Introduction to Communication Networks Spring 2007 Unit 4 Multiplexing, Framing, and some solutions... ©EECS 122 SPRING 2007 Acknowledgements – slides comming from: • Data and Computer Communication by Wiliam Stallings (our supplementary textbook). • Data Communications and Networking by B. Forouzan, Mc Graw Hill, 2004 ( a very nice-to-read book!) • Some figures have been used form the earlier issues of the EECS 122 tought by Prof Jean Walrand. • Introduction to Telephones & Telephone Systems by A. Michael Noll, Artech House, 1986 • Megabit Data Communication, John T. Powers, Henry H. Stair II, Prentice Hall • Digital Telephony by J. Bellamy: “”, J. Wiley & Sons, 2rd edition, 2000 Prof. Adam Wolisz 2of 63 MULTIPLEXING Prof. Adam Wolisz 3of 63 Multiplexing • General Problem: Several - n- different channels (voice, TV- channels) should be supported between a pair of locations. We would like to avoid usage of n physical links (cables). • Looking at the features of media you will easily see that the supported bandwidth exceeds by far the bandwidth needed for each channel... Prof. Adam Wolisz 4of 63 Variants of multiplexing • The dimensions of multiplexing – time (t) – frequency (f) – code (c) – space (si) – sometimes… • Care for separation: guard spaces, code orthognonality • Multiplexing can be – Synchronous (constant allocation) – Statistical (variable allocation) Prof. Adam Wolisz 5of 63 Frequency Multiplex • Separation of the whole spectrum into smaller frequency bands • A channel gets a certain band of the spectrum (in the synchronous case – for the whole time!) • Note: guard zones in frequency are needed!! Special case: Wave division Mux Prof. Adam Wolisz 6of 63 Schema for FDM [Forouzan] mod Prof. Adam Wolisz 7of 63 FDM of Three Voiceband Signals Prof. Adam Wolisz 8of 63 Example - Community Antenna TV (CATV) Currently used systems require about 6MHz /TV Channel Prof. Adam Wolisz 9of 63 Time Multiplex • The whole bandwidth is used all the time, but – alternatively – by different channels! Prof. Adam Wolisz 10 of 63 Time Multiplex: Interleaving of data segments [Forouzan] Prof. Adam Wolisz 11 of 63 Time and Frequency Multiplex [Schiller] • Combination of both methods • A channel gets a certain frequency band for a certain amount of time • Example GSM cellular telephony: FDM with TDD (8 bi- directional channels per frequency band) is used... k1 k2 k3 k4 k5 k6 c f t 2.18.1 Prof. Adam Wolisz 12 of 63 Time Division Duplex (TDD) and FDD Similarly a Frequency Division Duplex - FDD with two frequency Channels: for up-link and down-link respectively, can be defined Prof. Adam Wolisz 13 of 63 Bursty Data • Burstiness of data – In many data communication applications, data occur in bursts separated by idle periods – This type of data can often be transmitted more economically by statistical (or asynchronous) multiplexing... Prof. Adam Wolisz 14 of 63 Synchronous vs. Statistical TDM Note: Data slots must be addressed! Prof. Adam Wolisz 15 of 63 Statistical Multiplexing Gain [mod.from N.Mc Keown, Stanford] Comment: Synchronous Multiplexing would use 2C bits/s – statistical uses R<2C. But: how to define R? – The queue helps „smooth“ the load but there might be losses !!! A+B Rate 2C R < 2C A C R B time Statistical multiplexing gain = 2C/R Other definitions of SMG: The ratio of rates that give rise to a particular queue occupancy, or particular loss probability. Prof. Adam Wolisz 16 of 63 Example of Statistical Multiplexer Performance Prof. Adam Wolisz 17 of 63 Probability of Overflow and Buffer Size ρ- is the ratio of the offered load to the nominal service rate of the system – see Queuing (later) Prof. Adam Wolisz 18 of 63 FHSS (Frequency Hopping Spread Spectrum) I • Discrete changes of carrier frequency – sequence of frequency changes determined via pseudo random number sequence • Two versions – Fast Hopping: several frequencies per user bit – Slow Hopping: several user bits per frequency • Advantages – ROBUSTNESS: impact of frequency selective fading and interference limited to short period ! 2.32.1 Prof. Adam Wolisz 19 of 63 FHSS : Schema of the operation Fast hopping Slow hopping Prof. Adam Wolisz 20 of 63 FHSS – System overview narrowband spread signal transmit user data signal modulator modulator frequency hopping synthesizer sequence transmitter narrowband received signal signal data demodulator demodulator hopping frequency sequence synthesizer receiver 2.34.1 Prof. Adam Wolisz 21 of 63 CDMA: Code Division Multiple Access A Channel: a unique code in the same spectrum at the same time Prof. Adam Wolisz 22 of 63 Space division multiplexing...wireless... • Assume a sectorized antenna • Transmisison/receive in one of the sectors does not limit the usage of other sectors... Prof. Adam Wolisz 23 of 63 FRAMING Prof. Adam Wolisz 24 of 63 Framing • WHY: – The physical layer supports bit-synchronization. – Data units bigger than a single bit must be recognized... • HOW: – Time gaps (not good - might be squeezed within the physical layer), – Physical signaling - the physical layer has to support some control symbols, besides of a 0 and a 1, say a J and K. Example: the Manchester extension of the IEEE 802.5 - token ring. – Field Lenght marker at the beginnig of the field: Whole notion of unit lost if this lenght marker would get corrupted! – Specific symbols: Character oriented, bit oriented variants. – Clock based Prof. Adam Wolisz 25 of 63 Framing - delimiting symbols • Delimiting characters in character based transmission, i.e. the case when the transmitted information is composed of symbols - c.f.the ASCII code table. • SYN SYN - used for the synchronization • SOH......STX.......ETX the framing sequence for the header and for the text. – Transmission of binary information: • DLE STX .......binary information........ DLE ETX – What about a DLE inside the binary information?? • input binary information: ...................... DLE......... • transmitted binary information: .... DLE DLE......... • extracting the information: ........... DLE ................ – A) This scheme is closely tied to an 8 bit character representation. – B) A single error can cause a misinterpretation. Prof. Adam Wolisz 26 of 63 Framing - delimiting symbols (2) Example DLE STX A DLE BDLEETX (a) DLE STX A DLE DLE B DLE ETX (b) Stuffed DLE (c) DLE STX A DLE BDLEETX (a) Data sent by the network layer. (b) Data after being character stuffed by the data link layer. (c) Data passed to the network layer on the receiving side. Prof. Adam Wolisz 27 of 63 Bit oriented transmission-delimiting flag • Delimiting flags in bit oriented transmission, i.e. the case when the transmitted information is represented as a string of bits (the concept of octets is sometimes used to support the bit manipulation). – The usual flag pattern: 01111110 • Transmission transparency is assured via bit stuffing: – The transmitter always stuffs a 0 after 11111 – The receiver removes a 0 following 11111 • User data 01111110 are transmitted as 011111010 Combinations of solutions discussed above are frequently used to increase the power of framing - e.g. delimiting flags together with byte (symbol) count. See additional reading after bit error unit! Prof. Adam Wolisz 28 of 63 Bit-oriented Transmission The packet framed by two special bit patterns called flags. Bit stuffing is used to prevent a flag from occurring in the middle of a packet. The bit-stuffing procedure is illustrated in (b): a bit 0 is inserted after each pattern 011111 that appears in the packet. The reverse procedure (bit destuffing) is performed at the receiver to restore the original packet. Prof. Adam Wolisz 29 of 63 Clock based Idea: to have a sequence of markers in predefined positions – here the sequence 101 in proper distance all the elements are assumed to have equal length! Problem: This sequence, with the proper spacing, MIGHT appear just by chance in the content!!! Solution: Look several times – the random appearance will not be repeatable over numerous frame series! Prof. Adam Wolisz 30 of 63 Examples of transmission systems Prof. Adam Wolisz 31 of 63 Phone Backbone - FDM Carrier Standard – OLD! [Forouzan] Prof. Adam Wolisz 32 of 63 FDM Carrier Standards - OLD – some numbers Prof. Adam Wolisz 33 of 63 American Digital Hierarchy Each channel carries data (voice) digitized at a rate of 8000 samples per second with 8 bit per sample. A frame contains 24 channels plus one framing bit per frame. Thus, the required transmission rate for DS-1 is 8000 x (24 x 8 + 1) bits per second = 1.544 Mbit/s. Prof. Adam Wolisz 34 of 63 American Digital Hierarchy –synchronization [bellamy] • T1/D4 Superframe and D4 Channel Slots. – A super frame combines 12 frames of 193 bits each. – The framing bits of these frames produce the T1/D4 superframe pattern of 100011011100 Prof. Adam Wolisz 35 of 63 Extended Superframe Format Framing (1) • Extension of the super frame from 12 repetitions to 24 repetitions. • Framing bit positions take new functions and meanings (24 bits). • Framing (6 bits) • Error Checking (6 bits) • Maintenance Communications (12 bits) 193rd Bit 193rd Bit 125 µs 123456789101112131415161718192021222324 Bipolar format 10101101Binary code 5,2 µs Basic North American PCM framing and signaling format. Prof. Adam Wolisz 36 of 63 Some specific ways of using it... [Stallings] Prof. Adam Wolisz 37 of 63 American TDM Carrier Standard [Forouzan] Prof. Adam Wolisz 38 of 63 TDM Carrier Standards North American and International TDM Carrier Standards Prof. Adam Wolisz 39 of 63 Just for info – the International Frame... Prof. Adam Wolisz 40 of 63 Trunks Prof. Adam Wolisz 41 of 63 Mhmm... Not quite unified... Prof. Adam Wolisz 42 of 63 Instability of the timing.. • Cyclic changes of the data rate of sending • Systematic difference in timing between the sender and receiver. • What can we do? – cyclic- or fluctuating differences removed by elastic buffer... – Systematic difference has to be dealt with • Systematic differences. How? – Plesiochronous operation (T-hierarchy) – Synchronous operation (SONET) Prof. Adam Wolisz 43 of 63 Synchronization - Definition of terms: – Single discrete signal may be either • Isochronous : Constant frequency of signal changes (e.g.