servers to ensure the system integrity. board commands, allowing a terminal rather is local to each RMS. This is especially use- Security is ensured through the vigorous than computer-based software to connect to ful for the non-public emcomm RMS which updating of virus definitions and automatic an RMS via RF. Because of the inefficient may house valuable procedural information virus screening for all Internet mail and files. use of airtime, this method is discouraged pertinent to complex information or instruc- The system has the ability to block any user but may be used for the listing and deletion tion needed by specific agencies in any com- by both radio (by frequency band) and Inter- of messages only. munity emergency. net (by e-mail address) to prevent abuse of Airmail provides a super-fast replica of Multiple binary or text-based file attach- the system. Spam is controlled through the WL2K radio operations while directly con- ments of any type or number may be attached use of a secure “acceptance list” or “white nected through the Internet to one of the CMS to a message by simply selecting the file to be list” methodology. More information about servers. This method of obtaining messages sent from a Windows selection dialog in the the Winlink 2000 system is available on the over the Internet allows multiple attachments, user’s HF AirMail or the Winlink 2000 VHF/ Winlink website at www.winlink.org. catalog bulletins, and all other Winlink 2000 UHF Paclink server with a standard SMTP e- services normally available over radio chan- mail client. E-mail message attachments sent nels, but at Internet speeds. In order to use through the Winlink 2000 system must be 16.5.14 D-STAR this service, a user must currently be listed limited in size. Users are provided an option D-STAR (Digital Smart Technologies for as a radio user, and obtain the connection to allow this limit to be determined. When Amateur Radio) is a digital voice and data information for the closest CMS server. Both using the default B2F format, the protocol protocol specification developed as the result Paclink and Airmail support the TELNET chosen by the user usually determines the file of research by the Japan Amateur Radio client service. This operation allows regular size of an attachment. A user may also turn off League (JARL) to investigate digital use of the system with the same software the ability to receive file attachments. Certain technologies for Amateur Radio in 2001. While configuration at high speed, without using file attachment types are blocked from the there are other digital on-air technologies RF bandwidth and provides a full-featured system for the protection of the user from being used by amateurs that were developed mechanism to access the system where no virus attacks. for other services, D-STAR is one of the first RF connection is available. An Emergency The WL2K network administrator may on-air protocols to be widely deployed and Operations Center with Internet access can post notices that are delivered to all indi- sold by a major radio manufacturer that is use the TELNET path with RF as a fallback vidual WL2K users as a private message. designed specifically for amateur service use. with just a minor change in the software. This is a valuable tool for notifying users of D-STAR transfers both voice and data via a Similarly, an RV or marine user can use the system changes, outages, software upgrades, data stream over the 2 meter (VHF), 70 cm software from an Internet café or home via emergencies, etc. (UHF) and 23 cm (1.2 GHz) Amateur Radio TELNET and switch the software back to The integrated nature of the system makes bands either simplex or via repeater. the TNC when mobile, with no change in possible other services beyond just simple One of the interesting features about the functionality. messaging. The bulletin services mentioned D-STAR protocol is the fact the system above is beyond normal messaging, but uses Amateur Radio call signs not only as WINLINK FEATURES WL2K also provides rapid position report- an identifier, but also for signal routing. In To address the needs of mobile users for ing from anywhere in the world. This facility the most common configuration, the most near real-time data, WL2K uses an “on- is interconnected with the APRS, ShipTrak, vital part of a D-STAR system is the gateway demand” bulletin distribution mechanism. and YotReps networks. It supports weather server, which networks a single system into a (Note that such bulletins are not the same as reporting from cruising yachts at sea and an D-STAR network via a trust server. The trust traditional AX.25 Packet Bulletins.) Users interconnection with the YotReps network server provides a central, master database to must first select requested bulletins from an which is used by government forecasters for look up users and their associated system. available “catalog” list managed in Airmail. weather observations in parts of the world This allows Amateur Radio operators to When bulletin requests are received by an where no others are available. It also allows respond to calls made to them, regardless RMS station, a fresh locally cached copy of the maritime user to participate in the National of their location on the D-STAR network. the requested bulletin is delivered. If no fresh Weather Service’s NOAA MAROB a volun- While almost all documentation references locally-cached version is available, the RMS tary marine observation reporting program. the Internet as the connection point for accesses the Internet and finds the bulletin To ensure equitable access to the system a network, any IP network connectivity which is then downloaded to the RMS and individual users are assigned daily time lim- will work, depending on signal latency.

Digital Modes 16.27 Additionally, D-STAR can provide a zero infrastructure support system utilizing point- to-point “backbone” 10 GHz connections. D-STAR Repeater Controller Currently, the global D-STAR trust server is maintained by a group of dedicated D-STAR 23 cm DD Mode Data (128 kb) enthusiasts from Dallas, Texas — the Texas Interconnect Team. 23 cm DV Mode Voice Repeater The D-STAR protocol specifies two modes; Digital Voice (DV) and Digital Data (DD). In the protocol, the DV mode provides 70 cm DV Mode Voice Repeater both voice and low speed data channel on 2 meters, 70 cm and 23 cm over a 4800-bit/s data stream. In the protocol, the DV mode 2 m DV Mode Voice Repeater uses a data rate of 4800 bit/s. This data stream is broken down to three main packages: voice, Internet Gateway Server forward error correction (FEC) and data. The largest portion of the data stream is the voice package, which is a total of 3600 bits/s Local Application Server (Email, FTP, Chat, Web, etc.) with 1200 bit/s dedicated to forward error correction, leaving 1200 bit/s for data. This HBK0435 additional data contains various data flags as well as the data header, leaving about 950 bit/s Fig 16.13 — Full D-STAR system. available for either GPS or serial data. This portion of the data stream does not provide any type of error correction, which has been does exist and collisions do occur, the TR be implemented in standard software or overcome by implementing error correction switching is very fast and this effect is handled consumer routers and firewalls. in the application software. by TCP/IP as it is for WiFi access points. A D-STAR repeater system consists of While there are various techniques of The simplex channel eliminates the need for at least one RF module and a controller. encoding and transporting a DV signal, the duplexers at a repeater site if only the DD mode While any combination of RF modules can focus of D-STAR’s design was the most effi- system is installed. It is still recommended be installed, typically a full system includes cient way to conserve RF spectrum. While to have filtering, such as a band-pass filter, the three voice modules (2 meters, 70 cm D-STAR’s “advertised” occupied band- in place to reduce possible interference from and 23 cm) and the 23 cm DD mode module width is 6.25 kHz, tests reveal a band plan other digital sources close to the 23 cm band shown in Fig 16.13. A computer with dual of 10 kHz spacing is adequate to incorporate as well as to reduce RF overload from nearby Ethernet ports, running the Gateway software, the D-STAR signal as well as provide space RF sources. While some DD mode system is required for Internet access to the global for channel guards. owners would like more sensitivity or more network. An additional server, as shown in In addition to DV mode, the D-STAR output power (10 W), at the time of print, the diagram, can be incorporated for local protocol outlines the high speed Digital Data no manufacturer has developed pre-amps or hosting of e-mail, chat, FTP, web and other (DD) mode. This higher speed data, 128 kbit/s, RF power amplifiers with an adequate TR services. In a D-STAR system installation, is available only on the 23 cm band because switching time to boost the signals. the standard repeater components (cavities, it requires an advertised 130 kHz bandwidth, Radios currently providing DV mode isolators, antennas and so forth) are not shown only available at 23 cm in world-wide band data service use a serial port for low-speed but are required as with any analog system. plans. Unlike the DV mode repeaters, the DD data (1200 bit/s), while the DD mode radio Some groups have removed analog gear and mode module operates as an “access point” offers a standard Ethernet connection for replaced it with D-STAR components on operating in half duplex, switching quickly high-speed (128 kbit/s) connections, to allow the same frequency with no additional work on a single channel. easy interfacing with computer equipment. beyond connecting the power and feed lines. As with the DV mode, there is a portion of The DD mode Ethernet jack allows two More information about D-STAR systems the data stream used for signal identification radios to act as an Ethernet bridge without may be found in the Repeaters chapter and with the data header as well as various system any special software support required. This in the Digital Communications supplement flags and other D-STAR related items. Once allows standard file sharing, FTP, TELNET, on the Handbook CD. this portion of the data stream is taken into HTTP/web browsing, IRC chat or even consideration, the 128 kbit/s is reduced to remote desktop connections to function as USER-CREATED FEATURES approximately 100 kbit/s — still more than if connected by wire. AND TOOLS double a dial-up connection speed with In a Gateway configuration, all users must D-STAR has already benefitted from significant range. Another consideration is be registered in the network. This provides a strong user community that has been the data rate specified at 128 kbit/s is the the DD mode sysop a layer of authorization, developing applications, products and gross data rate. Therefore, the system meaning that if someone wants to use a DD upgrades at a rapid pace. The DPlus gateway developers are challenged by the area mode system, and they have not received add-on provides a number of new features, coverage/potential user issue. This means the authorization to use the gateway, their DD such as an echo test, voicemail, simulcast to all higher the elevation of the system, the more mode access will be denied. Any gateway modules, a linking feature, playback of voice potential users and the slower the system registered user, on the common network, or text messages and more. There are also will become as all the users split the data can use any DD mode system, even if the a number of “D-STAR Reflectors” around bandwidth. Finally, there is an issue from the registration was not made on that system. the world that act like conference bridges for days of packet radio. While technically, the While we are not able to use encryption in connecting multiple repeater systems. opportunity for “hidden transmitter” issues the Amateur Radio service, security can One of the other projects that came from

16.28 Chapter 16 the Open D-STAR project is the DV Dongle d*Chat is used to enable text-based Phase 1 completed in 1995. P25 Phase 1 by Robin Cutshaw, AA4RC. The DV Dongle communication between multiple stations on radio systems operate in 12.5 kHz analog, contains the DSVI AMBE codec chip and a a simplex frequency or through a repeater. digital or mixed mode. Phase 1 radios use USB interface to allow a computer user to talk More information can be found online at Continuous 4-level FM (C4FM) modulation with other D-STAR voice users much the way http://nj6n.com/dstar/dstar_chat.html. for digital transmissions at 4800 baud and EchoLink users can connect to the system Dan Smith, KK7DS developed D-RATS two bits per symbol, yielding 9600 bit/s with a PC. The DV Tool software is written with public service users in mind. In total channel throughput. In the case of data in Java to provide cross-platform support. addition to chat, D-RATS supports file/ transmission, data packets basically consist The DV Dongle is available at ham radio photo downloads bulletins, forms, email of a header, containing overhead information, dealers and the latest software and manual and a sophisticated mapping capability. It followed by data. In the case of digitized can be downloaded from www.opendstar. also has a “repeater” functionality that allows voice transmission, after the transmission org/tools. sharing a D-STAR radio data stream over of a header containing error protected Pete Loveall, AE5PL, created a DPRS- a LAN. The repeater feature is interesting overhead information, 2400 bit/s is devoted to-APRS gateway that moves D-STAR GPS in an environment where the radio would to periodically repeating the overhead data to the APRS-IS network. Although it be physically separated from the software information needed to allow for late entry doesn’t natively appear on the APRS RF user, like in an emergency operations center. (or the missed reception of the header). network (unless IGated specifically), it does Dan has current developments and downloads After evaluating several candidates, Project appear in all the online APRS tools. DPRS available at www.d-rats.com/wiki. 25 selected the IMBE (Improved MultiBand has been implemented in software with D-STAR TV was developed by John Excitation) vocoder from DVSI for phase 1, DStarMonitor running on many D-STAR Brown, GM7HHB, and provides still images operating at 4400 bit/s. An additional 2800 gateways and DPRS Interface available for over D-STAR DV mode and streaming video bit/s of forward error correction is added for most computers at www.aprs-is.net/dprs. over DD mode. The limited bandwidth of DV error correction of the digitized voice. An DPRS has been implemented in hardware mode data and the lack of error correction 88.9-bit/s low-speed data channel is provided with the µSmartDigi manufactured by Rich mean the images take a while and may have in the digitized voice frame structure and Painter, ABØVO. The µSmartDigi offers a some corruption (just like SSTV). The DD several forms of encryption are supported. compact, portable TNC designed to act as mode has much more bandwidth and can P25 Phase 2 requires a further effective a gateway for position packets between a support live video. More information is bandwidth reduction to 6.25 kHz by operating D-STAR digital network and a conventional available online at www.dstartv.com. as two-slot TDMA (Time Division Multiple analog APRS RF network via a D-STAR This is just a sample of the development Access) with support for trunking systems. radio and a conventional FM radio. More taking place and there will undoubtedly be This provides two channels from the same DPRS and µSmartDigi information is more applications coming as the user base 12.5 kHz spectrum when working through available at www.aprs-is.net/dprs and increases. The two websites with the most a repeater to provide time synchronization. www.usmartdigi.com respectively. current D-STAR information are www. The TDMA timing requirements does limit Satoshi Yasuda, 7M3TJZ/AD6GZ, created dstarinfo.com and www.dstarusers.org. range to a published 35 km but this is plenty a D-STAR node adapter or “hot spot” that for public safety users. It uses the newer connects to a standard simplex FM radio and AMBE (Advanced MultiBand Excitation) allows a D-STAR radio user to access the 16.5.15 APCO Project 25 codec from DVSI to accommodate the bit D-STAR network. The system requires the (P25) rate reduction to 4800 bit/s. Phase 2 radios FM rig be accurately configured for a clean Project 25 (P25) or APCO-25 is a suite of will still offer backward compatibility with signal and loses the D-STAR benefit of only standards for digital radio communications Phase 1. Phase 3 work has started and will consuming 6.25 kHz of bandwidth. It does for use by public safety agencies in North support access to the new 700 MHz band allow D-STAR users access to the network America. P25 was established by Association in the US. via RF where there is Internet access and of Public-Safety Communications Officials P25 radios offer a number of features of no repeater infrastructure available. More (APCO) to address the need for common interest to public service agencies such as an information can be found online at http:// digital public safety radio communications emergency mode, optional text messaging, d-star.dyndns.org/rig.html.en. standards for first responders and Home- over the air programming/deactivation. Brian Roode, NJ6N, wrote one of the land Security/Emergency Response profes­ More P25 information can be found at earliest end-user applications for D-STAR. sionals. In this regard, P25 fills the same role www.apco911.org/frequency/project25/ His d*Chat is a Windows-based keyboard as the European Tetra protocol, although not information.html#documents and www. to keyboard communication application that interoperable with it. tiaonline.org/standards/technology/ uses the D-STAR DV mode data capability. P25 has been developed in phases with project_25.

Digital Modes 16.29 FEC Error handling ARQ FEC FEC FEC FEC FEC/FEC+ARQ FEC/FEC+ARQ None None/FEC FEC FEC None None None FEC + ARQ None None FEC FEC FEC FEC ARQ FEC + ARQ FEC + ARQ None FEC FEC + ARQ None None None FEC/FEC+ARQ FEC + ARQ FEC None ARQ None FEC None 8FSK Modulation FSK FSK 36-QPSK 4FSK/QPSK DBPSK-DSSS 4-(2-16)DPSK/(2-4)DASK 8-(2-16)DPSK/(2-4)DASK 16FSK 18FSK 0.5 GMSK/QPSK/4PSK 0.5 GMSK/QPSK/4PSK FM, 1500-2300 Hz 15-QPSK 4FSK FSK, 170/200 shift ASK 44FSK 65FSK 16FSK 64-DPSK 32-FSK FSK, 200 Hz 2-DBPSK/PI-4DQPSK/8,16DPSK (2-18)-DBPSK/DQPSK BPSK QPSK 15-QPSK FM, 1200-2300 Hz FM, 1200-2300 Hz 9FSK/2-9FSK COFDM (n-QAM) (1-15)-QPSK/16QAM/4FSK 4FSK FM, 1200-2300 Hz (“N-” means multi-carrier) (“N-” FSK BPSK QPSK FSK, 170 Hz 125 Symbol rate Symbol rate 114 114 50 4,800 300 31.25 62.5 7.8/11/15.6 3.9-21.5 4800 128,000 120 lpm 50 441 100/200/300 122.5 21.53 2.7 15.625 5/10/20 15.63/31.25 100/200 100 100 31.25 31.25 83.33 240-line RGB 240-line RGB 1/2/4 31.25/62.5 1.46 120-line B/W (baud) 1200 62.5/125 62.5/125 45.45 375 Bit rate Bit rate 114 114 3600 9600 300 62.5-750 500-3000 31/44/62 15.63-86.13 4800 128,000 1450 882 80/160/240 122.5 116.8 16.1 62.5 320/640/1280 78.13/156.25 100/200 200-800 200-3600 31.25 62.5 2,500 62.5-3750 2.93 (bit/s) 1200 62.5/125 125/250 45.45 <375 Data rate Data rate 53 57 2400 6800 21.1/37.5 < 37.5-750 108-1994 31/44/62? (<15.63)-86.13 V:2400 D:960 72k-124k 1450 882 35/75/115 2.5 char/s 77.9 1.54 31.25 35/70/140 8.75/17.5 51.2/128 100-700 85-2722 31.25 31.25 300/1200/2400 114 s/frame 110 s/frame 10/20/40 wpm 0.45 8 s/frame 8 s/frame (bit/s) <1200 62.5/125 62.5/125 30.3 Data Principal Data Data Data Voice, Voice Keyboarding Data Data Keyboarding Keyboarding & data Voice Data Image Voice Meteor scatter Data Keyboarding Meteor scatter Moonbounce Data Keyboarding, Keyboarding Keyboarding Winlink e-mail Data, Winlink e-mail Data, Winlink e-mail Data, Keyboarding Keyboarding Data Image Image Keyboarding Data Voice, Winlink email signal beacon Weak Image Application Data Keyboarding Keyboarding contests Keyboarding, Principal HF HF HF VHF HF HF HF HF HF VHF UHF HF HF VHF HF HF 50 MHz V/UHF HF HF HF HF HF HF HF HF HF HF HF HF HF HF HF-VHF HF Freq VHF HF HF HF

ORX Ø Developer MIL-STD-188-141, HF FED-STD-1045 G3PLX G3PLX Corp AOR APCO IZ8BLY Hal Comm. Hal Comm. ZL2AFP ZL1BPU JARL JARL G3PLX/HB9TLK K1JT Kantronics Rudolf Hell K1JT K1JT ZL1BPU/IZ8BLY SP9VRC SP9VRC DL6MAA/DF4KV Sys. Comm. Spec. Sys. Comm. Spec. G3PLX G3PLX SP9VRC W Martin Emmerson Eddie Murphy G3PPT HB9TLK KN6KB K1JT Table 16.18 Table Digital Modulation Modes and Formats Used in Amateur Radio definitions and abbreviations of Chapter 16 for See the text March 2013 N7SS; Scott Honnaker, Alan Bloom, N1AL and edited by by Developed Mode or Name Format ALE AMTOR-A AMTOR-B AMBE AOR APCO P25 Chip64 CLOVER-II CLOVER-2000 Domino DominoEX D-Star (DV) D-Star (DD) Facsimile FDMDV FSK441 G-TOR Hellschreiber (Feld) JT6M JT65 MFSK16 MT63 Olivia (Bell202) Packet PACTOR-I PACTOR-II PACTOR-III PSK31 QPSK31 PSK63/125 QPSK62/126 (Baudot) RTTY SSTV (traditional) SSTV Martin M1 SSTV Scottie S1 Throb WinDRM WINMOR WSPR (MEPT-JT)

16.30 Chapter 16 16.6 Digital Mode Table Table 16.18 is a summary of common moonbounce and beaconing modes created no error handling which means errors are not digital modes used by amateurs as of early by Joe Taylor, K1JT as WSJT (see the section detected and must be addressed in a higher 2013 and their primary characteristics. The Structured Digital Modes). protocol layer (as in AX.25 packet). Forward following text is intended to make high-level There can be a significant difference error correction (FEC) makes a best effort to comparisons. For more information on these between data rate and bit rate in high reliability address errors in real time as part of the data modes see the earlier sections of this chapter. modes. The data rate is the amount of user sent in each packet. FEC can add substantial This page is also available as a PDF file on the data transmitted where the bit rate includes overhead to each packet and does not guarantee CD-ROM accompanying this book. the packet and error correction overhead. error-free delivery but does make the mode Many modes have a number of variants, Some of these rates are approximate and can robust in high noise environments. Automatic with the most common shown in the table. vary based on conditions and the variant of Retry Request (ARQ) can guarantee error-free High reliability, low data rate modes are the mode used. The symbol rate is a function delivery of data but has no ability to actually more common on HF. Higher data rates are of the modulation scheme and how many correct received data. The combination of FEC available on VHF/UHF, where the bands simultaneous carriers are used to transmit and ARQ allows minor errors to be corrected have less noise and require less effort to the data. in real time with major errors generating a make them reliable. Some modes have very The error handling mechanism is critical to retry request. specific intended uses like the meteor scatter, note when sending data. Some modes include

16.7 Glossary ACK — Acknowledgment, the control BER — Bit error rate. Code — Method of representing data. signal sent to indicate the correct receipt BERT — Bit-error-rate test. Codec — Algorithm for compressing and of a transmission block. Bit stuffing — Insertion and deletion of decompressing data. Address — A character or group of 0s in a frame to preclude accidental Collision — A condition that occurs when characters that identifies a source or occurrences of flags other than at the two or more transmissions occur at the destination. beginning and end of frames. same time and cause interference to the AFSK — Audio frequency-shift keying. Bit — Binary digit, a single symbol, in intended receivers. ALE — Automatic link establishment. binary terms either a one or zero. Compression — Method of reducing the Algorithm — Numerical method or Bit/s or bps — Bits per second. amount of data required to represent a process. Bitmap — See raster. signal or data set. Decompression is the APCO — Association of Public Safety Bit rate — Rate at which bits are method of reversing the compression Communications Officials. transmitted in bit/s or bps. Gross process. ARQ — Automatic Repeat reQuest, an (or raw) bit rate includes all bits Constellation — A set of points that error-sending station, after transmitting transmitted, regardless of purpose. Net represent the various combinations of a data block, awaits a reply (ACK or bit rate (also called throughput) only phase and amplitude in a QAM or other NAK) to determine whether to repeat the includes bits that represent data. complex modulation scheme. last block or proceed to the next. BLER — Block error rate. Contention — A condition on a ASCII — American National Standard BLERT — Block-error-rate test. communications channel that occurs Code for Information Interchange, a BPSK — Binary phase-shift keying in when two or more stations try to transmit code consisting of seven information which there are two combinations of at the same time. bits. phase used to represent data symbols. Control characters — Data values with AX.25 — Amateur packet-radio link-layer Byte — A group of bits, usually eight. special meanings in a protocol, used to protocol. Cache — To store data or packets in cause specific functions to be performed. Baud — A unit of signaling speed equal anticipation of future use, thus improving Control field — An 8-bit pattern in an to the number of discrete conditions routing or delivery performance. HDLC frame containing commands or or events per second. (If the duration Channel — Medium through which data is responses, and sequence numbers. of a pulse is 20 ms, the signaling rate transmitted. Convolution — The process of combining is 50 baud or the reciprocal of 0.02, Checksum — Data representing the sum or comparing signals based on their abbreviated Bd). of all character values in a packet or behavior over time. Baudot code — A coded character set in message. Cyclic Redundancy Check (CRC) — The which five bits represent one character. CLOVER — Trade name of digital result of a calculation representing all Used in the US to refer to ITA2. communications system developed by character values in a packet or message. Bell 103 — A 300-baud full-duplex Hal Communications. The result of the CRC is sent with modem using 200-Hz-shift FSK of tones COFDM — Coded Orthogonal Frequency a transmission block. The receiving centered at 1170 and 2125 Hz. Division Multiplex, OFDM plus coding station uses the received CRC to check Bell 202 — A 1200-baud modem standard to provide error correction and noise transmitted data integrity. with 1200-Hz mark, 2200-Hz space, immunity. Datagram — A data packet in a used for VHF FM packet radio. connectionless protocol.

Digital Modes 16.31 Data stream — Flow of information, either computer with applications programs MSK — Frequency-shift keying where the over the air or in a network. accessible by remote stations. shift in Hz is equal to half the signaling DBPSK — Differential binary phase-shift IA5 — International Alphabet, designating rate in bit/s. keying. a specific set of characters as an ITU MT63 — A keyboard-to-keyboard mode Dibit — A two-bit combination. standard. similar to PSK31 and RTTY. DQPSK — Differential quadrature phase- Information field — Any sequence of Nibble — A four-bit quantity. Half a byte. shift keying. bits containing the intelligence to be Node — A point within a network, usually Domino — A conversational HF digital conveyed. where two or more links come together, mode similar in some respects to Interleave — Combine more than one data performing switching, routine and MFSK16. stream into a single stream or alter the concentrating functions. DRM — Digital Radio Mondiale. data stream in such a way as to optimize OFDM — Orthogonal Frequency Division A consortium of broadcasters, it for the modulation and channel Multiplex. A method of using spaced manufacturers, research and characteristics being used. subcarriers that are phased in such a way governmental organizations which IP — Internet Protocol, a network protocol as to reduce the interference between developed a system for digital sound used to route information between them. broadcasting in bands between 100 kHz addresses on the Internet. OSI-RM — Open Systems Interconnection and 30 MHz. ISO — International Organization for Reference Model specified in ISO 7498 DV —Digital voice. Standardization. and ITU-T Recommendation X.200. Emission — A signal transmitted over the ITU — International Telecommunication PACTOR — Trade name of digital air. Union, a specialized agency of the communications protocols offered by Encoding — Changing data into a form United Nations. (See www.itu.int.) Special Communications Systems GmbH represented by a particular code. ITU-T — Telecommunication & Co KG (SCS). Encryption — Process of using codes Standardization Sector of the ITU, Packet — 1) Radio: communication using and encoding in an effort to obscure formerly CCITT. the AX.25 protocol. 2) Data: transmitted the meaning of a transmitted message. Jitter — Unwanted variations in timing or data structure for a particular protocol Decryption reverses the encryption phase in a digital signal. (see also frame.) process to recover the original data. Layer — In communications protocols, Parity (parity check) — Number of bits Error Correcting Code (ECC) — Code one of the strata or levels in a reference with a particular value in a specific used to repair transmission errors. model. data element, such as a byte or word Eye pattern — An oscilloscope display Least significant bit (LSB) — The bit in a or packet. Parity can be odd or even. A in the shape of one or more eyes for byte or word that represents the smallest parity bit contains the information about observing the shape of a serial digital value. the element parity. stream and any impairments. Level 1 — Physical layer of the OSI Pixel — Abbreviation for “picture Fast (FFT) — An reference model. element.” algorithm that produces the spectrum of Level 2 — Link layer of the OSI reference Primitive — An instruction for creating a a signal from the set of sampled value of model. signal or data set, such as in a drawing or the waveform. Level 3 — Network layer of the OSI for speech. FEC — Forward error correction. reference model. Project 25 — Digital voice system Frame — Data set transmitted as one Level 4 — Transport layer of the OSI developed for APCO, also known as P25. package or set. reference model. Protocol — A formal set of rules and Facsimile (fax) — A form of telegraphy for Level 5 — Session layer of the OSI procedures for the exchange of the transmission of fixed images, with or reference model. information. without half-tones, with a view to their Level 6 — Presentation layer of the OSI PSK — Phase-shift keying. reproduction in a permanent form. reference model. PSK31 — A narrow-band digital FCS — Frame check sequence. (see also Level 7 — Application layer of the OSI communications system developed by CRC) reference model. Peter Martinez, G3PLX. FDM — Frequency division multiplexing. Lossless (compression) — Method of QAM — Quadrature Amplitude FDMA — Frequency division multiple compression that results in an exact copy Modulation. A method of simultaneous access. of the original data phase and amplitude modulation. The FEC — Forward error correction, an Lossy (compression) — Method of number that precedes it, for example, error-control technique in which the compression in which some of the 64QAM, indicates the number of discrete transmitted data is sufficiently redundant original data is lost stages in each symbol. to permit the receiving station to correct MFSK16 — A multi-frequency shift QPSK — Quadrature phase-shift keying some errors. communications system in which there are four different FSK — Frequency-shift keying. Modem — Modulator-demodulator, a combinations of signal phase that Gray code — A code that minimizes the device that connects between a data represent symbols. number of bits that change between terminal and communication line (or Raster (image) — Images represented as sequential numeric values. radio). Also called data set. individual data elements, called pixels. G-TOR — A digital communications Most significant bit (MSB) — The bit in a Router — A network packet switch. In system developed by Kantronics. byte or word that represents the greatest packet radio, a network level relay station HDLC — High-level data link control value. capable of routing packets. procedures as specified in ISO 3309. Multicast — A protocol designed to RTTY — Radioteletype. Hellschreiber — A facsimile system for distribute packets of data to many users Sample — Convert an analog signal to a set transmitting text. without communications between the of digital values. Host — As used in packet radio, a user and data source.

16.32 Chapter 16 Shift — 1) The difference between mark Symbol — Specific state or change in of signal states that represent data. (see and space frequencies in an FSK or state of a transmission representing a also Viterbi coding and constellation.) AFSK signal. 2) To change between particular signaling event. Symbol rate Turnaround time — The time required character sets, such as between LTRS is the number of symbols transmitted per to reverse the direction of a half-duplex and FIGS in RTTY. unit of time (see also baud). circuit, required by propagation, modem SSID — Secondary station identifier. TAPR — Tucson Amateur Packet Radio reversal and transmit-receive switching In AX.25 link-layer protocol, a Corporation, a nonprofit organization time of transceiver. multipurpose octet to identify several involved in digital mode development. Unicast — A protocol in which information packet radio stations operating under the TDM — Time division multiplexing. is exchanged between a single pair of same call sign. TDMA — Time division multiple access. points on a network. State — Particular combination of signal Throb — A multi-frequency shift mode Varicode — A code in which the different attributes used to represent data, such as like MFSK16. data values are represented by codes with amplitude or phase. TNC — Terminal node controller, a device different lengths. Start (stop) bit — Symbol used to that assembles and disassembles packets Vector — Image represented as a collection synchronize receiving equipment at the (frames). of drawing instructions. beginning (end) of a data byte. Trellis — The set of allowed combination Word — Set of bits larger than a byte.

16.8 References and Bibliography A. Bombardiere, K2MO, “A Quick-Start munication Systems, Oxford University S. Ford, WB8IMY, HF Digital Handbook, Guide to ALE400 ARQ FAE,” QST, Jun Press, 1998. 4th edition, ARRL, 2007. 2010, pp 34-36. B. Sklar, Digital Communications, Fundamen- S. Ford, WB8IMY, VHF Digital Handbook, B.P. Lathi, Modern Digital and Analog Com- tals and Applications, Prentice Hall, 1988. ARRL, 2008.

Digital Modes 16.33