Book of Data, Communication, and Network
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Don't forget to check out the Online Learning Center, www.mhhe.com/forouzan for
additional resources!
Instructors and students using Data Communications and Networking, Fourth Edition
by Behrouz A. Forouzan will find a wide variety of resources available at the Online
Learning Center, www.mhhe.comlforouzan
Instructor Resources
Instructors can access the following resources by contacting their McGraw-Hill Repre-
sentative for a secure password.
a PowerPoint Slides. Contain figures, tables, highlighted points, and brief descriptions
of each section.
o Complete Solutions Manual. Password-protected solutions to all end-of-chapter
problems are provided.
a Pageout. A free tool that helps you create your own course website.
D Instructor Message Board. Allows you to share ideas with other instructors
using the text.
Student Resources
The student resources are available to those students using the book. Once you have
accessed the Online Learning Center, click on "Student Resources," then select a chap-
ter from the drop down menu that appears. Each chapter has a wealth of materials to
help you review communications and networking concepts. Included are:
a Chapter Summaries. Bulleted summary points provide an essential review of
major ideas and concepts covered in each chapter.
a Student Solutions Manual. Contains answers for odd-numbered problems.
o Glossary. Defines key terms presented in the book.
o Flashcards. Facilitate learning through practice and review.
a Animated Figures. Visual representations model key networking concepts, bringing
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tant ideas from the book.
a Web links. Connect students to additional resources available online.
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DATA
COMMUNICATIONS
AND
NETWORKING
Fourth Edition
Behrouz A. Forouzan
DeAnza College
with
Sophia Chung Fegan
• Higher Education
Boston Burr Ridge, IL Dubuque, IA Madison, WI New York San Francisco S1. Louis
Bangkok Bogota Caracas Kuala Lumpur Lisbon London Madrid Mexico City
Milan Montreal New Delhi Santiago Seoul Singapore Sydney Taipei Toronto
Halaman 13 dari 1.171
x CONTENTS
2.2 THE OSI MODEL 29
Layered Architecture 30
Peer-to-Peer Processes 30
Encapsulation 33
2.3 LAYERS IN THE OSI MODEL 33
Physical Layer 33
Data Link Layer 34
Network Layer 36
Transport Layer 37
Session Layer 39
Presentation Layer 39
Application Layer 41
Summary of Layers 42
2.4 TCP/IP PROTOCOL SUITE 42
Physical and Data Link Layers 43
Network Layer 43
Transport Layer 44
Application Layer 45
2.5 ADDRESSING 45
Physical Addresses 46
Logical Addresses 47
Port Addresses 49
Specific Addresses 50
2.6 RECOMMENDED READING 50
Books 51
Sites 51
RFCs 51
2.7 KEY lERMS 51
2.8 SUMMARY 52
2.9 PRACTICE SET 52
Review Questions 52
Exercises 53
Research Activities 54
PART 2 Physical Layer and Media 55
Chapter 3 Data and Signals 57
3.1 ANALOG AND DIGITAL 57
Analog and Digital Data 57
Analog and Digital Signals 58
Periodic and Nonperiodic Signals 58
3.2 PERIODIC ANALOG SIGNALS 59
Sine Wave 59
Phase 63
Wavelength 64
Time and Frequency Domains 65
Composite Signals 66
Bandwidth 69
3.3 DIGITAL SIGNALS 71
Bit Rate 73
Bit Length 73
Digital Signal as a Composite Analog Signal 74
Transmission of Digital Signals 74
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3.4 TRANSMISSION IMPAIRMENT 80
Attenuation 81
Distortion 83
Noise 84
3.5 DATA RATE LIMITS 85
Noiseless Channel: Nyquist Bit Rate 86
Noisy Channel: Shannon Capacity 87
Using Both Limits 88
3.6 PERFORMANCE 89
Bandwidth 89
Throughput 90
Latency (Delay) 90
Bandwidth-Delay Product 92
Jitter 94
3.7 RECOMMENDED READING 94
Books 94
3.8 KEYTERMS 94
3.9 SUMMARY 95
3.10 PRACTICE SET 96
Review Questions 96
Exercises 96
Chapter 4 Digital Transmission 101
4.1 DIGITAL-TO-DIGITAL CONVERSION 101
Line Coding 101
Line Coding Schemes 106
Block Coding 115
Scrambling 118
4.2 ANALOG-TO-DIGITAL CONVERSION 120
Pulse Code Modulation (PCM) 121
Delta Modulation (DM) 129
4.3 TRANSMISSION MODES 131
Parallel Transmission 131
Serial Transmission 132
4.4 RECOMMENDED READING 135
Books 135
4.5 KEYTERMS 135
4.6 SUMMARY 136
4.7 PRACTICE SET 137
Review Questions 137
Exercises 137
Chapter 5 Analog TranSl1'lission 141
5.1 DIGITAL-TO-ANALOG CONVERSION 141
Aspects of Digital-to-Analog Conversion 142
Amplitude Shift Keying 143
Frequency Shift Keying 146
Phase Shift Keying 148
Quadrature Amplitude Modulation 152
5.2 ANALOG-TO-ANALOG CONVERSION 152
Amplitude Modulation 153
Frequency Modulation 154
Phase Modulation 155
CONTENTS xi
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xii CONTENTS
5.3 RECOMMENDED READING 156
Books 156
5.4 KEY lERMS 157
5.5 SUMMARY 157
5.6 PRACTICE SET 158
Review Questions 158
Exercises 158
Chapter 6 Ba17chridth Utili::.ation: Multiplexing
and Spreading 161
6.1 MULTIPLEXING 161
Frequency-Division Multiplexing 162
Wavelength-Division Multiplexing 167
Synchronous Time-Division Multiplexing 169
Statistical Time-Division Multiplexing 179
6.2 SPREAD SPECTRUM 180
Frequency Hopping Spread Spectrum (FHSS) 181
Direct Sequence Spread Spectrum 184
6.3 RECOMMENDED READING 185
Books 185
6.4 KEY lERMS 185
6.5 SUMMARY 186
6.6 PRACTICE SET 187
Review Questions 187
Exercises 187
Chapter 7 Transmission Media 191
7.1 GUIDED MEDIA 192
Twisted-Pair Cable 193
Coaxial Cable 195
Fiber-Optic Cable 198
7.2 UNGUIDED MEDIA: WIRELESS 203
Radio Waves 205
Microwaves 206
Infrared 207
7.3 RECOMMENDED READING 208
Books 208
7.4 KEY lERMS 208
7.5 SUMMARY 209
7.6 PRACTICE SET 209
Review Questions 209
Exercises 210
Chapter 8 Svvitching 213
8.1 CIRCUIT-SWITCHED NETWORKS 214
Three Phases 217
Efficiency 217
Delay 217
Circuit-Switched Technology in Telephone Networks 218
8.2 DATAGRAM NETWORKS 218
Routing Table 220
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CONTENTS xiii
Efficiency 220
Delay 221
Datagram Networks in the Internet 221
8.3 VIRTUAL-CIRCUIT NETWORKS 221
Addressing 222
Three Phases 223
Efficiency 226
Delay in Virtual-Circuit Networks 226
Circuit-Switched Technology in WANs 227
8.4 STRUCTURE OF A SWITCH 227
Structure of Circuit Switches 227
Structure of Packet Switches 232
8.5 RECOMMENDED READING 235
Books 235
8.6 KEY TERMS 235
8.7 SUMMARY 236
8.8 PRACTICE SET 236
Review Questions 236
Exercises 237
Chapter 9 Using Telephone and Cable Networks for Data
Transm,ission 241
9.1 1ELEPHONE NETWORK 241
Major Components 241
LATAs 242
Signaling 244
Services Provided by Telephone Networks 247
9.2 DIAL-UP MODEMS 248
Modem Standards 249
9.3 DIGITAL SUBSCRIBER LINE 251
ADSL 252
ADSL Lite 254
HDSL 255
SDSL 255
VDSL 255
Summary 255
9.4 CABLE TV NETWORKS 256
Traditional Cable Networks 256
Hybrid Fiber-Coaxial (HFC) Network 256
9.5 CABLE TV FOR DATA TRANSFER 257
Bandwidth 257
Sharing 259
CM and CMTS 259
Data Transmission Schemes: DOCSIS 260
9.6 RECOMMENDED READING 261
Books 261
9.7 KEY TERMS 261
9.8 SUMMARY 262
9.9 PRACTICE SET 263
Review Questions 263
Exercises 264
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xiv CONTENTS
PART 3 Data Link Layer 265
Chapter 10 Error Detection and Correction 267
10.1 INTRODUCTION 267
Types of Errors 267
Redundancy 269
Detection Versus Correction 269
Forward Error Correction Versus Retransmission 269
Coding 269
Modular Arithmetic 270
10.2 BLOCK CODING 271
Error Detection 272
Error Correction 273
Hamming Distance 274
Minimum Hamming Distance 274
10.3 LINEAR BLOCK CODES 277
Minimum Distance for Linear Block Codes 278
Some Linear Block Codes 278
10.4 CYCLIC CODES 284
Cyclic Redundancy Check 284
Hardware Implementation 287
Polynomials 291
Cyclic Code Analysis 293
Advantages of Cyclic Codes 297
Other Cyclic Codes 297
10.5 CHECKSUM 298
Idea 298
One's Complement 298
Internet Checksum 299
10.6 RECOMMENDED READING 30 I
Books 301
RFCs 301
10.7 KEY lERMS 301
10.8 SUMMARY 302
10.9 PRACTICE SET 303
Review Questions 303
Exercises 303
Chapter 11 Data Link Control 307
11.1 FRAMING 307
Fixed-Size Framing 308
Variable-Size Framing 308
11.2 FLOW AND ERROR CONTROL 311
Flow Control 311
Error Control 311
11.3 PROTOCOLS 311
11.4 NOISELESS CHANNELS 312
Simplest Protocol 312
Stop-and-Wait Protocol 315
11.5 NOISY CHANNELS 318
Stop-and-Wait Automatic Repeat Request 318
Go-Back-N Automatic Repeat Request 324
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11.6
11.7
11.8
11.9
11.10
11.11
Selective Repeat Automatic Repeat Request
Piggybacking 339
HDLC 340
Configurations and Transfer Modes 340
Frames 341
Control Field 343
POINT-TO-POINT PROTOCOL 346
Framing 348
Transition Phases 349
Multiplexing 350
Multilink PPP 355
RECOMMENDED READING 357
Books 357
KEY TERMS 357
SUMMARY 358
PRACTICE SET 359
Review Questions 359
Exercises 359
332
CONTENTS xv
Chapter 12 Multiple Access 363
12.1 RANDOMACCESS 364
ALOHA 365
Carrier Sense Multiple Access (CSMA) 370
Carrier Sense Multiple Access with Collision Detection (CSMAlCD) 373
Carrier Sense Multiple Access with Collision Avoidance (CSMAlCA) 377
12.2 CONTROLLED ACCESS 379
Reservation 379
Polling 380
Token Passing 381
12.3 CHANNELIZATION 383
Frequency-Division Multiple Access (FDMA) 383
Time-Division Multiple Access (TDMA) 384
Code-Division Multiple Access (CDMA) 385
12.4 RECOMMENDED READING 390
Books 391
12.5 KEY TERMS 391
12.6 SUMMARY 391
12.7 PRACTICE SET 392
Review Questions 392
Exercises 393
Research Activities 394
Chapter 13 Wired LANs: Ethernet 395
13.1 IEEE STANDARDS 395
Data Link Layer 396
Physical Layer 397
13.2 STANDARD ETHERNET 397
MAC Sublayer 398
Physical Layer 402
13.3 CHANGES IN THE STANDARD 406
Bridged Ethernet 406
Switched Ethernet 407
Full-Duplex Ethernet 408
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CONTENTS xxi
22.5 RECOMMENDED READING 694
Books 694
Sites 694
RFCs 694
22.6 KEY lERMS 694
22.7 SUMMARY 695
22.8 PRACTICE SET 697
Review Questions 697
Exercises 697
Research Activities 699
PART 5 Transport Layer 701
Chapter 23 Process-fa-Process Delivery: UDp, TCp,
and SeTP 703
23.1 PROCESS-TO-PROCESS DELIVERY 703
Client/Server Paradigm 704
Multiplexing and Demultiplexing 707
Connectionless Versus Connection-Oriented Service 707
Reliable Versus Unreliable 708
Three Protocols 708
23.2 USER DATAGRAM PROTOCOL (UDP) 709
Well-Known Ports for UDP 709
User Datagram 710
Checksum 711
UDP Operation 713
Use ofUDP 715
23.3 TCP 715
TCP Services 715
TCP Features 719
Segment 721
A TCP Connection 723
Flow Control 728
Error Control 731
Congestion Control 735
23.4 SCTP 736
SCTP Services 736
SCTP Features 738
Packet Format 742
An SCTP Association 743
Flow Control 748
Error Control 751
Congestion Control 753
23.5 RECOMMENDED READING 753
Books 753
Sites 753
RFCs 753
23.6 KEY lERMS 754
23.7 SUMMARY 754
23.8 PRACTICE SET 756
Review Questions 756
Exercises 757
Research Activities 759
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xxii CONTENTS
Chapter 24 Congestion Control and Quality (~j'Service 767
24.1 DATA lRAFFIC 761
Traffic Descriptor 76]
Traffic Profiles 762
24.2 CONGESTION 763
Network Performance 764
24.3 CONGESTION CONTROL 765
Open-Loop Congestion Control 766
Closed-Loop Congestion Control 767
24.4 lWO EXAMPLES 768
Congestion Control in TCP 769
Congestion Control in Frame Relay 773
24.5 QUALITY OF SERVICE 775
Flow Characteristics 775
Flow Classes 776
24.6 TECHNIQUES TO IMPROVE QoS 776
Scheduling 776
Traffic Shaping 777
Resource Reservation 780
Admission Control 780
24.7 INTEGRATED SERVICES 780
Signaling 781
Flow Specification 781
Admission 781
Service Classes 781
RSVP 782
Problems with Integrated Services 784
24.8 DIFFERENTIATED SERVICES 785
DS Field 785
24.9 QoS IN SWITCHED NETWORKS 786
QoS in Frame Relay 787
QoS inATM 789
24.10 RECOMMENDED READING 790
Books 791
24.11 KEY TERMS 791
24.12 SUMMARY 791
24.13 PRACTICE SET 792
Review Questions 792
Exercises 793
PART 6 Application Layer 795
Chapter 25 DO/nain Name Svstem 797
25.1 NAME SPACE 798
Flat Name Space 798
Hierarchical Name Space 798
25.2 DOMAIN NAME SPACE 799
Label 799
Domain Narne 799
Domain 801
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25.3
25.4
25.5
25.6
25.7
25.8
25.9
25.10
25.11
25.12
25.13
25.14
DISTRIBUTION OF NAME SPACE 801
Hierarchy of Name Servers 802
Zone 802
Root Server 803
Primary and Secondary Servers 803
DNS IN THE INTERNET 803
Generic Domains 804
Country Domains 805
Inverse Domain 805
RESOLUTION 806
Resolver 806
Mapping Names to Addresses 807
Mapping Address to Names 807
Recursive Resolution 808
Iterative Resolution 808
Caching 808
DNS MESSAGES 809
Header 809
TYPES OF RECORDS 811
Question Record 811
Resource Record 811
REGISTRARS 811
DYNAMIC DOMAIN NAME SYSTEM (DDNS)
ENCAPSULATION 812
RECOMMENDED READING 812
Books 813
Sites 813
RFCs 813
KEY TERMS 813
SUMMARY 813
PRACTICE SET 814
Review Questions 814
Exercises 815
812
CONTENTS xxiii
Chapter 26 Remote Logging, Electronic Mail, and File Transfer 817
26.1 REMOTE LOGGING 817
TELNET 817
26.2 ELECTRONIC MAIL 824
Architecture 824
User Agent 828
Message Transfer Agent: SMTP 834
Message Access Agent: POP and IMAP 837
Web-Based Mail 839
26.3 FILE TRANSFER 840
File Transfer Protocol (FTP) 840
Anonymous FTP 844
26.4 RECOMMENDED READING 845
Books 845
Sites 845
RFCs 845
26.5 KEY lERMS 845
26.6 SUMMARY 846
Halaman 27 dari 1.171
xxiv CONTENTS
26.7 PRACTICE SET 847
Review Questions 847
Exercises 848
Research Activities 848
Chapter 27 WWW and HTTP 851
27.1 ARCHITECTURE 851
Client (Browser) 852
Server 852
Uniform Resource Locator 853
Cookies 853
27.2 WEB DOCUMENTS 854
Static Documents 855
Dynamic Documents 857
Active Documents 860
27.3 HTTP 861
HTTP Transaction 861
Persistent Versus Nonpersistent Connection 868
Proxy Server 868
27.4 RECOMMENDED READING 869
Books 869
Sites 869
RFCs 869
27.5 KEY 1ERMS 869
27.6 SUMMARY 870
27.7 PRACTICE SET 871
Review Questions 871
Exercises 871
Chapter 28 Network Management: SNMP 873
28.1 NETWORK MANAGEMENT SYSTEM 873
Configuration Management 874
Fault Management 875
Performance Management 876
Security Management 876
Accounting Management 877
28.2 SIMPLE NETWORK MANAGEMENT PROTOCOL (SNMP) 877
Concept 877
Management Components 878
Structure of Management Information 881
Management Information Base (MIB) 886
Lexicographic Ordering 889
SNMP 891
Messages 893
UDP Ports 895
Security 897
28.3 RECOMMENDED READING 897
Books 897
Sites 897
RFCs 897
28.4 KEY 1ERMS 897
28.5 SUMMARY 898
Halaman 29 dari 1.171
xxvi CONTENTS
Modern Round Ciphers 940
Mode of Operation 945
30.3 ASYMMETRIC-KEY CRYPTOGRAPHY 949
RSA 949
Diffie-Hellman 952
30.4 RECOMMENDED READING 956
Books 956
30.5 KEY TERMS 956
30.6 SUMMARY 957
30.7 PRACTICE SET 958
Review Questions 958
Exercises 959
Research Activities 960
Chapter 31 Network Security 961
31.1 SECURITY SERVICES 961
Message Confidentiality 962
Message Integrity 962
Message Authentication 962
Message Nonrepudiation 962
Entity Authentication 962
31.2 MESSAGE CONFIDENTIALITY 962
Confidentiality with Symmetric-Key Cryptography 963
Confidentiality with Asymmetric-Key Cryptography 963
31.3 MESSAGE INTEGRITY 964
Document and Fingerprint 965
Message and Message Digest 965
Difference 965
Creating and Checking the Digest 966
Hash Function Criteria 966
Hash Algorithms: SHA-1 967
31.4 MESSAGE AUTHENTICATION 969
MAC 969
31.5 DIGITAL SIGNATURE 971
Comparison 97I
Need for Keys 972
Process 973
Services 974
Signature Schemes 976
31.6 ENTITY AUTHENTICATION 976
Passwords 976
Challenge-Response 978
31.7 KEY MANAGEMENT 981
Symmetric-Key Distribution 981
Public-Key Distribution 986
31.8 RECOMMENDED READING 990
Books 990
31.9 KEY TERMS 990
31.10 SUMMARY 991
31.11 PRACTICE SET 992
Review Questions 992
Exercises 993
Research Activities 994
Halaman 119 dari 1.171
82 CHAPTER 3 DATA AND SIGNALS
Example 3.26
Suppose a signal travels through a transmission medium and its power is reduced to one-half.
This means that P2 = PI' In this case, the attenuation (loss of power) can be calculated as
Pz 0.5PI 1010glO - = 1010gl0 -- = 10 Ioglo0.5 = 10(-0.3) = -3 dB
PI PI
A loss of 3 dB (-3 dB) is equivalent to losing one-half the power.
Example 3.27
A signal travels through an amplifier, and its power is increased 10 times. This means that Pz=
1OPI' In this case, the amplification (gain of power) can be calculated as
Example 3.28
One reason that engineers use the decibel to measure the changes in the strength of a signal is that
decibel numbers can be added (or subtracted) when we are measuring several points (cascading)
instead ofjust two. In Figure 3.27 a signal travels from point 1 to point 4. The signal is attenuated
by the time it reaches point 2. Between points 2 and 3, the signal is amplified. Again, between
points 3 and 4, the signal is attenuated. We can find the resultant decibel value for the signal just
by adding the decibel measurements between each set of points.
Figure 3.27 Decibelsfor Example 3.28
I dB
:1 7dB 'I' -3 dB
Point 3 Transmission Point 4
medium
Transmission Point 2
medium
Point 1
1_:_---'--'-=-.3 dB __•
1
• ----'----
In this case, the decibel value can be calculated as
dB=-3+7-3=+1
The signal has gained in power.
Example 3.29
Sometimes the decibel is used to measure signal power in milliwatts. In this case, it is referred to
as dBm and is calculated as dBm = 10 loglO Pm' where Pm is the power in milliwatts. Calculate
the power of a signal if its dBm =-30.
Halaman 120 dari 1.171
SECTION 3.4 TRANSMISSION IMPAIRMENT 83
Solution
We can calculate the power in the signal as
dBm = 10 log10 Pm = -30
loglO Pm:= -3 Pm =10-3 rnW
Example 3.30
The loss in a cable is usually defined in decibels per kilometer (dB/km). If the signal at the
beginning of a cable with -0.3 dBlkm has a power of 2 mW, what is the power of the signal
at 5 km?
Solution
The loss in the cable in decibels is 5 x (-0.3)::: -1.5 dB. We can calculate the power as
Distortion
Distortion means that the signal changes its form or shape. Distortion can occur in a
composite signal made of different frequencies. Each signal component has its own
propagation speed (see the next section) through a medium and, therefore, its own
delay in arriving at the final destination. Differences in delay may create a difference in
phase if the delay is not exactly the same as the period duration. In other words, signal
components at the receiver have phases different from what they had at the sender. The
shape of the composite signal is therefore not the same. Figure 3.28 shows the effect of
distortion on a composite signal.
Figure 3.28 Distortion
Composite signal
sent
At the sender
Components,
in phase
Composite signal
received
Components,
out of phase
At the receiver
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84 CHAPTER 3 DATA AND SIGNALS
Noise
Noise is another cause of impairment. Several types of noise, such as thermal noise,
induced noise, crosstalk, and impulse noise, may corrupt the signal. Thermal noise is
the random motion of electrons in a wire which creates an extra signal not originally
sent by the transmitter. Induced noise comes from sources such as motors and appli-
ances. These devices act as a sending antenna, and the transmission medium acts as the
receiving antenna. Crosstalk is the effect of one wire on the other. One wire acts as a
sending antenna and the other as the receiving antenna. Impulse noise is a spike (a sig-
nal with high energy in a very short time) that comes from power lines, lightning, and so
on. Figure 3.29 shows the effect of noise on a signal. We discuss error in Chapter 10.
Figure 3.29 Noise
Transmitted
I
I
I
•
Point 1
Noise
Transmission medium
I
I
I
•
Point 2
Signal-to-Noise Ratio (SNR)
As we will see later, to find the theoretical bit rate limit, we need to know the ratio of
the signal power to the noise power. The signal-to-noise ratio is defined as
SNR =average signal power
average noise power
We need to consider the average signal power and the average noise power because
these may change with time. Figure 3.30 shows the idea of SNR.
SNR is actually the ratio of what is wanted (signal) to what is not wanted (noise).
A high SNR means the signal is less corrupted by noise; a low SNR means the signal is
more corrupted by noise.
Because SNR is the ratio of two powers, it is often described in decibel units,
SNRdB, defined as
SNRcm = lOloglo SNR
Example 3.31
The power of a signal is 10 mW and the power of the noise is 1 /lW; what are the values of SNR
and SNRdB?
Halaman 143 dari 1.171
106 CHAPTER 4 DIGITAL TRANSMISSION
At 1 Mbps, the receiver receives 1,001,000 bps instead of 1,000,000 bps.
1,000,000 bits sent 1,001,000 bits received 1000 extra bps
Built-in Error Detection It is desirable to have a built-in error-detecting capability
in the generated code to detect some of or all the errors that occurred during transmis-
sion. Some encoding schemes that we will discuss have this capability to some extent.
Immunity to Noise and Interference Another desirable code characteristic is a code I
that is immune to noise and other interferences. Some encoding schemes that we will
discuss have this capability.
Complexity A complex scheme is more costly to implement than a simple one. For
example, a scheme that uses four signal levels is more difficult to interpret than one that
uses only two levels.
Line Coding Schemes
We can roughly divide line coding schemes into five broad categories, as shown in
Figure 4.4.
Figure 4.4 Line coding schemes
Multitransition -- MLT-3
Multilevel -- 2B/IQ, 8B/6T, and 4U-PAM5
Line coding
Unipolar
Polar
Bipolar
--NRZ
NRZ, RZ, and biphase (Manchester.
and differential Manchester)
-- AMI and pseudoternary
There are several schemes in each category. We need to be familiar with all
schemes discussed in this section to understand the rest of the book. This section can be
used as a reference for schemes encountered later.
Unipolar Scheme
In a unipolar scheme, all the signal levels are on one side of the time axis, either above
or below.
NRZ (Non-Return-to-Zero) Traditionally, a unipolar scheme was designed as a
non-return-to-zero (NRZ) scheme in which the positive voltage defines bit I and the
zero voltage defines bit O. It is called NRZ because the signal does not return to zero at
the middle of the bit. Figure 4.5 show a unipolar NRZ scheme.
Halaman 144 dari 1.171
SECTION 4.1 DIGITAL-TO-DIGITAL CONVERSION 107
Figure 4.5 Unipolar NRZ scheme
Amplitude
v
1 f 0 f : 1 I 0 I
o 1------'1---1---1--+---.--__
I Time Nonnalized power
Compared with its polar counterpart (see the next section), this scheme is very
costly. As we will see shortly, the normalized power (power needed to send 1 bit per
unit line resistance) is double that for polar NRZ. For this reason, this scheme is nor-
mally not used in data communications today.
Polar Schemes
In polar schemes, the voltages are on the both sides of the time axis. For example, the
voltage level for 0 can be positive and the voltage level for I can be negative.
Non-Return-to-Zero (NRZ) In polar NRZ encoding, we use two levels of voltage
amplitude. We can have two versions of polar NRZ: NRZ-Land NRZ-I, as shown in
Figure 4.6. The figure also shows the value of r, the average baud rate, and the band-
width. In the first variation, NRZ-L (NRZ-Level), the level of the voltage determines
the value of the bit. In the second variation, NRZ-I (NRZ-Invert), the change or lack of
change in the level of the voltage determines the value of the bit. If there is no change,
the bit is 0; if there is a change, the bit is 1.
Figure 4.6 Polar NRZ-L and NRZ-I schemes
T=:= 1 Save "'NIl
p
0:~illdWidth oG""Iil""""'~I=-=-"""'r'-----'l..
o I 2 fIN
Time
Time
1 : 1 0
I
I
011I
I
NRZ-I f-----I----J---I---+--+--+----+----'--~
NRZ-L f--+--1---I---+--I---1------t----'--~
o No inversion: Next bit is 0 • Inversion: Next bit is 1
In NRZ-L the level of the voltage determines the value of the bit. In NRZ-I
the inversion or the lack of inversion determines the value of the bit.
Let us compare these two schemes based on the criteria we previously defined.
Although baseline wandering is a problem for both variations, it is twice as severe in
NRZ-L. If there is a long sequence of Os or Is in NRZ-L, the average signal power
Halaman 145 dari 1.171
108 CHAPTER 4 DIGITAL TRANSMISSION
becomes skewed. The receiver might have difficulty discerning the bit value. In NRZ-I
this problem occurs only for a long sequence of as. If somehow we can eliminate the
long sequence of as, we can avoid baseline wandering. We will see shortly how this can
be done.
The synchronization problem (sender and receiver clocks are not synchronized)
also exists in both schemes. Again, this problem is more serious in NRZ-L than in
NRZ-I. While a long sequence of as can cause a problem in both schemes, a long
sequence of 1s affects only NRZ-L.
Another problem with NRZ-L occurs when there is a sudden change of polarity in
the system. For example, if twisted-pair cable is the medium, a change in the polarity of
the wire results in all as interpreted as Is and all Is interpreted as as. NRZ-I does not
have this problem. Both schemes have an average signal rate ofNI2 Bd.
NRZ-L and NRZ-J both have an average signal rate ofNI2 Bd.
Let us discuss the bandwidth. Figure 4.6 also shows the normalized bandwidth for
both variations. The vertical axis shows the power density (the power for each I Hz of
bandwidth); the horizontal axis shows the frequency. The bandwidth reveals a very
serious problem for this type of encoding. The value of the power density is velY high
around frequencies close to zero. This means that there are DC components that carry a
high level of energy. As a matter of fact, most of the energy is concentrated in frequen-
cies between a and NIl. This means that although the average of the signal rate is N12,
the energy is not distributed evenly between the two halves.
NRZ-L and NRZ-J both have a DC component problem.
Example 4.4
A system is using NRZ-I to transfer 10-Mbps data. What are the average signal rate and mini-
mum bandwidth?
Solution
The average signal rate is S =NI2 =500 kbaud. The minimum bandwidth for this average baud
rate is Bnlin = S = 500 kHz.
Return to Zero (RZ) The main problem with NRZ encoding occurs when the sender
and receiver clocks are not synchronized. The receiver does not know when one bit has
ended and the next bit is starting. One solution is the return-to-zero (RZ) scheme,
which uses three values: positive, negative, and zero. In RZ, the signal changes not
between bits but during the bit. In Figure 4.7 we see that the signal goes to 0 in the mid-
dle of each bit. It remains there until the beginning of the next bit. The main disadvan-
tage of RZ encoding is that it requires two signal changes to encode a bit and therefore
occupies greater bandwidth. The same problem we mentioned, a sudden change of
polarity resulting in all as interpreted as 1s and all 1s interpreted as as, still exist here,
but there is no DC component problem. Another problem is the complexity: RZ uses
three levels of voltage, which is more complex to create and discern. As a result of all
these deficiencies, the scheme is not used today. Instead, it has been replaced by the
better-performing Manchester and differential Manchester schemes (discussed next).
Halaman 146 dari 1.171
SECTION 4.1 DIGITAL-TO-DIGITAL CONVERSION 109
Figure 4.7 Polar RZ scheme
Amplitude
o l
l
l
1 l
1
l
Time
p
o:lL
o I
o 1 2 fiN
Biphase: Manchester and Differential Manchester The idea of RZ (transition at
the middle of the bit) and the idea of NRZ-L are combined into the Manchester scheme.
In Manchester encoding, the duration of the bit is divided into two halves. The voltage
remains at one level during the first half and moves to the other level in the second half.
The transition at the middle of the bit provides synchronization. Differential Manchester,
on the other hand, combines the ideas of RZ and NRZ-I. There is always a transition at
the middle of the bit, but the bit values are determined at the beginning of the bit. Ifthe
next bit is 0, there is a transition; if the next bit is 1, there is none. Figure 4.8 shows
both Manchester and differential Manchester encoding.
Figure 4.8 Polar biphase: Manchester and differential Manchester schemes
( Ois L lis S )
0 I 1 I 0 I 0 I 1 I 1 I
I I I I I I
I I I I I I
..... I
r+- 1,...- I _, I 1 --4 I I I I
Manchester I I I I
Time l..+- I -I -+- I- I
I I
I I l I I I
r¢- I,.... 1,...- I
rt- I
I I
Differential I I I
Manchester I I I I
1 Time L...¢- I I
1
- I
1 I I I I I
o No inversion: Next bit is 1 • Inversion: Next bit is 0
p
11 Bandwidth
O.~~~
o 1
)0
2 fiN
In Manchester and differential Manchester encoding, the transition
at the middle of the bit is used for synchronization.
The Manchester scheme overcomes several problems associated with NRZ-L, and
differential Manchester overcomes several problems associated with NRZ-I. First, there
is no baseline wandering. There is no DC component because each bit has a positive and
Halaman 157 dari 1.171
120 CHAPTER 4 DIGITAL TRANSMISSION
maintain the even number of nonzero pulses after each substitution. The two rules can be
stated as follows:
1. If the number of nonzero pulses after the last substitution is odd, the substitution
pattern will be OOOV, which makes the total number of nonzero pulses even.
2. If the number of nonzero pulses after the last substitution is even, the substitution
pattern will be BOOV, which makes the total number of nonzero pulses even.
Figure 4.20 shows an example.
Figure 4.20 Different situations in HDB3 scrambling technique
First
substitution
Second Third
substitution substitution
t
Even
t t
Even Odd Even Even
There are several points we need to mention here. First, before the first substitu-
tion, the number of nonzero pulses is even, so the first substitution is BODY. After this
substitution, the polarity of the 1 bit is changed because the AMI scheme, after each
substitution, must follow its own rule. After this bit, we need another substitution,
which is OOOV because we have only one nonzero pulse (odd) after the last substitution.
The third substitution is BOOV because there are no nonzero pulses after the second
substitution (even).
HDB3 substitutes four consecutive zeros with OOOV or BOOV depending
on the number of nonzero pulses after the last substitution.
4.2 ANALOG-TO-DIGITAL CONVERSION
The techniques described in Section 4.1 convert digital data to digital signals. Some-
times, however, we have an analog signal such as one created by a microphone or cam-
era. We have seen in Chapter 3 that a digital signal is superior to an analog signal. The
tendency today is to change an analog signal to digital data. In this section we describe
two techniques, pulse code modulation and delta modulation. After the digital data are
created (digitization), we can use one of the techniques described in Section 4.1 to con-
vert the digital data to a digital signal.
Halaman 158 dari 1.171
SECTION 4.2 ANALOG-TO-DIGITAL CONVERSION 121
Pulse Code Modulation (PCM)
The most common technique to change an analog signal to digital data (digitization)
is called pulse code modulation (PCM). A PCM encoder has three processes, as shown
in Figure 4.21.
Figure 4.21 Components ofPCM encoder
Quantized signal
~'-'-T
! .• ··1······ :t i 'i
t=lJl::I::=t=J;.:;=~,
peM encoder
~. HSampling; IQuantizing J- Encoding J4 11 "'11°°1
Digital data
Analog signal
tuL I . I I.,
PAM signal
1. The analog signal is sampled.
2. The sampled signal is quantized.
3. The quantized values are encoded as streams of bits.
Sampling
The first step in PCM is sampling. The analog signal is sampled every Ts s, where Ts is
the sample interval or period. The inverse of the sampling interval is called the sam-
pling rate or sampling frequency and denoted by is, where is = IITs
' There are three
sampling methods-ideal, natural, and flat-top-as shown in Figure 4.22.
In ideal sampling, pulses from the analog signal are sampled. This is an ideal sam-
pling method and cannot be easily implemented. In natural sampling, a high-speed
switch is turned on for only the small period of time when the sampling occurs. The
result is a sequence of samples that retains the shape of the analog signal. The most
common sampling method, called sample and hold, however, creates flat-top samples
by using a circuit.
The sampling process is sometimes referred to as pulse amplitude modulation
(PAM). We need to remember, however, that the result is still an analog signal with
nonintegral values.
Sampling Rate One important consideration is the sampling rate or frequency. What
are the restrictions on Ts? This question was elegantly answered by Nyquist. According
to the Nyquist theorem, to reproduce the original analog signal, one necessary condition
is that the sampling rate be at least twice the highest frequency in the original signal.
Halaman 159 dari 1.171
122 CHAPTER 4 DIGITAL TRANSMISSION
Figure 4.22 Three different sampling methodsfor PCM
Amplitude Amplitude
,~Analog signal
...
" s
a. Ideal sampling
Amplitude
c. Flat-top sampling
Time
b. Natural sampling
/ Analog signal
...
According to the Nyquist theorem, the sampling rate must be
at least 2 times the highest frequency contained in the signal.
We need to elaborate on the theorem at this point. First, we can sample a signal
only if the signal is band-limited. In other words, a signal with an infinite bandwidth
cannot be sampled. Second, the sampling rate must be at least 2 times the highest fre-
quency, not the bandwidth. If the analog signal is low-pass, the bandwidth and the
highest frequency are the same value. If the analog signal is bandpass, the bandwidth
value is lower than the value of the maximum frequency. Figure 4.23 shows the value
of the sampling rate for two types of signals.
Figure 4.23 Nyquist sampling rate for low-pass and bandpass signals -------------------------------------
Amplitude
Nyquist rate = 2 x fm"x
Low-pass signal
frnin
Amplitude
o
Nyquist rate =2 x fmax
Bandpass signal
f min
f max Frequency
Frequency
Halaman 168 dari 1.171
SECTION 4.3 TRANSMISSION MODES 131
Adaptive DA1
A better performance can be achieved if the value of 0 is not fixed. In adaptive delta
modulation, the value of 0 changes according to the amplitude of the analog signal.
Quantization Error
It is obvious that DM is not perfect. Quantization error is always introduced in the pro-
cess. The quantization error of DM, however, is much less than that for PCM.
4.3 TRANSMISSION MODES
Of primary concern when we are considering the transmission of data from one device
to another is the wiring, and of primary concern when we are considering the wiring is
the data stream. Do we send 1 bit at a time; or do we group bits into larger groups and,
if so, how? The transmission of binary data across a link can be accomplished in either
parallel or serial mode. In parallel mode, multiple bits are sent with each clock tick.
In serial mode, 1 bit is sent with each clock tick. While there is only one way to send
parallel data, there are three subclasses of serial transmission: asynchronous, synchro-
nous, and isochronous (see Figure 4.31).
Figure 4.31 Data transmission and modes
Data transmission
Parallel Transmission
Binary data, consisting of Is and Os, may be organized into groups of n bits each.
Computers produce and consume data in groups of bits much as we conceive of and use
spoken language in the form of words rather than letters. By grouping, we can send
data n bits at a time instead of 1. This is called parallel transmission.
The mechanism for parallel transmission is a conceptually simple one: Use n wires
to send n bits at one time. That way each bit has its own wire, and all n bits of one
group can be transmitted with each clock tick from one device to another. Figure 4.32
shows how parallel transmission works for n =8. Typically, the eight wires are bundled
in a cable with a connector at each end.
The advantage of parallel transmission is speed. All else being equal, parallel
transmission can increase the transfer speed by a factor of n over serial transmission.
Halaman 169 dari 1.171
132 CHAPTER 4 DIGITAL TRANSMISSION
Figure 4.32 Parallel transmission
The 8 bits are sent together
,--" j/ ,/'-,.
I v \ I \ f < I \ 1
< 1 I
Sender I v I Receiver
I v j
v I \ \ < \ I A'\ /
'::/ // "-J
We need eight lines
But there is a significant disadvantage: cost. Parallel transmission requires n communi-
cation lines (wires in the example) just to transmit the data stream. Because this is
expensive, parallel transmission is usually limited to short distances.
Serial Transmission
In serial transmission one bit follows another, so we need only one communica-
tion channel rather than n to transmit data between two communicating devices (see
Figure 4.33).
Figure 4.33 Serial transmission
We need only
one line (wire).
o
1
1
Sender 0
o
o
1
o
The 8 bits are sent
one after another.
o
1
o 00010 1
-1-----------+1 g Receiver
o
1
o
Parallel/serial
converter
Serial/parallel
converter
The advantage of serial over parallel transmission is that with only one communi-
cation channel, serial transmission reduces the cost of transmission over parallel by
roughly a factor of n.
Since communication within devices is parallel, conversion devices are required at
the interface between the sender and the line (parallel-to-serial) and between the line
and the receiver (serial-to-parallel).
Serial transmission occurs in one of three ways: asynchronous, synchronous, and
isochronous.
Halaman 170 dari 1.171
SECTION 4.3 TRANSMISSION MODES 133
Asynchronous Transmission
Asynchronous transmission is so named because the timing of a signal is unimportant.
Instead, information is received and translated by agreed upon patterns. As long as those
patterns are followed, the receiving device can retrieve the information without regard
to the rhythm in which it is sent. Patterns are based on grouping the bit stream into
bytes. Each group, usually 8 bits, is sent along the link as a unit. The sending system
handles each group independently, relaying it to the link whenever ready, without regard
to a timer.
Without synchronization, the receiver cannot use timing to predict when the next
group will arrive. To alert the receiver to the arrival of a new group, therefore, an extra
bit is added to the beginning of each byte. This bit, usually a 0, is called the start bit.
To let the receiver know that the byte is finished, 1 or more additional bits are appended
to the end of the byte. These bits, usually Is, are called stop bits. By this method, each
byte is increased in size to at least 10 bits, of which 8 bits is information and 2 bits or
more are signals to the receiver. In addition, the transmission of each byte may then be
followed by a gap of varying duration. This gap can be represented either by an idle
channel or by a stream of additional stop bits.
In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more
stop bits (Is) at the end of each byte. There may be a gap between each byte.
The start and stop bits and the gap alert the receiver to the beginning and end of
each byte and allow it to synchronize with the data stream. This mechanism is called
asynchronous because, at the byte level, the sender and receiver do not have to be syn-
chronized. But within each byte, the receiver must still be synchronized with the
incoming bit stream. That is, some synchronization is required, but only for the dura-
tion of a single byte. The receiving device resynchronizes at the onset of each new byte.
When the receiver detects a start bit, it sets a timer and begins counting bits as they
come in. After n bits, the receiver looks for a stop bit. As soon as it detects the stop bit,
it waits until it detects the next start bit.
Asynchronous here means "asynchronous at the byte level;'
but the bits are still synchronized; their durations are the same.
Figure 4.34 is a schematic illustration of asynchronous transmission. In this exam-
ple, the start bits are as, the stop bits are 1s, and the gap is represented by an idle line
rather than by additional stop bits.
The addition of stop and start bits and the insertion of gaps into the bit stream
make asynchronous transmission slower than forms of transmission that can operate
without the addition of control information. But it is cheap and effective, two advan-
tages that make it an attractive choice for situations such as low-speed communication.
For example, the connection of a keyboard to a computer is a natural application for
asynchronous transmission. A user types only one character at a time, types extremely
slowly in data processing terms, and leaves unpredictable gaps of time between each
character.
Halaman 172 dari 1.171
SECTION 4.5 KEY TERMS 135
The advantage of synchronous transmission is speed. With no extra bits or gaps to
introduce at the sending end and remove at the receiving end, and, by extension, with
fewer bits to move across the link, synchronous transmission is faster than asynchro-
nous transmission. For this reason, it is more useful for high-speed applications such as
the transmission of data from one computer to another. Byte synchronization is accom-
plished in the data link layer.
We need to emphasize one point here. Although there is no gap between characters
in synchronous serial transmission, there may be uneven gaps between frames.
Isochronous
In real-time audio and video, in which uneven delays between frames are not accept-
able, synchronous transmission fails. For example, TV images are broadcast at the rate
of 30 images per second; they must be viewed at the same rate. If each image is sent
by using one or more frames, there should be no delays between frames. For this type
of application, synchronization between characters is not enough; the entire stream of
bits must be synchronized. The isochronous transmission guarantees that the data
arrive at a fixed rate.
4.4 RECOMMENDED READING
For more details about subjects discussed in this chapter, we recommend the following
books. The items in brackets [...] refer to the reference list at the end of the text.
Books
Digital to digital conversion is discussed in Chapter 7 of [Pea92], Chapter 3 of
[CouOl], and Section 5.1 of [Sta04]. Sampling is discussed in Chapters 15, 16, 17, and
18 of [Pea92], Chapter 3 of [CouO!], and Section 5.3 of [Sta04]. [Hsu03] gives a good
mathematical approach to modulation and sampling. More advanced materials can be
found in [Ber96].
4.5 KEY TERMS
adaptive delta modulation
alternate mark inversion (AMI)
analog-to-digital conversion
asynchronous transmission
baseline
baseline wandering
baud rate
biphase
bipolar
bipolar with 8-zero substitution (B8ZS)
bit rate
block coding
companding and expanding
data element
data rate
DC component
delta modulation (DM)
differential Manchester
digital-to-digital conversion
digitization
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