Overview

Before looking at the Open Systems Interconnections (OSI) model, let us examine the functions of a protocol. A protocol is a set of rules governing the way data are transmitted and received over data communications networks. Protocols must provide reliable, error-free transmission of user data as well as network management functions. Protocols packetize the user data into data envelopes. Some have fixed lengths and others have variable lengths depending on the protocol used.

Protocols are used whenever a serial bit stream is used. The protocol defines the order in which the bits will be sent and also appends information for use in routing and managing the network. This appended information is only used by the protocol and is transparent to the user.

Some protocols, such as Signaling System 7 (SS7), actually send predefined messages to the other nodes in the network. Messages can be used at any layer above layer 1 and are found commonly at layers 2 and 3. A typical example of a protocol message is the initial address message (IAM) sent by the SS7 protocol to establish a connection on a voice circuit between two end offices. Other messages exist for SS7 and will be discussed in greater detail in later chapters. Predefined messages are an excellent way to send network management functions and handle data error procedures.

Other functions of a protocol include the segmentation of blocks of data for easier transmission over the network and reassembly at the receiving node.