Communications Model

Communications Model

The 1970s and 1980s saw a merger of the fields of computer science and data

communications that profoundly changed the technology, products, and

companies of the now-combined computer-communications industry. Although

the consequences of this revolutionary merger are still being worked out, it

is safe to say that the revolution has occurred, and any investigation of the field of

data communications must be made within this new context.

The computer-communications revolution has produced several remarkable

facts:

There is no fundamental difference between data processing (computers) and

data communications (transmission and switching equipment).

There are no fundamental differences among data, voice, and video communications.

The lines between single-processor computer, multi-processor computer,

local network, metropolitan network, and long-haul network have blurred.

One effect of these trends has been a growing overlap of the computer and

communications industries, from component fabrication to system integration.

Another result is the development of integrated systems that transmit and process

all types of data and information. Both the technology and the technical-standards

organizations are driving toward a single public system that integrates all communications

and makes virtually all data and information sources around the world

easily and uniformly accessible.

It is the ambitious purpose of this lesson to provide a unified view of the broad

field of data and computer communications. The organization of the lesson reflects

an attempt to break this massive subject into comprehensible parts and to build,

piece by piece, a survey of the state of the art. This introductory lesson begins with

a general model of communications. Then, a brief discussion introduces each of the

four major parts of this lesson. Next, the all-important role of standards is introduced.

A Communication model.

We begin our study with a simple model of communications, illustrated by the block

diagram in Figure l.la.

The fundamental purpose of a communications system is the exchange of data

between two parties. Figure l.lb presents one particular example, which is the communication

between a workstation and a server over a public telephone network.

Another example is the exchange of voice signals between two telephones over the

same network. The key elements of the model are

Source. This device generates the data to be transmitted; examples are telephones

and personal computers.

FIGURE 1.1 Simplified communications model.

*Transmitter. Usually, the data generated by a source system are not transmitted

directly in the form in which they were generated. Rather, a transmitter

transforms and encodes the information in such a way as to produce electromagnetic

signals that can be transmitted across some sort of transmission system.

For example, a modem takes a digital bit stream from an attached device

such as a personal computer and transforms that bit stream into an analog signal

that can be handled by the telephone network.

*Transmission System. This can be a single transmission line or a complex network

connecting source and destination.

* Receiver. The receiver accepts the signal from the transmission system and

converts it into a form that can be handled by the destination device. For

example, a modem will accept an analog signal coming from a network or

transmission line and convert it into a digital bit stream.

* Destination. Takes the incoming data from the receiver.

This simple narrative conceals a wealth of technical complexity. To get some

idea of the scope of this complexity, Table 1.1 lists some of the key tasks that must

be performed in a data communications system. The list is somewhat arbitrary: Elements

could be added; items on the list could be merged; and some items represent

several tasks that are performed at different "levels" of the system.

TABLE 1.1 Communications tasks.

Transmission system utilization Addressing

Interfacing Routing

Signal generation Recovery

Synchronization Message formatting

Exchange management Security

Error detection and correction Network management

Flow control

The first item, transmission system utilization, refers to the need to make efficient

use of transmission facilities that are typically shared among a number of communicating

devices. Various techniques (referred to as multiplexing) are used to

allocate the total capacity of a transmission medium among a number of users.

Congestion control techniques may be required to assure that the system is not

overwhelmed by excessive demand for transmission services.

In order to communicate, a device must interface with the transmission system.

All the forms of communication discussed in this lesson depend, at bottom, on

the use of electromagnetic signals propagated over a transmission medium. Thus,

once an interface is established, signal generation is required for communication.

The properties of the signal, such as form and intensity, must be such that they are

(1) capable of being propagated through the transmission system, and (2) interpretable

as data at the receiver.

Not only must the signals be generated to conform to the requirements of the

transmission system and receiver, but there must be some form of synchronization

between transmitter and receiver. The receiver must be able to determine when a

signal begins to arrive and when it ends. It must also know the duration of each signal

element.

Beyond the basic matter of deciding on the nature and timing of signals, there

are a variety of requirements for communication between two parties that might be

collected under the term exchange management. If data are to be exchanged in both

directions over a period of time, the two parties must cooperate. For example, for

two parties to engage in a telephone conversation, one party must dial the number

of the other, causing signals to be generated that result in the ringing of the called

phone. The called party completes a connection by lifting the receiver. For data processing

devices, more will be needed than simply establishing a connection; certain

conventions must be decided upon. These conventions may include whether both

devices may transmit simultaneously or must take turns, the amount of data to be

sent at one time, the format of the data, and what to do if certain contingencies, such

as an error, arise.

The next two items might have been included under exchange management,

but they are important enough to list separately. In all communications systems,

there is a potential for error; transmitted signals are distorted to some extent before

reaching their destination. Error detection and correction are required in circumstances

where errors cannot be tolerated; this is usually the case with data processing systems.

For example, in transferring a file from one computer to another, it is

simply not acceptable for the contents of the file to be accidentally altered. Flow

control is required to assure that the source does not overwhelm the destination by

sending data faster than they can be processed and absorbed.

Next, we mention the related but distinct concepts of addressing and routing.

When a transmission facility is shared by more than two devices, a source system

must somehow indicate the identity of the intended destination. The transmission

system must assure that the destination system, and only that system, receives the

data. Further, the transmission system may itself be a network through which various

paths may be taken. A specific route through this network must be chosen.

Recovery is a concept distinct from that of error correction. Recovery techniques

are needed in situations in which an information exchange, such as a data

base transaction or file transfer, is interrupted due to a fault somewhere in the system.

The objective is either to be able to resume activity at the point of interruption

or at least to restore the state of the systems involved to the condition prior to the

beginning of the exchange.

Message formatting has to do with an agreement between two parties as to the

form of the data to be exchanged or transmitted. For example, both sides must use

the same binary code for characters.

Frequently, it is important to provide some measure of security in a data communications

system. The sender of data may wish to be assured that only the

intended party actually receives the data; and the receiver of data may wish to be

assured that the received data have not been altered in transit and that the data

have actually come from the purported sender.

Finally, a data communications facility is a complex system that cannot create

or run itself. Network management capabilities are needed to configure the system,

monitor its status, react to failures and overloads, and plan intelligently for future

growth.

Thus we have gone from the simple idea of data communication between

source and destination to a rather formidable list of data communications tasks. In

this lesson, we further elaborate this list of tasks to describe and encompass the

entire set of activities that can be classified under data and computer communications.