Unit 1 : Introduction to
Computer Network
Chandan Gupta Bhagat
Computer Network Definition
A computer network is a digital telecommunications network which allows
nodes to share resources (such as printers and CDs), exchange files, or
allow electronic communications.
 In computer networks, computing devices exchange data with each other
using connections (data links) between nodes.
 These data links are established over cable media such as wires or optic
cables, or wireless media such as Wi-Fi. Computer network connects two or
more autonomous
 computers.
 The computers can be geographically located anywhere.
Applications of Networks
•
Resource Sharing
•
•
Information Sharing
•
•
•
Hardware (computing resources, disks, printers) Software (application
software)
Easy accessibility from anywhere (files, databases) Search Capability
(WWW)
Communication
•
Email
•
Message broadcast
Remote computing Distributed
processing
What is Computer Network
•
•
A computer network is a set of devices (often referred to as nodes)
connected by communication links.
A node can be a computer, printer, or any other device capable of sending
or receiving data from the other node/device through the network.
Computer Network Criteria
1. Performance
 It can be measured in many ways and depends on the number of factors
• No of users
• Type of transmission medium
• Response time
• Transit time
• Hardware
• Software
1. Reliability
 This is measured by the following factors
• Frequency of failure
• The recovery time of a network after a failure.
• Damage
1. Security
 Network security issues include protecting data from the following
• Unauthorized access
• Viruses
Advantages of CN
 File Sharing: Networks offer a quick and easy way to share files directly.
 Resource Sharing: All computers in the network can share resources such as
printers, fax machines, modems and scanners.
 Communication: Those on the network can communicate with each other via email, messages etc.
Advantages of CN
 File Sharing: Networks offer a quick and easy way to share files directly.
 Resource Sharing: All computers in the network can share resources such as
printers, fax machines, modems and scanners.
 Communication: Those on the network can communicate with each other via email, messages etc.
 Flexible Access: Networks allow their users to access files from computers
throughout the network.
 Sharing of Information: Computer networks enable us to share data and
information with the computers that are located geographically large distance apart.
1.2 Network Topology
 The network topology defines the way in
which computers, printers, and other
devices are connected.
 A network topology describes the layout of the wire and devices as well as the
paths used by data transmissions.
 Network Topology refers to the logical arrangement or layout of a network and a
description of how various nodes(sender/receiver) are connected and communicate
with each other.
Bus Topology
 Bus topology is a network, in which all the computer nodes and network system are
connected to a single transmission channel.
 Linear Bus topology: when it has exactly two endpoints.
 Distributed Bus topology: when it has more than two endpoints.
 Features of Bus topology
– It transfers the data in a single direction.
– There is a single connection between the node/system and the channel.
Bus Topology (Contd.)
Advantages of Bus topology
1. It is easy to connect a device and handle
2. Take less time to set up
3. It is best-suited for small networks.
4. Easy to expand.
Disadvantages of Bus topology
1. If the backbone cable fails, then the whole network will be down.
2. No bi-directional communication.
3. Not suitable for heavy traffic data transmission as it increases the chance of
collision.
Star & Tree Topology
 The star topology is the most commonly used architecture in Ethernet LANs.
 When installed, the star topology resembles spokes in a bicycle wheel.
 Larger networks use extended star topology also called tree topology. When used
with network devices that filter frames or packets, like bridges, switches, and routers,
this topology significantly reduces the traffic on the wires by sending packets only to
the wires of the destination host.
Ring Topology
A frame travels around the ring, stopping at each node. If a node wants to transmit
data, it adds the data as well as the destination address to the frame.
 The frame then continues around the ring until it finds the destination
node, which takes the data out of the frame.
 Single ring – All the devices on the network share a single cable
 Dual ring – The dual ring topology allows data to be sent in both directions.
Mesh Topology
The mesh topology connects all devices (nodes) to each other for redundancy and
fault tolerance.
It is used in the WANs to interconnect LANs and for mission critical networks like
those used by Banks and financial institutions
Implementing the Mesh topology is expensive and difficult
1.3 Overview of Network Types
Types of Computer Networks (by their size / area/ geographical area)
A computer network is a group of computers linked to each other that enables the
computer to communicate with another computer and share their resources, data, and
applications.
A computer network can be categorized by their size.
Local Area Network (LAN)
Metropolitan Area Network (MAN)
Wide area network (WAN)
1.3 Overview of Network Types
LAN
MAN
CAN
PAN
WAN
Local Area Network (LAN)
Network in small geographical Area (Room, Building or a Campus) is called LAN
(Local Area Network)
Connecting two or more personal computers
Less costly
Data is transferred at an extremely faster rate
Provides higher security
Metropolitan Area Network (MAN)
Network in a City is call MAN (Metropolitan Area Network)
MAN network covers larger area by connections LANs
to a larger network of
computers.
The size of the MAN is larger than LANs and smaller
networks), a MANs covers the larger area of a city or town.
than WANs (wide area
Campus Area Network (CAN)
Campus Area Network (CAN) is a group of interconnected
Networks (LAN) within a limited geographical area
Local Area
like school campus,
university campus, military bases, or organizational campuses and corporate
buildings etc.
This Campus Area Network also called as Corporate Area Network.
Sometimes this network is also referred as Residential Network or ResNet as
it is only used by residents of specific campus only.
Personal Area Network (PAN)
Personal area network (PAN) is an interconnection
between different devices like smartphone, tablet, computer
and other digital devices.
PAN is used for a personal purpose like data sharing
among devices and it has a range of 10 meters. Some type
of personal area networks is wired like USB while others
are wireless like Bluetooth.
Wireless personal area network is also known as WPAN.
Wide Area Network (WAN)
A Wide Area Network is a network that extends over a large
geographical area such as states or countries.
A WAN is quite bigger network than the MAN.
A WAN is not limited to a single location, but it spans over a large
geographical area through a telephone line, fiber optic cable or satellite
links.
The internet is one of the biggest WAN in the world.
A WAN is widely used in the field of Business, government, and
education.
Networking Types
Client/Server
P2P
Client-Server
Centralized Computing
Network Computing Models (Client-Server Network)
A client-server network is where every client is connected to the
server .
Server or mainframe computer has huge storage and processing
capabilities.
Peer to Peer [P2P]
In peer to peer architecture all the computers in a computer network
are connected with every computer in the network.
Every computer in the network uses the same resources as other
computers.
There is no central computer that acts as a server
rather all
computers acts as a server for the data that is stored in them.
Peer to Peer [P2P]
Advantages of a Peer to Peer Architecture
Less costly is no central server that has to take the backup.
In case of a computer failure all other computers in the network are not affected and they will
continue to work as same as before the failure.
Installation of peer to peer architecture is quite easy as each computer manages itself.
Disadvantages of a Peer to Peer Architecture
Each computer has to take the backup rather than a central computer and the security
measures are to be taken by all the computers separately.
Scalability is an issue in a Peer to Peer Architecture as connecting each computer to every
computer is a headache on a very large network.
Overview of Protocols and Standards
Protocols:
A protocol is a set of rules that governs(control) data communications.
A protocol defines what is communicated, how is communicated, and when it is communicated.
The key elements of a protocol are :
Syntax
Semantics
Timing
Overview of Protocols and Standards
Elements of PROTOCOLS:
 Syntax
 Structure or format of the data.
 Indicates how to read the bits - field border or boundary.
 Syntax should be same in sender and receiver for to communicate.
 Semantics
 Interprets the meaning of the bits
 Knows which fields define what action
 Interpretation of the syntax should be same
 Timing
 When data should be sent and what
 Speed at which data should be sent or speed at which it is being received
Overview of Protocols and Standards
 Standards:
 Standards provide guidelines to manufacturers, vendors, government agencies,
and other service providers to ensure the kind of interconnectivity necessary in
today's marketplace and in international communications.
 Standards are essential in creating and maintaining an open and competitive
market for equipment manufacturers and in guaranteeing.
 Data communication standards fall into two categories:
 de facto (meaning "by fact" or "by convention")
 de jure (meaning "by law" or "by regulation").
Overview of Protocols and Standards
 Standards:
 Two Categories of Standards
 De facto: Standards that have not been approved by an organized body but
have been adopted as standards through widespread use are de facto standards.
De facto standards are often established originally by manufacturers who seek to
define the functionality of a new product or technology.
 De jure: Those standards by law or by regulation. These are the standards
recognized officially by an Organization.
Overview of Protocols and Standards
 Standards Organizations
•
Standards are developed through the cooperation of standards creation
committees, forums, and government regulatory agencies.
 Standards Creation Committees :
•
•
•
While many organizations are dedicated to the establishment of standards, data
telecommunications in North America rely primarily on those published by the
following:
International Organization for Standardization (ISO): The ISO is a multinational
body whose membership is drawn mainly from the standards creation committees
of various governments throughout the world. The ISO is active in developing
cooperation in the fields of scientific, technological, and economic activity.
International
Telecommunication
Union-Telecommunication
Standards
Sector (ITU-T): This committee was devoted to the research and establishment of
standards for telecommunications in general and for phone and data systems in
particular.
Overview of Protocols and Standards
 Standards Creation Committees :
 American National Standards Institute (ANSI): Despite its name, the American National
Standards Institute is a completely private, nonprofit corporation not affiliated with the U.S.
federal government.
 Institute of Electrical and Electronics Engineers (IEEE): It is the largest professional
engineering society in the world. International in scope, it aims to advance theory, creativity,
and product quality in the fields of electrical engineering, electronics, and radio as well as in all
related branches of engineering.
 Electronic Industries Association (EIA): Aligned with ANSI, It is a nonprofit organization
devoted to the promotion of electronics manufacturing concerns. Its activities include public
awareness education and efforts in addition to standards development. In the field of
information technology, the EIA has made significant contributions by defining physical
connection interfaces and electronic signaling specifications for data communication.
OSI Reference Model
 ISO- International Organizations for Standard
 OSI- Opens System Interconnections
 Stats developing in late 1970s
 Approved by 1984
 The term “Open” in Open System Interconnections denotes “to communicate with any 2 systems”
 There are 7 layers in OSI Reference model
 It is also called OSI layered architecture / OSI Protocol architecture
 The process of breaking up the functions or tasks of networking into layers reduces
complexity.
 Each layer provides a service to the layer above it in the protocol specification.
 Each layer communicates with the same layer’s software or hardware on other computers.
OSI Reference Model

The lower 4 layers are concerned with the flow of data
from end to end through the network an also knows as
lower layers.

The upper Three layers of the OSI model are orientated
more toward services to the applications an also known
upper layers.
OSI Reference Model
 Layer 7, Application: The Application layer provides services to the
through which the user requests network services. Your
software is not on the Application layer. This layer
software
computer application
isn’t about applications and
doesn’t contain any applications. In other words, programs such as Microsoft Word
or Corel are not at this layer, but browsers, FTP clients, and mail clients are.
 Layer 6, Presentation: This layer is concerned with data representation and code
formatting.
 Layer 5, Session: The Session layer establishes, maintains, and manages the
communication session between computers.
 Layer 4, Transport: The functions defined in this layer provide for the
reliable
transmission of data segments, as well as the disassembly and assembly of the
data before and after transmission.
OSI Reference Model
•
Layer 3, Network: This is the layer on which routing takes place, and, as a result,
is perhaps the most important OSI layer. The Network layer defines the processes
used to route data across the network and the
structure and use of logical
addressing.
•
Layer 2, Data Link: As its name suggests, this layer is concerned with the linkages
and mechanisms used to move data about the network, including the topology,
such as Ethernet or Token Ring, and deals with the ways in which data is reliably
transmitted.
•
Layer 1, Physical: The Physical layer’s name says it all. This layer defines the
electrical and physical specifications for the networking media that carry the data
bits across a network.
OSI Reference Model
The interaction between layers in the OSI model
OSI Reference Model
 Packaging the data:
 Each layer of the OSI model formats the data it receives to suit the functions to
be performed on that layer.
 In general, the package of data that moves through the layers is called a
 Protocol Data Unit (PDU).
 However, as the data is reformatted and repackaged, it takes on unique names
on certain layers. Table 1 lists the name each layer uses to refer to a message
OSI Layer
PDU Name
Application
Data
Presentation
Data
Session
Data
Transport
Segment
Network
Packet
Data Link
Frame
Physical
Bits
OSI Reference Model
Fig : Data Encapsulation
OSI Reference Model
Fig :PDU and Layered Addressing
OSI Reference Model
 At a transmitting side, the data encapsulation method works like this:
 User information is converted to data for transmission on the network.
 Data is converted to segments and a reliable connection is set up between the transmitting and
receiving hosts.
 Segments are converted to packets or data grams, and a logical address is placed in the header so
each packet can be routed through the internetwork.
 Packets or datagram are converted to frames for transmission on the
local network. Hardware
(Ethernet) addresses are used to uniquely identify hosts on a local network segment.
 Frames are converted to bits, and a digital encoding and clocking scheme is used.
TCP/IP Models and its comparison with OSI
 A reference model (OSI and TCP/IP)
is
a
conceptual
rule
of
how
communications should take place. It
addresses all the processes required
for
effective communication and
divides these processes into logical
groupings
called layers. When a
communication system is designed in
this manner, it is known as layered
architecture.
TCP/IP Models and its comparison with OSI
 Need of layered architecture
 It divides the network communication process into smaller and
simpler
components, thus aiding component development, design and troubleshooting.
 It allows multi-vendor development through standardization of
network
components.
 It allows various types of network software and hardware to communicate.
TCP/IP Models and its comparison with OSI
 OSI Model (Open System Inter Connection)
Layer 7
Layer 6
Application
· Provides a user interface
Presentation · Presents data
· Handles processing such as encryption/decryption
Layer 5
Session
· Keeps different applications data separate
Layer 4
Transport
· Provides reliable or unreliable delivery
· Performs error correction before retransmit
Layer 3
Network
·Provides logical addressing, which routes use for path determination
Layer 2
Data Link
Layer 1
Physical
·Combines packets into bytes provides access to media
address performs error detection, not correction.
· Specifies voltage wire speed and pin-out of cables.
using MAC
TCP/IP Models and its comparison with OSI
 TCP/IP (Transmission Control Protocol/IP) Model
Application
Process (FTP, SMTP)
Transport
Transport
Internet
Internet
Network Access
Network Access
TCP/IP Models and its comparison with OSI
 TCP/IP Layer
 Layer 4: Application Layer
 This layer defines TCP/IP application protocols and how host programs interface with Transport layer services
use the network.
 Layer 3: Transport Layer
 The purpose of this device is to permit devices on the source and destination hosts to carry on a conversation. It
defines the level of service and status of the connection used when transporting data.
 Layer 2: Internet Layer
 This layer packs data into data packets known as IP datagram, which contain source and destination address (IP
address) information that is used to forward the datagram between hosts and across networks.
 Layer 1: Network Access Layer
This layer defines details of how data is physically sent or optically signaled by hardware devices that interface
directly with a network medium, such as co-axial cable, optical fiber or twisted pair, copper wire.
TCP/IP Models and its comparison with OSI
PROTOCOL
 Stream Control Transmission Protocol (SCTP)
 TCP (Transmission Control Protocol)
 Address Resolution Protocol (ARP)
 UDP (User Datagram Protocol)
 Reverse Address Resolution Protocol (RARP)
 Virtual terminal (TELNET)
 Internet Group Message Protocol (IGMP)
 File transfer(FTP), and
 ICMP (Internet Control Message Protocol)
 Simple Mail Transfer Protocol/ electronic mail (SMTP)
 Domain Name System (DNS),
 HTTP (HyperText Transfer Protocol)
Networking Protocol: TCP/IP
Networking Protocol: TCP/IP
Fig: TCP/IP PROTOCOL SUITE (TCP/IP and OSI model)
Connection and Connection-Oriented Network
Services

A connection-oriented service is one that establishes a dedicated connection between the communicating
entities before data communication formed.
 It is modeled after the telephone system.
To use a connection-oriented service, the user first establishes a connection, uses it and then releases it.

In connection-oriented services, the data streams/packets are delivered to the receiver in the same order in
which they have been sent by the sender.
 Connection-oriented services may be done in either of the following ways −
 Circuit-switched connection: In circuit switching, a dedicated physical path or a circuit is established
between the communicating nodes and then data stream is transferred.
 Virtual circuit-switched connection: Here, the data stream is transferred over a packet switched network,
in such a way that it seems to the user that there is a dedicated path from the sender to the receiver. A virtual
path is established here. However, other connections may also be using this path.
Connection and Connection-Oriented Network
Services
 Network Core
 It means the approach to moving data through a network of links and switches.
There are two types:
 Circuit Switching
 Packet
Switching
Connection and Connection-Oriented Network
Services
 In circuit switched network, the resources needed along a path (buffer, link transmission rate) to provide for
communication between the end systems are reserved for the duration of the communication session between the
end systems.
 It reserves a constant transmission rate in the network‘s links for the duration of the communication. Since
bandwidth has been reserved for this sender-to- receiver. Connection, the sender can transfer the data to the receiver
at the guaranteed constant rate.
In packed switched network, the resources are not reserved for a sessions message use the resources on
demand, and as a consequence may have to wait (i.e. queue) for access to a communication link.
 The packet is sent into the network without reserving and bandwidth if one of the links is busy because other
packets need to be transmitted over the link at the same time, our packet will have to wait in a buffer at the sending
side of the transmission link, and suffer a delay
Connection and Connection-Oriented Network
Services
 There are two approaches in packet switched network
 Datagram Network
 Any network that forwards the packets according to the destination address is called a datagram
network.
 The routers in the internet forwards packets according to the destination address. Hence, internet is datagram
network.
 Virtual Circuit Network
 Any network that forwards the packets according to virtual circuit identifier (fixed route) is called a virtual circuit
network.
 Preplanned route established before packets sent.
ISPs, Backbone Network Overview
ISP:
 An Internet Service Provider (ISP) is a company such as AT&T, Verizon,
 Comcast, or BrightHouse that provides Internet access to companies, families,
and even mobile users. ISPs use fiber-optics, satellite, copper wire, and other forms
to provide Internet access to its customers.
Internet Service Provider (ISP) is a company which provides internet connection
to end user, but there are basically three levels of ISP. Such that Tier-1 ISP, Tier-2 ISP,
and Tier-3 ISP.
Networking Protocol: TCP/IP
ISPs, Backbone Network Overview
Tier-1 ISP:
These ISPs are at the top of the hierarchy and they have a global reach they do not pay for any internet traffic through
there network instead lower-tier ISPs have to pay a cost for passing there traffic from one geolocation to another which
is not under the reach of that ISPs. Generally, ISPs at the same level connect to each other and make allow free traffic
passes for each other these ISPs are called peers. Due to this cost is saved. They build infrastructure, such as the
Atlantic Internet sea cables, to provide traffic to all other Internet providers, not end users.
Some examples of tier 1 Internet providers:
 Cogent Communications,
 Hibernia Networks,
 AT&T
ISPs, Backbone Network Overview
Tier-2 ISP:
These ISPs is a service provider who connects between tier 1 and tier 3 ISPs. They have regional or country reach and
behave just like Tier-1 ISP for Tier-3 ISP.
Examples of tier 2 ISPs:
 Vodafone,
 Easynet,
 British Telekom
ISPs, Backbone Network Overview
Tier-3 ISP:
These ISPs are closest to the end users and helps them to connect to the internet by charging some amount. These ISPs
work on purchasing model. These ISPs have to pay some cost to Tier-2 ISPs based on traffic generated.
Examples of Tier-3 ISPs:
 Comcast.
 Deutsche Telekom.
 Verizon Communications
Backbone Network Overview
 Backbone is most important part of a system which provides the central support to the rest system, for example
backbone of a human body that balance and hold all the body parts.
 Similarly in Computer Networks a Backbone Network is as a Network containing a high capacity connectivity
infrastructure that backbone to the different part of the network.
 Actually a backbone network allows multiple LANs to get connected in a backbone network, not a single station is
directly connected to the backbone but the stations are part of LAN, and backbone connect those LANs.
Backbone Network Overview
 Backbone LANs:
 Because of increasing use of distributed applications and PCs, a new flexible strategy for LANs has been introduced. if a
premises wide data communication system is to be supported then we need a networking system which can span over the
required distance and which capable of interconnecting all the equipment in a single building or in a group of buildings.
 It is possible to develop a single LAN for this purpose but practically this scheme faces the following drawbacks:
 Poor Reliability:
 With a single LAN, the reliability will be poor since a service interruption even for a short duration can cause major
problem to the user.
Backbone Network Overview
 Capacity:
 There is a possibility that a single LAN may be saturated due to increase in number of devices beyond a certain
number
 Cost:
 A single LAN can not give its optimum performance for the diverse requirements of communication and interconnection.
 So the alternative for using a single LAN is to use low cost, low capacity, LANs in each building or department and then
interconnection all these LANs with high capacity LAN. such a network is called as Backbone LAN.
 The backbone itself is a LAN, it uses a LAN protocol such as ethernet, Hence each connection in the backbone is itself
another LAN. The two very common used architectures are: Bus backbone, Star backbone.
Backbone Network Overview
Bus Backbone:
 In Bus backbone the topology used for the backbone is bus topology.
 In Below figure the Bus backbone structure is used as a distribution backbone for connecting
different buildings in an organization.
 The structure is a bridge based (bridge is the connecting device) backbone with four LANs.
Backbone Network Overview
Working:
In Figure, structure if a station in LAN 2 wants to send
a frame to some other
station in Same LAN then
Bridge 2 will not allow the frame to pass to any other
LAN, hence this frame will not reach the backbone. If
a station from LAN 1 wants to send a frame to a
station in LAN 4 then Bridge 1 passes this frame to
the backbone. This frame is then received by Bridge
4 and delivered to the destination.
Backbone Network Overview
Star Backbone:
 The topology of this backbone is star topology.
 Figure
shows
the
Star
backbone
configuration, the backbone is
in
this
simply a switch
which is used to connect various LANs. The switch
does the job of backbone and connect the LANs
as well. This type of backbone are basically used
as distribution backbone inside a building.
Backbone Network Overview
 There is one more category of backbone network
is Interconnecting of Remote LANs:
 Interconnection of Remote control:
 In this type of backbone network the connection are
done through the bridge called remote bridges which
acts as connecting devices in connect LANs as point
to point network link.
 Example of point to point networks are leased
telephone lines or ADSL lines. Such a point to point
network can be considered as equivalent LAN without
stations.
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