A LOCAL AREA NETWORK PROTOCOL

To illustrate the application of timed CSP to the specification of real-time systems, we will show how the functions presented in chapter 6 may used to describe the behaviour of a communications protocol at two different levels of abstraction. The protocol chosen for this purpose is based upon the Ethernet communication protocol, a standard protocol for local area networks.

The Ethernet protocol is a broadcast protocol: signals sent by one station may reach all of the stations upon the network. It is a carrier-sense protocol: stations listen for a carrier signal on the broadcast medium and act accordingly. Another important feature is collision detection. Each station must monitor the broadcast medium during transmission, and cease immediately if it becomes apparent that another station is also transmitting.

The protocol specification is divided into two parts, corresponding to the data link and physical layers of the ISO reference model described by Tanenbaum (1981). This model consists of seven layers, each representing a different level of abstraction, from the hardware of the physical layer to the user software of the application layer. Each layer provides a service to the layer above, facilitating virtual communication between peer processes on different machines. In this chapter, we will concern ourselves with the bottom three layers of the model: the communication subnet of figure 7.1.

The physical layer is the lowest layer in the model hierarchy, and transmits data as bits between the stations, or nodes of the network.