In previous chapters we have introduced and illustrated a general concept of an event as an action without duration, whose occurrence may require simultaneous participation by more than one independently described process. In this chapter we shall concentrate on a special class of event known as a communication.

Chapter 4 at last introduces communication: it is a special case of interaction between two processes, one of which outputs a message at the same time as the other one inputs it. Thus communication is synchronised; if buffering is required on a channel, this is achieved by interposing a buffer process between the two processes.
An important objective in the design of concurrent systems is to achieve greater speed of computation in the solution of practical problems. This is illustrated by the design of some simple systolic (or iterative) array algorithms. A simple case is a pipe, defined as a sequence of processes in which each process inputs only from its predecessor and outputs only to its successor. Pipes are useful for the implementation of a single direction of a communications protocol, structured as a hierarchy of layers. Finally, the important concept of an abstract data type is modelled a a subordinate process, each instance of which communicates only with the block in which it is declared.