1 SIGNAL ENCODING TECHNIQUES Networks and Communication Department

1 SIGNAL ENCODING TECHNIQUES Networks and Communication Department NET 352

Lecture Contents 2 Introduction Digital data, Digital signals Key terms Encoding schemes � Nonreturn (NRZ-L to Zero , NRZI) � Multilevel Binary (Bipolar-AMI , Pseudoternary) � Biphase (Manchester 13 -Jan-22 , Differential Manchester) Networks and Communication Department

3 Introduction 13 -Jan-22 Networks and Communication Department

Introduction 4 13 -Jan-22 Networks and Communication Department

Introduction 5 q Digital data, digital signals: simplest form of digital encoding of digital data q q Digital data, analog signal: A modem converts digital data to an analog signal so that it can be transmitted over an analog medium q q Optical fiber and unguided media. Analog data, digital signals: Analog data, such as voice and video, are often digitized to be able to use digital transmission facilities. q q the equipment is less complex and less expensive than digital-to-analog modulation equipment Use of modern digital transmission and switching equipment. Analog data, analog signals: Analog data are modulated by a carrier frequency to produce an analog signal in a different frequency band, which can be utilized on an analog transmission system q Voice transmission and shift the bandwidth to other spectrum. 13 -Jan-22 Networks and Communication Department

6 Digital data, Digital Signal 13 -Jan-22 Networks and Communication Department

Digital Data, Digital Signal Ø Digital signal discrete, discontinuous voltage pulses l Each bit is a signal element l binary data encoded into signal elements l

Some Terms Ø Ø Ø Unipolar - signal elements have the same sign Polar - One logic state represented by positive voltage, other by negative duration or length of a bit modulation rate in (baud) mark and space

Key terms 9 Term Unit Definition Data element Bits A signal binary one or zero Data rate Bits per second (bps) The rate at which data elements are transmitted Signal element Digital: a voltage pulse of constant amplitude The part of a signal that occupies the shortest interval of a signaling code Analog: a pulse of constant frequency, phase, amplitude. Signaling rate or 13 -Jan-22 Modulation rate Signal elements per The rate at which Networks and Communication Department second signal elements are

Interpreting Digital Signals Ø Receiver needs to know timing of bits - when they start and end l signal levels l Ø factors affecting signal interpretation signal to noise ratio l data rate l bandwidth l encoding scheme – affects performance l

Comparison of Encoding Schemes Ø Ø Ø signal spectrum clocking error detection signal interference and noise immunity cost and complexity

Encoding Schemes

Nonreturn to Zero-Level (NRZ-L) Ø Ø two different voltages for 0 and 1 bits voltage constant during bit interval no transition i. e. no return to zero voltage l such as absence of voltage for zero, constant positive voltage for one l more often, negative voltage for one value and positive for the other l

Nonreturn to Zero Inverted (NRZI) Ø Ø Ø Non-return to zero, inverted on ones constant voltage pulse for duration of bit data encoded as presence or absence of signal transition at beginning of bit time l l Ø transition (low to high or high to low) denotes binary 1 no transition denotes binary 0 example of differential encoding since l l l data is represented by changes rather than levels more reliable detection of transition rather than level easy to lose sense of polarity in twisted-pair line (for NRZ-L)

NRZ Pros & Cons Ø Pros easy to engineer l make good use of bandwidth l Ø Cons dc component l lack of synchronization capability l Ø Ø used for magnetic recording not often used for signal transmission

Multilevel Binary Bipolar-AMI Ø Ø Use more than two levels Bipolar-AMI zero represented by no line signal l one represented by positive or negative pulse l ‘One’ pulses alternately in polarity l no loss of sync if a long string of ones l long runs of zeros still a problem l no net dc component l lower bandwidth l easy error detection l

Multilevel Binary Pseudoternary Ø Ø one represented by absence of line signal zero represented by alternating positive and negative no advantage or disadvantage over bipolar. AMI each used in some applications

Multilevel Binary Issues Ø synchronization with long runs of 0’s or 1’s l l Ø can insert additional bits, c. f. ISDN scramble data. not as efficient as NRZ l each signal element only represents one bit l l receiver distinguishes between three levels: +A, -A, 0 a 3 level system could represent log 23 = 1. 58 bits requires approx. 3 d. B more signal power for same probability of bit error

Scrambling Ø Ø use scrambling to replace sequences that would produce constant voltage these filling sequences must l l l Ø produce enough transitions to sync be recognized by receiver & replaced with original data be same length as original, no rate penalty design goals l l have no dc component have no long sequences of zero level line signal have no reduction in data rate give error detection capability

B 8 ZS and HDB 3

B 8 ZS Substitution Rules: • If an octet of all zeros occurs and the last voltage pulse preceding this octet was positive, then the eight zeros of the octet are encoded as 000+– 0–+. • If an octet of all zeros occurs and the last voltage pulse preceding this octet was negative, then the eight zeros of the octet are encoded as 000–+0+–.

HDB 3 Substitution Rules: 22 The fourth zero is replaced with a code violation. Successive violations are of alternate polarity Number of Bipolar Pulses (ones) since Last Substitution Polarity of Preceding Pulse + 13 -Jan-22 Odd 000000+ Even +00+ -00 - Networks and Communication Department

Bi. Phase Manchester Encoding Ø Ø Ø has transition in the middle of each bit period transition serves as clock and data low to high represents one high to low represents zero used by IEEE 802. 3 (Ethernet LAN)

Bi. Phase Differential Manchester Encoding Ø Ø Ø Mid-bit transition is clocking only transition at start of bit period representing 0 no transition at start of bit period representing 1 l Ø this is a differential encoding scheme used by IEEE 802. 5 (Token Ring LAN)

Biphase Pros and Cons Ø Con l l l Ø at least one transition per bit time and possibly two maximum modulation rate is twice NRZ requires more bandwidth Pros l l l synchronization on mid bit transition (self clocking) has no dc component has error detection

Modulation Rate

Problems Ø Ø Q 1. Assume a stream of ten 1’s. Encode the stream using the following schemes: NRZ-I, AMI, Manchester, Differential Manchester. How many transitions (vertical lines) are there for each scheme. Q 2. For the Manchester encoded binary stream of the following, extract the clock information and the data sequence.

Problems Ø Q 3. Consider a stream of binary data consisting of a long sequence of 1 s, followed by a zero, followed by a long sequence of 1 s. Preceding bit and level is indicated within parentheses. Draw the waveforms for NRZI (high), AMI (1 as negative voltage), and pseudo-ternary (0 as negative voltage).

Problems Q 4. The AMI waveform representing a sequence 0100101011 is transmitted over a noisy channel. The received waveform with a single error is shown in the following page. Locate the error with justification.

Problems Ø Ø Ø Q 5. For the received AMI bipolar sequence +000+ - 0 - + which has one violation, construct two possible transmitted pattern that might result in the same received pattern.

Digital data, Digital signal 31 Reference 13 -Jan-22 Data and Computer Communications, Ninth Edition by William Stallings, � Chapter 5 (5. 1) Networks and Communication Department