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 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-basedPeer-to-peer Centralised backing storageDistributed 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 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 ( Mo dulator/ Dem odulator) Telephones designed for speech (analogue signal) Modems at either end convert digital-analogue and then analogue-digital. modem digital analogue

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)