COSC 3213 – Computer Networks ISummer 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

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

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

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

1 0 1 0 1 1 0 01

UnipolarNRZ

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 = A2 / 4

Half the power used as compared to Unipolar NRZ with same distance between levels

Average value of signal = 0 Volts

1 0 1 0 1 1 0 01

UnipolarNRZ

NRZI

6

Line Coding (3)

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

1 0 1 0 1 1 0 01

UnipolarNRZ

NRZI

Pseudoternary

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 = A2 / 8 if bit 0 and 1 are equiprobable

Average value of signal = 0 Volts

1 0 1 0 1 1 0 01

UnipolarNRZ

NRZI

Pseudoternary

Bipolar AMI

8

Line Coding (5)

5. Manchester:

1 0 1 0 1 1 0 01

UnipolarNRZ

NRZI

Pseudoternary

Bipolar AMI

A/2-A/2

A/2-A/2

Manchester

9

Line Coding (6)

6. Differential Manchester: Always a transition in the middle of intervalBit 1: no transition at beginning of intervalBit 0: transition at beginning of interval

1 0 1 0 1 1 0 01

UnipolarNRZ

NRZI

Pseudoternary

Bipolar AMI

Manchester

DifferentialManchester

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

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

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0

0.2

0.4

0.6

0.8 1

1.2

1.4

1.6

1.8 2

f / R

pow

er d

ensi

ty

NRZL, NRZI

Multilevel binary (AMI,Bipolar, Pseudot.)

Biphase (Manchester, Diff. Manchester)

12

Comparison of Line Codes (3)

NRZNRZ Multilevel BinaryMultilevel Binary BiphaseBiphase

Bandwidth Smallest Same as NRZ Double of NRZ or Multilevel Binary

DC component Presence of DC component leads to power wastage

Zero DC component 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 Detection No capability No 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

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 positiveReplace an octet of eight zeros with if the last voltage level was negative

Bipolar- AMI

1 0 0 0 0 0 0 00 1

1 0 0 0 0 1

B8ZS

14

Digital Data, Analog Signal – ASK (1)

Amplitude Shift Keying (ASK):

Information 1 1 1 10 0

0 T 2T 3T 4T 5T 6TASK t

0binary

1binary

0

)2sin()(

tfAts c

15

Digital Data, Analog Signal – FSK (2)

Frequency Shift Keying (FSK):

Information 1 1 1 10 0

0 T 2T 3T 4T 5T 6TASK t

0 T 2T 3T 4T 5T 6TFSK t

0binary

1binary

)2sin(

)2sin()(

2

1

tfA

tfAts

16

Digital Data, Analog Signal – PSK (3)

Phase Shift Keying (PSK):

Information 1 1 1 10 0

0 T 2T 3T 4T 5T 6TFSK t

0 T 2T 3T 4T 5T 6TASK t

0 T 2T 3T 4T 5T 6TPSK t

0binary

1binary

)2sin(

)2sin()(

tfA

tfAts

c

c

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?

00binary

01binary

10binary

11binary

)4/72sin(

)4/52sin(

)4/32sin(

)4/2sin(

)(

tfA

tfA

tfA

tfA

ts

c

c

c

c

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.

levels of No.:PSK Multilevel

:PSK

:FSK

factor; rolloff:ASK

LRL

rB

RrB

fffRrfB

rrRrB

T

T

T

T

;log

)1(;)1(

;)1(2

10;)1(

2

12

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:

signal the infrequency maximum x 2 T) / (1 rate Sampling

Sampling

Analogue Signal:Defined for all timeCan have any amplitude

Discrete-time Signal:Defined for multiples of TCan have any amplitude

T

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 TAmplitude limited to a few levels

T

Discrete-time Signal:Defined for multiples of TCan have any amplitude

T

SNR introduced by Quantization: (20 log10 L + 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 voiceSampling rate (1 / T) >= 2 x 4000 or 8000 samples/secondSampling period (T) = 1 / 8000 = 125 microseconds

For digital telephony, no. of levels (L) used in the uniform quantizer are 256Number 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:

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

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:

index modulation:ModulationFrequency

index modulation:Modulation Phase

index modulation:Modulation Amplitude

ffcc

ppcc

aca

ntmntttfAts

ntmntttfAts

ntftxnts

);()()(2cos)(

);()()(2cos)(

2cos)](1[)(

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).

)(2

)1(2

2

tsA

nAn

BB

BB

m

f

mp

T

T

of value maximum the is and

FM for

PM forwhere

:FM / PM

:AM