Bt 72 computer networks

BT 0072
Computer Networks
Conte
n ts
Unit 1
Introduction 1
Unit 2
Physical Layer 33
Unit 3
Medium Access Sublayer 90
Unit 4
Data Link Layer – I 119
Unit 5
Data Link Layer – II 142
Uni t 6
Net work Layer – I 158
Unit 7
Network Layer – II 177
Unit 8
Transport Layer 207
Unit 9
Session Layer 241
Unit 10
Application Layer – I 257
Unit 11

Application Layer – II 283
Unit 12
Internet Security 309

Prof.V.B.Nanda Gopal
Director & Dean
Directorate of Distance Education
Sikkim Manipal University of Health, Medical & Technological Sciences (SMU DDE)
Board of Studies
Dr.U.B.Pavanaja (Chairman)
General Manager – Academics
Manipal Universal Learning Pvt. Ltd.
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Manipal Education
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Manipal Centre for Info. Sciences.
Manipal.
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Manipal Institute of Technology
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Sikkim Manipal University – DDE
Manipal.
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Edition: Spring 2009
This book is a distance education module comprising a collection of learning material for our
students. All rights reserved. No part of this work may be reproduced in any form by any
means without permission in writing from Sikkim Manipal University of Health, Medical and
Technological Sciences, Gangtok, Sikkim. Printed and published on behalf of Sikkim
Manipal University of Health, Medical and Technological Sciences, Gangtok, Sikkim by
Mr.Rajkumar Mascreen, GM, Manipal Universal Learning Pvt. Ltd., Manipal – 576 104.
Printed at Manipal Press Limited, Manipal.

SUBJECT INTRODUCTION
This book is a walkthrough of all the layers of the ISO OSI model which is a
preliminary requirement for a student to understand the world of networking.
Traditionally the area of networking has gone drastic changes in accordance
with the requirements of the user community. The layering in specific has
been modified according to the current requirements. The units in this
chapter provide a deep insight into the functionality of the networks along
with their services offered to a host of machines. The reader is taken
through a step-by-step approach to understand and implement those
functionalities in the real world scenario by understanding on a layer to layer
basis.
Unit 1: Introduction
This unit introduces computer networks and their applications in the real
world scenario. The network types and their structures are discussed. The
basic network architecture is presented. The base model of entire
networking, the ISO OSI model is discussed in detail. The services offered
by various types of networks are also discussed. The TCP/IP protocol is
described along with its features and architecture.
Unit 2: Physical Layer
This unit discusses the physical layer which is involved in raw data
transmission from source to destination machines. It describes various
representations of data in the form of analog and digital signals. It discusses
various possible errors during transmission called the Transmission
impairments. The various types of transmission media used, the
transmission and switching techniques are also highlighted. A detailed
discussion of Integrated Services Digital Network is also done at the end of
this unit.
Unit 3: Medium Access Sublayer
It starts with a detailed discussion on LANs and WANs. It describes the
basic LAN protocols. It describes the IEEE 802 standards for LANS. It
discusses the importance of Fiber Optic Networks and cabling used as
backbone for LAN connectivity.

Unit 4: Data Link Layer – I
This unit starts with the design issues of Data Link Layer and the application
or usage of this layer in the OSI model. It discusses about various error
detection and correction techniques. It describes the block coding
techniques, Cyclic codes, and Checksum used for either error correction or
error detection. It also describes the concept of Framing in Data Link Layer.
Unit 5: Data Link Layer – II
This unit describes the types of communication channels like Noisy and
Noiseless channels. It describes the HDLC control technique of the Data
Link Layer. It also describes the Point-to-Point Protocol. It discusses the
importance and applications of Channelization. It discusses the IEEE 802.11
standard for Wireless LANs. It also describes various connecting devices
used in LANs.
Unit 6: Network Layer – I
This unit starts with the design issues of the Network Layer. It describes the
principles of Routing in Network Layer. It explains various routing algorithms
adopted by the Network layer in message passing. At the end, a brief
comparison of various routing algorithms is done.
Unit 7: Network Layer – II
This unit describes the various addressing schemes used by the network
layer in identifying the hosts on a network. It describes both the IPV4 and
IPV6 addressing schemes along with their comparisons.
Unit 8: Transport Layer
This unit provides the reader with the concepts of of the TCP/IP protocols
like User Datagram Protocol (UDP), and the Transmission Control Protocol
(TCP) and their related details.
Unit 9: Session Layer
This unit provides the reader with an overview of Session Layer, its design
issues, services provided by it and the Remote Procedure Call Mechanisms.
Unit 10: Application Layer – I
This chapter provides an overview of the TCP/IP application protocols
dealing with electronic mail, including Simple Mail Transport Protocol
(SMTP), Sendmail, Multipurpose Internet Mail Extensions (MIME), Post
Office Protocol (POP), and Internet Message Access Protocol (IMAP).

Unit 11: Application layer – II
This unit introduces the components of DNS, the structure and architecture
of DNS, the difference between domains and zones, define recursive and
iterative queries and how DNS forward and reverse lookups work. It defines
the various roles of DNS servers.
Unit 12: Internet Security
This unit starts with the basic concepts of Internet Security. It deals with the
terminology associated with Internet Security. It then speaks about the
IPSec. It gives an insight into the topics like SSL / TLS, PGP, Firewalls and
so on relevant to the maintenance of Network Security. At the end it deals
with Firewalls.

Computer Networks
Unit 1
Unit 1
Introduction
Structure:
1.0 Objectives
1.1 Introduction
1.2 Network Structures
1.3 Network Architecture
1.4 OSI Reference Model: An Overview
1.5 Network Services
1.6 TCP/IP Protocol Suite
1.7 Summary
1.8 Self Assessment Questions
1.9 Terminal Questions
1.10 Answers to Self Assessment Questions
1.11 Answers for Terminal Questions
1.0 Objectives
The main objective of this unit is to make the reader understand the concept
of data communications and computer networks.
After completion of this unit the reader would be able to:
• Explain the importance of Networks
• Define and describe the ISO OSI Model
• Describe the functionalities of each and every layer of the OSI Model
• State the terminology associated with Network Services
• Describe various Network Services like connectionless and connection –
oriented
• State the relationship between Network Services and Network Protocols
• Describe TCP/IP Protocol Suite
Sikkim Manipal University Page No. 1

Computer Networks Unit 1
1.1 Introduction
Computer is an information tool. Networks enhance the computer‟s ability to
exchange, preserve, and protect information. Networks make it easier to
share expensive hardware and software. The type of information changes
from business to business. The way that information is stored and worked
with also varies.
Personal Computer (PC)

It is a fantastic information tool. A PC is shipped from the manufacturer with
minimum software to make it run and marginally useful; it is up to the user to
customize it for his / her own purpose.
Figure 1.1: A computer is a versatile tool that can perform many tasks when

configured with the software.
Information constantly flows through the business. A publishing house
collects market projections, receives manuscript drafts, delivers edited
proofs, requests corrections and elaboration, and finally a book is sent to be
printed. Before networks, people had to personally move the information
about, whether it was on paper, over the phone, or on floppy disk or
magnetic tape. When you use a computer not connected to a network, you
are working in a stand – alone environment. In this environment, you can
use software to produce data, graphics, spreadsheets, documents, and so

on, but to share information; you must print it out or put it on a floppy disk or
CD – ROM so that someone else can use it. That is, you are moving the
information about yourself rather than letting the computer do it for you.
The Network
Computers connected over a Network can make the information exchange
easier and faster. The information moves directly from computer to
computer rather than through a human intermediary. Due to this, people can
concentrate on getting their work done rather than on moving information
around the company. A group of computers and other devices connected
together is called a Network, and the concept of connected computers
sharing resources is called Networking.
LAN: (Local Area Network) A LAN is a number of computers connected to
each other by cables in a single location, usually a single floor of building or
all the computers in a small company. A simple cabling method, known as
the Bus Topology, allows about 30 computers on a maximum cable length
of about 600 feet.

Figure 1.2: A network is a number of computers linked together to share

resources.
Figure 1.2 shows a simple LAN.
LANs are perfect for sharing resources within a small geographical area
(Approx. 500 Meters), but they cannot be used to connect distant sites.

Another type of Network, called as Wide Area Network (WAN) addresses
this need. WAN is a set of connecting links between LANs.
The links in WAN can be made as follows:
♦ Over the telephone lines leased from various telephone companies.
♦ Using Satellite links
♦ Packet Radio Networks
♦ Microwave transceivers
Figure 1.3: A WAN links computers in different locations
Most WANs are private and are owned by the businesses that operate with
them. Many companies are forming private WANs, known as the Virtual
Private Networks (VPNs), through encrypted communication over the
Internet.
WANs suffer from extremely limited bandwidth. A typical LAN transmits at
10 Mbps, or 10 Million bits per second. A T1 line considered as a fast WAN

link, transmits at 1.5 Mbps, or 1 million bits per second, which makes the
sharing of resources over a WAN difficult.

In general, WAN links are used only for inter-process communications to
route short messages, such as e-mail or HTML traffic.
1.2 Network Structures
In any network there exists a collection of machines for running user (or
application) programs called Hosts. The Hosts are connected by the
communication subnet, or just subnet. The job of the subnet is to carry
messages from host to host. By separating the pure communication aspects
of the network (the subnet) from the application aspects (the hosts), the
complete network design is greatly simplified.
In most Wide Area Networks, the subnet consists of two distinct
components: Transmission Lines and Switching Elements. Transmission
Lines (also called as circuits, channels, or trunks) move bits between
machines.
The Switching Elements are specialized computers to connect two or more
transmission lines. When data arrives on an incoming line, the Switching
Element must choose an outgoing line to forward them.
The Switching Elements are also called as Interface Message Processors
(IMPs). Each host is connected to one (or occasionally several) IMPs. All
traffic to or from the host goes via its IMP.
There are two types of designs for communication subnets:
1. Point – to – Point Channels
2. Broadcast Channels
1. In Point-to-Point channels, the network contains numerous cables or
leased telephone line, each one connecting a pair of IMPs. If two IMPs
that do not share a cable nevertheless wish to communicate, they must
do this indirectly, via other IMPs. When a message in the form of a
packet is sent from one IMP to another IMP via one or more

intermediate IMPs, the packet is received at each intermediate IMP in its
entirety, stored there until the required output line is free, and then
forwarded. A subnet using this principle is called a point-to-point,
store-and-forward, or packet-switched subnet.
When a point-to-point subnet is used, an important design issue is what
the IMP interconnection topology should look like, LANs have a
symmetric topology, whereas WANs have asymmetric topology.
2. Broadcasting: Most LANs and a small number of WANs are of this
type. In a LAN, the IMP is reduced to a single chip embedded inside the
host, so that there is always one host per IMP, whereas in a WAN there
may be many hosts per IMP.
Broadcast Systems have a single communication channel shared by all
other machines on the network. Packets sent by any machine are
received by all the others. An address field within the packet specifies
for whom it is intended. Upon receiving a packet, a machine checks the
address field. If the packet is intended for some other machine, it is just
ignored.
Broadcast systems also support transmission to a subset of machines,
something known as Multicasting. A common scheme is to have all
addresses with high order bit set to 1. The remaining n-1 address bits
form a bit map corresponding to n-1 groups. Each machine can
subscribe to any or all of the n-1 groups.
1.3 Network Architecture
Modern computer networks are designed in a highly structured way. In the
following discussion, we examine some of the structuring techniques.
Protocol Hierarchies
To reduce their design complexity, most networks are organized as a series
of layers or levels, each one built upon its predecessor. The number of

layers, name, content, and function of each layer differs from network to
network. In all networks, the purpose of each layer is to offer certain
services to the higher layers, shielding those layers from details of how the
offered services are actually implemented.
Layer n on one machine carries on a conversation with layer n on another
machine. The rules and conventions used in this conversation are
collectively known as the Layer n protocol.
The entities comprising the corresponding layers on different machines are
called peer processes. It is the peer processes that communicate using the
protocol.
In reality, no data are directly transferred from layer n on one machine to
layer n on another machine. Instead, each layer passes data and control
information to the layer immediately below it, until the lowest layer is
reached. Below layer 1 is the physical medium through which the actual
communication occurs.
Between each pair of adjacent layers there is an interface. The interface
defines which primitive operations and services the lower layer offers to the
upper one. For the designers of a network one of the most important
considerations is defining clean interfaces between the layers. Doing so,
in turn, requires each layer perform a specific collection of well-understood
functions.
In addition to minimizing the amount of information that must be passed
between layers, clean-cut interfaces also make it simpler to replace the
implementation of one layer with a completely different implementation,
because all that is required in the new implementation is that it offer exactly
the same set of services as the to its upstairs neighbor as the old
implementation did.

The set of layers and protocols is called the Network Architecture. The
specification of the architecture must contain enough information to allow an
implementer to write the program or build the hardware for each layer so
that it correctly obeys the appropriate protocol.
Neither the details of the implementation nor the specification of the
interfaces are part of the architecture because these are hidden away inside
the machines and not visible from the outside. It is not even necessary that
the interfaces on all machines in a network be the same, provided that each
machine can correctly use all the protocols.
Design Issues for Layers
1. Every layer must have a mechanism for connection establishment. Since
a network normally has many computers, some of which have multiple
processes, a means is needed for a process on one machine to specify
with whom it wants to establish a connection. As a consequence of
having multiple destinations, some form of addressing is needed in order
to specify a specific destination.
2. Mechanisms for terminating the connections when they are no longer
needed.
3. Data Transfer Rules:
 Simplex Communication: In this type, data transfer occurs in only
one direction, i.e., either from source to destination or destination to
source machines.
 Half-duplex Communication: In this type, data transfer occurs in
either directions, but not simultaneously.
 Full-duplex Communication: In this type, data transfer occurs in
either directions simultaneously.
The protocol must also determine the number of logical channels per
connection along with their individual priorities. Many networks

provide at least two logical connections per channel, one for normal
data, and one for urgent data.
4. Error Control Mechanisms: It is one of the important issues since
physical communication circuits are not perfect. Many error-correcting
and detecting codes are known, but both ends of the connection must
agree on which one is being used. In addition, the receiver must have
some way of telling the sender which messages have been correctly
received and which have not.
5. Message Ordering: Not all communication channels preserve the
ordering of messages sent on them. To deal with a possible loss of
sequencing, the protocol must make explicit provision for the receiver to
allow the pieces to be put back together properly. An obvious solution is
to number the pieces, but this leaves open the question of what should
be done with pieces that arrive out of order.
6. An issue that occurs at every level is how to keep a fast sender from
swamping a slow receiver with data. All the proposed solutions have
some kind of feedback mechanisms, wherein the receiver informs its
current situation to the sender.
7. Another problem that must be solved at several levels is the inability of
all processes to accept arbitrarily long messages. This property leads to
mechanisms for disassembling, transmitting and then reassembling
messages. A related issue is what to do when processes insist upon
transmitting data in units that are so small that sending each one
separately is inefficient. Here the solution is to gather together several
small messages heading towards a common destination into a single
large message and dismember the large message at the other site.
8. When it is inconvenient to set up a separate connection for each pair of
communicating processes, the underlying layer may decide to use the

same connection for multiple, unrelated conversations. As long as this
multiplexing and de-multiplexing is done transparently, it can be used by
any layer.
9. When there are multiple paths between source and destination, a route
must be chosen. Sometimes this decision must be split over two or more
layers.
1.4 OSI Reference Model: An Overview
The layered model that dominated data communications and networking
literature before 1990 was the Open Systems Interconnection (OSI)
model. Everyone believed that the OSI model would become the ultimate
standard for data communications, but this did not happen. The TCP / IP
protocol suite became the dominant commercial architecture because it was
used and tested extensively in the Internet; the OSI model was never fully
implemented.
Established in 1947, the International Standards Organization (ISO) is a
multinational body dedicated to worldwide agreement on International
standards. An ISO standard that covers all aspects of network
communications is the OSI model, which was first introduced in 1970s.
Open System: A set of protocols that allows any two different systems to
communicate regardless of their underlying architecture.
Purpose of OSI Model: It shows how to facilitate communication between
different systems without requiring changes to the logic of underlying
hardware and software.
The OSI model is not a protocol; it is a model for understanding and
designing a network architecture that is flexible, robust, and interoperable.
The OSI Model is a layered framework for the design of network systems
that allows communication between all types of computer systems. It

consists of seven separate but related layers, each of which defines a part
of the process of moving information across a network.
Layered Architecture:
The OSI Model is composed of seven ordered layers:
♦ Layer 1 – The Physical Layer
♦ Layer 2 – The Data Link Layer
♦ Layer 3 – The Network Layer
♦ Layer 4 – The Transport Layer
♦ Layer 5 – The Session Layer
♦ Layer 6 – The Presentation Layer
♦ Layer 7 – The Application Layer
Figure 1.5 below shows the layers involved when a message is sent from
device A to device B. As the message travels from one device to another, it
may pass through several intermediate nodes or devices. These
intermediate nodes or devices usually involve only the first three layers of
the OSI model.

In modeling the OSI model, the designers distilled the process of
transmitting data to its most fundamental elements. They identified which
networking functions had related uses and collected those functions into
discrete groups that became the layers. Each layer defines a family of
functions distinct from those of the other layers. By defining and localizing
the functionality in this fashion, the designers created an architecture that is
both comprehensive and flexible.

Within a single machine, each layer calls upon the services of the layer
below it. Between machines, layer x on one machine communicates with
layer x on another machine. This communication is governed by an agreed-
upon series of rules and conventions called protocols. The processes on
each machine that communicates at a given layer are called peer-to-peer
processes. Communication between machines is therefore a peer-to-peer
process using the protocols appropriate to a given layer.
Peer-to-Peer Processes
At the physical layer, the communication is direct. In the figure above,
device A sends a stream of bits to device B (through intermediate nodes). At
the higher layers, communication must move down through the higher
layers on device A, over to device B, and then back up through the layers.
Each layer in the sending device adds its own information to the message it
receives from the layer just above it and passes the whole package to the
layer just below it.
At layer 1 the entire package is converted to a form that can be transmitted
to the receiving device. At the receiving machine, the message is
unwrapped layer by layer, with each process receiving and removing the
data meant for it.
Interfaces between Layers
The passing of the data and the information down through the layers of the
sending device and back up through the layers of the receiving device and
back up through the layers of the receiving device is made possible by an
interface between each pair of adjacent layers. Each interface defines the
information and services a layer must provide for the layers above it. Well-
defined interfaces and layer functions provide modularity to a network. As
long as a layer provides the expected services to the layer above it, the
specific implementation of its functions can be modified or replaced without
requiring changes to the surrounding layers.

Layer Organization
The seven layers can be thought of as belonging to three subgroups. Layers
1, 2, and 3 are the network support layers; they deal with the physical
aspects of moving data from one device to another. Layers 5, 6, and 7 can
be thought of as user support layers; they allow interoperability among
unrelated software systems.
Layer 4 links two subgroups and ensures that what the lower layers have
transmitted is in a form that the upper layers can use. The upper OSI layers
are almost always implemented in software, except for the physical layer
which is mostly implemented in hardware.
Layers in the OSI Model
This section discusses the functions of all the 7 layers of OSI model.
1. Physical Layer: This layer coordinates the functions required to carry a
bit stream over a physical medium. It deals with the electrical and
mechanical specifications of the interface and transmission medium. It
defines the procedures and functions that physical devices and
interfaces have to perform for transmission to occur.
2. Data Link Layer: This layer transforms the physical layer, a raw
transmission facility, to a reliable link. It makes the physical layer appear
error-free to the upper layer (to the Network layer). It is also responsible
for other functions such as framing, error control, flow control, physical
addressing, and access control mechanisms.
3. Network Layer: This layer is responsible for the source-to-destination
delivery of a packet, possibly across multiple networks (links). The Data
Link Layer oversees the delivery of the packet between two systems on
the same network (links), the network layer ensures that each packet
gets from its point of origin to its final destination. If two systems are
attached to the same link, there is no need for the network layer.

However, if the two systems are attached to different networks (links)
with connecting devices between the networks (links), there is often a
need for the network layer to accomplish source-to-destination delivery.
Other responsibilities of the Network layer include logical addressing,
and routing.
4. Transport Layer: This layer is responsible for process-to-process
delivery of the entire message. A process is an application program
running on the host. The Network layer oversees the source-to-
destination delivery of individual packets, it does not recognize the
relationship between those packets. It treats each packet independently,
as though each piece belonged to a separate message, whether or not it
does, The Transport layer, also ensures that the whole message arrives
intact and in order, overseeing both error and flow control at the source-
to-destination level.
5. Session Layer: This layer acts as the network dialog controller. It
establishes, maintains, and synchronizes the interaction among
communicating systems.
6. Presentation Layer: This layer is concerned with the syntax and
semantics of the information exchanged between two systems. The
specific responsibilities of this layer include Translation, Encryption, and
Compression.
7. Application Layer: This layer enables the user, whether human or
software to access the network. It provides user interfaces and support
for services such as electronic mail, remote file access and transfer,
shared database management, and other types of distributed
information services. Specific services offered by the Application layer
include: Provision of Network Virtual terminals, File transfer, access, and
management, mail services, and Directory Services.

Figure 1.6: Summary of Layers in OSI Model
Data Transmission in OSI Model
The sending process has some data it wants to send to the receiving
process. It gives the data to the application layer, which then attaches the
application header, AH (which may be null), to the front of it and give the
resulting item to the presentation layer.

The presentation layer may transform this item in various ways, where they
are actually transmitted to the receiving machine. On the machine various
headers are stripped off one by one as the message propagates up the
layers until it finally arrives at the receiving process.

The key idea throughout is although actual data transmission is vertical,
each layer is programmed as though it were really horizontal.
1.5 Network Services
The real function of each layer is to provide services to the layer above it.
i) Network Services Terminology
1. Entities: They are the active elements in each layer. An entity can be a
software entity (For example, a process) or a hardware entity (For
example, an intelligent I/O chip).
2. Peer Entities: Entities in the same layer on different machines
3. Service Provider and Service User: The entities in layer N implement
a service used by layer N+1. Layer N is called the Service Provider and
layer N+1 is called the Service User.
4. Service Access Points (SAPs): Services are available as SAPs. The
layer N SAPs are the places where layer N+1 can access the services
offered. Each SAP has an unique address that identifies it.
5. Interface Data Unit (IDU): For two layers to exchange information, there
has to be an agreed upon set of rules about the interface. At a typical
interface, the layer N+1 entity passes an IDU to the layer N entity
through the SAP.
6. SDU (Service Data Unit): The IDU consists of an SDU (Service Data
Unit) and some control information.
The SDU is the information passed across the network to the peer entity
and then up to layer N+1. The control information is needed to help the
lower layer do its job, but is not part of the data itself.
7. PDU (Protocol Data Unit): In order to transfer the SDU, the layer N
entity may have to fragment it into several pieces, each of which is given
a header and sent as a separate PDU (Protocol Data Unit) such as a

packet. The PDU headers are used by the peer entities to carry out their
peer protocol. They identify which PDUs contain data and which contain
control information, provide sequence number counts, and so on.
8. Connection – Oriented Service: Modeled after the telephone system.
To use a connection-oriented network service, the service user first
establishes a connection, uses the connection, and terminates the
connection.
9. Connection-less Service: Modeled after the Postal System. Each
message carries the full destination address, and each one is routed
through the system independent of all others.
Network Models
Computer networks are created by different entities. Standards are needed
so that these heterogeneous networks can communicate with one another.
The two best known standards are the OSI model and the Internet model.
The OSI model defines a seven-layer network; the Internet model defines a
five-layer network.
1.6 TCP / IP Protocol Suite
The TCP/IP protocol suite has become a staple of today's international
society and global economy. Continually evolving standards provide a wide
and flexible foundation on which an entire infrastructure of applications are
built. Through these we can seek entertainment, conduct business, make
financial transactions, deliver services, and much more.
The Transmission Control Protocol/Internet Protocol (TCP/IP) suite has
become the industry-standard method of interconnecting hosts, networks,
and the Internet. As such, it is seen as the engine behind the Internet and
networks worldwide.

Although TCP/IP supports a host of applications, both standard and
nonstandard, these applications could not exist without the foundation of a
set of core protocols. Additionally, in order to understand the capability of
TCP/IP applications, an understanding of these core protocols must be
realized.
Architecture, History, Standards, and Trends
Today, the Internet and World Wide Web (WWW) are familiar terms to
millions of people all over the world. Many people depend on applications
enabled by the Internet, such as electronic mail and Web access. In
addition, the increase in popularity of business applications places additional
emphasis on the Internet.
The Transmission Control Protocol/Internet Protocol (TCP/IP) protocol suite
is the engine for the Internet and networks worldwide. Its simplicity and
power has led to its becoming the single network protocol of choice in the
world today. In this section, we give an overview of the TCP/IP protocol
suite. We discuss how the Internet was formed, how it developed, and how
it is likely to develop in the future.
TCP/IP Architectural Model
The TCP/IP protocol suite is so named for two of its most important
protocols: Transmission Control Protocol (TCP) and Internet Protocol (IP). A
less used name for it is the Internet Protocol Suite, which is the phrase used
in official Internet standards documents.
Internetworking
The main design goal of TCP/IP is to build an interconnection of networks,
referred to as an internetwork, or internet, that provide universal
communication services over heterogeneous physical networks. The clear
benefit of such an internetwork is the enabling of communication between

hosts on different networks, perhaps separated by a large geographical
area.
The words „internetwork‟ and „internet‟ are simply a contraction of the phrase
interconnected network. However, when written with a capital “I”, the
Internet refers to the worldwide set of interconnected networks. Therefore,
the Internet is an internet, but the reverse does not apply. The Internet is
sometimes called the connected Internet.
The Internet consists of the following groups of networks:
• Backbones: Large networks that exist primarily to interconnect other
networks. Also known as network access points (NAPs) or Internet
Exchange Points (IXPs). Currently, the backbones consist of commercial
entities.
• Regional networks connecting, for example, universities and colleges.
• Commercial networks providing access to the backbones to
subscribers, and networks owned by commercial organizations for
internal use that also have connections to the Internet.
• Local networks, such as campus-wide university networks.
In most cases, networks are limited in size by the number of users that can
belong to the network, by the maximum geographical distance that the
network can span, or by the applicability of the network to certain
environments. For example, an Ethernet network is inherently limited in
terms of geographical size. Therefore, the ability to interconnect a large
number of networks in some hierarchical and organized fashion enables the
communication of any two hosts belonging to this internetwork.
Figure 1.7 below shows two examples of internets. Each consists of two or
more physical networks.

Figure 1.7: Internet examples: Two interconnected sets of networks, each
seen as one logical network.
Another important aspect of TCP/IP internetworking is the creation of a
standardized abstraction of the communication mechanisms provided by
each type of network. Each physical network has its own technology-
dependent communication interface, in the form of a programming interface
that provides basic communication functions (primitives). TCP/IP provides
communication services that run between the programming interface of a
physical network and user applications. It enables a common interface for
these applications, independent of the underlying physical network. The
architecture of the physical network is therefore hidden from the user and
from the developer of the application. The application need only code to the
standardized communication abstraction to be able to function under any
type of physical network and operating platform.
As is evident in figure 1.7, to be able to interconnect two networks, we need
a computer that is attached to both networks and can forward data packets

from one network to the other; such a machine is called a router. The term
IP router is also used because the routing function is part of the Internet
Protocol portion of the TCP/IP protocol suite.
To be able to identify a host within the internetwork, each host is assigned
an address, called the IP address. When a host has multiple network
adapters (interfaces), such as with a router, each interface has a unique IP
address. The IP address consists of two parts:
IP address = <network number><host number>
The network number part of the IP address identifies the network within the
internet and is assigned by a central authority and is unique throughout the
internet. The authority for assigning the host number part of the IP address
resides with the organization that controls the network identified by the
network number.
The TCP/IP protocol layers
Like most networking software, TCP/IP is modeled in layers. This layered
representation leads to the term protocol stack, which refers to the stack of
layers in the protocol suite. It can be used for positioning (but not for
functionally comparing) the TCP/IP protocol suite against others, such as
Systems Network Architecture (SNA) and the Open System Interconnection
(OSI) model. Functional comparisons cannot easily be extracted from this,
because there are basic differences in the layered models used by the
different protocol suites.
By dividing the communication software into layers, the protocol stack
allows for division of labor, ease of implementation and code testing, and
the ability to develop alternative layer implementations. Layers communicate
with those above and below via concise interfaces. In this regard, a layer
provides a service to the layer directly above it and makes use of services
provided by the layer directly below it. For example, the IP layer provides

the ability to transfer data from one host to another without any guarantee to
reliable delivery or duplicate suppression. Transport protocols such as TCP
make use of this service to provide applications with reliable, in-order, data
stream delivery.
Figure 1.8 shows how the TCP/IP protocols are modeled in four layers.

Figure 1.8: The TCP/IP protocol stack: Each layer represents a package of

Figure 1.9: Detailed Architectural Model
TCP/IP applications
The highest-level protocols within the TCP/IP protocol stack are application
protocols. They communicate with applications on other internet hosts and
are the user-visible interface to the TCP/IP protocol suite.
All application protocols have some characteristics in common:
• They can be user-written applications or applications standardized and
shipped with the TCP/IP product. Indeed, the TCP/IP protocol suite
includes application protocols such as:
– Telnet for interactive terminal access to remote internet hosts
– File Transfer Protocol (FTP) for high-speed disk-to-disk file transfers
– Simple Mail Transfer Protocol (SMTP) as an internet mailing system
These are some of the most widely implemented application
protocols, but many others also exist. Each particular TCP/IP
implementation will include a lesser or greater set of application
protocols.
• They use either UDP or TCP as a transport mechanism. Remember that
UDP is unreliable and offers no flow-control; so in this case, the
application has to provide its own error recovery, flow control, and

congestion control functionality. It is often easier to build applications on
top of TCP because it is a reliable stream, connection-oriented,
congestion-friendly, and flow control-enabled protocol. As a result, most
application protocols will use TCP, but there are applications built on
UDP to achieve better performance through increased protocol
efficiencies.
• Most applications use the client/server model of interaction.
IP Addressing
IP addresses are represented by a 32-bit unsigned binary value. It is usually
expressed in a dotted decimal format. For example, 9.167.5.8 is a valid IP
address. The numeric form is used by IP software. The mapping between
the IP address and an easier-to-read symbolic name, for example,
myhost.ibm.com, is done by the Domain Name System (DNS).
To identify a host on the Internet, each host is assigned an address, the IP
address, or in some cases, the Internet address. When the host is attached
to more than one network, it is called multihomed and has one IP address
for each network interface. The IP address consists of a pair of numbers:
IP address = <network number><host number>
The network number portion of the IP address is administered by one of
three Regional Internet Registries (RIR):
 American Registry for Internet Numbers (ARIN): This registry is
responsible for the administration and registration of Internet Protocol
(IP) numbers for North America, South America, the Caribbean, and
sub-Saharan Africa.
 Reseaux IP Europeans (RIPE): This registry is responsible for the
administration and registration of Internet Protocol (IP) numbers for
Europe, Middle East, and parts of Africa.

 Asia Pacific Network Information Centre (APNIC): This registry is
responsible for the administration and registration of Internet Protocol
(IP) numbers within the Asia Pacific region.
IP addresses are 32-bit numbers represented in a dotted decimal form (as
the decimal representation of four 8-bit values concatenated with dots). For
example, 128.2.7.9 is an IP address with 128.2 being the network number
and 7.9 being the host number. Next, we explain the rules used to divide an
IP address into its network and host parts.
The binary format of the IP address 128.2.7.9 is:
10000000 00000010 00000111 00001001
IP addresses are used by the IP protocol to uniquely identify a host on the
Internet (or more generally, any internet). Strictly speaking, an IP address
identifies an interface that is capable of sending and receiving IP datagrams.
One system can have multiple such interfaces. However, both hosts and
routers must have at least one IP address, so this simplified definition is
acceptable. IP datagrams (the basic data packets exchanged between
hosts) are transmitted by a physical network attached to the host. Each IP
datagram contains a source IPaddress and a destination IP address. To
send a datagram to a certain IP destination, the target IP address must be
translated or mapped to a physical address. This might require
transmissions in the network to obtain the destination's physical network
address.
Class-based IP addresses
The first bits of the IP address specify how the rest of the address should be
separated into its network and host part. The terms network address and
netID are sometimes used instead of network number. Similarly, the terms
host address and hostID are sometimes used instead of host number.

There are five classes of IP addresses as shown in Figure 1.10.
Figure 1.10: Assigned classes of IP addresses
Where:
Class A addresses: These addresses use 7 bits for the <network> and 24
bits for the <host> portion of the IP address. This allows for 27
-2
(126) networks each with 224
-2 (16777214) hosts – a total of more
than 2 billion addresses.
Class B addresses: These addresses use 14 bits for the <network>
and 16 bits for the <host> portion of the IP address. This allows for
214
-2 (16382) networks each with 216
-2 (65534) hosts – a total of
more than 1 billion addresses.
Class C addresses: These addresses use 21 bits for the <network>
and 8 bits for the <host> portion of the IP address. That allows for

221
-2 (2097150) networks each with 28
-2 (254) hosts – a total of
more than half a billion addresses.

Class D addresses: These addresses are reserved for multicasting (a sort
of broadcasting, but in a limited area, and only to hosts using the same
Class D address).
Class E addresses: These addresses are reserved for future or
experimental use.
Class A address is suitable for networks with an extremely large number of
hosts. Class C addresses are suitable for networks with a small number of
hosts. This means that medium-sized networks (those with more than 254
hosts or where there is an expectation of more than 254 hosts) must use
Class B addresses. However, the number of small- to medium-sized
networks has been growing very rapidly. It was feared that if this growth had
been allowed to continue unabated, all the available Class B network
addresses would have been used by the mid-1990s. This was termed the IP
address exhaustion problem.
The division of an IP address into two parts also separates the responsibility
of selecting the complete IP address. The network number portion of the
address is assigned by the RIRs. The host number portion is assigned by
the authority controlling the network. As shown in the next section, the host
number can be further subdivided: This division is controlled by the authority
that manages the network. It is not controlled by the RIRs.
Reserved IP addresses
A component of an IP address with a value of all bits 0 or all bits 1 has a
special meaning:
 All bits 0: An address with all bits zero in the host number portion is
interpreted as „this’ host (IP address with <host address>=0). All bits
zero in the network number portion is this network (IP address with
<network address>=0). When a host wants to communicate over a
network, but does not yet know the network IP address, it can send

packets with <network address>=0. Other hosts in the network interpret
the address as meaning this network. Their replies contain the fully
qualified network address, which the sender records for future use.
 All bits 1: An address with all bits 1 is interpreted as all networks or all
hosts. For example, the following means all hosts on network 128.2
(Class B address):
128.2.255.255
This is called a directed broadcast address because it contains both a
valid <network address> and a broadcast <host address>.
 Loopback: The Class A network 127.0.0.0 is defined as the loopback
network. Addresses from that network are assigned to interfaces that
process data within the local system. These loopback interfaces do not
access a physical network.
Special use IP addresses: RFC 3330 discusses special use IP addresses.
We provide a brief description of these IP addresses in Table 1.1.
Table 1.1: Special use IP addresses

1.7 Summary
This unit started with introduction to computer networks and their
applications in the real world scenario. The types of networks along with
their structures were discussed. The basic network architecture is
presented. The base model of entire networking, the ISO OSI model was
discussed in detail. The services offered by various types of networks are
also discussed. The latest TCP/IP protocol adopted by all networks is also
presented along with its features.
1. A group of computers and other devices connected together is called a
network, and the concept of connected computers sharing resources is
called _______.
2. A simple cabling method, known as the _____ Topology, allows about
30 computers on a maximum cable length of about 600 feet.
3. ____ is a set of connecting links between LANs.
4. A ____ line considered as a fast WAN link, transmits at 1.5 Mbps, or
1 million bits per second.
5. The ______ elements are specialized computers to connect two or more
transmission lines.
6. In ________, the network contains numerous cables or leased
telephone line, each one connecting a pair of IMPs.
7. The entities comprising the corresponding layers on different machines
are called _______ processes.
1. Write about different network structures in use.
2. Write about the ISO OSI Model
3. Write about TCP/IP Protocol suite

1. Networking
2. Bus
3. WAN
4. T1
5. Switching
6. Point-to-Point channels
7. peer
1.11 Answers for Terminal Questions
1. Refer to section 1.2

Computer Networks
Unit 2
Unit 2
Physical Layer

Structure:
2.0 Objectives
2.1 Analog and Digital Signals
2.2 Periodic Analog Signals
2.3 Transmission Impairments
2.4 Data Rate Limits
2.5 Transmission Media
2.6 Transmission and Switching
2.7 ISDN – Integrated Services Digital Network
2.8 Summary
2.12 Answers to Terminal Questions
2.0 Objectives
This unit is mainly intended to enable the reader understand how the data
transmission is done in a networked environment.
After completion of this unit you will be able to:
• Define and Distinguish between various kinds of signals
• Describe transmission impairments and their affects on data
transmission
• Define the limitations posed by data rates
• Describe the network performance with respect to various signaling
mechanisms
• Define and distinguish various transmission media
• Compare and contrast various switching techniques
• Define and describe ISDN and its services

2.1 Analog and Digital Signals
One of the major functions of the physical layer is to move data in the form
of electromagnetic signals across a transmission medium. The data usable
to a person or application is not in a form that can be transmitted over a
network. Fro example, a photograph must first be changed to a form that
transmission media can accept. Transmission media works by conducting
energy along a physical path.
Both data and the signals that represent them can be either analog or digital
in form.
– Analog Data: It refers to information that is continuous.
Example: An analog clock that has hour, minute, and second hands
gives information in a continuous form; the movement of the hands are
continuous.
– Digital Data: It refers to information that has discrete states.
Example: A digital clock that reports the hours and minutes will change
suddenly from 8:05 to 8:06.
Like the data they represent, signals can be either analog or digital.
– Analog Signal: It has infinitely many levels of intensity over a period of
time. As the wave moves from value A to value B, it passes through and
it includes an infinite number of values along its path.
– Digital Signal: It can have only limited number of defined values.
Although each value can be any number, it is often as simple as
0 or 1.
The simplest way to show signals is by plotting them on a pair of
perpendicular axes. The vertical axis represents the value or strength of a
signal. The horizontal axis represents time. Figure 2.1 below illustrates an
analog signal and a digital signal. The curve represents the analog signal
through an infinite number of points. The vertical lines of the digital signal,

however, demonstrate the sudden jump that the signal makes from value to
value.
Value
Value
Time
Time
a. Analog Signal b. Digital Signal
Figure 2.1: Comparison of Analog and Digital Signals
Periodic and Nonperiodic Signals
Both analog and digital signals can take one of two forms:
Periodic and Nonperiodic. A periodic signal completes a pattern
within a measurable time frame called a period and repeats that
pattern over subsequent identical periods. The completion of one
full pattern is called a cycle. A nonperiodic signal changes without
exhibiting a pattern or cycle that repeats over time.
Both analog and digital signals can be periodic or nonperiodic. In data
communications, we commonly used periodic analog signals
(since they need less bandwidth) and nonperiodic digital signals
(since they can represent variation in data).
2.2 Periodic Analog Signals
These signals can be classified as simple or composite. A simple
periodic analog signal, a sine wave, cannot be decomposed into
simpler signals. A composite periodic signal is composed of multiple
sine waves.

Sine Wave
The sine wave is the most fundamental form of a periodic analog signal.
When we visualize it as a simple oscillating curve, its change over the
course of a cycle is smooth and consistent, a continuous rolling flow. Figure
2.2 below shows a sine wave. Each cycle consists of a single arc above the
time axis followed by a single arc below it.
Value
Time
Figure 2.2: A Sine Wave
A Sine wave can be represented by three parameters:
1. Peak Amplitude: The Peak Amplitude of a signal is the absolute value
of its highest intensity, proportional to the energy it carries. For electric
signals, peak amplitude is normally measured in volts.
2. Frequency: Period refers to the amount of time in seconds, a signal
needs to complete one cycle. Frequency refers to the number of periods
in 1 s. Period is the inverse of frequency and so on. Period is formally
expressed in seconds. Frequency is formally expressed in Hertz (Hz).
Frequency is the rate of change with respect to time. Change in a short
span of time means high frequency. Change over a long span of time
means low frequency.
If a signal does not change at all, its frequency is zero. If a signal
changes instantaneously, its frequency is infinite.

Table 2.1 Units of period and frequency
Unit Equivalen
t
Unit Equivalent
Second (s) 1 s Hertz (Hz) 1 Hz
Milliseconds (ms) 10–3
s Kilohertz (kHz) 103
Hz
Microseconds ( µ s) 10–6
s Megahertz (MHz) 106
Hz
Nanoseconds (ns) 10–9
s Gigahertz (GHz) 109
Hz
Picoseconds (ps) 10–12
s Terahertz (THz) 1012
Hz
3. Phase: This term describes the position of the waveform relative to time
0. If we think of the wave as something that can be shifted backward or
forward along the time axis, phase describes the amount of that shift. It
indicates the status of the first cycle.
Phase is measured in degree or radians [3600
is 2∏ rad; 10 is 2/360 rad,
and 1 rad is 360/2]. A phase shift of 3600
corresponds to a shift
of a complete period; and a phase shift of 1800
corresponds to s shift
of one-half of a period; and a phase shift of 900
corresponds to a shift
of one-quarter of a period.
Wavelength: It is a characteristic of a signal traveling through a
transmission medium. Wavelength binds the period or the
frequency of a simple sine wave to the propagation speed of the
medium.
While the frequency of a signal is independent of the
medium, the wavelength depends on both the frequency and the
medium. It is a property of any type of signal. In data
communications, we often use wavelength to describe the
transmission of light in an optical fiber. This wavelength is the
distance a simple signal can travel in one period.
It can be calculated if one is given the propagation speed (the speed
of light) and the period of the signal. However, since period and
frequency are related to each other, if we represent the wavelength

by, propagation speed by c (speed of light), and frequency by f, we
get:

Wavelength = propagation speed X period = Propagation Speed / frequency
c
i.e., λ =
f
The wavelength is normally measured in micrometers (micro)
instead of meters.
Time and Frequency Domains
A sine wave is comprehensively defined by its amplitude,
frequency, and phase. The time domain plot of a sine wave
shows changes in signal amplitude with respect to time. Phase is
not explicitly shown on a time domain plot.
To show the relationship between amplitude and frequency, we
can use what is called a frequency-domain plot. A frequency
domain plot is concerned with only the peak value and the
frequency. Changes of amplitude during the period are not shown.

Figure 2.3: The time-domain and frequency domain plots of a sine wave
A complete sine wave is represented by one spike. The position of the
spike shows the frequency, the height shows the peak amplitude.

Composite Signals
A composite signal is made of many sine waves. Fourier showed that any
composite signal is actually a combination of simple sine waves with
different frequencies, amplitudes and phases.
A composite signal can be periodic or nonperiodic. A periodic composite
signal can be decomposed into a series of simple sine waves with discrete
frequencies – frequencies that have integer values (1,2,3, and so on). A
nonperiodic composite signal can be decomposed into a combination of an
infinite number of simple sine waves with continuous frequencies,
frequencies that have real values.
Figure 2.4: A composite periodic signal
Figure below shows the result of decomposing the above signal in both time
and frequency domains.

Figure 2.5: Decomposition of a composite periodic signal in time and
frequency.
The amplitude of the sine wave with frequency f is almost the same as the
peak amplitude of the composite signal. The amplitude of the sine wave with
frequency 3f is one-third of that of the first, and the amplitude of the sine
wave with frequency 9f is one-ninth of the first. If the frequency of the sine
wave with frequency f is the same as the frequency of the composite signal;
it is called the fundamental frequency or first harmonic. If the sine wave with
frequency 3f has a frequency of 3 times the fundamental frequency; it is
called the third harmonic. The sine wave with frequency 9f has a frequency
of 9 times the fundamental frequency; it is called the ninth harmonic.
Bandwidth
The range of frequencies contained in a composite signal is its bandwidth.
The bandwidth is normally a difference between two numbers. For example,
if a composite signal contains frequencies between 1000 and 5000, its
bandwidth is 5000 – 1000 or 4000.
Figure below shows the concept of bandwidth. The figure below depicts two
composite signals, one periodic and the other nonperiodic. The bandwidth of
the periodic signal contains all integer frequencies between 1000 and 5000.

The bandwidth of the nonperiodic signals has the same range, but the
frequencies are continuous.

Figure 2.6: The bandwidth of periodic and nonperiodic composite signals
Digital Signals
Information can also be represented by digital signals. For example, a 1 can
be encoded as a positive voltage and a 0 as zero voltage. A digital signal
can have more than two levels. In this case, we can send more than 1 bit for
each level.
Figure 2.7 shows two signals, one with two levels and other with four levels.

Figure 2.7: Two digital signals: One with two signal levels and one with four
signal levels.
We send one bit per level in part ‘a’ of the figure and 2 bits per level in part b
of the figure. In general, if a signal has L levels, each level needs log2
L bits.

Bit Rate The number of bits sent in 1s, expressed in bits per
second (bps).
Bit Length The distance one bit occupies on the transmission medium.
Bit length = propagation speed x bit duration
Digital Signal as a Composite Analog Signal
Based on Fourier Analysis, a digital signal is a composite analog
signal. The bandwidth of such a signal is infinite. A digital signal in
the time domain comprises connected vertical and horizontal line
segments. A vertical line in

the time domain means a frequency of infinity (sudden change in time); a
horizontal line in the time domain means a frequency of zero (no change in
time). Going from a frequency of zero to a frequency of infinity (and vice
versa) implies all frequencies in between are part of the domain.
Fourier analysis can be used to decompose a digital signal. If the digital
signal is periodic, which is rare in data communications, the decomposed
signal has a frequency-domain representation with infinite bandwidth and
discrete frequencies. If the digital signal is nonperiodic, the decomposed
signal has an infinite bandwidth, but the frequencies are continuous.

Figure 2.8: Time and Frequency domains of digital Signals
Transmission of Digital Signals
How can we send a digital signal from point A to point B? We can transmit a
digital signal by using one of two different approaches: Baseband
Transmission or Broadband Transmission (using Modulation).
Baseband Transmission: It means sending a digital signal over a channel
without changing the digital signal to an analog signal. Figure 2.9 below
shows baseband transmission.

Digital Signal
Channel
Figure 2.9: Baseband Transmission
Baseband transmission requires that we have a low-pass channel, a
channel with a bandwidth that starts from zero. This is the case if we have a
dedicated medium with a bandwidth constituting only one channel. For
example, the entire bandwidth of a cable connecting two computers is one
single channel.

Figure 2.10 below shows two low-pass channels: one with a narrow
bandwidth and the other with a wide bandwidth.
Figure 2.10: Bandwidth of two low pass channels

2.3 Transmission Impairments
Analog signals consist of varying voltage with time to represent an
information steam. If the transmission media were perfectly, the receiver
could receive exactly the same signal that the transmitter sent. But
communication lines are usually not perfect, so the receive signal is not the
same as the transmitted signal. For digital data this difference can lead to
errors. Transmission lines suffers from three major problems
1. Attenuation distortion
2. Delay distortion
3. Noise
Attenuation distortion
It is the loss of energy as the signal propagates outward. The amount of
energy depends on the frequency. The signal attenuates as shown in figure
2.11 as it propagates. If the attenuation is too much, the receiver may not be
able to detect the signal at all, or the signal may fall below the noise level.
For reliable communication, the attenuation and delay over the range of
frequencies of transmission should be constant.

Figure 2.11: Signals loose power at it travels time
Issues
1. Signals must be sufficiently strong so that the receiver will be able to
detect and interpret them
2. They should maintain a sufficient high level to make them
distinguishable from noise
3. Too strong signals can overload the circuitry of the transmitter and result
in distortion.

in
log
4. They should take into account that attenuation increases with the
frequency.
Attenuation is measured in Bel as
Pow er
10
Pow er
out
Bel

It can be also expressed in decibel (dB) as
o
in
Pow e
20 * log
10 Pow er
r
ut
Decibel
Decibels are commonly used because
1. Signal strengths often fall off logarithmically
2. Cascade losses and gains can be calculated with simple
additions and subtractions.
Delay distortion
The second transmission impairment is delay distortion. Communication
lines have distributed inductance and capacitance, which
distort the amplitude of signals and also delay the signals at
different frequencies by different amounts. It is caused by the fact
that different Fourier components travel at different speed. The delay
distortion is illustrated as shown in figure 2.12.
It is due to velocity of propagation the frequency varies. Thus, various
frequency components of a signal arrive at the receiver at different times.
Figure 2.12: Delay distortion

For digital data, fast components from one bit may catch up and over take
slow component from bit ahead, mixing the two bits and increasing the
probability of incorrect reception. Thus it is very critical in particular for
digital data, that is because signal components of bit positions spill into
other bit positions, and so limiting the allowed rate of transmission.
Dispersion

Figure 2.13: Signal is dispersed
Signals tend to spread as they travel, with the amount of spreading
dependent on the frequency. It is illustrated as shown in figure 2.13.
Noise
Noise is a third impairment. It can be define as unwanted energy from

Noise is a third impairment. It can be define as unwanted energy from
sources other than the transmitter. Thermal noise is caused by the random
motion of the electrons in a wire and is unavoidable. Consider a signal as
shown in figure 2.14, to which a noise shown in figure 2.15 is added may be
in the channel.
Figure 2.14: Signal

Figure 2.15: Noise

Figure 2.16: Signal + Noise
At the receiver, the signal is recovered from the received signal and is
shown in figure 2.16. That is signals are reconstructed by sampling.
Increased data rate implies "shorter" bits with higher sensitivity to noise

Source of Noise
Thermal:
Agitates the electrons in conductors, and is a function of the temperature. It
is often referred to as white noise, because it affects uniformly the different
frequencies.
• The thermal noise in a bandwidth W is N = kTW
Where T=temperature, and
k= Boltzmann's constant = 1.38 10-23 Joules/degrees Kelvin.
S
• Signal to noise ratio: : SNR(dB) =
N
dB
Noise Pow er
SNR(dB) = 10 * log10
signal Pow er
It is typically measured at the receiver, because it is the point where the
noise is to be removed from the signal.
Intermodulation:
Results from interference of different frequencies sharing the same medium.
It is caused by a component malfunction or a signal with excessive strength

is used. For example, the mixing of signals at frequencies f1 and f2 might
produce energy at the frequency f1 + f2. This derived signal could interfere
with an intended signal at frequency f1 + f2.
Cross talk:
Similarly cross talk is a noise where foreign signal enters the path of the
transmitted signal. That is, cross talk is caused due to the inductive coupling
between two wires that are close to each other. Sometime when talking over
the telephone, you can hear another conversation in the background. That is
cross talk.
Impulse:
These are noise owing to irregular disturbances, such as lightning, flawed
communication elements. It is a primary source of error in digital data.
2.4 Data Rate Limits
A very important consideration in data communications is how fast we can
send data, in bits per second, over a channel. Data rate depends on three
factors:
1. The bandwidth available
2. The level of the signals we use
3. The quality of the channel (the level of noise)
Two theoretical formulae were developed to calculate the data rate: one by
Nyquist for a noiseless channel, another by Shannon for a noisy channel.
Noiseless channel: Nyquist Rate
For a noiseless channel, the Nyquist bit rate formula defines the theoretical
maximum bit rate.
BitRate = 2 x bandwidth x log2
L

Bandwidth is the banwidth of the channel. L is the number of signal levels
used to represent data, and BitRate is the bit rate in bits per second.
Noisy Channel: Shannon Capacity
In reality, we cannot have a noiseless channel; the channel is always noisy.
In 1944, Claude Shannon introduced a formula called the Shannon
Capacity, to determine the theoretical highest data rate for a noisy channel.
Capacity = bandwidth x log2
(1 + SNR)
In the above formula, bandwidth is the bandwidth of the channel,
SNR is the signal-to-noise ratio, and capacity is the capacity of the
channel in bits per second. This formula defines the characteristic
of the channel, not the method of transmission.
2.5 Transmission medium
Transmission media: Transmission media is the physical path
between the transmitter and receiver. It can be guided or unguided.
Guided & Unguided Transmission medium
Guided media provides a guided (by a solid medium) path for
propagation of signals such as twisted pairs, coaxial cables, optical
fibers etc. Unguided media employ an antenna for transmitting
through air, vacuum or water. This form of transmission is
referred to as wireless transmission. For example Broadcast radio,
satellite etc.
Selection of transmission Media depends on the characteristics and
quality of data transmission which are in turn determined by
characteristics of the medium and signal. For guided media the
medium itself in determining the limitations of transmission. For
Unguided media BW of the signal produced at the transmitting
antenna is more important than characteristics of the transmission

characteristics.

In general, signals at lower frequencies are omni directional (all directions)
and at higher frequencies are directional (focused).
The key concern in design of data transmission system is Data Rate and
Distance: The greater the data rate and distance, the better.
Factors used to determine data rate and distance:
• Bandwidth (BW): Greater the BW of the signal, higher the data rate that
can be achieved.
• Transmission impairment: These limit the distance. Twisted pair
suffers more impairment than coaxial cable which in turn suffers more
than optical fiber.
• Interference: Overlapping frequency bands can distort/wipeout a signal.
It is of more concern for unguided media than guided.
For guided it can be caused due to nearby cables. Proper shielding of
cables can minimize this problem.
• Number of receivers: Point to point links are used or a shared link is
used with multiple attachments.
In a shared link, each attachment introduces some attenuation and
distortion on the line limiting the distance and/or data rate.
• For guided media the transmission capacity depends on data rate or BW
and depends critically on the distance (whether medium is p-p or
multipoint)
Twisted pair
They are least expensive and most widely used. They are easier to work
with but limited in terms of data rate and distance.

Physical Description

Figure 2.17: a) CAT 3 UTP and b) CAT 5 UTP
It consists of two insulated cu wires arranged in regular spiral pattern as
shown in Figure 2.17. Wire pair acts like a communication link. Usually
numbers of these pairs are bundled together in a protective sheath into a
cable. Twisting tends to decrease the crosstalk. On long distance links, the
twist length typically varies from 5-15 cm. The thickness of wires may be
0.4-0.9mm. Over long distance, cables may contain hundreds of pairs. It is
most common for both analog and digital signals. It is commonly used in
telephone network and is the workhouse for communication within buildings.
Example: Individual residential telephone or in an office building.
These were designed to support voice traffic using analog signaling.
However it can handle digital data traffic at modest data rates. It is also
commonly used for digital signaling with the use of a digital switch or digital
PBX with data rate of 64kbps commonly. It is for LAN supporting PC’s with
commonly 10Mbps (now a days may 1Gbps also is possible). For long
distance Twisted pair 4Mbps or more is used.
Transmission characteristics
Twisted pair can be used for both analog and digital transmission. For
analog signals, amplifiers are required about every 5-6km. For digital
transmission (analog and digital signals), repeaters are required every 2-
3kms. Attenuation is a very strong function of frequency. Other impairments

are also severe for twisted pair. It is susceptible to interference and noise.
Impulse noise can also intrude easily.
Application as LAN Cables
Unshielded Twisted Pair Cable (UTP) which is typically 1mm thick with a
minimum number of twist per foot. Twisting reduces electrical interference.
They run several kilometers without amplifications, repeaters are needed.
They are used for transmitting either analog or digital signals. There are
different categories of UTP’s that are used. They are:
• CAT3 Cable: They are less expensive. It consists of 4 pairs grouped in
plastic sheet to protect the wires. They are mostly used in office
buildings. Bandwidth of 16 to 100 MHz signals can be handled
• CAT5 Cable: They are similar to CAT3, but are with more twists/cm.
They have less crosstalk and provide better quality over long distance.
They are suitable for high speed computer communication. Bandwidth
16 to 100 MHz signals can be handled.
• CAT6 & CAT7 Cable: They are still more improved version than CAT 5.
They are suitable for higher bandwidth of 250MHz and 600MHz
Coaxial Cable
Coaxial cable has traditionally been an important part of the long distance
telephone network. Today, it faces increasing competition from optical fiber,
terrestrial microwave, and satellite. Using frequency-division multiplexing
(FDM), a coaxial cable can carry over 10,000 voice channels
simultaneously.

Physical Description
Figure 2.18: Coaxial cable
Coaxial cable, like twisted pair, consists of two conductors, but constructed
differently to permit it to operate over a wider range frequency. It consists of
hollow outer cylindrical conductor that surrounds a single inner wire
conductor. The inner conductor is held in place by either regularly spaced
insulating rings or a solid dielectric material. The outer conductor is covered
with jacket or shield. The physical description is illustrated in Figure 2.18. A

single coaxial cable has a diameter of from 1 to 2.5cm. Because of its
shielded, concentric construction, coaxial cable is much less susceptible to
interference and crosstalk than twisted pair. Coaxial cable can be used over
long distances and support more stations on a shared line than twisted pair.
Coaxial cable is perhaps the most versatile transmission medium and is
enjoying widespread use in a wide variety of applications. The most
important of these are used in television distribution and long-distance
telephone transmission. Also they find applications in Short-run computer
system links and Local Area Networks.
Coaxial cable is spreading rapidly as a means of distributing TV signals to
individual homes-cable TV. From its modest beginnings as Community
Antenna Television (CATV), designed to provide service to remote areas,
cable TV will eventually reach almost as many homes and offices as the
telephone. A cable TV system can carry dozens or even hundreds of TV
channels at ranges up to a few tens of kilometers. Coaxial cable is also

commonly used for short range connections between devices. Using digital
signaling, coaxial cable can be used to provide high-speed I/O channels on
computer systems.
Transmission Characteristics
Coaxial cable is used to transmit both analog and digital signals. Coaxial
cable has frequency characteristics that are superior to those of twisted pair,
and can hence be used effectively at higher frequencies and data rates.
Because of its shielded, concentric construction, coaxial cable is much less
susceptible to interference and crosstalk than twisted pair.
The principal constraints on performance are attenuation, thermal noise,
and inter-modulation noise. The latter is present only when several channels
(FDM) or frequency bands are in use on the cable. For long-distance
transmission of analog signals, amplifiers are needed every few kilometers,
with closer spacing, if higher frequencies are used. The usable spectrum for
analog signaling extended to about 500MHz. For digital signaling, repeaters
are needed every kilometer or so, with closer spacing needed for higher
data rates.
Application of LAN Cables
Co-axial cable has better shielding than twisted pairs and can span longer
distance at higher speeds. There are two types of coaxial cable that are
used in LAN and are illustrated in figure 2.19.
• Thick coax: They are used for Ethernets but are difficult to work with
and are expensive. It has greater degree of noise immunity and is
strong. It requires vampire tap and a drop cable to connect to the
network.
• Thin coax: They are easier to work and less expensive. It carries
signals over shorter distance and is preferred over thick coax as it needs

simple BNC connector. They are flexible, cheaper, soft and ideal for

Figure 2.19: Thin and Thick coaxial cable
Optical Fiber
An optical fiber is a thin, flexible medium capable of guiding an optical ray.
Total internal reflection is the basic principle on which the transmission of
data takes place through fibers. If the angle of incidence is sufficiently large,
then the light in the fiber will reflect repeatedly in the interface between the

Figure 2.20: Total internal reflection

Figure 2.21: (a) Side view of a single fiber (b) view of sheath with three fibers
An optical fiber has a cylindrical shape and consists of three concentric
sections; the core, the cladding and the jacket. The core is the inner most
sections and consists of one or more very thin strands or fibers, made of
glass or plastic and is as shown in Figure 2.21. The core has a diameter in
the range of 8 to 100µm. Each fiber is surrounded by its own cladding, a

glass or plastic coating that has optical properties different from those of the
core. Various glasses and plastics can be used to make optical fibers. The
lowest losses have been obtained using fibers of ultra-pure fused silica.
Ultra pure fiber is difficult to manufacture; higher loss multi component glass
fibers are more economical and still provide good performance. Plastic fiber
is even less costly and can be used for short-haul links, for which
moderately high losses are acceptable.
The interface between the core and cladding acts as a reflector to confine
light that would otherwise escape the core. The outermost layer,
surrounding one or a bundle of cladded fibers, is the jacket. The jacket is
composed of plastic and other material layered to protect against moisture,
abrasion, crushing and other environmental dangers. One of the most
significant technological breakthroughs in data transmission has been the
development of practical fiber optic communication systems. Optical fiber
already enjoys considerable use in long-distance telecommunications, and
its use in military applications is growing.
Sikkim Manipal University
The continuing improvements in
Page No. 57

performance and decline in prices, together with the inherent advantages of
optical fiber, have made it increasingly attractive for LAN.
2.6 Transmission and Switching
Optical fiber transmits a signal encoded beam of light by means of total
internal reflection. Total internal reflection can occur in any transparent
medium that has a higher index of refraction than the surrounding medium.
The principle is described in Figure 2.22. In effect, the optical fiber acts as a
waveguide for frequencies in the range of about 1014 to 1015 Hz. This
covers portions of infrared and visible spectra.
Light from a source enters the cylindrical glass or plastic core. Rays at
shallow angles are reflected and propagated along the fiber; other rays are
absorbed by the surrounding material. This form of propagation is called
Step-index multimode referring to the variety of angles that will reflect.
With multimedia transmission, multiple propagation paths exist, each with a
different path length and hence time to traverse the fiber. This causes
signal elements (light pulses) to spread out in time, which limits the rate at
which data can be accurately received. In other words, the need to leave
spacing between the pulses limits data rate. This type of fiber is best suited
for transmission over very short distances. When the fiber core radius is
reduced, fewer angles will reflect. By reducing the radius of the core to the
order of a wavelength, only a single angle or mode can pass: the axial ray.
This single mode propagation provides superior performance for the
following reason, because there is a single transmission path with single
mode transmission. The distortion found in multimode cannot occur. Single-
mode is typically used for long distance applications, including telephone
and cable television.

Finally, by varying the index of refraction of the core, a third type of
transmission, known as graded index multimode, is possible. This type is
intermediate between the other two in characteristics. The higher refractive
index at the center makes the light rays moving down the axis advance
more slowly than those curves helically because of the graded index,
reducing its travel distance. The shortened path and higher speed allows
light at the periphery to arrive at a receiver at about the same time as the
straight rays in the core axis. Graded index fibers are often used in LANs.
Applications
The advantages of optical fiber over twisted pair and coaxial cable become
more compelling as the demand for all types of information (voice, data,
image and video) increases. Five basic categories of applications have
become important for optical fiber. They are: Long-haul trunks, Metropolitan
trunks, Rural exchange trunks, Subscriber loops, and Local Area Networks.
Long-haul transmission:
It is becoming increasingly common in the telephone network. Long-haul
routes average about 1500kms in length and offer higher capacity (typically
20,000 to 60,000 voice channels). These systems compete economically
with microwave. Undersea optical fiber cables also enjoy increasing use.
Metropolitan trunking:
These circuits have an average length of 12km and may have as many as
1,00,000 voice channels in a trunk group. Most facilities are installed in
underground conduits and are repeater-less, joining telephone exchange in
a metropolitan or city area. Included in this category are routes that link
long haul microwave facilities that terminate at a city perimeter to the main
telephone exchange building downtown.

Rural exchange trunks:
These have circuit lengths ranging from 40 to 60km and link towns and
villages. In the United States, they often connect the exchanges of different
telephone companies. Most of these systems have fewer than 5000 voice
channels. The technology used in these applications competes with
microwave facilities.
Subscriber loop circuits:
These are fibers that run directly from the central exchange to a subscriber.
These facilities are beginning to displace twisted pair and coaxial cable links
as the telephone networks evolve into full service networks capable of
handling not only voice and data, but also image and video.
Application as LAN Cable
Standards have been developed and products introduced for optical fiber
networks that have a total capacity of 100Mbps to 1 Gbps. Recent
Achievable bandwidth is in excess of 50,000Gbps i.e 50Tbps but Current
limit is 10Gbps. It can support hundreds or even thousand of stations in a
large office building of a complex of buildings.
Comparison of fiber optics and copper wire
The following characteristics distinguish optical fiber from twisted pair or
coaxial cable:
Greater capacity:
The potential bandwidth, and hence data rate, of optical fiber is immense,
data rates of hundred of Gbps over tens of kilometers have been
demonstrated. Compare this to the practical maximum of hundreds of Mbps
over about 1km for coaxial cable and just a few Mbps over 1km or up to
100Mbps to 1Gbps over a few tens of meters for twisted pair.

Smaller size and lighter weight:
Optical fibers are considerably thinner than coaxial cable or bundled
twisted–pair cable at lest an order of magnitude thinner for comparable
information transmission capacity. For cramped conduits in buildings and
underground along public rights-of-way, the advantage of small size is
considerable. The corresponding reduction in weight reduces structural
support requirements.
Lower attenuation:
Attenuation is significantly lower in optical fiber than in coaxial cable or
twisted pair and is constant over a wide range.
Electromagnetic Isolation:
Optical fiber systems are not affected by external electromagnetic fields.
Thus the systems are not vulnerable to interference, impulses noise or
crosstalk. By the same token, fibers do not radiate energy, so there is little
interference with other equipment and there is a high degree of security
from eavesdropping. In addition, fiber is inherently difficult to tap.
Greater repeater spacing:
Fewer repeaters mean lower cost and fewer sources of error. The
performance of optical fiber systems from this point of view has been
steadily improving. Repeater spacing in the tens of kilometers for optical
fiber is common, and repeater spacing of hundreds of kilometers have been
demonstrated. Coaxial and twisted pair systems generally have repeaters
every few kilometers.
Switching
For transmission of data beyond local area, communication is typically
achieved by transmitting data from source to destination through a network
of intermediate switching nodes. The switching nodes are not concerned
with the content of data. Rather their purpose is to provide a switching

facility that will move the data from node to node until it reaches the
destination. Circuit switching and packet switching techniques are more
commonly used and are as shown in Figure 4.5 (a) and (b) respectively.
Circuit Switching
A circuit switching network is one that establishes a dedicated circuit (or
channel) between nodes and terminals before the users may communicate.
Each circuit that is dedicated cannot be used by other callers until the circuit
is released and a new connection is set up. Even if no actual communication
is taking place in a dedicated circuit, then that channel still remains
unavailable to other users. Channels that are available for new calls to be
set up are said to be idle. Circuit switching is used for ordinary telephone
calls. It allows communications equipment and circuits, to be shared among
users. Each user has sole access to a circuit (functionally equivalent to a
pair of copper wires) during network use.

Figure 2.22: (a) circuit switching (b) packet switching
For call setup and control (and other administrative purposes), it is possible
to use a separate dedicated signaling channel from the end node to the

network. ISDN is one such service that uses a separate signaling channel.
The method of establishing the connection and monitoring its progress and
termination through the network may also utilize a separate control channel.
Circuit switching can be relatively inefficient because capacity is wasted on
connections which are set up but are not in continuous use (however
momentarily). On the other hand, the connection is immediately available
and capacity is guaranteed until the call is disconnected
Communication using circuit switching involves three phases discussed
below:
1. Connection establishment: Before any signal can be transmitted, an
end to end circuit must be established.
2. Data transfer: Information can now be transmitted from source through
the network to the destination using the dedicated path established.
3. Termination: After some period of data transfer, the connection is
terminated
Consider communication between two points A and D in a network as
shown in Figure 2.23. The connection between A and D is provided using
(shared) links between two other pieces of equipment, B and C.

Figure 2.23: A four node and 3 link network

Network use is initiated by a connection phase during which a circuit is set
up between source and destination and terminated by a disconnect phase
as listed above. These phases, with associated timings, are illustrated in the
Figure 2.24.

Figure 2.24: A circuit switched connection between A and D
(Information flows in two directions. Information sent from the calling end is
shown in grey and information returned from the remote end is shown in
black)
After a user requests a circuit, the desired destination address must be

communicated to the local switching node (B). In a telephony network, this
is achieved by dialing the number. Node B receives the connection request
and identifies a path to the destination (D) via an intermediate node (C).
This is followed by a circuit connection phase handled by the switching
nodes and initiated by allocating a free circuit to C (link BC), followed by
transmission of a call request signal from node B to node C. In turn, node C
allocates a link (CD) and the request is then passed to node D after a similar
delay.

The circuit is then established and may be used. While it is available for use,
resources (i.e. in the intermediate equipment at B and C) and capacity on
the links between the equipment are dedicated to the use of the circuit.
After completion of the connection, a signal confirming circuit establishment
(a connect signal in the diagram) is returned; this flows directly back to node
A with no search delays since the circuit has been established. Transfer of
the data in the message then begins. After data transfer, the circuit is
disconnected; a simple disconnect phase is included after the end of the
data transmission.
Delays for setting up a circuit connection can be high, especially if ordinary
telephone equipment is used. Call setup time with conventional equipment
is typically on the order of 5 to 25 seconds after completion of dialing. New
fast circuit switching techniques can reduce delays. Trade-offs between
circuit switching and other types of switching depend strongly on switching
times.
Message switching
Message switching was the precursor of packet switching, where messages
were routed in their entirety and one hop at a time. It was first introduced by
Leonard Kleinrock in 1961. Message switching systems are nowadays
mostly implemented over packet-switched or circuit-switched data networks.
Hop-by-hop Telex forwarding are examples of message switching systems.
E-mail is another example of a message switching system. When this form
of switching is used, no physical path is established in advance in between
sender and receiver. Instead, when the sender has a block of data to be
sent, it is stored in the first switching office (i.e. router) then forwarded later
at one hop at a time.
Each block is received in its entity form, inspected for errors and then
forwarded or re-transmitted. It is a form of store-and-forward network. Data

is transmitted into the network and stored in a switch. The network transfers
the data from switch to switch when it is convenient to do so, and as such
the data is not transferred in real-time. Blocking can not occur, however,
long delays can happen. The source and destination terminal need not be
compatible, since conversions are done by the message switching
networks.
Again consider a connection of a network shown in Figure 2.23. For
instance, when a telex (or email) message is sent from A to D, it first passes
over a local connection (AB). It is then passed at some later time to C (via
link BC), and from there to the destination (via link CD). At each message
switch, the received message is stored, and a connection is subsequently
made to deliver the message to the neighboring message switch. Message
switching is also known as store-and-forward switching since the messages
are stored at intermediate nodes en route to their destinations.

Figure 2.25: Message switching to communicate between A and D

The Figure 2.25 illustrates message switching; transmission of only one
message is illustrated for simplicity. As the figure indicates, a complete
message is sent from node A to node B when the link interconnecting them
becomes available. Since the message may be competing with other
messages for access to facilities, a queuing delay may be incurred while
waiting for the link to become available. The message is stored at B until the
next link becomes available, with another queuing delay before it can be
forwarded. It repeats this process until it reaches its destination.
Circuit setup delays are replaced by queuing delays. Considerable extra
delay may result from storage at individual nodes. A delay for putting the
message on the communications link (message length in bits divided by link
speed in bps) is also incurred at each node enroute. Message lengths are
slightly longer than they are in circuit switching, after establishment of the
circuit, since header information must be included with each message; the
header includes information identifying the destination as well as other types
of information. Most message switched networks do not use dedicated
point-to-point links.
Packet switching
Packet switching splits traffic data (for instance, digital representation of
sound, or computer data) into chunks, called packets. Packet switching is
similar to message switching. Any message exceeding a network-defined
maximum length is broken up into shorter units, known as packets, for
transmission. The packets, each with an associated header, are then
transmitted individually through the network. These packets are routed over
a shared network. Packet switching networks do not require a circuit to be
established and allow many pairs of nodes to communicate almost
simultaneously over the same channel. Each packet is individually
addressed precluding the need for a dedicated path to help the packet find
its way to its destination.

Packet switching is used to optimize the use of the channel capacity
available in a network; to minimize the transmission latency (i.e. the time it
takes for data to pass across the network), and to increase robustness of
communication.
Again consider the same network as shown in figure 2.23. Now the
message of Figure 2.25 is broken into three small units called packets and
labeled 1-3 as illustrated in Figure 2.26.

Figure 2.26: Packet-switched communication between A and D
The most well-known use of packet switching is the Internet. The Internet
uses the Internet protocol suite over a variety of data link layer protocols.
For example, Ethernet and Frame relay are very common. Newer mobile
phone technologies (e.g., GPRS, I-mode) also use packet switching. Packet
switching is also called connectionless networking because no connections
are established.

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