Previous Table of Contents Next


Chapter 4
Communication Protocols

Questions Answered in This Chapter

What is a communication protocol?

What is the OSI communication protocol model?

How do ATM and ADSL relate to the OSI model?

What are some communication protocol examples?

What is a frame and why is it important to ATM/ADSL?

Introduction

What is a protocol? Diplomatic protocols are the established ceremonial forms and courtesies used by diplomats in the exercise of diplomacy. Deviations in the established protocol are considered serious breaches of convention and can lead to unfortunate consequences. Similarly, communication protocols define acceptable queries and responses between users of a particular network type. The objective of course, is to provide for the orderly and predictable behavior of hardware and software used in the network.

While governing bodies have established communication protocols to be machine independent, setting down rules and guidelines for the general cases without regard to the specific platforms performing the communication tasks, protocols, in a sense, are very environment oriented. Designers must consider the specific equipment and their peculiarities, or uniqueness, used in assembling network components. A DOS-based machine will not behave identically to a UNIX-based machine in a network unless such behavior differences are accounted for in the design of the network. Currently, the most common method of accounting for machine dependencies in network communications is to purchase third-party software and hardware. Protocols are concerned with every aspect of communications, from equipment physical interfaces, data formats, and communication speeds to high-level software interfaces. The objective is to provide a communications service to users that is reliable.

A protocol is an agreed-upon set of rules relative to specific objectives that are meaningful to the parties that agreed upon the rules in the first place. Any particular protocol, diplomatic or communication, may not necessarily be understood by others only indirectly related to the process. Communication protocols are rules that attempt to define specific events which are intended to control, in some acceptable and understandable manner relative to the objective to be accomplished, the devices that are involved in a communications network. The events of interest and the sequence or manner in which they occur define the protocol. There are numerous communications protocols existing today. Some such protocols are IEEE 488, RS-232, and IEEE 803.

Why does the world need communications protocols? Variety. Someone said variety is the spice of life. I do not know the context of the statement but I can certainly see how it may be applicable in some settings and totally inappropriate in others. Variety is certainly the norm in communications. There are many different ways to transport information from end to end and there are many different ways of encoding information for transport. For any two similar transport or encoding systems to work, a defined set of rules must specify all the particulars of any importance. Two similar transport or encoding systems will provide gibberish, i.e., garbage, results unless all the rules are followed within any specified tolerances. Just as humans need varied behavioral protocols to define acceptable behavior in varied social settings so that human actions may be interpreted with a certain degree of accuracy, so do communication systems need protocols to define acceptable behavior, that all communications between end users of a similar system will be understandable.

What if a rebel, with or without a cause, decides to flaunt convention and attempt to communicate outside the established protocol? A communication outside the established protocol may go unrecognized or result in a message to the sender, and perhaps to the intended recipient, that an unacceptable or unrecognizable communiqué was attempted. Totally unexpected results may occur or, in poorly designed systems, equipment may crash.


Figure 4-1.   Open Systems Interconnection (OSI) model

A protocol template to use for modeling communication systems is beneficial that we might more easily visualize the underlying concepts. ITU-T (formerly CCITT) developed the Open Systems Interconnection (OSI) model that serves the purpose of a protocol template very well. See Figure 4-1. The OSI model represents a stack of seven layers that can be considered as gradations from simpler to more complex functionality as one goes up the stack. This does not mean that one layer is simpler than another. In fact, each layer in its own right is a complex interworking of rules, hardware, software, and human endeavor.

The OSI model, especially above the physical layer, is really a hierarchical software model that provides for coherent communications between groups of software procedures and/or programs. Each layer can be represented by a software program that communicates only with the layer above and below it. Typically, each layer is comprised of software routines that are called by the next higher layer, when appropriate. The software routines are written for specific communication applications. The interface between higher and lower layer routines is performed by procedure calls known as application programming interface (API) calls.

The first three layers, the Physical Layer, the Data Link Layer, and the Network Layer, are closely linked together. Kind of a “you can’t have one without the other” sort of thing.

The three layers comprise the minimum structure required to establish and maintain communications between two entities (nodes). These three layers pass from node to node in the transmission path and contain, besides the data, address and error-detection information necessary to ensure error-free routing to the correct destination. The control and routing information is called Operations Administration and Management (OAM) data and is added to the user data stream in quantities called headers and trailers. Each layer adds it own headers and trailers to the data “packet.”

The control and routing information is interpreted by switches, routers, and bridges along the transmission path. The control information and data are bound together until received by the destination, where the control information and data are separated. Generally, the first three layers represented by the OSI model comprise the fundamental ISDN/ BISDN software package running on a host processor responsible for the physical switching duties, with the user providing the application specific software package, comprising the remaining OSI layers, that typically runs on a remote host computer.


Figure 4-2.   OSI Physical Layer


Previous Table of Contents Next