Other Standards The Application Layer

110 Communication Networks Copyright © 2005 PragSoft • LAN protocol stack . This implements the LAN protocol layers. This usually takes the form of a hardware card inside the user station, containing a microprocessor and firmware which implements the non-physical protocols. • Physical Interface Unit . This directly interfaces the user station-based LAN hardware to the LAN physical medium. The exact form of the PIU is highly dependent on the LAN physical medium. Coaxial cable connectors and cable TV taps are common examples. • Physical Medium . This provides a physical path for signals to travel between stations. Coaxial cable, optical fiber, and infra red light are examples. The LAN protocol stack will be the main focus of our attention in this chapter.

9.1.1. Topologies and Access Protocols

There are two general categories of LAN topologies: bus and ring see Figure 9.94. The bus topology uses a broadcast technique, hence only one station at a time can send messages and all other station listen to the message. A listening station examines the recipient address of the message and if it matches its own address, copies the message; otherwise, it ignores the message. The ring topology uses a closed, point-to-point-connected loop of stations. Data flows in one direction only, from one station to the next. As with the bus topology, transmission is restricted to one user at a time. When a station gains control and sends a message, the message is sent to the next station in the ring. Each receiving station in the ring examines the recipient address of the message and if it matches its own address, copies the message. The message is passed around the ring until it reaches the originator which removes the message by not sending it to the next station. Figure 9.94 LAN topologies. PIU Bus Ring PIU PIU PIU PIU PIU PIU PIU Given that access to the bus or ring is restricted to one station at a time, some form of arbitration is needed to ensure equitable access by all stations. Arbitration is imposed by access protocols. A number of such protocols have been devised: • Carrier Sense . This protocol is applicable to a bus topology. Before a station can transmit, it listens to the channel to see if any other station is already