TH ADV. COMMUNICATION LAB 6 SEM E&C • the line-coded signal can directly be put on a transmission line, in the form of variations of the voltage or current (often using differential signaling). • the line-coded signal (the "base-band signal") undergoes further pulse shaping (to reduce its frequency bandwidth) LINE CODING and then modulated (to shift its frequency bandwidth) to create the "RF signal" that can be sent through free space. Line coding consists of representing the digital signal to be • the line-coded signal can be used to turn on and off a light transported by an amplitude- and time-discrete signal that is in Free Space Optics, most commonly infrared remote optimally tuned for the specific properties of the physical channel control. (and of the receiving equipment). The waveform pattern of • the line-coded signal can be printed on paper to create a voltage or current used to represent the 1s and 0s of a digital bar code. data on a transmission link is called line encoding. The common • the line-coded signal can be converted to a magnetized types of line encoding are unipolar, polar, bipolar and spots on a hard drive or tape drive. Manchester encoding. • the line-coded signal can be converted to a pits on optical For reliable clock recovery at the receiver, one usually imposes a disc. maximum run length constraint on the generated channel Unfortunately, most long-distance communication sequence, i.e. the maximum number of consecutive ones or channels cannot transport a DC component. The DC zeros is bounded to a reasonable number. A clock period is component is also called the disparity, the bias, or the DC recovered by observing transitions in the received sequence, so coefficient. The simplest possible line code, called unipolar that a maximum run length guarantees such clock recovery, because it has an unbounded DC component, gives too while sequences without such a constraint could seriously many errors on such systems. hamper the detection quality. Most line codes eliminate the DC component — such codes After line coding, the signal is put through a "physical channel", are called DC balanced, zero-DC, zero-bias or DC equalized either a "transmission medium" or "data storage medium". etc. There are two ways of eliminating the DC component: Sometimes the characteristics of two very different-seeming channels are similar enough that the same line code is used for • Use a constant-weight code. In other words, design each them. The most common physical channels are: transmitted code word such that every code word that contains some positive or negative levels also contains enough of the opposite levels, such that the average level • AMI over each code word is zero. For example, Manchester • Modified AMI codes : B8ZS, B6ZS, B3ZS, HDB3 code and Interleaved 2 of 5. • 2B1Q • Use a paired disparity code. In other words, design the • 4B5B receiver such that every code word that averages to a • 4B3T negative level is paired with another code word that • 6b/8b encoding averages to a positive level. Design the receiver so that • Hamming Code either code word of the pair decodes to the same data • 8b/10b encoding bits. Design the transmitter to keep track of the running • 64b/66b encoding DC buildup, and always pick the code word that pushes • 128b/130b encoding the DC level back towards zero. For example, AMI, 8B10B, • Coded mark inversion (CMI) 4B3T, etc. • Conditioned Diphase • Eight-to-Fourteen Modulation (EFM) used in Compact Disc Line coding should make it possible for the receiver to • EFMPlus used in DVD synchronize itself to the phase of the received signal. If the • RZ — Return-to-zero synchronization is not ideal, then the signal to be decoded will • NRZ — Non-return-to-zero not have optimal differences (in amplitude) between the various • NRZI — Non-return-to-zero, inverted digits or symbols used in the line code. This will increase the • Manchester code (also variants Differential Manchester & error probability in the received data. Biphase mark code) • Miller encoding (also known as Delay encoding or Modified It is also preferred for the line code to have a structure that will Frequency Modulation, and has variant Modified Miller enable error detection. encoding) • MLT-3 Encoding Note that the line-coded signal and a signal produced at a • Hybrid Ternary Codes terminal may differ, thus requiring translation. • Surround by complement (SBC) A line code will typically reflect technical requirements of the • TC-PAM transmission medium, such as optical fiber or shielded twisted Optical line codes: pair. These requirements are unique for each medium, because each one has different behavior related to interference, • Carrier-Suppressed Return-to-Zero distortion, capacitance and loss of amplitude. • Alternate-Phase Return-to-Zero [EDIT ] COMMON LINE CODES NON-RETURN-TO-ZERO mechanisms for bit synchronization when a separate clock signal is not available. From Wikipedia, the free encyclopedia NRZ-Level itself is not a synchronous system but rather an Jump to: navigation, search encoding that can be used in either a synchronous or asynchronous transmission environment, that is, with or without an explicit clock signal involved. Because of this, it is not strictly necessary to discuss how the NRZ-Level encoding acts "on a clock edge" or "during a clock cycle" since all transitions happen in the given amount of time representing the actual or implied integral clock cycle. The real question is that of sampling--the The binary signal is encoded using rectangular pulse amplitude high or low state will be received correctly provided the modulation with polar non-return-to-zero code transmission line has stabilized for that bit when the physical line level is sampled at the receiving end. In telecommunication, a non-return-to-zero (NRZ) line code is a binary code in which 1's are represented by one significant However, it is helpful to see NRZ transitions as happening on the condition (usually a positive voltage) and 0's are represented by trailing (falling) clock edge in order to compare NRZ-Level to some other significant condition (usually a negative voltage), other encoding methods, such as the mentioned Manchester with no other neutral or rest condition. The pulses have more code, which requires clock edge information (is the XOR of the energy than a RZ code. Unlike RZ, NRZ does not have a rest clock and NRZ, actually) and to see the difference between NRZ- state. NRZ is not inherently a self-synchronizing code, so some Mark and NRZ-Inverted. additional synchronization technique (for example a run length limited constraint, or a parallel synchronization signal) must be CONTENTS used to avoid bit slip. • 1 Unipolar Non-Return-to-Zero Level For a given data signaling rate, i.e., bit rate, the NRZ code • 2 Bipolar Non-Return-to-Zero Level • requires only half the bandwidth required by the Manchester 3 Non-Return-to-Zero Space • 4 Non-Return-to-Zero Inverted code. (NRZI) • 5 See also When used to represent data in an asynchronous communication • scheme, the absence of a neutral state requires other 6 References [EDIT ] UNIPOLAR NON-RETURN-TO-ZERO LEVEL Main article: On-off keying An example of this is RS-232, where "one" is −5V to −12V and "zero" is +5 to +12V. "One" is represented by one physical level (such as a DC bias on the transmission line). [EDIT ] NON-RETURN-TO-ZERO SPACE "Zero" is represented by another level (usually a positive voltage). Non-Return-to-Zero Space "One" is represented by no change in physical level. In clock language, "one" transitions or remains high on the "Zero" is represented by a change in physical level. trailing clock edge of the previous bit and "zero" transitions or remains low on the trailing clock edge of the previous bit, or just In clock language, the level transitions on the trailing clock edge the opposite. This allows for long series without change, which of the previous bit to represent a "zero." makes synchronization difficult. One solution is to not send bytes without transitions. Disadvantages of an on-off keying are the This "change-on-zero" is used by High-Level Data Link Control waste of power due to the transmitted DC level and the power and USB. They both avoid long periods of no transitions (even spectrum of the transmitted signal does not approach zero at when the data contains long sequences of 1 bits) by using zero- zero frequency. See RLL bit insertion. HDLC transmitters insert a 0 bit after five contiguous 1 bits (except when transmitting the frame delimiter [EDIT ] BIPOLAR NON-RETURN-TO-ZERO LEVEL '01111110'). USB transmitters insert a 0 bit after six consecutive 1 bits. The receiver at the far end uses every transition — both "One" is represented by one physical level (usually a negative from 0 bits in the data and these extra non-data 0 bits — to voltage). maintain clock synchronization. The receiver otherwise ignores these non-data 0 bits. "Zero" is represented by another level (usually a positive voltage). [EDIT ] NON-RETURN-TO-ZERO INVERTED (NRZI) In clock language, in bipolar NRZ-Level the voltage "swings" from positive to negative on the trailing edge of the previous bit clock cycle. Example NRZI encoding From Wikipedia, the free encyclopedia Jump to: navigation, search In telecommunication, Manchester code (also known as Phase NRZ-transition occurs for a zero Encoding, or PE) is a line code in which the encoding of each data bit has at least one transition and occupies the same time. Non return to zero, inverted (NRZI) is a method of mapping a It therefore has no DC component, and is self-clocking, which binary signal to a physical signal for transmission over some means that it may be inductively or capacitively coupled, and transmission media.