Network Environments

Communications Networks LAN – Local Area Network

Collection of Computers and peripherals with a common connection in one building or site.

WAN – Wide Area Network Groups of networks connected together over a

larger geographical area. Ultimate WAN ??

Network topologies The topology of a network is its physical layout

— the way in which the computers and other units (commonly referred to as nodes) are connected.

Common topologies: Star Bus Ring

LAN - Advantages Shared resources

Disk storage, printers, modems, scanners etc. Shared Information Efficient Storage

Programs can be stored on one computer Local Email Easier Set-up procedures Different types of computers can be connected

LAN - Disadvantages Dependence on a single server machine.

Large systems will have a back-up server. Hardware/software access depends on network System Manager dependent

System must be properly organised for efficiency Difficult to make system secure.

Vulnerable to hackers – particularly via modems Performance degrades as use increases

Types of LAN Server-based – Client-Server Architecture

Devices on networks are either clients or servers. Workstations would be clients. They request a service from

servers. For example, retrieve a file from the file server, or print a document on a printer which is a server device

Peer-to-peer Alternative architecture for small businesses (3 or 4

computers) Each workstation can communicate directly with every other

workstation with no central server.

Network Maintenance Tasks Any network requires the management of:

Adequate response times Necessary hardware upgrades Software availability Software upgrades Provision of information for users Back-up procedures Network Security

Comparison

Server-based Peer-to-peerCentralised backing storage Distributed storage

Software held centrally and shared. Server manages distribution of data and software to clients. Some processing may be handled locally but most done by file server. More speed and power but additional cost and complication.

Copies of software held on individual machines. Basic network services to enable sharing of data, software and printers are provided. Generally easier to set up and maintain than servers. OK for small businesses.

User Ids, passwords and access levels are controlled centrally

No central security

Centralised back-up facilities. Back-up responsibility is devolved to individual users.

If server goes down, all users affected. No central server dependence

Central node computer controls the communication protocol. Computers may be of a different type.

Individual computers must all have the same software loaded to control communication with other computers.

Wide Area Networks LAN is connected by cables. WAN is spread over a wide geographical area Mode of communication may be:

Telephone line, microwave, satellite link Use of global networks has increased due to:

Telephone networks: analogue to digital Reduced connection and network costs Improved compression techniques

Communications links Communications

satellite

Mainframe in Norwich

Mainframe in London

Mainframe in USA

modem modem

Possible

Configuration

Communications media Twisted pair (copper cable)

used in telephone network and LANs Coaxial cable

high quality, well insulated cable Fibre optic cable

sends pulses of light rather than electricity Microwave

almost line of sight (30 miles apart) Communications satellite

in geosynchronous orbit

Telephone Network Service BANDWIDTH determines speed of data transfer and is

defined as the range of frequencies that a line can carry. The following high bandwidth services are capable of sending voice, video and computer data.

ISDN Lines Integrated Services Digital Network

ADSL Lines Asymmetric Digital Subscriber Line

Other Linking Devices Bridge

A connection between 2 LANs Gateway

Connects LANs with WAN or mainframe Modem (Modulator/Demodulator)

Telephones designed for speech (analogue signal) Modems at either end convert digital-analogue and

then analogue-digital.modem modem

digital digitalanalogue

Transmission Asynchronous

Sends 1 character at a time, with each character preceded by a start bit and followed by a stop bit. A parity bit is also usually included to combat incorrect transmission. Used by PCs, fast and economical for small amounts of data.

Synchronous Transmission controlled by timing signals – no start

and stop bits. Less error-prone than asynchronous.

Standardisation In order that equipment from different suppliers can be

linked or networked, standard sets of rules or PROTOCOLS have been devised to ensure compatibility. Any equipment using the same protocol can be linked together.

Where necessary, protocol converters can be used to overcome: Different types of transmission – asynchronous(PC) or

synchronous(mainframe). Different character representations – ASCII(PC) or

EBCDIC(mainframe). Different error checking methods.

Factors affecting rate of data transmission The speed of the modem

Different modems vary in data transmission rates, typically from 9K to 56K bps (bits per second).

The nature of the transmission line A digital line such as an ISDN line has a much higher transmission

speed than an analogue line. The type of cable used

Twisted pair has a transfer rate of 10Mbps; fibre optic cable is about 10 times as fast.

The type of transmission synchronous or asynchronous (see Ch.60)