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A L L A H

Introduction to Computer Networks

INTRODUCTION TO COMPUTER NETWORKS

Computer Networks

Computer network connects two or more autonomous computers.

The computers can be geographically located anywhere.

Introduction to Computer Networks

LAN, MAN & WAN

Introduction to Computer Networks

Network in small geographical Area (Room, Building or a Campus) is called LAN (Local Area Network)

Network in a City is call MAN (Metropolitan Area Network)

Network spread geographically (Country or across Globe) is called WAN (Wide Area Network)

Applications of Networks

Introduction to Computer Networks

Resource SharingHardware (computing resources, disks, printers)Software (application software)

Information SharingEasy accessibility from anywhere (files, databases)Search Capability (WWW)

CommunicationEmailMessage broadcast

Remote computingDistributed processing (GRID Computing)

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.

Introduction to Computer Networks

Bus Topology

Commonly referred to as a linear bus, all the devices on a bus topology are connected by one single cable.

Introduction to Computer Networks

Star & Tree Topology

Introduction to Computer Networks

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 the 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

Introduction to Computer Networks

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 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.

Introduction to Computer Networks

Network Components

Introduction to Computer Networks

Physical MediaInterconnecting DevicesComputersNetworking SoftwareApplications

Networking Media

Networking media can be defined simply as the means by which signals (data) are sent from one computer to another (either by cable or wireless means).

Introduction to Computer Networks

Networking Devices

Introduction to Computer Networks

HUB, Switches, Routers, Wireless Access Points, Modems etc.

Computers: Clients and ServersIn a client/server network arrangement, network services are located in a dedicated computer whose only function is to respond to the requests of clients.

The server contains the file, print, application, security, and other services in a central computer that is continuously available to respond to client requests.

Introduction to Computer Networks

Networking Protocol: TCP/IP

Introduction to Computer Networks

Models of the Internet

OSI/ISO Reference Model TCP/IP Reference Model

Open Systems Interconnection (International Standards Office)

ApplicationPresentationSessionTransportNetworkData LinkPhysical

Model Layers

Application

TransportInternetHost-to-network

Transmission Control Protocol/Internet Protocol

1) Physical layer

• Purpose: Necessary infrastructure.• Think "wires in the ground and switches

connecting them".• This is the physical hardware of the internet.• Wires/optical cables/wireless links and other

technologies provide a way for transmission of raw bits (0s and 1s).

• Routers and switches connect these cables and direct the traffic.

2) Data link layer

• Purpose: Provides basic connection between two logically connected machines.

• Think: “I stuff packets down a wire to my neighbour”

• Send raw packets between hosts.• Basic error checking for lost data.• In TCP/IP the "Physical layer" and the "Data

Link" layer are grouped together and called the host-to-network layer.

3) Network Layer/Internet Layer

• Purpose: Provide end-to-end communication between any two machines.

• Think: “I try to get a packet to its destination”• Tells data which link to travel down.• Addresses the problem known as routing.• Deals with the question "where do I go next to

get to my destination?"• Ensures packets get from source A to

destination B.

4) Transport Layer

• Purpose: Ensure that data gets between A and B.• Think: “From the source and destination, I make

sure that the data gets there”.• Ensures a data gets between source and

destination.• If necessary ensure that connection is lossless

(resend missing data).• Provides flow control if necessary (send data

faster or slower depending on the network conditions).

5) Session Layer (not TCP/IP)

• Purpose: Provides a single connection for one application.

• Think: “I am in charge of the entire message.”• This connection may be two way or may be

synchronised.• Not discussed much as it is never implemented.

6) Presentation Layer (Not TCP/IP)

• Purpose: Provides commonly used functions for applications.

• Think: “I meet internationalisation standards”.• The main job of the presentation layer is to

ensure that character sets match – e.g. that Chinese characters are correctly received by the sends.

• Again not discussed much as it is never implemented.

7) Application layer

• Purpose: The computer programs which actually do things with the network.

• Think: “I deliver the mail, browse the web etc.”• For example, your email client program which will talk

to the email server at the other end.• At this layer, we have many protocols (http, snmp,

smtp, ftp, telnet) which different bits of software use.• We often talk in terms of client and server architecture

for the software.

TCP/IP model in summary

Internet (IP) addresses

richard@manor.york.ac.uk (email)http://www.apoptygma.eu.org (www)ftp://ftp.uk.debian.org (file transfer)telnet://towel.blinkenlights.nl (telnet)144.32.100.24 148.122.211.110195.224.53.3962.250.7.101

These are the “real” IP addressesof the above sites. IP addressesare 32 bits grouped into 4 octets.(Octet = 8 bits – a number from0-255)

IP Networks(1)

• IP addresses use less significant bits first to indicate sub-networks.

• IP address: 123.45.67.89• Netmask:255.255.255.0 (no holes allowed)• If two IP addresses are the same when bitwise

AND’d against the netmask then they are on the same subnet.

• 123.45.67.?? is always on the same subnet in the above example.

IP Networks(2)

• IP networks were originally subdivided into class A, B, C, D and E networks.

Start End Networks Hosts/network

A 1.0.0.0 127.255.255.255 126 16 million

B 128.0.0.0 191.255.255.255 16,382 64K

C 192.0.0.0 223.255.255.255 2 million 254

D 224.0.0.0 239.255.255.255 Multicast

E 240.0.0.0 247.255.255.255 Reserved

Hub

Switch

Switch? Router?

Basic Definitions: Host, Router, Switch, Source, Destination

• Host: A machine which is a point on a network which packets travel through – a node in a graph.

• Router: A host which finds a route for packets to travel down – an intermediate point in a journey.

• Switch: Often used interchangeably with router but implies that the routes are “fixed”.

• Source: Where data is coming from.• Destination: (or sink) Where data is going to.

Applications

E-mailSearchable Data (Web Sites)E-CommerceNews GroupsInternet Telephony (VoIP)Video ConferencingChat GroupsInstant Messengers Internet Radio

Introduction to Computer Networks