Data Communication Networking
We begin our study with a simple model of communications, illustrated by the block diagram in figure 1.a
The fundamental purpose of a communications system is the exchange of data between two parties. Another example is the exchange of voice signals between two telephones over the same network. The key elements of the model are as follows.
Source.
This device generates the data to be transmitted, examples are telephones and personal computers.
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 bitstream from an attached device such as a personal computer and transforms that bitstream 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 bitstream.
Destination.
Takes the incoming data from the receiver.
List of the key tasks that must be performed in a data communications system.
Addressing
Routing
Recovery
Message formatting
Security
Network management
Transmission system utilization
Interfacing
Synchronization
Flow control
Exchange management
Signal generation
To communicate, a device must interface with the transmission system. All the forms of communication discussed in this book depend 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 the signal is capable of being propagated through the transmission system, and 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 also there must be some form of synchronization between the transmitter and the 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.
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 system involved to the condition prior to the beginning of the exchange.
Finally, a data communications facility is a complex system that cannot create or run itself. Networking 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 the data communication between source and destination to a rather formidable list of data communications tasks. In this book, we elaborate on this list of tasks to describe and encompass the entire set of activities that can be classified under data and computer communications.
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