This document provides an introduction to data communications and computer networks. It discusses key topics such as: - The components of a basic data communication system including a transmitter, receiver, and transmission medium. - Different types of computer networks including local area networks (LANs) designed for short distances, wide area networks (WANs) for long distances, and metropolitan area networks (MANs) within a city. - Common network topologies like bus, star, ring, and hybrid configurations and their advantages and disadvantages. - Protocols that define rules for communication including syntax, semantics, and timing. Standards help ensure interconnectivity.

1. 1.1
Chapter 1
Introduction
 DCN

2. 1.2
1-1 DATA COMMUNICATIONS1-1 DATA COMMUNICATIONS
The termThe term telecommunicationtelecommunication means communication at ameans communication at a
distance. The worddistance. The word datadata refers to information presentedrefers to information presented
in whatever form is agreed upon by the parties creatingin whatever form is agreed upon by the parties creating
and using the data.and using the data. Data communicationsData communications are theare the
exchange of data between two devices via some form ofexchange of data between two devices via some form of
transmission medium such as a wire cable.transmission medium such as a wire cable.
 Components of a data communications system
 Data Flow
Topics discussed in this section:Topics discussed in this section:

3. 1.3
FOUR FUNDAMENTAL
CHARACTERISTICS of Data
Communication Network
Delivery
Accuracy
Timeliness
Jitter

4. 1.4
Figure 1.1 Components of a data communication system

5. 1.5
Wireless Channel
Transmitt
er
Wireless
Channel
Receive
r

6. 1.6
Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)

7. 1.7
1-2 NETWORKS1-2 NETWORKS
AA networknetwork is a set of devices (often referred to asis a set of devices (often referred to as nodesnodes))
connected by communicationconnected by communication linkslinks. A node can be a. A node can be a
computer, printer, or any other device capable of sendingcomputer, printer, or any other device capable of sending
and/or receiving data generated by other nodes on theand/or receiving data generated by other nodes on the
network. A link can be a cable, air, optical fiber, or anynetwork. A link can be a cable, air, optical fiber, or any
medium which can transport a signal carryingmedium which can transport a signal carrying
information.information.
 Network Criteria
 Physical Structures
 Categories of Networks
Topics discussed in this section:Topics discussed in this section:

8. 1.8
Network Criteria
 Performance
 Depends on Network Elements
 Can be measured in terms of transit time and response time.
 Measured in terms of Delay and Throughput( Max Rate)
 Reliability
 Failure rate of network components
 Measured in terms of availability/robustness
 Security
 Data protection against corruption/loss of data due to:
 Errors
 Malicious users

9. 1.9
Physical Structures
 Type of Connection
 Point to Point - single transmitter and receiver( television and
infrared remote control)
 Multipoint - multiple recipients of single transmission
 Physical Topology
 Connection of devices
 Type of transmission - unicast, mulitcast, broadcast

10. 1.10
Figure 1.3 Types of connections: point-to-point and multipoint

11. 1.11
Figure 1.4 Categories of topology

12. 1.12
Figure 1.5 A fully connected mesh topology (five devices)
In a mesh topology we need n(n-1)/2
duplex mode links

13. 1.13
Advantages of mesh
Dedicated links guarantees that each connection can carry its own
load.
Mesh topology is robust.
Privacy and security.
Fault identification and fault isolation easy.
Disadvantages
Amount of cabling and no of input and output ports.
Installation and reconnection are difficult.
Bulk of wiring
Hardware required to connect each link can be expensive.

14. 1.14
Figure 1.6 A star topology connecting four stations
High speed LANs

15. 1.15
Advantages of STAR
Less expensive than mesh.
Easy to install and reconfigure.
Robustness (one link fail . Only that link is affected.)
Less cabling needs.
Disadvantages
Dependency of whole topology on one single point ,the hub.
If the hub goes down, the whole system is dead.

16. 1.16
Figure 1.7 A bus topology connecting three stations
Multipoint, One long cable act as backbone .There is a limit on the no
of Taps a bus can support. ,As a signal travels along the backbone,
some of its energy is transformed into heat.

17. 1.17
Advantages of BUS
Ease of installation.
Less cabling than STAR and MESH
Disadvantages
Difficulty in reconnection and fault isolation.
A fault or break in bus cable stop all transmission
.

18. 1.18
Figure 1.8 A ring topology connecting six stations

19. 1.19
Advantages of RING
Ease of installation.
ADD and DELETE is Easy.
Generally a signal is circulating all the time. If one device does
not receive a signal within a specified period , it can issue an
alarm.
Disadvantages
A break in a ring can disable entire network.

20. 1.20
Figure 1.9 A hybrid topology: a star backbone with three bus networks

21. 1.21
Categories of Networks
 Local Area Networks (LANs)
 Short distances
 Designed to provide local interconnectivity
 Wide Area Networks (WANs)
 Long distances
 Provide connectivity over large areas
 Metropolitan Area Networks (MANs)
 Provide connectivity over areas such as a city, a campus

22. 1.22
Figure 1.10 An isolated LAN connecting 12 computers to a hub in a closet
LANS are designed to allow resources to be shared
between personal computer or workstations.

23. 1.23
Figure 1.12 A heterogeneous network made of four WANs and two LANs

24. 1.24
1-3 THE INTERNET1-3 THE INTERNET
TheThe InternetInternet has revolutionized many aspects of our dailyhas revolutionized many aspects of our daily
lives. It has affected the way we do business as well as thelives. It has affected the way we do business as well as the
way we spend our leisure time. The Internet is away we spend our leisure time. The Internet is a
communication system that has brought a wealth ofcommunication system that has brought a wealth of
information to our fingertips and organized it for our use.information to our fingertips and organized it for our use.
Organization of the Internet
Internet Service Providers (ISPs)
Topics discussed in this section:Topics discussed in this section:

25. 1.25
1-4 PROTOCOLS1-4 PROTOCOLS
A protocol is synonymous with rule. It consists of a set ofA protocol is synonymous with rule. It consists of a set of
rules that govern data communications. It determinesrules that govern data communications. It determines
what is communicated, how it is communicated and whenwhat is communicated, how it is communicated and when
it is communicated. The key elements of a protocol areit is communicated. The key elements of a protocol are
syntax, semantics and timingsyntax, semantics and timing
 Syntax
 Semantics
 Timing
Key elements of Protocol:Key elements of Protocol:

26. 1.26
Elements of a Protocol
 Syntax
 Structure or format of the data
 Indicates how to read the bits - field delineation
 Semantics
 Interprets the meaning of the bits
 Knows which fields define what action
 Timing
 When data should be sent and what
 Speed at which data should be sent or speed at which it is
being received.

27. STANDARDS
 Standards are essential in creating and maintaining an open and
competitive market for equipment manufacturers and in
guaranteeing national and international interoperability of data
and telecommunication technology and processes.
 Standards provide guidelines to manufacturers , vendors, and
other providers to ensure the kind of interconnectivity necessary
in todays markets.
 De facto standards ( By fact , not approved by organized body)
 De Jure Standards (Legislated by officially recognized body )
1.27

28. STANDARD
ORGANISATIONS
 ISO
 ITU-T
 ANSI
 IEEE
 EIA
1.28