Computer Network

Compiled by GF Kalumuna

January 2014

Computer Network

• A way of connecting two or more computers together in order to gain access to different information or services

Features of a Network

• Communication channel• Hardware• Software• Network topology

Communications Channel

• Wires that connect the network• Node- a computer or hardware in a

network• Why many communication channels?

•Need for Bandwidth- capacity of a communication channel to carry information

•Transmission speed

Communications Hardware

• These are nodes• Examples of Nodes• Server – a computer that handles traffic

and dispenses files/information (Cf Host)• Client- computer that uses services and

information from the server• Bridge- connects similar networks together• Controller- Handles incoming dialup calls

and forwards them

•Multiplexer- splits a channel so that many nodes can use it

•Router- Determines where to send or forward data on the network

•Switch- selects the path for sending data

Communications SoftwareThe network operating system (NOS)• Helps to share different hardware

(printers, applications and files)• Manages people on the network (cf

security)The server software• Determines which computer can

participate in the network• Ensures proper paths for

communications to take placeThe client software• Helps to obtain information and use the

services of servers

Network Topology

• Also called configuration• The layout of computers and other

devices and how they are connected• Bus Topology• Star Topology• Ring Topology• Hierarchical Topology

Bus Topology • Also called Ethernet• Each device handles its own

communication control • No host computer• All communications travel along a

common connecting cable, a bus.• As info passes, it is examined by

each device if relevant for it• Ideal for few computers• Less expensive• Not as efficient as star network

Star Network

• Several devices are linked to a central unit• The central unit may be a host computer

or a file server• Each device asked “Polled” if it has a

message to send, then allow to send it• Several users can share resources from

the central computer• Ideal for linking several computers

(organisation database)

Ring Network• Each device is connected to two other

devices, hence, forming a ring• No central file server or computer• Messages pass around the ring until they

reach the correct destination• Mainly used to link mainframes to cover a

wide geographical area• Perform their own processing, but may

rarely share with other mainframes• Ideal for decentralised organisations

Hierarchical Network

• Also called hybrid network• Several computers are linked to a central

host computer (cf star network)• These other computers are also hosts to

other computers or peripheral devices• Levels/Hierarchy

• Mainframes (Top)• Minicomputers (Sub)• Microcomputers (Sub sub)

• Allows various computers to share databases, processing power, & different output devices

•Ideal in centralised organisations

Network Types

• Three Important Types

• Local Area Networks (LANs)

• Metropolitan Area Networks (MANs)

• Wide Area Networks (WANs)

Local Area Networks• Connects with computers and peripheral

devices in close physical proximity• Linked by cable e.g. telephone• Often use a bus design• Users can share equipment (e.g printer,

file server)• Other equipment can be added to the LAN

e.g mini/mainframe computers, optical storage devices

• It can be linked to another LAN

Metropolitan Area Networks

• Link between office buildings in a city

• Covers a larger area than LAN

• Used by many individuals and organisations

• They use wires/cables

• Can extend up to 50 kms

Wide Area Networks

• Countrywide and worldwide networks

• Use microwave relays and satellites to reach users over long distances

• The widest WAN is the Internet (entire globe)