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Chapter 7
Fundamentals of Data Communications

I submit to the public a small machine by my invention, by means of which you alone may, without any effort, perform all the operations of arithmetic, and may be relieved of the work which has often times fatigued your spirit.

Blaise Pascal, 1623–1662

Little did the humble Pascal understand the complexity of computing systems which would develop over the next 300 or so years, much less the requirement for networking them. The ability to communicate with a remote calculating machine first was demonstrated in 1940 by Dr. George Stibitz at Dartmouth College. Stibitz explained, and demonstrated the use of a calculator newly developed by Bell Labs. The calculator was located in New York City and communications took place over standard telegraph lines [7-1]. Contemporary computers, even the most lowly Personal Computer (PC), are shipped from the factory with communications capabilities which would astound the experts of even ten years past.

Up to this point, we have explored the world of voice communications. During that process, we learned that many of the devices and circuits in the Public Switched Telephone Network are digital in nature. In large part the contemporary PSTN is a data network, transporting and switching voice data. This chapter will introduce the basic concepts of data data. The balance of this book largely and truly is data-oriented.

In order to comprehend the intricacy of contemporary data communications systems and networks, it is necessary to develop a solid understanding of the certain basic concepts. These definitions and concepts extend across all technologies and service offerings, from the historical to even the most contemporary.

This chapter first addresses the concept of functional domains, as we did in Chapter 6 which dealt with the world of PSTN voice communications. Terminal equipment, communications equipment and communication software all will be explained, with detailed discussion of modems and DSUs/CSUs. The concept of protocols will be examined at length, with discussion of basic protocol dimensions and issues. Computer network architectures will be explored, with emphasis on IBM’s SNA and OSI, the international standard reference model. Finally, we will pause to consider the importance of security in data systems and networks.

Functional Domains

Functional domains comprise the spheres of influence exerted by the various network elements which perform specific tasks in a data network. Data terminal equipment, data communications equipment, communications software, switches and transmission facilities all are physical elements of such a network, with each performing specific functions, and with all supported by a signaling and control system. Since the general nature of circuit switches was explored in Chapters 2 and 6, that discussion will not be repeated, nor will signaling systems, which were discussed in Chapter 6.

Data Terminal Equipment (DTE)

The data equivalent of CPE (Customer Premise Equipment) in the voice world, DTE is the computer transmit and receive equipment, including a wide variety of dumb terminals, or terminals without embedded intelligence in the form of programmed logic. Such terminals are devices that merely provide a user interface to a more capable host computer; examples of dumb terminals include the Hewlett-Packard HP2521P and the Televideo 950. Semi-intelligent terminals (IBM 317x and 327x) possess a limited amount of intelligence, allowing them to perform certain, limited processes, independent of the intelligence contained in the host. Intelligent terminals generally are in the form of personal computers (PCs), which are networked to a host computer. Such devices are highly capable in their own right, although and in this context, they often are linked across a network to an even more capable host. At the top of the terminal food chain are client workstations, highly intelligent and capable devices that access a more capable server in a client/server environment. In such an operating environment, the clients’ requirements for access to files, applications and network communications software are satisfied by a server which typically is accessed across a LAN. As a result, the client workstation can perform certain appropriate functions (e.g., screen formatting) related to the specific user task at hand, while the server’s memory and processing power is dedicated to the performance of tasks that are accomplished more effectively on a centralized basis.

DTE also is in the form of host computers such as mainframes and midrange (minicomputer) computers. Host computers, also known as host nodes, are highly capable devices with substantial processing power and storage memory. Hosts also, at least theoretically, are carefully administered to ensure that they operate successfully and reliably. They also serve as effective information repositories, with the data backed up and archived on external storage media such as magnetic tapes.

Data Communications Equipment (DCE)

Also known as Data Circuit Terminating Equipment (DCTE), DCE is the equipment that interfaces the DTE to the network, in the process resolving any issues of incompatibility between those domains. Incompatibility issues can include digital versus analog, voltage level, transmission speed and bit density. DCE includes modems, DSUs and CSUs, and Front-End Processors (FEPs), all of which will be discussed in greater detail later in this chapter.


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