Transcript Lecture 4
What Is a Network? • A group of linked computers whose users can share: – Information, e.G. Corporate databases, group-ware documents, emails – 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 University 1 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 University 2 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 University 3 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 University 4 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 University 5 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 University 6 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 University 7 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 University 8 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 University 9 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 University 10 Semester One 2001/2002 Sheffield Hallam University 11 Semester One 2001/2002 Sheffield Hallam University 12 Three Network Topologies Star Bus Ring Semester One 2001/2002 Sheffield Hallam University 13 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 University 14 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 peerto-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 University 15 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 University 16 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 University 17 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. Semester One 2001/2002 Sheffield Hallam University 18