1. COSC 3213 – Computer Networks I Summer 2003 Topics: 1. Line Coding (Digital Data, Digital Signals) 2. Digital Modulation (Digital Data, Analog Signals) 3. PCM and Delta Modulation (Analog Data, Digital Signals) 4. Analog Modulation (Analog Data, Analog Signals) Stallings: Sections 5.1 – 5.4

2. 2 Overview 1.Recall Data: information that needs to be transmitted, e.g. voice signal, binary file. Signal: waveform used to transmit data, e.g., sine wave or line codes Some pre-processing stage is needed to convert data into a signal. 2.In this chapter, we will cover four different types of pre-processing techniques a)Digital Data, digital signal b)Analog Data, digital signal Encoding: appropriate representation of data into signals Decoding: inverse of encoding

3. 3 Overview (2) c)Digital Data, analog signal d)Analog Data, analog signal Modulation: is a process of shifting the frequency content of the signal to a higher frequency. Allows multiple users to use the same channel simultaneously by selecting a different carrier frequency for each user (broadband) Demodulation: is the inverse of modulation.

4. 4 Line Coding (1) Line Coding: Converts digital data (a binary sequence) into a digital signal  NRZ-L:Bit 0 is represented by a higher level (+A Volts) Bit 1 is represented by a lower level (0 Volts) Average transmitted power per pulse = 1/2 x (A 2 ) + 1/2 x (0) = A 2 / 2 Average value of signal = A / 2 Volts 101 0 11001 Unipolar NRZ

5. 5 Line Coding (2) 2.NRZI (Nonreturn to Zero Inverted):Bit 0: No transition at beginning of interval Bit 1: Transition at beginning of interval Average transmitted power per pulse = A 2 / 4 Half the power used as compared to Unipolar NRZ with same distance between levels Average value of signal = 0 Volts 101 0 11001 Unipolar NRZ NRZI

6. 6 Line Coding (3) 3.PseudoternaryBit 0: positive or negative voltage, alternating for successive 0’s Bit 1: no line signal Average transmitted power per pulse = A 2 / 8 if bit 0 and 1 are equiprobable Average value of signal = 0 Volts 101 0 11001 Unipolar NRZ NRZI Pseudoternary

7. 7 Line Coding (4) 4.Bipolar AMI:Bit 0: no line signal Bit 1: positive or negative voltage, alternating for successive 0’s Average transmitted power per pulse = A 2 / 8 if bit 0 and 1 are equiprobable Average value of signal = 0 Volts 101 0 11001 Unipolar NRZ NRZI Pseudoternary Bipolar AMI

8. 8 Line Coding (5) 5.Manchester: 101 0 11001 Unipolar NRZ NRZI Pseudoternary Bipolar AMI A/2 -A/2 A/2 -A/2 Manchester

9. 9 Line Coding (6) 6.Differential Manchester: Always a transition in the middle of interval Bit 1: no transition at beginning of interval Bit 0: transition at beginning of interval 101 0 11001 Unipolar NRZ NRZI Pseudoternary Bipolar AMI Manchester Differential Manchester

10. 10 Comparison of Line Codes (1) Following are the important selection criterion: 1.Signal Spectrum:  Lesser Bandwidth is preferable  Lack of dc component is preferable  More spectral power in the middle of the spectrum rather than at the edges is preferable 2.Synchronization: locate the beginning and end of the pulse from the line codes 3.Error detection: Built some error-detection capability in line codes 4.Signal Interference: Make line codes less susceptible to distortion introduced by a second signal sharing the medium. 5.Noise Immunity: Minimize the effect of noise 6.Complexity: Make the encoder and decoder simpler to implement. Low signaling rate typically means lower cost.

11. 11 Comparison of Line Codes: Spectrum (2) Power spectra of different line coding schemes: NRZ: Used for lowpass channels. Limitation: DC component Multilevel Binary: No DC, Same bandwidth required as NRZ, energy concentrated in mid frequencies. Biphase: No DC; Double BW required

12. 12 Comparison of Line Codes (3) NRZ Multilevel Binary Biphase BandwidthSmallestSame as NRZ Double of NRZ or Multilevel Binary DC component Presence of DC component leads to power wastage Zero DC component Synchronization String of continuous 0s (and 1s) leads to loss in synchronization String of continuous 0s (or 1s) leads to loss of synchronization Transition at middle of pulse allows synchronization Error DetectionNo capability Built in capability because of transition Maximum Modulation Rate Same as data rate. NRZL: for 1010… NRZI: for 1111… Same as data rate. Bipolar: for 1111… Pseudo: for 0000… Double of data rate. For 000…

13. 13 Scrambling Techniques  Recall that Manchester codes are best in terms of synchronization and error detection capabilities but require twice the bandwidth as compared to Multilevel Binary.  Is it possible to include synchronization bits within the waveforms of Multilevel Binary?  Yes! Sequences that result in a constant voltage level are replaced in part by filling segments that provide transitions (Scrambling).  Example: Bipolar AMI – Replace strings of 0’s that result in 0 volts (B8ZS or HDB3)  Bipolar with 8 zeros substitution (B8ZS): Replace an octet of eight zeros with  if the last voltage level was positive Replace an octet of eight zeros with  if the last voltage level was negative Bipolar- AMI 1000000001 1000  01 B8ZS

14. 14 Digital Data, Analog Signal – ASK (1) Amplitude Shift Keying (ASK): Information111 100 0 T 2T2T 3T3T 4T4T5T5T 6T6T ASK t

15. 15 Digital Data, Analog Signal – FSK (2) Frequency Shift Keying (FSK): Information111 100 0 T 2T2T 3T3T 4T4T5T5T 6T6T ASK t 0 T 2T2T 3T3T 4T4T5T5T 6T6T FSK t

16. 16 Digital Data, Analog Signal – PSK (3) Phase Shift Keying (PSK): Information111 100 0 T 2T2T 3T3T 4T4T5T5T 6T6T FSK t 0 T 2T2T 3T3T 4T4T5T5T 6T6T ASK t 0 T 2T2T 3T3T 4T4T5T5T 6T6T PSK t

17. 17 Digital Data, Analog Signal – QPSK (4) Quadrature Phase Shift Keying (QPSK): Example: (1) Draw the waveform for the information bits 0011101101 if the string is coded using QPSK? What is the bit rate of QPSK scheme if the data rate is R bps? (2) How can PSK scheme be extended so that each waveform encodes 3 bits at a time? What is the bit rate of the extended PSK scheme (8-ary PSK) if the data rate is R bps?

18. 18 Digital Data, Analog Signal – Comparison (5)  Comparison of Shift keying schemes is performed on the basis of the transmission bandwidth ( B T ) which is a function of the transmission rate R = 1/ T.  Another parameter used is the bandwidth efficiency ( B T / R ) defined as the ratio of the bandwidth ( B T ) and the transmission rate R = 1/ T.

19. 19 Analog Data, Digital Signals – PCM (1) There are two steps involved in converting analog data to a digital signal: 1.Sampling: obtain the value of signal every T seconds.  Choice of T is determined by how fast a signal changes, i.e., the frequency content of the signal  Nyquist Sampling theorem says: Sampling     Analogue Signal: Defined for all time Can have any amplitude     Discrete-time Signal: Defined for multiples of T Can have any amplitude T

20. 20 Analog Data, Digital Signals – PCM (2) There are two steps involved in converting an analogue signal to a digital signal: 2.Quantization: approximate signal to certain levels. Number of levels used determine the resolution. Quantization     Digital Signal (PCM): Defined for multiples of T Amplitude limited to a few levels T     Discrete-time Signal: Defined for multiples of T Can have any amplitude T SNR introduced by Quantization: (20 log 10 L + 1.76) dB where L = # levels = 2 n

21. 21 Analog Data, Digital Signals – PCM (3) Example: PCM signal obtained for voice signal Voice:maximum frequency = 4 kHz voice Sampling rate (1 / T) >= 2 x 4000 or 8000 samples/second Sampling period (T) = 1 / 8000 = 125 microseconds For digital telephony, no. of levels (L) used in the uniform quantizer are 256 Number of bits (n) required to represent a level = log 2 (L) = log 2 (256) = 8 bits Data rate = 8000 x 8 or 64 kbps Question: Repeat for stereo music system that contains a maximum frequency of 22 kHz. The number of levels used by the uniform quantizer are 64K. Remember there are 2 channels (L & R) in a stereo system. How much data will be generated in one hour?

22. 22 Analog Data, Digital Signals – Delta Modulation (1)  Delta modulation is a scheme used to improve the performance of PCM.  An analog signal is approximated by a staircase function as follows: 1.Start the approximated signal at a quantized level close to the analog signal 2.At the next sampling interval, if the level of the analog signal: a.increases, the amplitude of the approximated signal is increased by . b.Decreases, the amplitude of the approximated signal is decreased by . 3.If the output of (2a) results in , represent the delta modulated signal by bit 1. If the output of (2a) results in , represent the delta modulated signal by bit 0.

23. 23 Analog Data, Digital Signals – Delta Modulation (2) Example:

24. 24 Analog Data, Digital Signals – Delta Modulation (3)

25. 25 Analog Data, Analog Signals: Analog Modulation (1)  Analog Modulation: defined as a process of combining an input signal m(t) and a carrier of frequency f c to produce a signal s(t) whose bandwidth is centered at f c.  There are three different forms of analog modulation:

26. 26 Comparison (2)

27. 27 Bandwidth of Analog Modulation Schemes (3)  In terms of Bandwidth, FM and PM requires higher bandwidth than AM.  Baseband versus Broadband Schemes: Baseband do not modulate the frequency of the information signals. e.g., Line codes, PCM / Delta modulation. Broadband shift the frequency of the information signals to a higher frequency. e.g., FSK/PSK/ASK (digital modulation schemes) or AM/PM/FM (analog modulation schemes).