COSC 3213 – Computer Networks I Summer 2003 Topics:

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. Overview • 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. • In this chapter, we will cover four different types of pre-processing techniques • Digital Data, digital signal • Analog Data, digital signal • Encoding: appropriate representation of data into signals • Decoding: inverse of encoding

3. Overview (2) • Digital Data, analog signal • 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. 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 (A2) + 1/2 x (0) = A2 / 2 • Average value of signal = A / 2 Volts 0 1 0 1 1 1 1 0 0 Unipolar NRZ

5. 0 1 0 1 1 1 1 0 0 Unipolar NRZ NRZI Line Coding (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 = A2 / 4 Half the power used as compared to Unipolar NRZ with same distance between levels Average value of signal = 0 Volts

6. 0 1 0 1 1 1 1 0 0 Unipolar NRZ NRZI Line Coding (3) Pseudoternary Bit 0: positive or negative voltage, alternating for successive 0’s Bit 1: no line signal Average transmitted power per pulse = A2 / 8 if bit 0 and 1 are equiprobable Average value of signal = 0 Volts Pseudoternary

7. NRZI Pseudoternary Line Coding (4) Bipolar AMI: Bit 0: no line signal Bit 1: positive or negative voltage, alternating for successive 0’s Average transmitted power per pulse = A2 / 8 if bit 0 and 1 are equiprobable Average value of signal = 0 Volts 0 1 0 1 1 1 1 0 0 Unipolar NRZ Bipolar AMI

8. A/2 A/2 -A/2 -A/2 NRZI Pseudoternary Manchester Line Coding (5) Manchester: 0 1 0 1 1 1 1 0 0 Unipolar NRZ Bipolar AMI

9. NRZI Pseudoternary Manchester Differential Manchester Line Coding (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 0 1 0 1 1 1 1 0 0 Unipolar NRZ Bipolar AMI

10. Comparison of Line Codes (1) • Following are the important selection criterion: • 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 • Synchronization: locate the beginning and end of the pulse from the line codes • Error detection: Built some error-detection capability in line codes • Signal Interference: Make line codes less susceptible to distortion introduced by a second signal sharing the medium. • Noise Immunity: Minimize the effect of noise • Complexity: Make the encoder and decoder simpler to implement. Low signaling rate typically means lower cost.

11. 1.2 NRZL, NRZI Multilevel binary (AMI,Bipolar, Pseudot.) Biphase (Manchester, Diff. Manchester) 1 0.8 0.6 power density 0.4 0.2 f / R 0 1 0 2 0.6 0.8 1.2 1.4 0.2 0.4 1.6 1.8 -0.2 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. Comparison of Line Codes (3)

13. 1 1 0 0 0 0 0 0 - 0 + 0 0 0 0 + 0 - 1 1 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 000 + - 0 - + if the last voltage level was positiveReplace an octet of eight zeros with 000 - + 0 + - if the last voltage level was negative Bipolar- AMI B8ZS

14. 1 0 1 1 0 1 6T 2T 4T 5T 3T T 0 Digital Data, Analog Signal – ASK (1) • Amplitude Shift Keying (ASK): Information t ASK

15. 1 0 1 1 0 1 6T 6T 2T 2T 4T 4T 5T 5T 3T 3T T T 0 0 t ASK t FSK Digital Data, Analog Signal – FSK (2) • Frequency Shift Keying (FSK): Information

16. 1 0 1 1 0 1 6T 6T 6T 2T 2T 2T 4T 4T 4T 5T 5T 5T 3T 3T 3T T T T 0 0 0 t ASK t FSK t PSK Digital Data, Analog Signal – PSK (3) Phase Shift Keying (PSK): Information

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

19. T 5D/2 5D/2 D/2 D/2 Sampling -D/2 -D/2 -5D/2 -5D/2 Analogue Signal:Defined for all timeCan have any amplitude Discrete-time Signal: Defined for multiples of T Can have any amplitude Analog Data, Digital Signals – PCM (1) • There are two steps involved in converting analog data to a digital signal: • 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:

20. T 5D/2 D/2 T -D/2 Quantization -5D/2 5D/2 Digital Signal (PCM):Defined for multiples of TAmplitude limited to a few levels D/2 -D/2 -5D/2 Discrete-time Signal:Defined for multiples of TCan have any amplitude Analog Data, Digital Signals – PCM (2) There are two steps involved in converting an analogue signal to a digital signal: Quantization: approximate signal to certain levels. Number of levels used determine the resolution. SNR introduced by Quantization: (20 log10L + 1.76) dB where L = # levels = 2n

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 = log2(L) = log2 (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. 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: • Start the approximated signal at a quantized level close to the analog signal • At the next sampling interval, if the level of the analog signal: • increases, the amplitude of the approximated signal is increased by d. • Decreases, the amplitude of the approximated signal is decreased by d. • If the output of (2a) results in +d, represent the delta modulated signal by bit 1. If the output of (2a) results in -d, represent the delta modulated signal by bit 0.

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

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

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 fc to produce a signal s(t) whose bandwidth is centered at fc. • There are three different forms of analog modulation:

26. Comparison (2)

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).