passed to the next device in a predetermined sequence. The two most influential bodies in determining LAN standards are the Institute of Electrical and Electronic Engineers, (IEEE), and the 56.4.4.4 Network topology International Standards Organisation, (ISO). The IEEE Local Network Standards Committee, (Project 802), is Several network topologies, (physical layout), can be used to design developing LAN access standards and protocols in a layered ap- a Local Area Network, the most common being Bus, Ring, Star and proach similar to the ISO's Open Systems Interconnection (OSI) Star-Shaped Ring. (See Figure 56.12.): Reference Model. The OSI Reference Model (see Chapter 12) represents the rela- 1. Bus network is the most prevalent LAN configuration, used in tionship between a network and the services it can support by a both Ethernet (CSMA/CD) and Token Passing Bus. All devices hierarchy of seven protocol layers. Each layer uses the services of connect to the bus by means of a tap, (transceiver). the lower layers in conjunction with its own functions to create new In Ring topologies, all networked devices are connected in a services which are then made available to the higher layers. (See closed loop. Figure 56.13.) For Star topologies the networked devices are connected di- Sub groups within the IEEE 802 Committeè are producing the rectly to a central network device, which is usually active. following LAN standards: 56/10 Local and Wide Area Networking APPLICATION Includes user programmes PRESENTATION Data formatting and compression Establishment/termination of SESSION transport connections Provides end to end service between TRANSPORT layers 1-3 and the higher layers NETWORK Includes routing and switching functions Includes synchronisation and lower layer DATA LINK error control PHYSICAL Includes signal transmission Figure 56.13 The OSI Reference Model 1. IEEE 802.1, covering architecture, addressing, internetworking The IEEE 802 series are standards covering access methods and and management. as such are applicabl