data communication presentation
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 is 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 on.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 seem 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 are the related but distinct concepts of addressing and routing. When more than two devices share a transmission facility, a source system must 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 database 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,such as the 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 receiver 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 actually come from the purported sender. Finally,a data communications facility is a complex system that cannot create or run itself.Network managementcapabilities 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 book,we elaborate this list of tasks to describe and encompass the entire set of activities that can be classified under data and computer communications.
