Introduction to protocols

This chapter provides an overview of protocols used in data communication. A protocol can be defined as a set of rules governing the exchange of data between a transmitter and a receiver over a communications link or a network. Data communications systems, which follow the open systems interconnection (OSI) reference model, are made up of several hierarchical layers. Each of these layers contains working software or hardware elements referred to as entities. One of the elements in each layer is a protocol entity, which has its own specification, and broadly speaking is a protocol in its own right. The purpose of the protocol entity is to determine how messages are transferred across a network to a peer entity in another node. Actual transfers across the physical link in OSI model are determined by the data link layer protocol. Two most popular types of flow control protocols: XON/XOFF and ETX/ACK are discussed in detail. The chapter also describes two modes of binary synchronous protocols: point-to-point and multipoint. File transfer protocols and automatic repeat-request (ARQ) protocols are also elaborated in the chapter.