Semester One 2001/2002 Sheffield Hallam University1 What Is a Network? A group of linked computers whose users can share: –Information, e.G. Corporate databases, group-ware documents, s –Software, e.G. Applications like word and excel on the networks at SHU –Hardware, e.G. Printers and scanners These connections are usually by cable, but radio communications are used in wireless networks

Semester One 2001/2002 Sheffield Hallam University2 Network Components two or more computers cables, or some other means of communicating information between the networked computers (next two slides) a network card on the motherboard (next week) of each computer, the physical interface between computer and network network Operating System - e.g. Windows NT or Novell NetWare - to manage file handling across the network

Semester One 2001/2002 Sheffield Hallam University3 Types of Network LAN - local area network. Uses purpose built cables to connect computers within a small area, such as a building or group of buildings. SHU uses local area networks. WAN - wide area network. Uses phone lines, ISDN links (integrated services digital networks), microwaves, satellites and WAP technologies to connect computers across large geographic areas.

Semester One 2001/2002 Sheffield Hallam University4 Connecting via Cables Fibre optic, hair thin strands of glass in a sleeve. Fast and efficient, high bandwidth, but costly. Will carry a signal two kilometres without loss of information. Coaxial, as in TV systems. OK up to 50 metres. Twisted pair, as in telephone systems: 1 wire to send, 1 to receive. Twisting lowers risk of information loss through noise.

Semester One 2001/2002 Sheffield Hallam University5 Fibre Optic Transmits light signals, as oppsed to electrical signals It is more expensive than copper cable Very high bandwidth - up to 2 Gbps Much lower attenuation EMI resistance (Electromagnetic Interference) Lighter than copper

Semester One 2001/2002 Sheffield Hallam University6 Coaxial Cable Copper wire as core, surrounded by a plastic insulation and a second conductor (shield) EMI resistance better than unshielded cable More difficult to install Much higher bandwidth than unshielded, 10Mbps Two types: –Thin coax, thinnet, thin ethernet –Thick coax, thicknet, thick ethernet

Semester One 2001/2002 Sheffield Hallam University7 Twisted Pair Two insulated copper wires, twisted together like ‘bell wire’ Widely used in telephone system Unshielded - Category 5 UTP (Unshielded Twisted Pair) 100 Mbps Shielded - 155MBps currently used in IBM Token Ring Networks at 16 Mbps Suffers from attenuation Some resistance to EMI because of twists

Semester One 2001/2002 Sheffield Hallam University8 Connecting without Cables Wireless connections expensive Used for: –LAN, extended LAN, mobile computing Infra-red –within 100 feet, 10Mbps. Won’t pass through solid objects but will bounce with attenuation Laser based –line of sight Radio –frequency restrictions, low bandwidth –can travel massive distances Satellite Microwave –line of sight, in excess of 50Km

Semester One 2001/2002 Sheffield Hallam University9 Architectures, Topologies and Protocols A network’s architecture refers to the means of storing information for common access across the network Network topologies describe the physical means of arranging computers and peripheral devices to enable communication between them all Network protocols manage data exchange. (They handle situations like two computers wanting simultaneous access to the same file or device.)

Semester One 2001/2002 Sheffield Hallam University10 Network Architectures How do LANs store information in such a way as to support common access? There are two basic architectures, which differ in how and where the information is stored. These are: –Peer-to-Peer: a simple and cheap way of sharing information between a small number (typically ten or less) of computers. –Client/Server: more expensive but much more efficient, and the only practical architecture for connecting large numbers of computers

Semester One 2001/2002 Sheffield Hallam University11

Semester One 2001/2002 Sheffield Hallam University12

Semester One 2001/2002 Sheffield Hallam University13 Three Network Topologies Star Bus Ring

Semester One 2001/2002 Sheffield Hallam University14 Star Topology With this topology all computers and peripheral devices (printers etc) link to a central computer. The only way one device can talk to any other is through the central computer. Peer-to-peer architectures are not an option with this topology. Star networks are still used, but hark back to the mainframe days of heavily centralised computing.

Semester One 2001/2002 Sheffield Hallam University15 Bus Topology The commonest topology: all devices link along a single cable. Data from one device to another is sent over the whole network; software ensures only the targeted device actually receives it. Bus networks can use either peer- to-peer or client/server architectures. Data exchange protocols for most bus networks use Ethernet, a proprietary technology which transmits data at between 10 and 1000 megabits per second. One problem is data collision, when two devices transmit at the same time. The need to re-send when this occurs can make bus networks slow at times.

Semester One 2001/2002 Sheffield Hallam University16 Ring Topology Devices are linked on a closed loop, and each device can talk directly to any other. This topology allows token ring protocols. An electronic signal, or token, continuously travels around the loop. A device that is ready to communicate checks the token as it passes. If the token free, the message is attached and no other device can access it. On receiving the message, the destination device releases the token, allowing it to resume its circulation.

Semester One 2001/2002 Sheffield Hallam University17 Repeaters Data sent from a computer can only travel so far before it degrades. (See, for example, slide 4.) If signals are required to travel further they must be boosted by a relay device known as a repeater. Repeaters can be very simple, doing no more than strengthening the signal, but some are capable of performing error checks that will establish whether or not degradation has already occurred.

Semester One 2001/2002 Sheffield Hallam University18 Network unto Network Even with repeaters, LAN’s cannot be infinitely expanded. Distance, number of users, and sheer volume of traffic all constrain how big a network can be. The ability to use switches, routers and gateways to link networks overcomes these constraints - and makes possible the internet, which is no more and no less than a world wide linking up of hundreds of thousands of LANs and WANs.