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Data, Network
&Internet
Communications
Technology
Elahi &Elahi

Ata Elahi, Ph.D
Southern Connecticut State University
Mehran Elahi, Ph.D
Elizabeth City State University
THOIVISON
~
'T' Australia • Canada· Mexico • Singapore • Spain. United Kingdom · United States
DELMAR LEARNING

xi
THOIVISON
~
--r
DELMAR LEARNING
Data, Network, and Internet Communications Technology
Ata Elahi and Mehran Elahi
Vice President, Technology and Marketing Director: Senior Production Manager:
Trades SBU: Dave Garza Larry Main
Alar Elken
Senior Channel Manager: Production Editor:
Editorial Director: Dennis Williams Benj Gleeksman
Sandy Clark
Marketing Coordinator: Art/Design Coordinator:
Senior Acquisitions Editor: Stacey Wiktorek Francis Hogan
Stephen Helba
Production Director: Senior Editorial Assistant:
Senior Development Editor: Mary Ellen Black Dawn Daugherty
Michelle Ruelos Cannistraci
COPYRIGHT © 2006 Thomson Delmar Learning. For permission to use material from the text or product,
Thomson and Delmar Learning are trademarks used contact us by
herein under license. Tel. (800) 730-2214
Fax (800) 730-2215
Printed in the United States of America www.thomsonrights.com
1 2 3 4 5 XX 07 06 05
Library of Congress Cataloging-in-Publication Data:
For more information contact Thomson Delmar Learning Elahi, Ata
Executive Woods . Data, network, and Internet communications
5 Maxwell Drive, PO Box 8007, technology / by Ata Elahi, Mehran Elahi.
Clifton Park, NY 12065-8007 p. cm.
Or find us on the World Wide Web at Includes bibliographical references and index.
www.delmarlearning.com ISBN (invalid) 0-401-87269-7
1. Computer Networks-Textbooks. 2. Data
ALL RIGHTS RESERVED. No part of this work covered
transmission systems--Textbooks. I. Elahi, Mehran.
by the copyright hereon may be reproduced in any form
II. Title.
or by any means--graphic, electronic, or mechanical,
including photocopying, recording, taping, Web distri TK5105.5.E3882005
bution, or information storage and retrieval systems 004.6-dc22 2005041005
without the written permission of the publisher.
ISBN: 1401872697
NOTICE TO THE READER
Publisher does not warrant or guarantee any of the products described herein or perform any independent analysis in
connection with any of the product information contained herein. Publisher does not assume, and expressly disclaims,
any obligation to obtain and include information other than that provided to it by the manufacturer.
The reader is expressly warned to consider and adopt all safety precautions that might be indicated by the activities
herein and to avoid all potential hazards. By following the instructions contained herein, the reader willingly assumes
all risks in connection with such instructions.
The publisher makes no representation or warranties of any kind, including but not limited to, the warranties of fit
ness for particular purpose or merchantability, nor are any such representations implied with respect to the material
set forth herein, and the publisher takes no responsibility with respect to such material. The publisher shall not be li
able for any special, consequential, or exemplary damages resulting, in whole or part, from the readers' use of, or re
liance upon, this material.
Preface
1.1
1.2
1.3
1.4
1.5
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
2.10
2.11
3.1
3.2
3.3
3.4
3.5
3.6
3.7

s Technology
Senior Production Manager:
Larry Main
Production Editor:
Benj Gleeksman
Art/Design Coordinator:
Francis Hogan
Senior Editorial Assistant:
Dawn Daugherty
;e material from the text or product,
14
15
i.com
Cataloging-in-Publication Data:
ld Internet communications
Elahi, Mehran Elahi.
phical references and index.
01-87269-7
works-Textbooks. 2. Data
!ms-Textbooks. I. Elahi, Mehran.
OS
2005041005
perform any independent analysis in
not assume, and expressly disclaims,
'y the manufacturer.
might be indicated by the activities
herein, the reader willingly assumes
not limited to, the warranties of fit
implied with respect to the material
aterial. The publisher shall not be Ii
part, from the readers' use of, or re-
Preface xi
Introduction to Communications Network 1
Introduction 1
1.1 Computer Networks 2
1.2 Network Models 3
1.3 Network Components 5
1.4 Network Topology 5
1.5 Types of Networks 11
Summary 14
Review Questions 15
Data Communications 17
Introduction 18
2.1 Analog Signals 18
2.2 Digital Signals 21
2.3 Binary Numbers 22
2.4 Coding Schemes 24
2.5 Transmission Modes
2.6 Transmission Methods
2.7 Communication Modes
2.8 Signal Transmission
2.9 Digital Signal Encoding
2.10 Error Detection Methods
2.11 Error Correction 39
Summary 43
Review Questions 45
26
30
28
29
30
33
Introduction to Computer Architecture 49
Introduction 49
3.1 Components of a Microcomputer 50
3.2 Memory Hierarchy 58
3.3 Disk Controller 59
3.4 Microcomputer BUS 60
3.5 Plug and Play 62
3.6 FireWire 63
3.7 Intel Microprocessor Family 63

Iv • CONTENTS
3.8 Itanium Architecture 65
Summary 67
Review Questions 68
Communication Channels and Media 71
Introduction 71
4.1 Conductive Media 72
4.2 Fiber-Optic Cable 74
4.3 Wireless Transmission 79
4.4 Transmission Impairment 81
4.5 Bandwidth, Latency, throughput and Channel Capacity 83
Summary 87
Review Questions 89
Multiplexer and Switching Concepts 93
Introduction 93
5.1 Types of Multiplexers 95
5.2 Telephone System Operation 101
5.3 Digitizing Voice 101
5.4 T1 Links 102
5.5 Switching Concepts 103
Summary 107
Review Questions 108
Standard Organizations and OSI Model 111
Introduction 111
6.1 Communication Protocols 112
6.2 The Open System Interconnection Model 113
6.3 Error and Flow Control 117
6.4 Frame Transmission Methods 122
6.5 IEEE 802 Standards Committee 124
Summary 127
Modem, Digital Subscriber Line (DSL), Cable Modem
" and ISDN 133
Introduction 133
7.1 Modem 134
7.2 Digital Subscriber Line (DSL) 140
7.3
7.4
8.1
8.2
8.3 Et
9.1
9.2
9.3
9.4
9.5
9.6
10.1
10.2
10.3
10.4
11.1
11.2
11.3
11.4

71
Channel Capacity 83
111
113
Modem
CONTENTS • v
7.3 Cable Modem 143
7.4 Integrated Services Digital Network (ISDN) 148
Summary 151
Ethernet Technology 157
Introduction 157
8.1 Ethernet Operation 158
8.2 IEEE 802.3 Frame Format 159
8.3 Ethernet Characteristics, Cabling, and Components 161
Summary 167
Token Ring and Token Bus Networking Technology 173
Introduction 173
9.1 Token Ring Operation 174
9.2 Physical Connections 175
9.3 Ring Management 177
9.4 Token and IEEE 802.5 Frame Format 178
9.5 Token Ring NIC and Cable Specifications 181
9.6 Token Bus (IEEE 802.4) 183
Summary 184
Fast Ethernet Technology 187
Introduction 187
10.1 Fast Ethernet Media Types 188
10.2 Fast Ethernet Repeaters 192
10.3 Fast Ethernet Network Diameter 192
10.4 Expanding Fast Ethernet 195
Summary 195
LAN Interconnection Devices 199
Introduction 200
11.1 Repeaters 200
11.2 Bridges 200
11.3 Routers 202
11.4 Gateways 204

vi • CONTENTS
11.5 CSU/DSU 205
11.6 Switches 205
11.7 Virtual LAN (VLAN) 210
Summary 214
Gigabit and 10 Gigabit Ethernet 221
Introduction 221
12.1 Gigabit Ethernet Standards 222
12.2 Characteristics of Gigabit Ethernet 222
12.3 Gigabit Ethernet Protocol Architecture 223
12.4 10 Gigabit Ethernet 228
12.5 10 Gigabit Ethernet Physical Layer 230
Summary 232
Fiber Distributed Data Interface (FOOl) 237
Introduction 237
13.1 FDDI Technology 238
13.2 FDDI Layered Architecture 238
13.3 FDDI Ports 240
13.4 FDDI Access Method 241
13.5 FDDI Fault Tolerance 241
13.6 FDDI Bit Transmission 242
13.7 FDDI Frame and Token Formats 243
13.8 FDDI Backbone 244
Summary 246
17.1
Frame Relay 249
17.
Introduction 249 17.:i
14.1 Frame Relay Network 250 17.
14.2 Components of Frame Relay 251 17.S
14.3 Frame Relay Frame Format 251 17.6
14.4 Frame Relay Operation 253 17.7
Summary 255 17.8

221
222
ernet 222
-hitecture 223
Layer 230
FDDI) 237
238
ts 243
251
251
CONTENTS • vII
Synchronous Optical Transmission (SONET) 259
Introduction 259
15.1 SONET Characteristics 260
15.2 SONET Components 260
15.3 SONET Signal Rates 261
15.4 SONET Frame Format 262
15.5 SONET Multiplexing 262
15.6 Virtual Tributaries 264
Summary 265
Internet Protocols (Part I) 269
Introduction 270
16.1 The Internet Architecture Board (lAB) 272
16.2 TCP/IP Reference Model 274
16.3 TCP/IP Application Level 275
16.4 Transport Level Protocols: UDP and TCP 277
16.5 Internet Level Protocols: IP and ARP 281
16.6 IPv4 Addressing 286
16.7 Assigning IP Addresses 289
16.8 Demultiplexing Information 290
16.9 TCP Connection and Disconnection 291
16.10 Internet Protocol Version 6 (IPv6) 292
16.11 Internet II 297
Summary 299
Internet Protocols (Part II) and MPlS 305
Introduction 305
17.1 Dynamic Host Configuration Protocol (DHCP) 306
17.2 Internet Control Message Protocol (ICMP) 310
17.3 Point-to-Point Protocol (PPP) 312
17.4 Virtual Private Network (VPN) 317
17.5 IP Security (IPsec) Protocol 318
17.6 Routing 321
17.7 Networking Diagnostic Commands 324
17.8 Multi-Protocol Label Switching (MPLS) 328
Summary 331

viii • CONTENTS
Wireless Local Area Network (WLAN) 335
Introduction 336
18.1 WLAN Components 336
18.2 WLAN Topologies 338
18.3 Wireless LAN Technology 339
18.4 WLAN Standards (IEEE 802.11) 341
18.5 Wireless LAN Physical Layers 341
18.6 Physical Layer Architecture 348
18.7 WLAN Medium Access Control (MAC) 350
18.8 MAC Frame Format 352
18.9 WLAN Frame Types 354
18.10 Station]oining a Basic Service Set (BSS) 356
18.11 Roaming 356
18.12 Wi-FI Certification 356
18.13 WLAN Signal Distortion 357
18.14 WLAN Security 357
Summary 360
Bluetooth Technology 365
Introduction 365
19.1 Bluetooth Topology 366
19.2 Bluetooth Protocol Architecture 367
19.3 Bluetooth Physical Links 371
19.4 Bluetooth Error Correction 372
19.5 Bluetooth Packet Types 372
Summary 374
Wireless MAN 379
Introduction 379
20.1 Wireless MAN Topology 380
20.2 Wireless MAN Protocol Architecture 381
20.3 Wireless MAN MAC Layers 381
20.4 Wireless MAN Physical Layers 386
20.5 IEEE 802.16a Physical Layers 389 2:{.1
20.6 WiMAX 390 23.
Summary 391 23.3
Review Questions 392 23.4

CONTENTS • Ix
Voice Over IP Protocol (VoIP) 395
335
Introduction 396
21.1 Voice Quality 396
21.2 Applications of VoIP 397
21.3 VoIP Operation 399
341
41 21.5 H.323 Components 400
21.4 VoIP Protocols and Standards 400
I
8 21.6 H.323 Protocols 401
AC) 350 21.7 H.323 Call Setup 404
21.8 Session Initiation Protocol (SIP) 406
21.9 SIP Components 406
(BSS) 356 1
_ 21.10 SIP Request and Response Commands 408
21.11 SIP Addressing 409
21.12 SIP Connection Operation 409
21.13 VoIP Bandwidth Calculation 412
Summary 413
Asynchronous Transfer Mode (ATM) 417
Introduction 418
22.1 ATM Network Components and Characteristics 419
22.2 ATM Forum 419
I~.
367 22.3 Types of ATM Connection 420
22.4 ATM Switch Operation 421
22.5 ATM Switch Architecture 422
22.6 ATM Connection Setup through Signaling 423
22.7 ATM Cell Format 424
22.8 ATM Protocol 425
22.9 Types of Adaptation Layers 429
22.10 Comparing ATM Network with Gigabit Ethernet
Network 433
Summary 434
e 381
Network Security 439
86
9 Inltroduction 439
23.1 Elements of Network Security 440
23.2 Introduction to Cryptography 440
23.3 Digital Signatures 447
23.4 Kerberos 449

x • CONTENTS
23.5 Certificates 450
23.6 Secure Socket Layer (SSL) Protocol 452
23.7 IEEE 802.1X Extensible Authentication Protocol (EAP) 454
23.8 Firewalls 456
Summary 458
Universal Serial BUS (USB) and PCI-Express 463
Introduction 463
24.1 USB Architecture 464
24.2 USB Packet Format 466
24.3 USB Device Configuration Process 469
24.4 USB Transaction 469
24.5 Split Transaction 472
24.6 PCI-Express BUS 474
24.7 PCI-Express Architecture 474
24.8 PCI-Express Protocol Architecture 475
Summary 476
Appendix A: Computers and Networks Connectors 481
Appendix B: Answers to Odd Numbered Questions 489
Appendix C: Acronyms 523
Appendix D: Glossary 533
Bibliography 551
Web Links 553
Index 555
dents malor!
ing. The con
colleges, whll
level courses.
We have opt__
each technolo
a brief introdu
ply another p
Chapter 1 Is
topologies and t

52
Protocol (EAP) 454
463
69
5
This book is the result of teaching the course, Data Communications and
Computer Networks, at Southern Connecticut State University since
1987. The book covers the technology aspect of networks, rather than
the theory of networks.
The beta version of this textbook was tested in undergraduate-level
computer networks courses at Southern Connecticut State University. The
textbook covers networking using a direct and practical approach that
explains the technology in simple terms. This book covers the latest topics
in networking technology, including DSL, cable modem, ATM, fast ether
net, LAN switching, gigabit ethernet, frame relay, SONET, wireless LAN,
wireless MAN, Voice over IP, and network security.
Intended Audience
This book is written primarily as an introduction to networking for stu
dents majoring in computer science, electronics technology, or engineer
ing. The content is clear and easy to understand for those in technical
colleges, while the broad range of topiCS is appealing for those in higher
level courses. For students in business applications and CIS courses, the
instructor may want to omit part of text.
Organization
The material in this textbook is presented practically rather than taking a
theoretical and mathematical approach. Therefore, no specialized back
ground is required to understand the material; the first three chapters of
the book form the foundation for the rest of the text.
We have opted to focus on the technology aspect of networks, so
each technology is presented in a separate chapter. In addition, we offer
a brief introduction to computer architecture, because networks are sim
ply another part of the computer.
Chapter 1 is an introduction to computer networks, covering network
topologies and types of networks.
Chapter 2 covers basic data communications, including analog sig
nal, digital signal, binary numbers, serial and parallel transmission,

xII • PREFACE
communication modes, digital encoding, error detection methods, and
error correction used in networking. This chapter gives students the
basic knowledge required for the rest of the textbook.
Chapter 3 presents an overview of computer architecture. The net
work is part of the computer; therefore, the reader should have a basic
knowledge of microcomputer architecture. This chapter covers the basic
components of a microcomputer: the CPU, types of memory, and com
puter buses.
Chapter 4 covers data communications media such as twisted-pair
cable, coaxial cable, fiber-optic cable, and wireless communications. It
also covers channel bandwidth, channel capacity, and throughput.
Chapter 5 covers the types of multiplexers (TDM, FDM, SPM, CDM,
and WDM), T1 link architecture, and switching concepts.
Chapter 6 explains the function of standards organizations, and lists
some of the computer protocols, followed by a summary of the open sys
tem model, which explains the function of each layer. IEEE 802 commit
tee standards are also presented.
Chapter 7 presents modem technology, modulation methods, digital
subscriber line (DSL) technology, and cable modem technology.
Chapter 8 covers the Ethernet network, from operation to technical
specifications and cabling. It serves as the foundation for the material on
Fast Ethernet and Gigabit Ethernet.
Chapter 9 explains the operation of the Token Ring network, Token
Ring technical specifications, and Token Bus operation.
Chapter 10 presents Fast Ethernet technology, Fast Ethernet repeaters,
and different types of media used for Fast Ethernet.
Chapter 11 presents networking interconnection devices such as
repeaters, bridges, routers, gateways, and switches. It also explains switch
operations and applications for VLAN.
Chapter 12 explains Gigabit Ethernet technology and the types of
media used for Gigabit Ethernet, followed by an explanation of the
applications of Gigabit Ethernet. It also covers 10 Gigabit Ethernet tech
nology and the type of media used for 10 Gigabit Ethernet.
Chapter 13 covers Fiber Distributed Data Interface (FDOI) technology
and its applications.
Chapter 14
Chapter 15
Chapter 16 covers In
Control Protocol (TCP),
Chapter 17 explains
Internet Message Protoc
Private Network (VPN), f
Chapter 18 presents
topologies, WLAN Med
and WLAN security.
Chapter 19 covers BI
ture, and Bluetooth
Chapter 21 covers
nents of VoIP, Voice over
sion Initiation Protocol
Chapter 22 presents
ture, and ATM Adaptation
Chapter 23 covers
signatures, Kerberos,
802.1x extensible
Chapter 24 covers
mats, USB operations, USB
Features
• Each chapter begin
the chapter Outline
highlighted in bold
chapter. A bulleted
Summary. At the en
questions for a quick
of more challenging
• Each chapter focuse
ing the text flexible

PREFACE • xIII
methods, and Chapter 14 covers Frame Relay technology, operations, and components.
r gives students the
Chapter 15 presents SONET components and architecture.
ok.
Chapter 16 covers Internet architecture, IP addressing, Transmission
chitecture. The net
Control Protocol (TCP), Internet protocol, IPv6, and Internet II.
should have a basic
pter covers the basic Chapter 17 explains Dynamic Host Configuration Protocol (DHCP),
f memory, and com- Internet Message Protocol (ICMP), Point-to-Point Protocol (PPP), Virtual
Private Network (VPN), IP security, and routing tables.
' uch as tWisted-pair Chapter 18 presents wireless LAN (IEEE 802.11 a, b, and g), WLAN
communications. It topologies, WLAN Medium Access Control (MAC), Wi-PI certification,
throughput. and WLAN security.
Chapter 19 covers Bluetooth topology, Bluetooth protocol architec
ture, and Bluetooth Physical link.
nizations, and lists Ch~pter 20 explains wireless MAN (IEEE 802.16) topology, wireless
of the open sys MAN applications, and IEEE 802.16 protocol architecture.
. IEEE 802 commit-
Chapter 21 covers application of Voice over IP Protocol (VoIP), compo
nents of VoIP, Voice over IP protocols and standards, H.323, RTP, and Ses
sion Initiation Protocol (SIP).
Chapter 22 presents ATM network components, ATM switch architec
ture, and ATM Adaptation layer.
Chapter 23 covers elements of network security, cryptography, digital
Signatures, Kerberos, certificates, Secure Socket Layer protocol (SSL), IEEE
network, Token 802.1x extensible authentication protocol (EAP), and firewalls.
Chapter 24 covers architecture of Universal Serial Bus (USB), pact for
mats, USB operations, USB transactions, and PCI Express Bus architecture.
Features
• Each chapter begins with a quick look of topics to be covered in
the chapter Outline and Objectives. Within the text, key terms are
highlighted in bold repeated for reinforcement at the end of the
chapter. A bulleted list of important pOints is listed in the chapter
Summary. At the end of each Chapter is a set of multiple-choice
questions for a quick review of chapter concepts, followed by a set
of more challenging short answer questions.
• Each chapter focuses on a separate networking technology, mak
I) technology
ing the text flexible for readers to pick and choose selected topics.

xlv • PREFACE
• The book covers the latest emerging topics such as Bluetooth tech
nology, which is covered in Chapter 19, Chapter 20 explores
wireless MANs and Chapter 21 introduces Voice over IP Protocol
(VoIP) and Chapter 23 explores security topiCS, such as cryptogra
phy and encryption.
• Chapters 16 and 17 are both devoted to internet topiCS and
TCP/IP.
• Numerous examples and illustrations are clear and help make it
easy to learn some of the more complicated concepts.
• Appendix A provides a list of computers and networks connectors.
• Appendix Bcontains answers to odd-numbered questions.
• Appendix C and 0 includes a list of acronyms and glossary terms.
Supplements
e.resource CD
The following instructional aids are available all on one CD.
• Instructor's Manual contains answers to all end-of-chapter review
questions.
• Exam View Testbank includes over 700 review questions, includ
ing true/false, multiple-choice and short answer.
• PowerPoint Presentation Slides highlight key concepts from
each chapter and can be used as handouts or as a springboard for
lecture and discussion.
• Image Library includes all images from the textbook so instruc
tors can customize handouts, tests, and power point presenta
tions.
Acknowledgments
Many people contributed to the development of this book. We would like
to express our deep appreCiation to Professor Greg Simmons of DeVry
University for his in-depth review of the manuscript, and valuable sugges
tions and comments that enabled us to improve the quality of this text
book. We would like to thank Thomas Sadowski and Megan Daman.
Many thanks to the staff of Thomson Delmar Learning particularly our
Acquistions Editor, Steve Helba, Development Editor Michelle Ruelos
Cannistraci, Production Editor Benj Gleeksman and Marketing Director
Dave Garza.
We would like to thank Dr. Edward Harris Dean of the School of
Communication, Information and Library Science and Professor Winnie
Yu, Chairperson of
University; Dr. Carol
demic Affairs), Dr. Ron.....
Technology), and PlrtlM
ment of Techn()logy)
agement and support
The author would 11
earlier version of N ptwftll
Mike Awwad, 1
Lynette Garctz,
Anu Gokhalc,
Nebojsa Jaosl..,
Steve Kuchler, IV
Dr. William Ll
Indianapolis, IN
Clifford Prescnt,
Richard Rouse, I
The Authors and
reviewers for the flnal VMtWI
tions Technology.
Phil Dumas, DeVry

·s such as Bluetooth tech
9, Chapter 20 explores
?S Voice over IP Protocol
opics, such as cryptogra
to internet topics and
=clear and help make it
~d concepts.
nd networks connectors.
bered questions.
yms and glossary terms.
on one CD.
111 end-of-chapter review
eview questions, includ
nswer.
ght key concepts from
5 or as a springboard for
:he textbook so instruc
power point presenta
his book. We would like
reg Simmons of DeVry
pt, and valuable sugges
the quality of this text
ki and Megan Daman.
arning particularly our
Editor Michelle Ruelos
md Marketing Director
Dean of the School of
e and Professor Winnie
PREFACE • xv
Yu, Chairperson of Computer Science at Southern Connecticut State
University; Dr. Carolyn Mahoney (Provost and Vice Chancellor for Aca
demic Affairs), Dr. Ronald Blackmon (Dean of School of Math, Science &
Technology), and particularly Dr. Akbar Eslami (Chairman of the Depart
ment of Technology) at Elizabeth City State University for their encour
agement and support.
The author would like to thank the following reviewers who reviewed the
earlier version of Network Communications Technology.
Mike Awwad, DeVry University, North Brunswick, NJ
Omar Ba-Rukab, Florida Institute of Technology, Melbourne, FL
Robert Diffenderfer, DeVry University, Kansas City, MO
Enrique Garcia, Laredo Community College, Laredo, TX
Dr. Rafiqul Islam, DeVry University, Alberta, Canada
Lynette Garetz, Heald College, Hayward, CA
Anu Gokhale, Illinois State University, Normal, IL
Nebojsa Jansic, DeVry University, Columbus, OH
Steve Kuchler, IVY Tech, Indianapolis, IN
Dr. William Lin, PhD, Indiana University-Purdue University,
Indianapolis, IN
Clifford Present, DeVry University, Pomona, CA
Glenn Rettig, Owens Community College, Findlay, OH
Richard Rouse, DeVry University, Kansas city, MO
Mohammed Mehdi Shahsavari, Florida Institute of Technology,
Melbourne, FL
Lowell Tawney, DeVry University, Kansas City, MO
Julius Willis, Heald College, Hayward, CA
The Authors and Thomson Delmar Learning would like to thank our
reviewers for the final version of Data, Network and Internet Communica
tions Technology.
Silver Dasgupta, Wentworth Institute of Technology
Phil Dumas, DeVry University, Orlando, FL

xvi • PREFACE
Bob Gill, British Columbia Institute of Technology, Canada
Bob Gruber, Norhtern Virginia Community College, Manassas, VA
William Lin, Indiana University / Purdue University at Indianapolis, IN
John Lyon, Onondaga Community College, Syracuse, NY
William Routt, Wake Technical Community College, Raleigh, NC
Gregory Simmons, DeVry, Columbus, OH
Randy Winzer, Pittsburgh State University, Pittsburgh, KS

ology, Canada
:ollege, Manassas, VA
ersity at Indianapolis, IN
;yracuse, NY
Dedication
:::ollege, Raleigh, NC
This book is dedicated to
our parents, Rahim and Shayesteh.
ttsburgh, KS -Ata & Mehran
my children Shabnam and Aria.
-Ata

Introduction "
to
Communications Networks
_ OUTLINE 1.1 Computer Networks
1.2 Network Models
1.3 Network Components
1.4 Network Topology
1.5 Types of Networks
After completing this chapter, you should be able to:
• Explain the components of a data communication system
• Explain the advantages of computer networks
• Describe the components of a network
• Discuss the function of a client/server model
• Explain various networking topologies
• Describe different types of networks in terms of their advantages and
disadvantages
In order to transfer information from a source to a destination, some hardware
I J RODUCTION
components are required. Figure 1.1 shows the components of a data com
munication model for transmitting information from source to destination.
Source: The source station can be a computer and its function is to
pass information to a transmitter for subsequent transmission.

CHAPTER 1
1.1
FIGURE 1.1 Transmission
Adata com Link
munication
model
Source Destination
Transmitter: The function of a transmitter is to accept information
from the source and change the information such that it is com
patible with the transmission link. Information is then transmit
ted over the communication link. Modem and network cards are
examples of transmitters.
Transmission: The function of a transmission link is to carry
information from a transmitter to a receiver. The transmission link
can be a conductor, a fiber optic cable, or wireless media (air).
Receiver: The receiver accepts information from the transmission
link and information is then converted to the proper form so that
it is acceptable to the destination.
Destination: The receiver passes information to the destination for
processing.
In networking technology both the receiver and transmitter come in one
unit and are usually installed inside the computer, such as network cards
or modems.
Computer Networks
computer networking is a business tool for companies: for example,
a bank can transfer funds between branches by using a network. People
can access their bank accounts using automatic teller machines via a net
work. Travel agencies are using networks to make airline reservations. We
can even make banking transactions and shop online using a network.
Networking technology is growing fast and will enable students to access
the Internet in any campus location using their laptop computers.
Networking is a generic term. Several computers connected together
are called a computer network. A network is a system of computers and
related equipment connected by communication links in order to share
data. The related equipment may be printers, fax machines, modems,
1.2
Peer-to-Peer
Model (Work
the other stations in the network. Indlvr.
copiers, and so forth. Thl' folluwl
computer networks:
Resource sharing: cnmput
as data, printers, <
.Iil.k drlv
Reliability: Since computers ,
computers on the Ill,twork c......
found on other computeI'! I
Cost: Microcomputl'rs arc.' In
Instead of using scv(,rill rnlll
frame as a server, with ~ev
server as clients. This creat
Communication: Users con
or other messaging SYStC01;a,
Acomputer in a network can be
is a computer on the network th
operating system (NOS) that
for a client computer to accen t
computer must request informatl
transmit the information request
Three models are used, b
required:
1. Peer-to-peer network (work
2. Server-based network
3. Client/server network
In a peer-to-peer model there Is n
and a network operating system.
Group)
and/or as a client. In this model, eAch
ing and upgrading the software of h
centralized station to manage n~tw
cally used for a network of few~r th
peer-to-peer network model.

i
'stination
rmation
tis com
ransmit
cards are
to carry
sion link
air).
smission
so that
ation for
lle in one
ark cards
xample,
. People
ia a net
-ions. We
etwork.
to access
rs.
together
lters and
to share
odems,
1.2
Peer-to-Peer
Model (Work
Group)
INTRODUCTlON TO COMMUNICATIONS NETWORKS • 3
copiers, and so forth. The following are some of the benefits of using
computer networks:
Resource sharing: Computers in a network can share resources such
as data, printers, disk drives, and scanners.
Reliability: Since computers in a network can share data, if one of the
computers on the network crashes, a copy of its resources might be
found on other computers in the network.
Cost: Microcomputers are much less expensive than mainframes.
Instead of using several mainframes, a network can use one main
frame as a server, with several microcomputers connected to the
server as clients. This creates a client/sever relationship.
Communication: Users can exchange messages via electronic mail
or other messaging systems, and they can transfer files.
Network Models
Acomputer in a network can be either a server, a client, or a peer. Aserver
is a computer on the network that holds shared files and the network
operating system (NOS) that manages the network operations. In order
for a client computer to access the resources on the server, the client
computer must request information from the server. The server will then
transmit the information requested to the client.
Three models are used, based on the type of network operation
required:
1. Peer-to-peer network (work group)
2. Server-based network
3. Client/server network
In a peer-to-peer model there is no special station that holds shared files
and a network operating system. Each station can access the resources of
the other stations in the network. Individual stations can act as a server
and/or as a client. In this model, each user is responsible for administrat
ing and upgrading the software of his or her station. Since there is no
centralized station to manage network operations, this model is typi
cally used for a network of fewer than ten stations. Figure 1.2 shows a
peer-to-peer network model.

H
Al'rr" ,
erver-Based
Network
ient/Server
Model
FIGURE 1.2
Peer-to-peer
network
In a file server model, a server stores all of the network's shared files and
applications such as word processor documents, compilers, database
applications, and the network operating system that manages network
operations. A user can access the file server and transfer shared files to his
or her station.
Figure 1.3 shows a network with one file server and three users, or
clients. Each client can access the resources on the server as well as the
resources of other clients. Clients that are connected on a network may
be able to freely exchange information with one another. The following
are widely used servers:
FIGURE 1.3
Network with
one server and
three clients
Server Client
Mail server: A mail server stores all the clients' electronic mail. The
client can access the server and transfer incoming mail to its sta
tion. The client can also use the mail server to transfer outgoing
mail to the network.
Print server: Clients can submit files to the server for printing.
Communication server: The server is used by clients to communicate
with other networks via communication links.
In the client/server model, a client submits its task to the server; the
server then executes the client's task and returns the results to the re
questing client station. This method of information sharing is called the
client/server model and is depi...'ll'd In
less information travels through th
model, rnqk.ing more efficient usc
FIGURE 1.4
Client/server
model
1.3
1.
2.
computers together a
the computers on th
are twisted pair cable,
3. Network Operating S)'ItM
and provides services t
file, network adminlstr.....
puter operating syst~m
1.4
The topology of a network dcs~'t~
together. Topology is a malor
bility. The following is a list of co..
networking:
• Star
• Ring
• Bus
• Mesh
• Tree
• Hybrid

shared files and
pilers, database
anages network
hared files to his
three users, or
er as well as the
a network may
.r. The following
ronic mail. The
mail to its sta
ansfer outgoing
'r printing.
to communicate
the server; the
esults to the re
__ 11_..l ..Ll-_
INTRODUCTION TO COMMUNICATIONS NETWORKS • 5
client/server model and is depicted in Figure 1.4. In a client/server model,
less information travels through the network compared to the server-based
model, rnaking more efficient use of the network.
FIGURE 1.4
Client/server
model
Server Clients
1.3 Network Components
A computer network has several basic components:
1. Network Interlace Card (NIC): Each computer in a network re
qUires a network interface card, which allows the stations on the
network to communicate with each other.
2. Transmission Medium: The transmission medium connects the
computers together and provides a communication link between
the computers on the network. Some of the more common types
are twisted pair cable, coaxial cable, fiber optic cable, and wireless.
3. Network Operating System (NOS): The NOS runs on the server
and provides services to the client such as login, password, print
file, network administration, and file sharing. Most modern com
puter operating systems have NOS functionality.
1.4 Network Topology
The topology of a network describes the way computers are connected
together. Topology is a major design consideration for cost and relia
bility. The following is a list of common topologies found in computer
networking:
• Star
• Ring
• Bus
• Mesh
• Tree
• Hvhrirl

6 • CHAPTER 1
Star Topology
Ring Topology
In a star topology, all stations are connected to a central controller or hub,
as shown in Figure 1.5. For any station to communicate with another
station, the source must send information to the hub, and then the hub
must transmit that information to the destination station. If station #1
wants to send information to station #3, it must send information to the
hub; the hub must pass the information to station #3.
FIGURE 1.5 Station 1 Station 2
Star topology
Station 4 Station 3
The disadvantage of star topology is that the operation of the entire
network depends on the hub. If the hub breaks down, the entire network
is disabled. The advantages of star topology are as follows:
• It is easy to set up the network.
• It is easy to expand the network.
• If one link to the hub breaks, only the station using that link is
affected.
It is possible for a network to have one topology electrically but ano
ther topology physically. For example, Ethernet with UTP cable uses star
topology physically but the stations are connected using a bus topology.
IBM invented ring topology, which is well known as IBM Token Ring. In
a ring topology all the stations are connected in cascading order to make a
ring, as shown in Figure 1.6. The source station transfers information to
the next station on the ring, which checks the address of the information.
If the address of Hw I"torm
l"opies the information an,4 .......
repeats the proCI!SS ami p
so on, until the informatl
removes the informutlon r
the direction in which th
FIGURE 1.6
Ring topology
The disadvantages of rl
• Complex hardwar
expensive).
• Adding a new client
The advantages of ring
to expand.
A bus network is a multl-poln
Topology
to a single cable called a /1W """...
In the bus topology, all statl
used in LAN networking. P.th"Pft.~.
use bus topolOgy.
The advantages of bUll toMMI
expansion of the network.
breakdown in the bus cable b

ns are connected to a central controller or hub,
)r any station to communicate with another
nd information to the hub, and then the hub
lation to the destination station. If station #1
to station #3, it must send information to the
,information to station #3.
Station 2
Station 3
r topology is that the operation of the entire
b. If the hub breaks down, the entire network
of star topology are as follows:
!e network.
.he network.
b breaks, only the station using that link is
ork to have one topology electrically but ano
r example, Ethernet with UTP cable uses star
stations are connected using a bus topology.
y, which is well known as IBM Token Ring. In
ns are connected in cascading order to make a
'i. The source station transfers information to
I which checks the address of the information.
Bus Topology
INTRODUCTION TO COMMUNICATIONS NE1WORKS • 7
If the address of the information matches the station's address, the station
copies the information and passes it to the next station; the next station
repeats the process and passes the information on to the next station, and
so on, until the information reaches the source station. The source then
removes the information from the ring. The arrows in Figure 1.6 indicate
the direction in which the information flows.
FIGURE 1.6
Ring topology
1 1
•
The disadvantages of ring topology are as follows:
• If a link or a station breaks down, the entire network is disabled.
• Complex hardware is required (the network interface card is
expensive).
• Adding a new client disrupts the entire network.
The advantages of ring topology are that it is easy to install and easy
to expand.
A bus network is a multi-point connection in which stations are connected
to a single cable called a bus. The bus topology is depicted in Figure 1.7.
In the bus topology, all stations share one medium. Bus topology is widely
used in LAN networking. Ethernet is one of the most popular LANs that
use bus topology.
The advantages of bus topology are simplicity, low cost, and easy
expansion of the network. The disadvantage of bus topology is that a
breakdown in the bus cable brings the entire network down.

CHAPTER 1
FIGURE 1.7
Bus topology
sh Topology Mesh topology can be full or partial. In a full mesh topology each station
is directly connected to every other station in the network, as shown in
Figure 1.8.
The advantage of a fully connected topology is that each station has
a dedicated connection to every other station; therefore this topology
offers the highest reliability and security. If one link in the mesh topol
ogy breaks, the network remains active.
FIGURE 1.8
Full mesh topology
Topology
A major disadvantage of a fully connected topology is that it uses
many connections and therefore requires a great deal of wiring, espe
cially when the number of stations increases. Consider, for example,
a fully connected network with
require 99 network connection
99. The number of connection
the number of stations in th
used because it is not cost Cff..,..IU..
In partial mesh topology,
stations, but some of th(' stall
with which they exchangl' th~
topology.
Internet Service
FIGURE 1.9
Partial mesh topology
The tree topology uses an acllv,
together. The hub is one of th
because it links stations in the netUlAIllt
is to accept information from on
other stations on the hub, as shown

partial. In a full mesh topology each station
yother station in the network, as shown in
connected topology is that each station has
very other station; therefore this topology
and security. If one link in the mesh topol
Lins active.
~
fully connected topology is that it uses
)fe requires a great deal of wiring, espe.
Ltions increases. Consider, for example,
a fully connected network with 100 workstations. Workstation #1 would
require 99 network connections to connect it to workstations 2 through
99. The number of connections is determined by N(N - 1)/2, where N is
the number of stations in the network. This type of topology is seldom
used because it is not cost effective.
In partial mesh topology, some of the stations are connected to other
stations, but some of the stations are connected only to those stations
with which they exchange the most data. Figure 1.9 shows partial mesh
topology.
Internet Service
Provider
.J ~.-...-..-............tt. .1!.•.••..•.•....-..•.•..~
•••{t )~
/.. ~~ _. ',
= .................;/.
tj iJ······,·····.. / .........,...
: .........><....... // .1 ......,.... / ............
~/ '-._,{'i ! -c1)"--"-'~
%--'-~d"" / /,// !, ,</!
UP / ~___..~ -'-.
,l/:~----;4~ ,/L~"<~
j
~'-.'~<:~~3~~::::::.i/L])
FIGURE 1.9
Partial mesh topology
The tree topology uses an active hub or repeater to connect stations
together. The hub is one of the most important elements of a netwo.rk
because it links stations in the network together. The function of the hub
is to accept information from one station and repeat the information to
other stations on the hub, as shown in Figure 1.10.

HAPTER 1
Hybrid
Topology
FIGURE 1.10
Tree topology
The advantage of this topology is that when one hub breaks, only
stations connected to the broken hub will be affected. There are several
types of hubs:
Manageable hub: Intelligent hubs are defined as manageable hubs;
that is, each of the ports on the hub can be enabled or disabled by
the network administrator using software.
Standalone hub: A standalone hub is used for workgroups of com
puters that are separate from the rest of the network. They cannot
be linked together logically to represent a larger hub.
Modular hub: A modular hub comes with a chassis or card cage and
the number of ports can be extended by adding extra cards.
Stackable hub: A stackable hub looks like a standalone hub, but sev
eral of them can be stacked or connected together in order to
increase the number of ports.
Hybrid topology is a combination of different topologies connected
together by a backbone cable, as shown in Figure 1.11. Each network is
connected to the backbone cable by a device called a bridge.
~, '
FIGURE 1.11
Hybrid topology
1.5
Local Area
Network (LAN)
Metropolitan
Area Network
(MAN)
The distance between co mpllt ~r
mines the type of network, such
politan Area Network (MAN), on
A Local Area Network (LAN) Is
computers and other data com
small geographic area such as an
a multi-story building. Several I.AN
ing or campus to extend the co
network. The most popular LAN
and Gigabit Ethernet.
A Metropolitan Area Network (MAN
miles, connecting multiple nctwor
tions of a city or town. The com
owned by a network service provld
Area Network.

ology is that when one hub breaks, only
ken hub will be affected. There are several
gent hubs are defined as manageable hubs;
s on the hub can be enabled or disabled by
tor using software.
jalone hub is used for workgroups of COffi
from the rest of the network. They cannot
ally to represent a larger hUb.
hub comes with a chassis or card cage and
be extended by adding extra cards.
e hub looks like a standalone hub, but sev
acked or connected together in order to
orts.
ation of different topologies connected
as shown in Figure 1.11. Each network is
Ie by a device called a bridge.
1.5
Local Area
Network (LAN)
Metropolitan
Area Network
(MAN)
Backbone
FIGURE 1.11
Hybrid topology
Types of Networks
The distance between computers that are connected as a network deter
mines the type of network, such as a Local Area Network (LAN), Metro
politan Area Network (MAN), and Wide Area Network (WAN).
A Local Area Network (LAN) is a high-speed network designed to link
computers and other data communication systems together within a
small geographic area such as an office, a department, or a single floor of
a multi-story building. Several LANs can be connected together in a build
ing or campus to extend the connectivity. A LAN is considered a private
network. The most popular LANs in use today are Ethernet, Token Ring,
and Gigabit Ethernet.
A Metropolitan Area Network (MAN) can cover approximately 30 to 100
miles, connecting multiple networks that are located in different loca
tions of a city or town. The communication links in a MAN are generally
owned by a network service provider. Figure 1.12 shows a Metropolitan
Area Network.

INTRODU
2 • CHAPTER 1
Metropolitan
IGURE 1.12
etropolitan Area Network (MAN)
Wide Area A Wide Area Network (WAN) is used for long-distance transmission of
etwork (WAN) information. WANs cover a large geographical area, such as an entire
country or continent. WANs may use leased lines from telephone com
panies, Public Switch Data Networks (PSDN), or satellites for communi
cation links.
Internet The Internet is a collection of networks located all around the world that
are connected by gateways, as shown in Figure 1.13. Each gateway has a
routing table containing information about the networks that are con
nected to the gateway. Several networks may be connected to one gate
way. The gateway accepts information from the source network and
checks its routing table to see if the destination is in a network connected
to the gateway. If the destination station is in a network connected to the
gateway, it transmits the information to the destination network. Other
wise, it passes the information to the next gateway, which performs
the same operation; this process continues until the information reaches
its destination.

for long-distance transmission of
phical area, such as an entire
leased lines from telephone com
(PSDN), or satellites for communi
located all around the world that
in Figure 1.13. Each gateway has a
about the networks that are con
ks may be connected to one gate
Ion from the source network and
tion is in a network connected
on is in a network connected to the
to the destination network. Other-
he next gateway, which performs
inues until the information reaches
>
~
<lJ
....
C':!
(j
(;
~
::J
....
U
Q)
M .~
... ..c
• u
... ....
C':!
loY ....
III: Q)
;) E
_....
'" Q)
""".E:

• CHAPTER 1
Summary Review Questions
• A group of several computers connected by communication media is
called a computer network.
• Some of the applications of computer networks are file sharing, relia
bility, hardware sharing, and electronic mail.
• In the client/server model of networking, the client submits the infor
mation to the server, which processes the information and returns the
results to the client station.
• The components of a network are the Network Interface Card (NIC),
the Network Operating System (NOS), and the communication link
(transmission medium).
• Computers can be connected in the form of star, ring, bus, mesh, tree,
and hybrid topologies.
• The types of networks are LAN, MAN, WAN, and the Internet.
Key Terms
Bus Network
Client
File Server Model
Hub
Hybrid Topology
Internet
Local Area Network (LAN)
Mesh Topology
Metropolitan Area Network (MAN)
Network Interface Card (NIC)
Network Operating System (NOS)
Client/Server Model
Computer Network
Peer-to-Peer Model
Ring Topology
Server
Star Topology
Topology
Transmission Medium
Tree Topology
Wide Area Network (WAN)
• Multiple Choice Quest!
1. Several computers
a. computer network
b. client
2. In a --
then the server
client station.
a. peer-to-peer
b. file server-based
3. A computer in a m·tw
a. client, server
b. client, user
4. A stores all of t
a. file server
b. print server
S. A
a. mail server
6. In a topology,
troller or hub.
a. star
b.ring
7.Ina __
a. star
b. ring
8. A topology Is
nected together by a h " ..I"~
a. star
b.ring
9. The network topology th
a. ring
b. bus
10. The type of topology thot
a. bus
b. star

,
( INTRODUCTION TO COMMUNICATIONS NETWORKS • 15
Review Questions
.cl by communication media is • Multiple Choice Questions
1. Several computers connected together are called a ___
etworks are file sharing, relia a. computer network c. server
rnail. b. client d. hub
,.g, the client submits the infor 2. In a network, the client submits a task to the server, and
pe information and returns the then the server executes and returns the result to the requesting
client station.
a. peer-to-peer c. client/server
Network Interface Card (NIC),
b. file server-based d. all of the above
and the communication link
3. A computer in a network can function as a or as a ___
a. client, server c. a and b
( 1Il of star, ring, bus, mesh, tree,
b. client, user d. all of the above
4. A stores all of the client's electronic mail.
WAN, and the Internet.
a. file server c. communication server
b. print server d. mail server
5. A uses a modem or other type of communication link to
enable clients to communicate with other networks.
a. mail server c. a and b
b. communication server d. none of the above
6. In a topology, all stations are connected to a central con
troller or hub.
a. star c. bus
b. ring d. mesh
7. In a topology, all stations are connected in cascade.
a. star c. tree
b. rtng d.bus
8. A topology is a combination of different topologies con
nected together by a backbone cable.
a. star c. bus
Area Network (WAN)
b. ring d. hybrid
9. The network topology that uses a hub is ___.
a. ring c. star
b. bus d. mesh
10. The type of topology that uses multi-point connections is a ___
a. bus c. ring
b. star d. full mesh

16 • CHAPTER 1
11. A fully connected network with five stations requires ___'
a. 5 c. 20
b. 10 d. 15
12. The network used for office buildings is ___
a. LAN c. WAN
b. MAN d. Internet Data Comm
13. The topology used for Ethernet is a ___'
a. bus c. ring
b. star d. a and b
14. The Internet is a collection of LANs connected together by ___
, , OUTLINE 2.1 Analog Signals
a. routers c. gateways
b. switches d. repeaters 2.2 Digital Signals
15. Computers on a campus are connected by ___ 2.3 Binary Numbers
a. LAN c. MAN 2.4 Coding Schemes
b. WAN d. Internet
2.5
2.6
• Short Answer Questions
2.7 Communication
1. What are the components of a communication model?
2.8 Signal Transmissl
2. Explain the function of a server.
2.9 Digital Signal En
3. What is the function of the client in a file server model?
2.10 Error Detection
4. Explain the term client/server model.
2.11
S. What are the advantages of a client/server model?
. ___OBJECTIVES
6. What are the network components?
7. A network operating system runs on the ___.
8. List the networking topologies.
9. What is the disadvantage of a fully connected topology?
vice versa
10. What is a hub?
• Represent characters and
11. What are the types of networks?
• Compare serial, parallel, llIVl'l~'"
12. What does MAN stand for?
• List the communication m
13. Explain the Internet operation.
14. What does WAN stand for?
15. What are the advantages of using bus topology?

'e stations requires ___
c. 20
d. 15
gsis ___.
c. WAN
j. Internet
a ___
· ring
j. a and b
connected together by ___
· gateways
1 repeaters
'ted by ___,
. MAN
· Internet
unication model?
1 a file server model?
1.
server model?
the __
:onnected topology?
:; OUTLINE
_ OBJECTIVES
s topology?
Data Communications
2.1 Analog Signals
2.2 Digital Signals
2.3 Binary Numbers
2.4 Coding Schemes
2.6 Transmission Methods
2.7 Communication Modes
2.8 Signal Transmission
2.9 Digital Signal Encoding
2.10 Error Detection Methods
2.11 Error Correction
After completing this chapter, you should be able to:
• Distinguish between analog and digital signals
• Distinguish between periodic and non-periodic signals
• Convert decimal numbers to binary, binary to hexadecimal, and
vice versa
• Represent characters and decimal numbers in the 7-bit ASCII code
• Compare serial, parallel, asynchronous, and synchronous transmission
• List the communication modes

• CHAPTER 2
TRaDUCTION
2.1
• Explain the different types of digital encoding methods
• Calculate a Block Check Character (BCC)
• Calculate a Frame Check Sequence (FCS), which is used for error
detection in networking
• learn different error detection methods
In order to understand network technology it is important to know how
information is represented for transmission from one computer to another.
Information can be transferred between computers in one of two ways: by
an analog signal or a digital signal.
Analog Signals
An analog signal is a signal whose amplitude is a function of time and
changes gradually as time changes. Analog signals can be classified as
non-periodic or periodic signals.
Non-periodic signal. In a non-periodic signal there is no repeated
pattern in the signal, as shown in Figure 2.1.
Voltage
Time
o
FIGURE 2.1
Representation of a non-periodic analog signal
Periodic signal. A signal that repeats a pattern within a measurable time
period is called a periodic signal and completion of a full pattern is called a
cycle. The Simplest periodic signal is a sine wave, which is shown in Figure
2.2. In the time domain, sine wave amplitude aCt) can be represented
mathematically as aCt) = A sin(wt + e) where A is the maximum amplitude,
w is the angular frequency, and eis the phase angle.
h.racteristics
of an Analog
Signal
FIGURE 2.2
Time domain representation of
A periodic signal can also be
the horizontal axis is the freqU8lnmr
of signal. Figure 2.3 shows the
wave signal.
Amplitude
A
w
FIGURE 2.3
Frequency representation of a sin
Usually an electrical signal reprt'lenUIl
sound is made of multiple wavef,
tal frequency and multiple f .." .",..'".....
The characteristics of a periodic anll_
and phase.
Frequency. Frequency (F) is th
F = +
, represented in Hz (Hertz).
repeated everyone second, the frl!oulftl

DATA COMMUNICATIONS • 19
Voltage
tal' encoding methods
:r (SCC)
:e (FCS), which is used for error
!thods
!chnology it is important to know how
.mission from one computer to another.
een computers in one of two ways: by
amplitude is a function of time and
s. Analog signals can be classified as
eriodic signal there is no repeated
'gure 2.1.
..Time
ignal
a pattern within a measurable time
mpletion of a full pattern is called a
ine wave, which is shown in Figure
amplitude aCt) can be represented
here A is the maximum amplitude,
phase angle.
Characteristics
of an Analog
Signal
I.' • Time
FIGURE 2.2
TIme domain representation of a sine wave
A periodic signal can also be represented in the frequency domain where
the horizontal axis is the frequency and the vertical axis is the amplitude
of signal. Figure 2.3 shows the frequency domain representation of a sine
wave signal.
Amplitude
A
(j)
w
FIGURE 2.3
Frequency representation of a sine wave
Usually an electrical signal representing voice, temperature, or a musical
sound is made of multiple waveforms. These signals have one fundamen
tal frequency and multiple frequencies that are called harmonics.
The characteristics of a periodic analog signal are frequency, amplitude,
and phase.
Frequency. Frequency (F) is the number of cycles in one second;
F = t, represented in Hz (Hertz). If each cycle of an analog signal is
repeated everyone second, the frequency of the signal is one Hz. If each

CHAPTER 2
cycle of an analog signal is repeated 1000 times every second (once
millisecond) the frequency is;
1 1
f = - = - = 1000 Hz = 1 kHz
T 10- 3
Table 2.1 shows different values for frequency and their correspond.
ing periods.
TABLE 2.1 Typical Units of Frequency and Period
Units of Frequency INumerical I Units of Period
Value
Hertz (Hz) 1 Hz Second (s) 1 s
Kilo Hertz (kHz) 103 Hz Millisecond (ms) 10-3 s
Mega Hertz (MHz) 106 Hz Micro Second (/-1s) 10-6 s
Giga Hertz (GHz) 109
Hz Nanosecond (ns) 10-9 s
Tera Hertz (THz) 1012 Hz Pico Second (ps) 10-12 s
Amplitude. The amplitude of an analog signal is a function of time
(as shown in Figure 2.4) and may be represented in volts (unit of volt
age). In other words, the amplitude is the signal's voltage value at any
given time. At the time of t1
, the amplitude of signal is Vl'
aCt)
Vl=+A
3T/4 T
t1=T/4
-A I ,:......
FIGURE 2.4
Asine wave signal over one cycle
Phase. Two signals with 11ll' !tum
means that one of the signals 'it
one. This difference Gill be rcprl'sentl&l
or by radians where 360
0
211 rldl••'
sented by equation the (/( t)
tude; w (omega) is freqllcm:y In
and eis the phase angle. Cyc1t:
w according to ,. = r1T' A phase
time t = 0 and a phase angl~ or
degrees as shown in Figure 2.5.
2.2
Signal A Signal B
j
2700
FIGURE 2.S
Three sine waves with different ph
Example 2.1. Find the cqunU
of 10Hz, maximum amplitude
Modern computers communlcat
are represented by two voltage,:
in binary and the other voltagt rept'lllfl
example of a digital signal Is shoW
sents 0 in binary and +S volts .....,redIltI

eated 1000 times every second (once every
1
= 10-3 = 1000 Hz = 1 kHz
falues for frequency and their correspond
luency and Period
erical I Units of Period
Second (s) 1 s
[z Millisecond (ms) 10-3 s
[z Micro Second (fLS) 10-6 s
[z Nanosecond (ns) 10-9 s
1Z Pico Second (ps) 10-12 s
f an analog signal is a function of time
may be represented in volts (unit of volt
Etude is the signal's voltage value at any
le amplitude of signal is VI.
2 3T/4 T
Phase. Two signals with the same frequency can differ in phase. This
means that one of the signals starts at a different time from the other
one. This difference can be represented by degrees, from 0 to 360 degrees
or by radians where 3600
= 21T radians. A sine wave signal can be repre
sented by equation the aCt) = A sin(wt + 8) where A is the peak ampli
tude; w (omega) is frequency in radians per second; t is time in seconds;
and 8 is the phase angle. Cyclic frequency f can be expressed in terms of
w according to f =:r1T. A phase angle of zero means the sine wave starts at
time t = 0 and a phase angle of 90 degrees mean the signal starts at 90
degrees as shown in Figure 2.5.
Signal A Signal B Signal C
I I I
I
I
I
/
2700
FIGURE 2.5
Three sine waves with different phases
Example 2.1. Find the equation for a sine wave signal with frequency
of 10Hz, maximum amplitude of 20 volts and phase angle of zero.
rad
w = 21Tf= 2 x 3.1416 x 10 = 62.83 sec
aCt) = 20sin(62.83t)
2.2 Digital Signals
Modern computers communicate by using digital signals. Digital signals
are represented by two voltages: one voltage represents the number 0
in binary and the other voltage represents the number 1 in binary. An
example of a digital signal is shown in Figure 2.6, where 0 volts repre
sents 0 in binary and + 5 volts represents 1.

- - - -
CHAPTER 2
FIGURE 2.6 Voltage
Digital signal
+5booI 1 1 1 I0 0:Time
o
2.3
Binary, or base-2, numbers are represented by 0 and 1. A binary digit, 0 or
I, is called a bit. Eight bits are equal to one byte. Two or more than
bytes is called a word. The hexadecimal number system has a base of 16,
and therefore has 16 symbols (0 through 9, and A through F). Table 2.2
shows the decimal numbers, their binary values from 0 to IS, and their
hexadecimal equivalents.
TABLE 2.2 Decimal Numbers with Binary and Hexadecimal Equivalents
Binary (Base 2) Hexadecimal
(Base 16) or HEX
I Decimal
0000
0 0
1 0001 1
2 0010 2
3 0011 3
4 0100 4
5 0101 S
6 0110 6
7 0111 7
8 1000 8
9 1001 9
10 1010 A
11 1011 B
12 1100 C
13 1101 D
14 1110 E
15 1111 F
- -
nverting
from Hex
to Binary
Converting
Binary to
Decimal
Table 2.2 can also be uSl'd to c
binary, and from binary to hcxICSICUIII
Example 2.2. Convert thl' bin
mal. Each 4 bits are grouped fr
4-bit group can be converted to I
0010
2
Example 2.3. Convert (3DS),
in binary is
:~
()()1l
The resulting binary number Is:
Example 2.4. Convert 6DB
Table 2.2, the result in binary I
6
0110
The resulting binary numot'r Is:
In general, any binary numher
where
a is a binary digit or bit (eithcr 0
j
Equation 2.1 can be convertcd to cllCla
(as a4
a3 az a1 ao·a_ 1 a
~ rl
Integer
Example 2.5. To convert (110111. .vNIIJII
(1l0111.101h = 1*20
+ 1*21
+
+ 1*25* + .
= 55.625

J 1 1 1 I 0 0:Time
Converting
from Hex
to Binary
represented by 0 and 1. A binary digit, 0
equal to one byte. Two or more than
xadecimal number system has a base of 1
(0 through 9, and A through F). Table 2
their binary values from 0 to 15, and their
s with Binary and Hexadecimal Equivalents
'ase 2) Hexadecimal
(Base 16) or HEX
o
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F
onverting
Binary to
Decimal
Table 2.2 can also be used to convert a number from hexadecimal to
binary, and from binary to hexadecimal.
Example 2.2. Convert the binary number 001010011010 to hexadeci
mal. Each 4 bits are grouped from right to left. By using Table 2.2, each
4-bit group can be converted to its hexadecimal equivalent.
0010 1001 1010
2 9 A
Example 2.3. Convert (3D5)16 to binary. By using Table 2.2, the result
in binary is
3 D 5
0011 1101 0101
The resulting binary number is: 001111010101.
Example 2.4. Convert 6DB from hexadecimal to binary. By using
Table 2.2, the result in binary is
6 D B
0110 1101 1011
The resulting binary number is: 011011011011.
In general, any binary number can be represented by equation 2.1.
(as a4 a3 a2 a1ao' a_I a_2 a-3)2 (2.1)
where
is a binary digit or bit (either 0 or 1).
aj
Equation 2.1 can be converted to decimal number by using equation 2.2.
20
(as a4 a3 a2 a, ao' a_I a_2 a-3)2 = ao x + a1 x 21 + a2 x 22
"------
~
~ '-y---J + a x 23 + ...... + a x 2-1
3 ,
Integer Fraction + a_2
x 2-2 + ..... (2.2)
Example 2.5. To convert (110111.101)2 to decimal:
(110111.101)2 = 1*20
+ 1*21 + 1*22 + 0*23
+ 1*24
+ 1*2s* + 1*2-1 + 0*2-2 + 1*2-3
= 55.625

• CHAPTER 2
2.4
•
Since computers can understand only binary numbers (0 or I), all
mation (such as numbers, letters, and symbols) must be represented as
binary data. One commonly used code to represent printable and
printable characters is the American Standard Code for Informa
Interchange (ASCII).
ASCII Code Each character in ASCII code is represented by 8 bits; the most significant
bit is used for parity bit. Table 2.3 shows the ASCII code and its hexa
decimal eqUivalent.
Characters from hexadecimal 00 to IF and 7F are control characters,
which are nonprintable characters, such as NUL, SOH, STX, ETX, ESC,
and DLE (data link escape).
Example 2.6. Convert the word "Network" to binary and show the
result in hexadecimal. By using Table 2.3 each character is represented
"'" III I II? I STX . 0100010
ETX 0100011
04 , EOT 0100100
OS ENQ
-
06
by seven bits and results in:
1001110 1100101 1110100
N e t
or in hexadecimal
4E 65 74
1110111 1101111 1110010 1101011
w 0 r k
77 6F 72 6B
09 HT 0101001 29
OA LF 0101010 2A
OR VT 0101011 2B
OC FF 0101100
OD CR 0101101
OE SO 0101110
OF SI 0101111 2F /
10 : DLE 0110000 30 0
11 DCI 0110001 31 1
12 DC2 0110010 32 2
13 I DC3 0110011 33 3
14 DC4 0110100 34 4
NACK 0110101 35 S
15
SYN 0110110 36 6
16
17 ETB 0110111 I 37 7
18 CAN 0111000 38 8
19 EM 0111001 39 9
, lA SUB 0111010 3A
1B ESC 0111011 3B
) 1C FS 0111100 3C
1D GS 0111101 3D I =
) IE RS 0111110 3E I <
3f I
Iniversal Code Unicode is a new 16-bit character encoding standard for representing
or Unicode characters and numbers in languages such as Greek, Arabic, Chinese, and
Japanese. The ASCII code uses eight bits to represent each character in
Latin, and it can represent 256 characters. The ASCII code does not
support mathematical symbols and scientific symbols. Unicode uses 16
bits, which can represent 65536 characters or symbols. A character
in unicode is represented by16-bit binary, equivalent to four digits in
hexadecimal. For example, the character B in unicode is U0042H (U rep
resents unicode). The ASCII code is represented between (00)16 to (FF)16.
For converting ASCII code to unicode, two zeros are added to the left side
of ASCII code; therefore, the unicode to represent ASCII characters is
between (0000)16 to (00FF)16. Table 2.4 shows some of the unicode for
Latin and Greek characters. Unicode is diVided into blocks of code, with
each block assigned to a specific language. Table 2.5 shows each block of
unicode for some different languages.
I
I IF US 0111111 ?

DATA COMMUNICATIONS • 2S
American Standard Code for Information Interchange (ASCII)
ly binary numbers (0 or I), all infor
nd symbols) must be represented as
ode to represent printable and non
an Standard Code for Information
sented by 8 bits; the most significant
shows the ASCII code and its hexa
to IF and 7F are control characters,
such as NUL, SOH, STX, ETX, ESC,
"Network" to binary and show the
Ie 2.3 each character is represented
111 1101111 1110010 1101011
o k
6F 72 68
.ncoding standard for representing
such as Greek, Arabic, Chinese, and
. bits to represent each character in
aracters. The ASCII code does not
scientific symbols. Unicode uses 16
haracters or symbols. A character
Jinary, equivalent to four digits in
cter B in unicode is UOO42H (U rep
epresented between (00)16 to (FF)w
, two zeros are added to the left side
ie to represent ASCII characters is
~.4 shows some of the unicode for
is divided into blocks of code, with
uage. Table 2.5 shows each block of
0001011
11001100
()OOl101
0001110
0001111
(J010000
()010001
0010010
0010011
0010100
0010101
0010110
0010111
0011000
0011001
0011010
0011011
0011100
0011101
0011110
0011111
Hex I Char Binary Hex I Chari Binary Hex 1Char Binary Hex I Char
00 1 NUL 01 20 I SP 1 1000000 40 1 @ 60
01 I SOH 0100001 21 1000001 41 I A 1100001 61 I a
02 I STX 0100010 22 " 1000010 42 I B 1100010 62 I b
03 I ETX 0100011 23 # 1000011 43 I C 1100011 63 I c
04 I EOT 0100100 I 24 $ I 1000100 I 44 D I 1100100 I 64 d
05 I ENQ 0100101 I 25 % I 1000101 1 45 E I 1100101 I 65 e
06 lACK 0100110 I 26 N I 1000110 I 46 F I 1100110 I 66 f
07 I BEL 0100111 1 27 1000111 1 47 G I 1100111 I 67 g
08 I BS 0101000 I 28 ( I 1001000 I 48 H I 1101000 I 68 h
09 I HT 0101001 I 29 ) 1001001 I 49 I I 1101001 I 69 i
OA I LF 0101010 I 2A * 1001010 I 4A J 1 1101010 I 6A
OB I VT 0101011 I 2B + 1001011 I 4B K I 1101011 I 6B k
OC IFF 0101100 I 2C 1001100 I 4C L I 1101100 I 6C I
OD I CR 0101101 I 2D 1001101 1 4D M I 1101101 I 6D m
OE 1 SO 0101110 I 2E 1001110 I 4E N I 1101110 1 6E n
OF lSI 0101111 1 2F / 1001111 I 4F o 1 1101111 I 6F o
10 IDLE 0110000 I 30 o 1010000 I SO P I 1110000 I 70 p
11 I DCI I 0110001 I 31 I I 1010001 I 51 Q I 1110001 I 71 I q
12 I DC2 I 0110010 I 32 2 I 1010010 I 52 R 1 1110010 I 72 I r
13 I DC3 1 0110011 I 33 3 1 1010011 1 53 S I 1110011 I 73 I s
14 I DC4 I 0110100 1 34 4 1 1010100 I 54 T I 1110100 1 74 I t
15 I NACK I 0110101 1 35 5 I 1010101 1 55 U I 1110101 I 75 I u
16 I SYN 1 0110110 1 36 6 1 1010110 1 56 v I 1110110 1 76 I v
17 1 ETB I 0110111 1 37 711010111157 W I 1110111 I 77 I w
18 I CAN 1 0111000 I 38 8 I 1011000 I 58 x 1 1111000 I 78 I x
19 I EM I 0111001 I 39 9 I 1011001 I 59 Y I 1111001 I 79 I Y
1A 1 SUB I 0111010 I 3A 1011010 I 5A z I 1111010 I 7A I z
1B I ESC I 0111011 I 3B 1011011 I 5B [ I 1111011 I 7B
1C I FS I 0111100 I 3C < I 1011100 I 5C I 1111100 I 7C I
1D 1 GS I 0111101 I 3D 1011101 I 5D 1111101 I 7D II
IE I RS I 0111110 I 3E < I 1011110 I 5E " I 1111110 I 7E
IF I US I 0111111 I 3F ? I 1011111 I 5F 1111111 I 7F IDEL

• CHAPTER 2
TABLE 2.4 Unicode Values for some Latin and Greek Characters
I Character
A
B
C
0
8
latin
Code (Hex) Character
U0041 'P
U0042 CY
U0043 'Y
U0030 /-1
U0038 f3
Greek
Code (Hex)
U03C6
U03B1
U03B3
U03BC
U03B2
TABLE 2.5 Unicode Block Allocations
Start Code (Hex)
UOOOO
U0080
U0370
U0530
U0590
U0600
U01AO
End Code (Hex)
U007F
UOOFF
U03FF
U058F
U05FF
U06FF
U10FF
Block Name
Basic Latin
Latin supplement
Greek
Armenian
Hebrew
Arabic
Georgian
When data is transferred from one computer to another by digital
signals, the receiving computer has to distinguish the size of each signal
to determine when a signal ends and when the next one begins. For
example, when a computer sends a signal as shown in Figure 2.7, the
receiving computer has to recognize how many ones and zeros are in
the signal. Synchronization methods between source and destination
devices are generally grouped into two categories: asynchronous and
synchronous.
FIGURE 2.7
Digital signal
Asynchronous transmission u
serial communication, slIl"h II
nous transmission l'ach daU,
start of the character, and on
character, as shown in Flgur
the data bits may be a parity
detection. After the parity bit I
one) for at least one hit time: t
start bit serves as an indicator t
is coming and allows thl' ren-Ivl
receiver and transmitter dock
transmitter uses the start bit t
the transmitter clock. Also, th
number of bits in each charact
o
Start
flit
DO Dl D2 D3
LSB
Some applications require.' t
file from disk or a print jnh fr
transmission is an efficient rncth
using time intervals for sym:hroru_
One method of sym:hron
through the use of an external
clock pulse represents the data
and is used to determine the sp
Figure 2.9 reads the data as 011 01
FIGURE 2.9 Transmitter
Synchronous
transmission

Greek
lex) Character Code (Hex)
<p U03C6
a U03B1
'Y U03B3
fL U03BC
f3 U03B2
Asynchronous transmission occurs character by character and is used for
~s for some Latin and Greek Characters
serial communication, such as by a modem or serial printer. In asynchro
nous transmission each data character has a start bit that identifies the
start of the character, and one or two bits that identify the end of the
character, as shown in Figure 2.8. The data character is 7 bits. Following
the data bits may be a parity bit, which is used by the receiver for error
detection. After the parity bit is sent, the signal must return to high (logic
one) for at least one bit time to identify the end of the character. The new
start bit serves as an indicator to the receiving device that a data character
is coming and allows the receiving side to synchronize its clock. Since the
receiver and transmitter clock are not synchronized continuously, the
transmitter uses the start bit to reset the receiver clock so that it matches
the transmitter clock. Also, the receiver is already programmed for the
number of bits in each character sent by the transmitter.
Allocations
ct Code (Hex) Block Name
07F Basic Latin
OFF Latin supplement
3FF Greek
58F Armenian
SFF Hebrew
5FF Arabic
)FF Georgian
l 0 I
, Start
Bit DO D1 D2 D3 D4 DS D6' Parity, Stop Stop
Bit Bit Bit
LSB MSB
One Character
FIGURE 2.8
A'ynchronous transmission
Some applications require transferring large blocks of data, such as a
file from disk or a print job from a computer to a printer. Synchronous
transmission is an efficient method of transferring large blocks of data by
using time intervals for synchronization.
lone computer to another by digital One method of synchronizing the transmitter and receiver is
las to distinguish the size of each signal through the use of an external connection that carries a clock pulse. The
:is and when the next one begins. For clock pulse represents the data rate of the signal, as shown in Figure 2.9,
ds a signal as shown in Figure 2.7, the and is used to determine the speed of data transmission. The receiver of
~nize how many ones and zeros are in Figure 2.9 reads the data as 01101.
thods between source and destination
rlto two categories: asynchronous and
FIGURE 2.9 Data
Transmitter Receiver
Synchronous 01 1 1
&
~
&
transmission
~~ ~~
Clock

28 • CHAPTER 2
2.6
Figure 2.9 shows that an extra connection is required to carry the
pulse for synchronous transmission. In networking, one medium is u
for transmission of both information and the clock pulse. The two
are encoded in such a way that the synchronization signal is
into the data. This can be done with Manchester encoding or Differential
Manchester encoding, which are discussed later in the chapter.
There are two types of transmission methods used for sending digital sig
nals from one station to another across a communication channel: serial
transmission and parallel transmission.
Serial In serial transmission, information is transmitted one bit at a time over
Transmission one wire, as shown in Figure 2.lO.
Parallel
Transmission
FIGURE 2.10
Serial transmission
Ground Line
In parallel transmission, multiple bits are sent simultaneously, one byte or
more at a time, instead of bit by bit as in serial transmission. Figure 2.11
shows how computer A sends eight bits of information to computer Bat
FIGURE 2.11
Parallel
transmission
Computer A
&
~~
I
m Computer B
0
m
0
&
[1]
III
0
III
t
Clock
2.7
Mode
Duplex
Mode
thl..' same tim,,' hy lIslnK ~'
than serial transmissIon, at
A communication mod
and receive data by det
"
l
connections. There ar
half-duplex, and full-du
In simplex mode, tra
shown in Figure 2.12. A
broadcast; the sending
ing device.
FIGURE 2.12
Simplex transmission
In half-duplex mode, tw
Figure 2.13; however,
only one direction at
(CB) or ham radio; a u
cannot talk at the saml' tl
FIGURE 2.13
Half-duplex transmission
II.Duplex
Mode
In full-duplex mode, both
simultaneously, as shown
our modern telephone Sf
at the same time, with thrl
the phone lines.

nection is required to carry the clock
In networking, one medium is used
and the clock pulse. The two signals
synchronization signal is embedded
Manchester encoding or Differential
2.7
ussed later in the chapter.
nethods used for sending digital sig
ss a communication channel: serial Simplex Mode
n.
s transmitted one bit at a time over
QJl~
•
t
Ground Line
Half-Duplex
Mode
are sent simultaneously, one byte or
s in serial transmission. Figure 2.11
its of information to computer B at
[1] ~ Computer B
0
•
[1]
0
[1]
[1] .
•
~&
~~
Full-Duplex
Mode
0
•
[1J ~
t
Clock
the same time by using eight different wires. Parallel transmission is faster
than serial transmission, at the same clock speed.
Communication Modes
A communication mode specifies the capability of a device to send
and receive data by determining the direction of the signal between two
connections. There are three types of communication modes: simplex,
half-duplex, and full-duplex.
In simplex mode, transmission of data goes in one direction only, as
shown in Figure 2.12. A common analogy is a commercial radio or TV
broadcast; the sending device never requires a response from the receiv
ing device.
FIGURE 2.12
Simplex transmission jbJ ~bl
Transmit
~ ~
Sender Receiver
In half-duplex mode, two devices exchange information, as shown in
Figure 2.13; however, information can be transmitted across the channel
only one direction at a time. A common example is Citizen Band radio
(CB) or ham radio; a user can either talk or listen, but both parties
cannot talk at the same time.
FIGURE 2.13
Half-duplex transmission
-' -
.-
&0
. 4 •
&0
~~ -~
Transmitting in both directiOns,
but only one direction at a time
In full-duplex mode, both computers can send and receive information
simultaneously, as shown in Figure 2.14. An example of full-duplex is
our modern telephone system, in which both users may talk and listen
at the same time, with their voices carried two ways simultaneously over
the phone lines.

o • CHAPTER 2
2.8
Baseband
Transmission
Mode
Broadband
Transmission
Mode
2.9
FIGURE 2.14
Full-duplex transmission
1CJ: : lbl
~ ~
There are two methods used to transfer information over media: b
band and broadband transmission.
When the entire bandwidth of a cable is used to carry only one si
the cable operates in baseband mode. Many digital signals use baseuauI
transmission. An example is Local Area Network.
When the bandwidth of a cable is used to carry several signals si
neously, the cable operates in broadband mode. For example, cable
transmission works in broadband mode because it carries multiple ch
nels using multiple signals over the cable. Broadband mode frequen
uses analog signals.
Digital signal encoding is used to represent binary values in the form
digital signals. The receiver of the digital signal must know the timing
each signal, such as the start and end of each bit. The following are
methods used to represent digital signals:
• Unipolar Encoding
• Polar Encoding
• Bipolar Encoding
• Non-Return to Zero (NRZ)
• Non-Return to Zero Inverted (NRZ-I)
• Manchester and Differential Manchester Encoding
Manchester and Differential Manchester, and Non-Return to Zero
Inverted (NRZ-I) encoding schemes are used in LANs, and Non-Return to
Zero is used in WANs. Each encoding technique is described next.
In unipolar encoding only nnlltt....
fl' rHcscnt binary 0 ami I. F
zero volts represents 0, us sh
FIGURE 2.15
Unipolar encoding signals
In polar encoding positive
binary zero and one. For exam.
-s volts represent binary zero,
FIGURE 2.16
Polar encoding signals
In bipolar encoding, signal
and negative voltage. On
methods is alternate mark In"AftIAilI
In AMI encoding, binary
is represented by alternatl'!I he.....
shown in Figure 2.17.
FIGURE 2.17 Voltag
Bipolar encoding signals
-s
NRZ is a simple form of polar 8ft~
representing 1 and 0, with bin
binary 1 represented by negatlv

~
--'I $0
11
..-
~;o.., "'*-
o transfer information over media: base
ion.
f a cable is used to carry only one signal,
lmode. Many digital signals use baseband
cal Area Network.
Il' is used to carry several signals simulta
roadband mode. For example, cable TV
ld mode because it carries multiple chan
r the cable. Broadband mode frequently
represent binary values in the form of
digital signal must know the timing of
end of each bit. The following are some
signals:
Manchester Encoding
Manchester, and Non-Return to Zero
are used in LANs, and Non-Return to
technique is described next.
Unipolar
Encoding
Polar
Encoding
Bipolar
Encoding
Non-Return to
Zero Encoding
(NRZ)
In unipolar encoding only positive voltage or negative voltage is used to
represent binary 0 and 1. For example, +5 volts represents binary 1 and
zero volts represents 0, as shown in Figure 2.15.
VOltage
FIGURE 2.15
Unipolar encoding signals
+s
1 0 0 1_ Time
In polar encoding positive and negative voltages are used to represent
binary zero and one. For example +5 volts represents binary one and
-5 volts represent binary zero, as shown in Figure 2.16.
FIGURE 2.16 Voltage
Polar encoding signals
~brJl I t • Tim'
-s
In bipolar encoding, signal voltage varies in three levels: positive, zero,
and negative voltage. One of the most popular bipolar encoding
methods is alternate mark inversion (AMI).
In AMI encoding, binary 0 is represented by zero volts and binary 1
is represented by alternates between positive and negative voltages, as
shown in Figure 2.17.
FIGURE 2.17 Voltage
Bipolar encoding signals
s
o
-s
NRZ is a simple form of polar encoding, using two voltage levels for
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binary 1 represented by negative voltage, as shown in Figure 2.1S.

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and the fields are going out of cultivation. They certainly would do a
great deal better in Corfu, Cephalonia, and the other islands than
they would elsewhere; and if they were landed in small batches they
might find work. I expect most of them have got a little money, and
as living is very cheap, if you were to give them a couple of pounds
a head it would enable them to live a long time while they are
looking for work. Besides, there are committees on those islands for
helping refugees; so I do think it would be better to land all those
who have no friends in Greece, or any particular wish to go there, in
our islands. I should say Zaimes and Marco might go round among
them in the morning and ask if any of them have friends in the
Greek islands or the mainland, and to put it to the others, that
though they can be landed in Greece if they like, they will probably
be better off and certainly much more free from anxiety and danger,
in the Ionian Isles.”
“I think that that would be a very good plan,” Mr. Beveridge said.
“When are you going to get under sail again, Captain Martyn?”
“As soon as I have finished this cup of coffee, Mr. Beveridge, we will
get a boat lowered and find the buoy and pick up the anchor Miller
slipped this morning. I don’t want to lose that, and the chain. As
soon as we have got it on board we will be off. There is not much
breeze here after dark, but we may as well get what benefit we can
from it. I have no fear of the other Turkish frigate looking in here on
her way back; and if she did, now that we have got all our crew on
board, I have no doubt we could give a good account of her. But I
want to be under weigh. There will be no comfort on board till we
have got rid of our passengers. Whereabout do you think the buoy is
lying, Miller?”
“I fancy we were anchored a couple of hundred yards or so farther
out, and a quarter of a mile astern. You know where you landed last
night. You had to march along the beach some little distance before
you came to the path on the hills.”

“That is so, Miller. I am afraid we shall have some little trouble in
finding it. However, we will have a try. It is just eight bells now, and
it won’t be light for another six hours. I don’t want to waste that
time if I can help it.”
“Well, I will take one of the gigs, and Tarleton can take the other. We
will take some blue lights with us, and I expect we shall soon find it.”
“Very well. Directly you do, hang on to the buoy-rope and get the
end of the chain into your gig. Hail me, and send Tarleton back. We
will get up her anchor at once, and the gig and the long-boat shall
tow the schooner up to you. Then you can pass the end of the chain
on board, and we will get it round the capstan and have the anchor
up in no time. Now, Mr. Beveridge, if you will take my advice you will
turn in at once. You only got a couple of hours’ sleep last night in
that orchard, and have had twenty-four hours’ really hard work.”
“I will take your advice, Martyn;” and Mr. Beveridge touched the
hand-bell beside him. “Marco, you must help me to my cabin, for I
am so stiff I don’t think I could get out of my chair by myself.”
“We will help you in, sir,” Martyn said; and he and Miller raised Mr.
Beveridge from his chair and almost carried him into his cabin. Then
they lit their pipes and went on deck.
The buoy was found after a few minutes’ search, and in another ten
minutes the schooner was under-weigh and stealing out from the
land.
“I will take the watch,” Miller said. “You had better all turn in. I will
put a couple of the hands who remained with me at the wheel, and
let all the rest lie down. As they will be on deck one can rouse them
up in a minute if they are wanted.”
The next day the two Greeks went among the fugitives and
questioned the heads of each family as to the number of their party,
the means they possessed, and whether they had any friends in
Greece. Most of them possessed a little money, the proceeds of their

last harvest and vintage, and some eight or ten had sums varying
from a hundred to four hundred pounds, besides the jewels of their
females, which, in their cases, were of considerable value. Some of
the poorer ones had literally nothing beyond the clothes in which
they stood and a few almost worthless trinkets. There were not half
a dozen of the whole number who had friends or connections in
Greece. Some thirty of the unmarried men expressed their desire to
join the Greek army and fight against the Turks; the rest thankfully
embraced the offer of being landed on islands under the protection
of the British flag. It took a whole day to ascertain all these
particulars, and on the following day the exiles were asked to divide
themselves into parties according to the villages from which they
came, in order that acquaintances and relations should be landed
together.
When this had been done, Zaimes distributed, in the name of Mr.
Beveridge, to the head of each family a sum amounting to two
pounds for each of its members, except to those whose resources
were sufficient to maintain them for a considerable time.
The wind was very light, and it was six days after they weighed
anchor before they entered the port of Zante. Another week was
spent in landing the fugitives among the Ionian Islands, each party
being in proportion to the size of the island and the facilities of
obtaining employment there. The gratitude of the poor people to Mr.
Beveridge, and indeed to all on board the schooner, was very great,
but they were all much depressed on landing. At first their delight at
having escaped with their lives was unbounded. But as the days
went on, and the feeling that they had lost all else, were separated
for ever from their birthplace and home, and were in future to live
among strangers, overwhelmed them.
Mr. Beveridge went a great deal among them, and endeavoured to
cheer them with the assurance that the war could not last very long,
and that at its termination, whenever that might be, there would
certainly be a general amnesty, and that all fugitives would then be
permitted to return to their homes. He therefore advised them to

keep this always in mind, and to lay by every penny they could spare
of their earnings, so that they would eventually be able to return to
Cyprus and resume their former life. When the Misericordia left
Cyprus there remained on board only some half a dozen families
who had friends in Greece, and the young men who intended to join
the Greek army. Never did a vessel undergo a more thorough
washing and cleaning up than the schooner on her voyage round to
Athens. The deck was scrubbed and holy-stoned twice a day; the
lower deck was equally cleaned, and, in addition, the woodwork
received two coats of fresh paint, after having been thoroughly
fumigated.
“The Greeks may have their virtues,” Martyn remarked to Miller, “but
cleanliness on board ship is marked by its absence.”
“There is no doubt about that,” Miller agreed. “I have always heard
that a cargo of Mohammedan pilgrims to Mecca was about the most
painful experience a sailor could have; but I back the Greeks against
them. I don’t think the schooner herself liked it. She seemed to have
lost all her liveliness and to be depressed at being turned into a
human pig-stye. I don’t believe it was worse between decks when
she had a cargo of slaves on board.”
“Mr. Beveridge has just told me,” Martyn said, “that I am to tell the
crew that at the next pay he shall give three pounds a head to each
man as a reward for their work at Cyprus and the inconveniences
they have been since put to.”
“They will appreciate that,” Miller said. “They certainly have been put
about a good deal, and they will be pleased at the recognition of it
as much as with the money. Besides, the same thing may happen
again, and it is a good thing to keep them all in a good humour,
especially as at present there hasn’t been any chance whatever of
prize-money.”
“What are the next orders, sir?” Martyn asked Mr. Beveridge when
they had finished supper.

“There will be nothing particular going on for some time, I should
imagine, Captain Martyn. The Turkish army does not seem to be
ready to advance, and the Greeks are not troubling themselves to
get up an army at all. After the last affair every man made off with
the booty he had gathered to his own village; and there, I am afraid,
they are all likely to stay till a Turkish army invades them. Athens
and Nauplia may hold out for some time longer—for weeks, perhaps,
possibly for months. Therefore, for the present I leave it entirely
with you to cruise where you think best.”
“Then, sir, we will go south. Since we have come out we have not
taken a prize worth having; and I think that as prize-money was
certainly one of the inducements held out to the sailors when they
joined, we might as well try to pick up a few Turkish merchantmen.
There is no doubt that the ships from Smyrna and all the Syrian
ports, as well as from the islands, keep near land, and that even
those bound for Alexandria and the African ports coast round there
also. Some of these no doubt carry rich cargoes, and many will be
taking Greek slaves to Alexandria and Tunis; so we shall be carrying
out your object by releasing them, as well as picking up some prize-
money. I think the men well deserve a little indulgence in this way.
Their work has not been altogether pleasant for some time. They
have been turned out of their quarters, and have had to sleep under
the awning forward. I have heard no grumbling among them, for I
am sure they were glad to do all they could to help the poor
creatures we have had on board. Still, they will be glad of a chance
of what they would consider legitimate business.”
“Very well, Captain Martyn, let it be so. I quite agree with you as to
the excellent conduct of the men. They have certainly had a good
deal of hardship to put up with, for everything has been very
uncomfortable since our visit to Cyprus.”
In a few minutes the boatswain’s whistle was heard, followed by the
tramp of the men round the capstan and the stir of getting up sail.
Then the watch was set, and the schooner sped along under a
gentle breeze towards the south.

For the next two months the Misericordia cruised on the coast of
Syria. Scarce a day passed without some vessel being overhauled.
Many of these were small coasters laden only with grain or other
cargoes of small value. These were permitted to proceed on their
way without interference. Of the larger vessels some contained
mixed cargoes. In the cases where no Greek captives were on
board, the valuable portion of the cargo was transferred to the
schooner, and the ship was then permitted to proceed on her
voyage. Where Greek slaves were found on board, the captain was
given the choice of having the vessel burned, or giving a bond for an
amount equal to half her estimated value and that of the cargo,
signed by himself, the representative of the owners, if there was one
on board, and the principal passengers.
These bonds could not, perhaps, have been enforced in any court;
but Mr. Beveridge had confidence in the honesty of the Turks, and in
every case the amounts were duly forwarded to the agents he
named. Seven ships contained valuable cargoes of silks, tobacco,
and wine. These were all bound for Alexandria and Tunis, and
carried a considerable number of Greek women and children, the
survivors of massacres in towns in Asia Minor. In these cases the
Turks were all placed in their boats within two or three miles of land,
and the vessels with prize crews on board were consigned to Greeks
at Corinth and Athens, who had undertaken to act as Mr. Beveridge’s
agents, and who were to dispose of them and their cargoes to Greek
merchants.

T
CHAPTER XI
IN THE HANDS OF THE TURKS
OWARDS the end of the cruise the schooner had just returned
to the coast of Asia Minor after having run across to Athens and
taken on board the officers and men who had sailed the last prizes
taken there. On the day after they took up their place on their
cruising ground they fell in with a large polacca brig. The vessel
mounted ten small guns, and fought with some obstinacy, and it was
not until Martyn placed the schooner so that she could rake the
brig’s decks, which were crowded with men, that she hauled down
her flag.
“Lower two boats, Miller. You take charge of one and Tarleton the
other. By the look of those fellows I don’t believe they are Turks at
all. I believe they are from Algiers or Tunis; pirates at ordinary times,
but who have come here to pick up slaves cheap. They are
treacherous beggars, so be on your guard. There is a very strong
crew. Don’t row alongside till I lay the schooner broadside on.”
In five minutes Miller hailed from the deck of the prize, “You are
right, sir, they are Algerines, and as cut-throat a looking lot as ever I
came across. She is crowded below with Greek women and girls,
and as far as I can see at present she has no cargo of any sort. I
have sent one of the boats for Marco. He can speak to the women,
who are making a fearful hubbub down below.”
“Have you disarmed the crew, Mr. Miller?”
“Mr. Tarleton has just finished that. We have had to knock a good
many of the scoundrels down. They are as savage as wildcats.”
The schooner was brought alongside the polacca and lashed there.
The deck of the prize showed that the fire of the schooner had been

terribly destructive. Over twenty bodies lay scattered about,
principally round the guns.
“Are they all dead?” Martyn asked as he stepped on board.
“They are all dead now, but they were not when we boarded her.
But as they lay there they fired their pistols among us. Two or three
pretended to be dead, and then sprang up, knife in hand, and
several of the men have got nasty cuts; so that was soon put a stop
to. Some of the fellows below made quite a fight of it, and the men
had to use their cutlasses pretty freely. However, they are all
disarmed and bound now. I have no doubt they are Algerine pirates,
and deserve to be hung to the yard-arm every man-jack of them.”
“Have you overhauled the hold yet?”
“Yes, sir. It is filled with these unhappy slaves. She evidently came
merely in ballast, with money to buy them.”
“Well, no doubt these fellows have been pirates, Mr. Miller, but as we
have no means to prove it we must let them go as we have the
others, though it is a nuisance, for they only warn the people at the
ports against us. We won’t put them on the mainland this time, but
land them on one of the little islands. They may be some time in
getting a craft to take them to the mainland, and then they will find
it rough work making along the coast. However, we can settle upon
that later. The first thing to do is to get the decks roughly cleaned
and the dead bodies thrown overboard.”
A dozen men were set to work with mops and buckets, while others
fastened shot to the feet of the Algerines and dropped them
overboard. As soon as this was done Marco was sent below to tell
the captives that they could come on deck.
As the women poured up, looking almost dazed at their sudden
release, and at the bright sunlight after the stifling atmosphere of
the dark hold in which they had been confined for six days, Horace
saw one of them, a woman of some five-and-thirty years of age, to

whose side a girl of fifteen was clinging, looking round with an air of
excitement, in strong contrast to the comparative apathy of the
others. She glanced round at him and the men engaged in tidying
up the deck, and then with a cry sank fainting on the deck. He
hurried up to her, and partly raised her, when he was struck by the
cry of the girl, “Oh, mother, mother!” He looked at her in
astonishment.

THE GREEK CAPTIVES ARE BROUGHT ON DECK
“Are you English?” he exclaimed.
“Yes,” she cried, “we are English; but we have been seized and
carried away by these horrid Turks. Mother said she fancied she
heard some shouts in English, but she thought she must have been
mistaken, as only a Greek came down and spoke to us in the hold,
and she did not think it possible that it could be English. And have
you rescued us out of the hands of the Turks, sir? Mother said they
were taking us away to sell us as slaves.”
“Yes, we have rescued you,” Horace said. “You are free now. If you
will hold your mother’s head for a moment I will fetch the doctor; we
have one on board.”
“If you would get a little water, sir, she will soon come round. She
has fainted several times since we were captured.”
Horace, however, caught sight of Macfarlane.
“Doctor, here is an English lady among the captives. She has fainted.
Please see to her. I will run to get some water;” and he sprang over
the bulwark on to the deck of the schooner.
“Bring some brandy with you too,” Macfarlane said as he hurried to
the side of the fainting woman.
Horace rushed down to the cabin, and returned with a jug of water,
a decanter of brandy, and a tumbler. The doctor sprinkled some
water on the lady’s face, poured a few drops of spirits between her
lips, and in a minute or two she opened her eyes.
“It is all right now, madam,” he said as she looked round in a
confused way. “You are safe among friends and British sailors.”

“Thank God for His mercies!” she murmured, while tears fell down
her cheeks. “It seems almost too great happiness to be true.”
In a few minutes she was well enough to be assisted down to the
cabin of the schooner, where she was left to the care of her
daughter for a time. Half an hour later she was able to relate her
story to Mr. Beveridge. She was, she said, the wife of an English
merchant at Smyrna. They lived a short distance out of the town,
and had, since the troubles began, gone but little abroad, for
although it was only the Greeks who had been involved in the
massacre that had taken place there some months before, there was
a good deal of hostility upon the part of the lower class of the
population against all Christians. One evening she had been with her
daughter in the garden, her husband being engaged till late at his
business in the town. It was just getting dark, and she was about to
re-enter the house, when five or six ruffians of the lowest class
rushed into the garden, seized her and her daughter in spite of their
shrieks, threw thick cloths over their heads, and then carried them
away. They were taken for some distance, when they stopped, and
she heard an animated conversation and the clink of money. Then
they were placed in a boat, and presently carried up on to the deck
of a ship and taken below.
When their mufflings were removed they found they were in the
hold of a vessel with a large number of Greek captives. She
endeavoured in vain to make herself understood by the sailors who
came below, and who, she perceived at once, were not Turks. She
told them that she was English, and that her husband would pay a
large sum if she and her daughter were set on shore unharmed. No
attention was paid to her entreaties, but on her persisting she was
brutally knocked down, and in a short time a man, who was
evidently an officer, came down and forced them both to take off
their European dresses and put on others that some of the Greek
women were ordered to hand over to them. It was now evident to
her that they had been seized by some of the ruffians of the town
and sold to the Algerines, who were in no way particular as to the

nationality of their slaves, and that they were destined to be sold in
the slave-market of either Tunis or Algiers.
A few hours after they were taken on board they heard the anchor
run up, and could soon tell by the ripple of the water against the
planks that they were under weigh. All hope now left them, and they
had passed a terrible six days, overcome by despair, and half
suffocated by the foul air of the hold. Hope had again sprung up
when a gun was fired overhead, and it was soon evident that the
vessel was engaged in an encounter with an enemy. At last the firing
ceased, then there was a sound of shouting and the clashing of
swords on the deck above their heads. Presently the hatchways had
been opened and a Greek had come down and told them that the
vessel had been captured from the Turks, and that they were free.
She fancied that she heard English voices, but until she had reached
the deck and saw the faces and uniforms of the sailors, she thought
that she must be mistaken. After that she remembered no more until
she heard the doctor’s voice.
“I am rejoiced indeed that I have been enabled to save you and your
daughter from the horrors of slavery,” Mr. Beveridge said. “We have
had the pleasure of rescuing many hundreds of Greek women and
children from the hands of the Turks, but I never expected to find a
countrywoman among them. This cabin will be at your disposal,
except that we must, I fear, take our meals here. The cabin
adjoining will be wholly yours. In the course of a week I hope to
land you at Corfu, thence you will be able to write to your husband
and arrange either for joining him again at Smyrna, or taking a
passage for England, which would, I should think in the present
state of things, be the wisest course. My purse will be entirely at
your disposal. I am the owner of this schooner, which is called the
Misericordia, and although we fight under the Greek flag, and have
come out to assist them to obtain their independence, we are
principally devoting ourselves to saving the unhappy victims of this
war.”

The lady, whose name was Mrs. Herbert, expressed her deep
gratitude, and Mr. Beveridge at once took possession of Miller’s
cabin, as the lieutenant would, he had no doubt, remain in charge of
the prize. When the capture was made, the schooner was some
eighty miles to the east of Rhodes, and after talking the matter over
with Miller, Martyn decided to land the Algerines on Caxo, an islet
lying some fifty miles to the south-west of Rhodes. Miller and
Tarleton were for the present to continue on board the prize. The
prisoners, forty-eight in number, were transferred into the schooner.
The next evening they arrived off Caxo, where the Algerines were
landed in boats. Martyn then went on board the polacca.
“I have been thinking, Miller, that as we seem to have frightened all
the Turks into remaining in port for the present, I will leave you and
Tarleton on board the polacca, and give you twenty men and let you
cruise on your own account, while we take these women and
children round to the Ionian Isles. We will shift two of the eighteen-
pounders on board this craft. No one will suspect you, and you will
have a good chance of picking up some more prizes, while the sight
of our white sails sends everything running into port as far off as
they can be seen. We can rendezvous here again this day fortnight.”
“I should like that very much,” Miller said, “and I think it is a capital
plan. I must ask Mr. Beveridge to let me have Marco, or I shall have
no means of making myself understood either by Turk or Greek.”
A fortnight later the schooner returned to the island. She had had
rough weather for the last three days of her voyage, but the sky had
now cleared again.
“There is the island,” Martyn said, as Horace came up at six o’clock
in the morning to take charge of the watch, for he had now
command of the starboard watch, and Tom Burdett had the port.
“There is the island, but there is no sign of the polacca yet. I wonder
Miller is not here first. If we had been having calms I should not
have been the least surprised at his not turning up, but with this

strong southerly wind there is no reason why he should not have
been here. Go up to the main-top, Horace, and take a look round.”
But Horace could see no sail in sight.
“You are not uneasy about Miller surely,” Mr. Beveridge said at
breakfast, seeing that Martyn was not in his usual spirits.
“Well, I am rather uneasy, sir. Miller would be more likely to be a day
too soon than too late, and with the wind from the south he could
have calculated his time here from wherever he happened to be,
within an hour or two. The wind has been strong with us, and for
aught I know it may have been blowing a gale more to the east. We
don’t know much about the sailing qualities of the polacca, certainly
she was very light in ballast, and if she has been caught off a lee
shore in a heavy gale she may not have been able to claw off,
especially if she happened to be embayed when it came on. Of
course we must give him twenty-four hours more, but if he does not
come then we will shape our course north-east and cruise along the
coast; as we get eastward we may pick up some fishing craft or
small coaster and hear what the weather has been there, possibly
even get news of the polacca. If Miller gets here after we have left,
he will guess what course we have taken. Very likely he will land a
boat and learn that we have been here, and the course we took
when we sailed away, and would then be guided by circumstances.
At any rate, if nothing has happened to him, we are sure to meet
sooner or later.”
“Do just as you think best, Captain Martyn. I most sincerely trust
that there are no grounds for your uneasiness. Hitherto everything
has gone well with us, and it would be terrible indeed if anything
should have happened to our two friends and so many of our brave
fellows.”
The day passed slowly. A look-out was kept in the top, but until the
sun went down no sail was seen above the horizon. The crew shared
the anxiety of their captain, and gathering in groups, discussed what

could have occurred to prevent their consort arriving at the
rendezvous.
“I don’t believe as the Turks have caught them,” one of the sailors
said. “You won’t never gammon me into taking in such a yarn as
that. I don’t believe as there is a Turk living would get the weather
gauge of Lieutenant Miller. As to tempests, that is different. We don’t
care for tempests one way or the other on board the schooner, but
then she is a craft such as you don’t see twice in a v’yage round the
world. If they had been in her I shouldn’t have felt noways uneasy;
but seeing as how they are in a outlandish brig whose ways they
don’t understand, it may be that if they was caught off a lee shore
by a heavy gale, even the first lieutenant with our men at his back
couldn’t get her out of the mess.”
“I said all along,” another sailor put in, shaking his head, “as there
was bad luck coming. Three days ago I dreamed of a black cat, and
everyone as knows anything knows as there ain’t nothin’ more
unlucky to dream about than a black cat.”
“Surely, Bill,” another said, shaking his head gravely.
“Well, mates, it is my opinion,” Tom Burdett said gruffly, “as there is
something in dreams, but in nine cases out of ten it is something as
has gone afore and not what comes after. I know once when I came
back from a v’yage I had written a letter to tell my old woman what
time I should arrive. I reckoned to be in to dinner. Well, the coach
broke down and I did not get in till nine o’clock. The old woman had
made a plum-duff pretty nigh as big as my head, knowing as I was
fond of it, and she was in such a taking at my not having been in to
eat it at dinner that I sat down and I finished that there pudding
cold for supper. Well, I dreamt of about ten million black cats and
about as many sharks mixed up together, but if you will believe me
nothing came of it; and ever since that I have held to the opinion
that when you have a bad dream, what you have got to think about
when you wake ain’t what it means in the future, but what you have
been having for supper.

“Now, I expect if Bill there was to turn his mind back he would
remember that the night as he had that dream, he had been filling
hisself up with fruit or such like trash afore he turned in. I don’t say
as nothing has happened to Lieutenant Miller and our mates, but I
am cocksure as that black cat Bill said he dreamt on hadn’t nothing
to do with it either way. Why, bless me, in my village there is
hundreds of women as thinks of nothing but dreams and tokens. It
is no matter what you dream of, they have got a ’terpretation of it,
and if the ’terpretation happens to be a bad one they bother their
husbands and brothers and sons, as the case may be, not to put to
sea, and there is many a good fisherman whose cupboard is bare
half the year, through listening to them. I may have my ideas as to
whether harm are come to that polacca or not, but if every seaman
on board the ship was to dream of a black tom-cat and his wife and
family, it wouldn’t make not so much as a shade of difference, in my
opinion.”
Martyn did not wait for daylight, but when the middle watch was
relieved sail was made, and the schooner bore away to the north-
east. Land was sighted about four o’clock, and by nightfall they were
coasting along at the distance of about a mile. When it became dark
they stood on and off the shore, as Martyn wished to examine every
inlet and bay as they went on. As soon as it was daylight the
schooner proceeded on her way. The sails of several craft were
made out seaward during the course of the day, but none of these
resembled the canvas of the polacca, and attention was
concentrated upon the shore, every rock being closely scanned with
glasses, and a sharp look-out kept for signals of any kind.
In the evening a small fishing-boat was overhauled as it made its
way into a village. The fishermen were interrogated by Zaimes, who
understood a little Turkish. They had seen nothing of any craft
answering to a description of the polacca. Interrogated about the
weather, they replied that the storm four days before had been an
exceptionally severe one, coming on very suddenly and blowing with
tremendous force for some hours.

The next morning they were at the mouth of the Gulf of Adalia.
“If Miller has gone to grief anywhere,” Martyn said to Horace, “it is
as likely as not to be somewhere in this bay. He might very well have
been cruising about in here to pick up anything coming out of
Adalia, which is the principal port along this part of the coast. It is a
large bay, you see, and if he happened to be well up it when he was
caught in that sudden gale it is probable enough that he would not
be able to beat out in that craft. I see on the map there are three or
four small towns between this Cape and Adalia. I don’t want to show
ourselves inside the cape, for the probability is the schooner would
be recognized directly. What I think will be the best plan would be
for you and Zaimes to take one of the boats and coast along close in
to the cape. There is a place called Grambusa a mile or two around
the corner, and another place called Yanar a little farther on. I want
you either to board a fishing-boat and find out whether they have
news of a wreck between this and Adalia, or have heard of any
Greek or European prisoners being brought there from farther east.
If you can’t succeed in getting hold of a fishing-boat, Zaimes might
land and try to pick up the news at some cottage in the outskirts of
the village. There are Greeks in all these sea-side villages, for most
of the fishing is in their hands, and though in the towns there were
massacres I don’t suppose they would be disturbed in quiet villages
where they had been settled for generations.”
Zaimes was summoned, and agreed at once to land, as both Martyn
and Horace were of opinion that there was more probability of their
getting trustworthy information that way than from fishermen, who
would be scared at finding their boat suddenly overhauled.
Accordingly, taking a gig with six men Horace and Zaimes started for
the shore, while the schooner turned her head west.
“I shall cruise backward and forward,” Martyn said. “I sha’n’t go
more than four miles from the cape; so when you come out again
you will only have to lie on your oars till I come back for you.”

They rowed direct to shore, crept along close to it till they saw the
village half a mile ahead, and then rowed in and landed Zaimes. He
was absent an hour, and his walk assured Horace that he had bad
news even before he reached the side of the boat.
“I am afraid you have bad news, Zaimes.”
Zaimes shook his head. “Very bad; it could hardly be worse. There
are several Christians in the village, and I learned from them that
four days ago a brig that was caught in the storm was driven ashore
close to Adalia. It was found that she was a Turkish vessel which
had been captured by pirates. The people would have torn them to
pieces, but the pasha, who had come down to the shore with a body
of troops to try and save those on board the ship when she was
seen to be driving ashore, protected them from the mob and lodged
them in prison. They say that he has sent off to Smyrna, where the
governor of Anatolia resides, to ask for instructions, and it is
expected that orders will come for their execution in a day or two.”
“Stretch to your oars, men,” Horace said. “The others have been
wrecked and captured by the Turks, and the sooner we are on board
with the news the better.”
The men bent to their oars and made the boat fly through the water,
and when they rounded Cape Khelidonia they saw the schooner a
quarter of a mile away in the act of going about. They were seen
almost as soon as they caught sight of her, and she remained
thrown up in the wind until they got alongside. Martyn and Mr.
Beveridge were both on deck, and as soon as Zaimes had told his
story they went down into the cabin for a consultation.
“What on earth is to be done?” Martyn said; “Adalia is a large town.
Zaimes says there are troops there, likely enough a whole regiment.
It would be hopeless to try to attack it with thirty men. The only
thing I can see at present would be for us to sail right in, anchor off
the town, and threaten to bombard it with red-hot shot if they don’t
give up the prisoners. The objection is that they are likely to have

some batteries there, and in that case we might get the worst of it.
Besides, it is likely enough that they might hang Miller and the rest
of them at the first shot we fired.”
“No, that is not to be thought of,” Mr. Beveridge said. “It seems to
me that we might anchor within sight of the place, send a boat
ashore with a white flag, and offer to pay any ransom they might fix
for the prisoners. I would rather pay ten thousand pounds than that
harm should come to them. What do you think, Horace?”
“If we could have got at the pasha before he sent off to Smyrna that
might have done, father; but having once referred the case to
Smyrna, I am afraid he might consider it too risky to let them go.
But we might try that if everything else fails.”
“But what else is there, Horace?”
“Well, I should say, father, the best thing would be to land Zaimes
and myself again. He has already made some acquaintances in the
village here, and no doubt they could rig us both up in dresses like
their own. Then we could go boldly on to Adalia, find out exactly
how things stand, what sort of a place they are imprisoned in, how
strong is the guard, and how close the barrack of the troops is to the
prison. I should suggest that you sail away west, so that if, as it is
likely enough, the schooner has been noticed by any of the peasants
in the villages scattered about among the hills and word sent to
Adalia, the report may also go that it has sailed right away. Then you
should capture a small Turkish craft; a large fishing-boat would do.
Leave ten men on board the schooner, and sail in the prize nearly up
to Adalia. If you anchor, say a couple of miles this side of the town,
and hoist a little flag, say a red flag over a white, to your mast-head
we should recognize you and come down to the beach.
“If it is in the daytime you will make us out with your glasses easily
enough, and send a boat ashore for us. If it is nighttime we will
empty out a little powder, moisten it, and flash it off; then you can
send ashore for us. I should order the schooner to come every night,

keeping three or four miles off shore, sailing up nearly to Adalia, and
then returning so as to be round the cape again before daylight. In
that way we could communicate with her and go on board again
when we liked. Till we examine the place there is no saying whether
there is a possibility of rescue or not. If we find that there is no
possibility of anything being done in that direction we can embark on
board the schooner again, and carry out the plan you suggested:
anchor off Adalia, and send in to offer a ransom, with the alternative
that if it is not accepted we will bombard the place about their ears.
In that way, you see, we shall anyhow lose nothing by this
expedition of Zaimes and myself ashore.”
“I think your plan is an excellent one, Horace,” Martyn said, and Mr.
Beveridge equally approved of it.
“I don’t think there will be any great danger about it, Martyn. There
seems no reason why any suspicion should fall upon him and Zaimes
if they are dressed in the same way as the Greeks in these villages.”
“No, I don’t see why there should. Of course they will only speak
with other Greeks. I certainly think the plan of our getting hold of a
small native craft and anchoring near the town is a capital one. It
will save a great deal of time, for it is somewhere about fifty miles
from the cape to the town, and it would, in fact, save a whole day,
as, if they come off to us in the evening we could do what there is to
do that night, whereas, if they had to walk all the way down the
coast to the cape and come on board there it would be too late to
do anything that night, and we should have to wait until the next.”
Zaimes was called in, and eagerly embraced the proposal when it
was explained to him. He was passionately fond of his brother, from
whom he had never been separated, and was ready to dare anything
to attempt his rescue. It was agreed they had better wait till dark
before they landed. Accordingly the schooner sailed west for some
hours and did not return to the cape until after darkness had fallen.
Then Zaimes and Horace were landed, and as soon as the boat
returned the schooner again sailed away. Before leaving the ship

Horace had dressed himself as a Greek, and on landing they walked
to the village.
“You had best remain outside for a few minutes, Mr. Horace,” Zaimes
said, “while I see the man I conversed with this morning. I told him
then that my brother was on board the polacca that was wrecked,
and that I should endeavour to get the ear of some person of
importance at Adalia. He said that he was sure that I could do
nothing, but anything he could do to help me he would, for his
people came years ago from Naxos, which, as you know, is our
native place. I will just go in first to see if he is alone and to tell him
that I have a friend with me. As soon as I see that he is in the same
mood I will call you in.”
In three or four minutes the door of the cottage opened again and
Horace was called in.
“This is the young friend who accompanies me,” Zaimes said to the
man. “He is not a relation, but he has been with my brother ever
since he was born, and is willing to join me in the effort to save
him.”
“It is quite hopeless,” the peasant said. “You are only risking your
lives. Still, that is your business. You are ready, you say, to buy of
me two suits of our clothes. I have one suit belonging to my son,
who is at present away in a coasting ship, and I have a suit of my
own that I can let you have.”
Zaimes and Horace had both brought on shore a considerable
amount of gold stowed in belts beneath their clothes, in case they
should find any opportunity of bribing a prison official, and had in
their pockets an ample sum for any ordinary expenditure. As the
peasant only asked about three times the amount which the clothes
would cost new, they paid for them without bargaining, and at once
put them on.
“I have a brother at Adalia,” the man said, well pleased with the
bargain he had made; “and if you go to him and say that you come

from me, his brother Alexis, of this village, I am sure he will be glad
to lodge you, especially when you tell him that you too belong to
Naxos.”
After receiving instructions as to how to find the man’s brother in
Adalia they started at once upon their journey. They lay down for
three hours in the middle of the night in a wood, and entered Adalia
at eight o’clock in the morning. They went straight to the address
the peasant had given them. It was a small house with but two
rooms, and its master was a cobbler. As soon as Zaimes mentioned
his brother’s name, and said that they were ready to pay for the
accommodation, the shoemaker agreed at once to receive them. He
was a chatty fellow, and was very anxious to hear news about affairs
in Greece, when they told him that they had but lately arrived from
there.
“Now,” he said, “what is your business? Of course I can see that you
do not belong to us. You are from Naxos, as you say; I notice a few
turns of speech such as my father used to use. But what have you
come here for? and why have you bought my brother’s clothes from
him, for I recognized them directly you came in? I like to know
things, not because I am inquisitive, but because I do not want to
have the pasha’s executioner suddenly coming in at the door and
taking off my head, without even explaining the reason why.”
“I am what I told you, a Greek of Naxos,” Zaimes said; “and as I
explained to your brother, I have a brother who is one of the crew of
that ship that was wrecked here six days ago; and I have come to
see whether, by greasing the palms of some of the officials, I can
manage to get him out.”
“That you can’t,” the man said decidedly. “If he were in the civil
prison it might be done; but the pasha, guessing perhaps that many
of us Christians would sympathize with them, or possibly having an
idea that the mob might rise, handed them over to the soldiers, and
they are confined in a room in the military prison in the centre of the
barracks, where there are lots of sentries. The gates have been

closed since they were taken there, and no civilian is allowed to
enter under any pretence. So you see there is no bribing to be done.
Of course the sentries are changed frequently. There is no knowing
what officer has the prisoners specially under his charge. And even if
he were bribed, there would be no getting them past the sentries.
So you can give up the idea altogether of getting your brother out.”
“How long does it take for a messenger to go from here to Smyrna?”
Zaimes asked, with a slight glance at Horace to show that he was
changing the conversation purposely.
“By ordinary travelling some two weeks; but a mounted messenger,
with relays of horses, can do it in four days.”
“Then in another three days the answer may come from Smyrna?”
“That is so. I wonder myself that the pasha took the trouble of
sending to the governor of Anatolia, instead of hanging the prisoners
at once.”
“I suppose he thought that the governor might like to have them
sent to him, so that he could forward them to Constantinople.”
“Are you thinking of delaying the messenger’s return? That might be
done, you know.” And the man drew his finger across his throat
significantly.
“I don’t see that the delay would be of any use,” Zaimes replied. “If
there is no chance of getting my brother out, it matters not whether
the messenger arrives to-day or a fortnight hence. However, it is a
matter that may be worth thinking over later. At any rate we will go
out and have a look at the barracks. Will you go with us? I am not
without money, and can make it well worth your while to aid us by
your advice.”
“I am ready enough,” the man said. “Trade is dull, and a man must
live; and besides, I would gladly save a Christian and a native of my
own island from the Turks.”

“I would not trust him too far,” Zaimes said in an undertone to
Horace when the man went into the apartment behind to speak to
his wife. “He is now inclined to help us, especially if he thinks that
he will be well paid for it. But we had better not let him know
anything of our plans. When he saw there was danger, what with
fear as to his own safety and the hope of a bigger reward than he
could expect to get from us, he might decide to turn traitor. We had
better let him suppose that we have given up all hope.”
“I agree with you, Zaimes. His hint about the messenger may be a
useful one. I don’t mean, of course, that we should cut the poor
beggar’s throat; but we might bind him and fasten him up for a few
days if we find there is need of time to make our preparations.”
“I am afraid time will not help us,” Zaimes said. “The fellow can have
no motive for lying; and if what he says is a fact, I don’t see a
shadow of a chance of our getting them out, even if we had all the
crew of the schooner here.”
“We shall know more about it when we have seen the place, Zaimes.
I expected they would be securely locked up, and it is not much
worse than I looked for. It is hard if we can’t hit on some plan for
getting them out.”

E
CHAPTER XII
PLANNING A RESCUE
VEN Horace was obliged to admit, when he with Zaimes and
their guide had walked round the barracks, that he saw no
chance whatever of being able to get the prisoners out by force. The
barracks consisted of an old castle, a portion of which was, as the
shoemaker told them, now used as a military prison; and round this
at some distance ran a strong wall some fifteen feet high, loopholed
for musketry. The troops were lodged in huts between this wall and
the castle.
“There you see,” the guide said, “what I said was true. You could not
get a bird out of that place, much less a man.”
“That is so,” Zaimes agreed. “Well, what cannot be done, cannot.
However, we will talk it over this evening at your house. Now let us
walk about and view the city. Truly it is a fine one.”
Few towns, indeed, have a finer situation than Adalia, standing as it
does at the head of a noble bay, a great portion of which is fringed
with lofty and precipitous cliffs. The town, which at that time
contained some ten thousand inhabitants, stands on ground sloping
upwards from the sea in terraces rising one above another. It was
surrounded by a ditch and a double wall of massive construction,
with square towers every fifty yards. Beyond the walls stretched
gardens and groves of orange, lemon, and mulberry trees. Ten
mosques with their domes and minarets reared themselves above
the houses, and there were several churches belonging to the
Christian population, which was, the guide told them, about two
thousand in number, the great proportion of whom spoke only the
Turkish language. “I can talk equally well in both, for it is but fifty
years since my father settled here, and we always talked Greek in

the family as long as he lived. Now I always speak Turkish; it is
safer, and does not remind the Turks continually that we are of
Greek race.”
“Where does the pasha reside?” Horace asked presently.
“I will show you his place; it is at the lower corner of the north wall.
His gardens stretch down to the wall by the water, and another high
wall on this side separates them from the town.”
Passing through several streets they arrived opposite the residence
of the pasha of the sanjak of Tekeh, of which Adalia is the chief
town. The residence itself stood at the angle of the two walls
dividing the garden from the town. It was a massive building. Some
soldiers sat on benches at either side of the gate that opened into
the court-yard, and townspeople and officials passed in and out.
“The public offices are in the court-yard,” the guide said. “The
pasha’s private dwelling and his harem lie behind it.”
“I suppose we can walk in?”
“Certainly,” the guide said; and they passed through the gates into
the court-yard. On one side was a guard-room, stables, and other
offices; on the other were the rooms of the secretaries and officials
and that in which the pasha transacted business and received
visitors. The portion of the house facing the gates was blank on the
basement story, except that a door faced the gateway. Above were a
line of windows, all closed with jalousies. “That is the dwelling-
house,” their guide said. “I believe all the apartments of the family
face the garden. Those windows you see there are only those of the
apartments of the servants and slaves.”
After leaving the pasha’s they walked down to the bottom of the
town, where two gates with strong flanking powers opened upon the
port, which was smaller than Horace had expected to find it.
However, he was glad to see that there were several craft anchored
in the roadstead, some near the port, some at a distance, showing

that vessels did not come in unless for shelter in bad weather or to
discharge heavy cargoes. Whatever the craft, then, in which the
crew of the schooner might arrive, it would not attract attention by
anchoring outside the port, as arranged. They returned with their
guide to his house and had a meal there. Zaimes was profoundly
discouraged. He saw no prospect whatever of rescuing his brother or
the other prisoners, and the strength of the walls and the guns that
were mounted upon them—a step which, the host told him, had
been taken a few months before to defend the town against the
Greek fleet, should it make its appearance there—showed that there
was no prospect of the Turks being alarmed by the appearance or
threats of a craft like the schooner.
“It seems altogether hopeless,” Zaimes said to the Greek.
The latter shook his head, “I can see no possible way,” he replied. “If
it had been an ordinary prisoner in the jail it could be managed
without difficulty. I could have got one of our countrymen of some
influence to have approached the prison officers, or I myself could
have worked with the warders; a small sum of money would have
done it. But now it seems to me hopeless, and even if we stop the
messenger and gain another eight days while the pasha sends again
to Smyrna, we should only run some risk and gain nothing.”
Zaimes assented mournfully.
“You had better make the man a present, Zaimes,” Horace said when
they were alone for a minute after the meal was finished. “Tell him
that it seems to us to be hopeless, and that we shall probably go
right away; but that if, thinking it over, we can hit upon any possible
plan we will be back again this evening and sleep here.”
Zaimes carried out the suggestion, gave their host a gold coin, and
said that they saw no use in staying longer, but would think it over in
every way and might return that evening.
“If you go outside the town you must be back by sunset,” the man
said; “the gates are closed at that hour.”

“We will not forget, but I do not think you will see us again.”
“Even if our people don’t arrive this evening, Zaimes, I think it will
be just as well not to go back into the town,” Horace said as they
issued out through the gates into the country. “I don’t say for a
moment that the man is not honest, but it is just as well not to put
temptation in his way. He knows that we are friends of the prisoners,
and he, no doubt, guesses that we belong to the craft that captured
the polacca that was wrecked. No doubt he would not openly betray
us; that would bring him into discredit with all the Christians in the
town. But a few words whispered to some Turk, and an agreement
to share any reward that may be given for our capture, would
answer the purpose just as well. I don’t say he would do it, you
know, but it would be just as well not to run the risk.”
On issuing from the gate, Horace saw that there was a narrow road
running between a deep dry ditch at the foot of the city walls and
the outlying gardens and orchards.
“This will be our shortest way down to the water, Zaimes, let us
follow it.”
The Greek turned without question. When they had gone half-way
down between the gate and the bottom of the hill, Horace stopped.
“Now, let us have a good look at this place. On the other side of that
wall is the garden of the pasha’s house. I counted the number of
steps up from the house to the cross-road leading to the gateway,
and I have counted them coming down again; we are about fifty
yards below the upper wall of the garden.”
“I daresay it is so,” Zaimes replied listlessly.
“This ditch is about ten feet deep, and from the bottom of the ditch
to the top of that first wall is from five-and-twenty to thirty; between
that wall and the higher one inside it is about fifteen feet; and the
inner wall is about fifteen feet higher than the outer one; those
square towers form junctions between the two walls. Now, we may
be quite sure that there are no sentries either on the wall or on the

square towers. I don’t suppose there are sentries anywhere except
in the batteries on the water-face, but there certainly won’t be here,
for they would command a view down into the pasha’s garden; so
we may quite conclude that except for the trouble of scaling the
walls there is nothing to prevent our getting over. A couple of rope-
ladders and one or two twenty-foot planks with bits nailed across
them to give a foothold would take us on to the inner wall; then we
should need another long ladder to get down into the garden. That
would be about thirty-five feet, I should say.”
“Yes, I see all that,” Zaimes, whose face had again become animated
as he listened, agreed; “but what would be the good of getting into
the pasha’s garden?”
“No good at all, if we were by ourselves, Zaimes, but with Martyn
and twenty men from the schooner a good deal of good, I should
say. We have only got to make a sudden rush into the house, which
will, of course, be open to the garden, seize the pasha, and carry
him and some of his wives and children off to the craft that our
fellows come in, and then on to the schooner. Then we can send
ashore to say that unless the prisoners are sent off in a boat to us
by twelve o’clock in the day we shall hang the pasha. Maybe when
we get hold of the pasha there will be no occasion to carry him and
his women off; the mere threat of it might be enough. We can tell
him that it will be painful to us to have to hoist them up to the top of
the wall in sacks, but that we shall be obliged to do it unless he
signs an order for the prisoners’ release, and sends it off at once by
an officer to the jail. A handsome bribe that will enable him to make
his peace with his superior at Smyrna may help to quicken his
perception.”
Zaimes seized Horace’s hand with fervour, shook it wildly, clasped his
hands on his breast, raised them to heaven, and poured forth a
stream of exclamations of delight. The quiet habits of many years
had been thrown to the winds in a moment, and the excitable Greek
nature burst through all restraints. “You have given me new life,” he
exclaimed as soon as he had calmed down a little. “Just now there

did not seem even a shadow of hope. Now there is a chance that
once again I may clasp my brother in my arms. Your plan is difficult,
it is dangerous, and yet we may succeed. It is a desperate
undertaking, but what is that? I would give my life for my brother,
and your sailors would all risk theirs for their comrades.”
“Let us sit down here quietly for a few minutes, Zaimes, and take a
good look at these walls. It is evident by the look of this road that it
is very little used, and even if anyone did come up they would only
think that we had been working in the orange groves behind us and
were taking a quiet smoke. It is lucky that there is a moon to-night;
it would be an awfully difficult job to get over those walls and into a
place we know nothing of if it were a dark night. There will be no
difficulty in throwing up a grapnel and getting on to the first wall.
The greatest difficulty will be in crossing from that one to the one
behind it. Of course with a regular gangway it would be easy
enough, but we should not be able to get materials for making one.
However, with a couple of stout spars put up a foot apart with ropes
between them a foot from each other so as to make ratlings, we
could get up, though it wouldn’t be a very easy job passing women
down. Still, I hope it won’t come to that. I should think if we capture
the pasha and his children, if he has any—and I suppose with half-a-
dozen wives he will be sure to have some—we might leave the
women alone, though, of course, we should threaten to take them.
But I’ll tell you what we shall want, and that is a man who can speak
Turkish well, so as to explain exactly to the pasha the fix he is in.”
“Yes, we shall want such a man,” Zaimes agreed.
“Very well, Zaimes, then I think you had better go back to our friend
at once. Even if he did mean treachery, he would have taken no
steps yet, as he won’t expect us back till the evening if we come at
all. Tell him that you want a service of him in which he will run no
personal danger—for you know we can dress him up in some of our
things, and put a bit of black cloth as a mask half over his face—and
that he will be paid twenty pieces of gold for a night’s work. That will
be a fortune to him.”

“That will be the best plan,” Zaimes said. “Where shall we meet
you?”
“I will go down the hill to the bottom to see what sort of a road
there is along the sea, and I will wait there for you. If the road is
exposed to the view of the sentries on the batteries at the sea wall
we must make our way through the orchards to this point; if not, we
will move along there.”
“Do you think that Captain Martyn is sure to be here this evening?”
“He is quite certain to be. He knows that every hour is of
importance, and he will get hold of some craft or other early this
morning even if he has to go into a fishing port to get it.”
Zaimes retraced his steps up the hill, while Horace sauntered down
until he came out on to the road leading to the port along the shore.
A good many small houses were scattered along by its side, and
some fishing-boats drawn up on the beach. At the angle of the wall
there was a battery. Three guns pointed along the road and the
Turkish sentry was leaning against the parapet by the side of them.
“We shall have to make our way through the orchards,” he said to
himself. “There will be no getting along this road with the moon up.
The sentry would notice us a quarter of a mile away. Besides, the
tramp of so many feet would be certain to bring people to their
doors. And we must come early if we can, so as to catch the pasha
before he goes to bed.”
In half an hour Zaimes and the cobbler came up.
“It is agreed,” the former said in English; “twenty pounds will make
him what he considers rich, and he declares he is ready to run any
risk for a single night’s work in order to gain it. I think he is an
honest fellow. I watched him closely when I went in, and if he had
any thought whatever of betraying us, I think I should have seen it
in his face.”

It was now four o’clock in the afternoon, and they soon made out a
small brigantine anchored a quarter of a mile out, and about a mile
and a half along the shore.
“I expect that is her,” Horace said. “She has only just come in, for
there are some men upon the yards stowing away the sails, and that
is just the position we agreed she should take up.”
When they had gone a mile farther they could see that she had
small red and white flags at her mast-head. When they got opposite
to her they went down to the water’s edge. Horace waved a white
handkerchief for a moment and then sat down. A minute later the
boat towing behind the brigantine was hauled up. Two men got into
her and rowed leisurely to the shore. They were dressed as Turkish
sailors, but Horace recognized them as they came close as two of
the crew. They stepped in at once, and the boat rowed out again.
“Have you any news of Mr. Miller and the others, Mr. Horace,” one of
them said, “if I might make so bold as to ask?”
“Certainly you may. They are in prison, and there is no possibility of
getting them out with the strength we have got; it would need three
or four hundred men at least. But we have another plan, which we
hope will be successful.”
“You will find the captain down in the cabin with your father, Mr.
Horace. Everyone is keeping below except three or four of our
chaps, who are got up, like us, in the clothes of the crew of the
craft.”
“Come along, Zaimes,” Horace said as he stepped on board. “You
had better come with me. This man is going to help us, Davidson, so
make him as comfortable as you can till Zaimes comes out again.”
Horace found his father, Martyn, and the doctor in the little cabin. He
was heartily welcomed back, and eagerly questioned as to his news.
He first told them of the impossibility of doing anything to effect the

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Data Network Internet Communications Technology 1st Edition Ata Elahi

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