Data and Computer Communications by William Stallings Eighth Edition

1. Signal Encoding Techniques Data and Computer Communications by William Stallings Eighth Edition Click to edit Master subtitle style Chapter 5 Networks and Communication Department

2. Introduction • Digital data, digital signals: simplest form of digital encoding of digital data • the equipment is less complex and less expensive than digital-to-analog • Using Line coding Technique. • Digital data, analog signal: A modem converts digital data to an analog signal so that it can be transmitted over an analog medium. • Analog data, digital signals: Analog data, such as voice and video, are often digitized to be able to use digital transmission facilities. • 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 • Eg:Voice transmission • Q)What type of signal we should use? • It depends on the situation and available bandwidth

3. Signal Encoding Techniques

4. Digital Data, Digital Signal • Digital signal • A digital signal is a sequence of discrete, discontinuous voltage pulses • Each bit is a signal element • binary data encoded into signal elements • the equipment for encoding digital data into a digital signal is less complex and less expensive than digital-to-analog modulation equipment

5. Some Terms • Data rate - Rate of data (R) transmission in bits per second • Duration or length of a bit - Time taken for transmitter to emit the bit (1/R) • Modulation rate -Rate at which the signal level changes, measured in baud = signal elements per second. Depends on type of digital encoding used. • Mark and Space - Binary 1 and Binary 0 respectively

6. Some Terms • Line coding schemes: • Unipolar – It uses only one voltage level • Polar – use two voltage levels ,one positive and the other one negative • Bipolar-use three voltage levels ,positive voltage ,0 voltage and negative voltage

7. Interpreting Digital Signals • Receiver needs to know • timing of bits - when they start and end • signal levels • Four factors affecting signal interpretation • signal to noise ratio • data rate • Bandwidth • An increase in data rate increases bit error rate (BER). • An increase in SNR decreases bit error rate. • An increase in bandwidth allows an increase in data rate. • encoding scheme – affects performance

8. Comparison of Encoding Schemes • Ways of evaluating or comparing the various encoding techniques: • 1.Signal Spectrum - Lack of high frequencies reduces required bandwidth, lack of dc component provide isolation. • 2.Number of signal levels: two levels (for binary) , or multilevel • Two data levels, two signal levels Two data levels , three signal levels • 3.Clocking - need for synchronizing transmitter and receiver either with an external clock or with a sync mechanism based on signal • 4.Error detection - useful if can be built in to signal encoding • 5.Signal interference and noise immunity - some codes are better than others • 6.Cost and complexity - Higher signal rate (& thus data rate) lead to higher costs,

9. Encoding Schemes • Encoding scheme is the mapping from data bits to signal element. • They include: • Nonreturn to Zero-Level (NRZ-L) • Nonreturn to Zero Inverted (NRZI) • Bipolar -AMI • Pseudoternary • Manchester • Differential Manchester

10. Encoding Schemes

11. Nonreturn to Zero-Level (NRZ-L) • two different voltages for 0 and 1 bits • A positive voltage means 0 ,while negative means 1 • voltage constant during bit interval • no transition i.e. no return to zero voltage • Lack of synchronization when the data contain a long streams of 0s or 1s.

12. Nonreturn to Zero- Inverted(NRZ-I) • A 0 bit represented by no change, while inverted on 1 • data encoded as presence or absence of signal transition at beginning of bit time • data is represented by changes rather than levels • transition (low to high or high to low) denotes binary 1 • no transition denotes binary 0 • Loss of synchronization when the data contain a long streams of 0s

13. NRZ Pros & Cons • Pros • easy to engineer • make good use of bandwidth • Cons • dc component • lack of synchronization capability

14. Multilevel Binary Bipolar-AMI • Bipolar-AMI • Use three voltage levels (Multilevel) • 0 represented by zero level • 1 represented by alternating positive and negative pulse • no loss of sync if a long string of 1s. • but Loss of synchronization when the data contain a long streams of 0s • no net dc component • Less bandwidth than the bandwidth for NRZ • easy error detection

15. Multilevel Binary Pseudoternary • Bipolar • Use three voltage levels (Multilevel) • 1 represented by zero level • 0 represented by alternating positive and negative pulse • no loss of sync if a long string of 0s. • but Loss of synchronization when the data contain a long streams of 1s • no advantage or disadvantage over bipolar-AMI

16. Multilevel Binary Issues • synchronization with long runs of 0’s or 1’s • not as efficient as NRZ • each signal element only represents one bit • receiver distinguishes between three levels: +A, -A, 0

17. Manchester Encoding • overcomes the limitations of NRZ codes • has transition in the middle of each bit period • there is a transition at the middle of each bit period • Mid-bit transition is used for both synchronization(clocking)and data representation • low to high represents 1 • high to low represents 0

18. Differential Manchester Encoding • overcomes the limitations of NRZ codes • Mid-bit transition is used for synchronization(clocking) only transition at start of bit period representing 0 • no transition at start of bit period representing 1

19. Biphase Pros and Cons • Con • requires more bandwidth • Pros • synchronization on mid bit transition , the receiver can synchronize on that transition, known as self-clocking codes. • has no dc component • has error detection: The absence of an expected transition can be used to detect errors

20. Problems • Q1. Assume a stream of ten 1’s. Encode the stream using the following schemes: • a)unipolar ,b)NRZ-L,c) NRZ-I, e)Manchester,f)Differential Manchester.

21. Problems • Q2. For the Manchester encoded binary stream of the following page, extract the clock information and the data sequence. 1 0 1 0 0 1 1 1 0 0 1

22. Digital Data, Analog Signal • Modulation: The process by which some characteristics of the carrier, ie(amplitude/frequency/phase) is varied in accordance with the instantaneous value of the modulating signal • main use is public telephone system • has freq range of 300Hz to 3400Hz • use modem (modulator-demodulator) • encoding techniques • Amplitude shift keying (ASK) • Frequency shift keying (FSK) • Phase shift keying (PSK)

23. Modulation Techniques

24. Amplitude Shift Keying • Values represented by two different amplitudes of the carrier. • usually have one amplitude zero • susceptible to sudden gain changes • inefficient • used for • up to 1200bps • very high speeds over optical fiber

25. Binary Frequency Shift Keying • most common is binary FSK (BFSK) • two binary values represented by two different frequencies • less susceptible to error than ASK • used for • up to 1200bps • high frequency radio transmission • higher frequency on LANs using coaxial cable

26. Phase Shift Keying • phase of carrier signal is shifted to represent data • binary PSK • two phases represent two binary digits • differential PSK • phase shifted relative to previous transmission rather than some constant reference signal • a binary 0 is represented by sending a signal burst of the same phase as the previous signal burst sent. • A binary 1 is represented by sending a signal burst of opposite phase to the preceding one.

27. DPSK

28. Quadrature PSK • get more efficient use if each signal element represents more than one bit • e.g. shifts of /2 or (90) • each element represents two bits

29. Analog Data, Digital Signal • digitization is conversion of analog data into digital data which can then: • be transmitted using NRZ-L • be transmitted using code other than NRZ-L • be converted to analog signal using one of the modulation techniques • Codec (coder-decoder) : device used for converting analog data to digital form . two principal techniques used in codecs: • pulse code modulation • delta modulation

30. Digitizing Analog Data

31. Pulse Code Modulation (PCM) • Pulse code modulation (PCM) is based on the sampling theorem • sampling theorem: • “If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all information in original signal” • e.g. if voice data is limited to frequencies below 4000 Hz , then requires 8000 sample per sec • Strictly: these are analog samples called: • Pulse Amplitude Modulation (PAM) • To convert to digital, each of these analog samples must be assigned a binary code.

32. PCM Example

33. PCM Block Diagram

34. Non-Linear Coding

35. Delta Modulation • analog input is approximated by a staircase function • can move up or down one level () at each sample interval • has binary behavior • since function only moves up or down at each sample interval • hence can encode each sample as single bit • 1 for up or 0 for down

36. Delta Modulation Example

37. PCM verses Delta Modulation • DM has simplicity compared to PCM • but has worse SNR • PCM preferred to DM for analog signals

38. Summary • looked at signal encoding techniques • digital data, digital signal • digital data, analog signal • analog data, digital signal