[1990a, 1990b, 1990c] covers a broad array of standards in this area. Other books that have been found to be pecially useful by practitioners are those by rose [ 1990] and Black [1992] The latest developments are, of course, covered in the current literature, conference proceedings, and the notes of standards meetings. A pedagogically oriented magazine that specializes in computer communications networks is IEEE Network, but IEEE Communications and IEEE Computer often also contain interesting articles communications include the IEEE INFOCOM and ACM SIGCOMM series, which are heate or .often in this area. ACM Communications Review, in addition to presenting pedagogically oriented articles, often presents very useful summaries of the latest standards activities. Major conferences that specialize in computer We will begin our discussion with a brief statement of how computer networking came about and a capsule description of the networks that resulted from the early efforts. Networks of this generic class, called wide-area networks(WANs), are broadly deployed today, and there are still a large number of unanswered questions with respect to their design. The issues involved in the design of those networks are basic to the design of most networks, whether wide area or otherwise. In the process of introducing these early systems, we will describe and contrast three basic types of communication switching: circuit, message, and packet. We will next turn to a discussion of computer communication architecture, which describes the structure of communication-oriented processing software within a communication processing system. Our discussion is limited to the International Standards Organization/Open Systems Interconnection(ISO/OSI) reference model(ISORM) because it provides a framework for discussion of some of the modern developments in communications in general and communication networking in particular. This discussion is necessarily sim plified in the extreme, thorough coverage requiring on the order of several hundred pages, but we hope our brief description will enable the reader to appreciate some of the issues Having introduced the basic architectural structure of communication networks, we will next turn to a discussion of an important variation on this architectural scheme: the local-area network(LAN). Discussion ecause it helps to illustrate what the the architecture of LANs illustrates how the ISORM can be adapted for specialized purposes. Specifically, early network architectures anticipate networks in which individual node pairs are interconnected via a single link onnections through the network are formed by concatenating node-to-node connections LAN architectures, on the other hand, anticipate all nodes being interconnected in some fashion over the same communication link(or medium). This, then, introduces the concept of adaption layers in a natural way. It also illustrates that if the services provided by an architectural layer are carefully defined, then the can be used to implement virtually any service desired by the user, possibly at the price of some ine After discussing LANS, we will conclude our article with a discussion of two of the variants in packet transmission technology: frame relay and a recent development in basic transmission technology called the asyn chronous transfer mode, which is a part of the larger broadband integrated services digital network effort. These technologies are likely to be important building blocks for the computer co ication networks of the future General Networking Concepts Data communication networks have existed since about 1950. The early networks existed primarily for the purpose of connecting users of a large computer to the computer itself, with additional capability to provide communications between computers of the same variety and having the same operating software. The lessons learned during the first twenty or so years of operation of these types of networks have been valuable in preparing ne way for modern networks. For the purposes of our current discussion, however, we will think of commu- nication networks as being networks whose purpose is to interconnect a set of applications that are implemented on hosts manufactured by possibly different vendors and managed by a variety of operating systems Networking capability is provided by software systems that implement standardized interfaces specifically designed for the exchange of information among heterogeneous computers. buring the late 1960s, many forward-looking thinkers began to recognize that significant computing resources(that is, supercomputers)would be expensive and unlikely to be affordable by many of the researchers needing this kind of computer power. In addition, they realized that significant computing resources would number of research sites, then the cost of the resource could be shared by its uscm tce could be shared by a not be needed all of the time by those having local access. If the computing resor e 2000 by CRC Press LLC© 2000 by CRC Press LLC [1990a, 1990b, 1990c] covers a broad array of standards in this area. Other books that have been found to be especially useful by practitioners are those by Rose [1990] and Black [1992]. The latest developments are, of course, covered in the current literature, conference proceedings, and the notes of standards meetings. A pedagogically oriented magazine that specializes in computer communications networks is IEEE Network, but IEEE Communications and IEEE Computer often also contain interesting articles in this area. ACM Communications Review, in addition to presenting pedagogically oriented articles, often presents very useful summaries of the latest standards activities. Major conferences that specialize in computer communications include the IEEE INFOCOM and ACM SIGCOMM series, which are held annually. We will begin our discussion with a brief statement of how computer networking came about and a capsule description of the networks that resulted from the early efforts. Networks of this generic class, called wide-area networks (WANs), are broadly deployed today, and there are still a large number of unanswered questions with respect to their design. The issues involved in the design of those networks are basic to the design of most networks, whether wide area or otherwise. In the process of introducing these early systems, we will describe and contrast three basic types of communication switching: circuit, message, and packet. We will next turn to a discussion of computer communication architecture, which describes the structure of communication-oriented processing software within a communication processing system. Our discussion is limited to the International Standards Organization/Open Systems Interconnection (ISO/OSI) reference model (ISORM) because it provides a framework for discussion of some of the modern developments in communications in general and communication networking in particular. This discussion is necessarily sim￾plified in the extreme, thorough coverage requiring on the order of several hundred pages, but we hope our brief description will enable the reader to appreciate some of the issues. Having introduced the basic architectural structure of communication networks, we will next turn to a discussion of an important variation on this architectural scheme: the local-area network (LAN). Discussion of this topic is important because it helps to illustrate what the reference model is and what it is not. In particular, the architecture of LANs illustrates how the ISORM can be adapted for specialized purposes. Specifically, early network architectures anticipate networks in which individual node pairs are interconnected via a single link, and connections through the network are formed by concatenating node-to-node connections. LAN architectures, on the other hand, anticipate all nodes being interconnected in some fashion over the same communication link (or medium). This, then, introduces the concept of adaption layers in a natural way. It also illustrates that if the services provided by an architectural layer are carefully defined, then the services can be used to implement virtually any service desired by the user, possibly at the price of some inefficiency. After discussing LANs, we will conclude our article with a discussion of two of the variants in packet switching transmission technology: frame relay and a recent development in basic transmission technology called the asyn￾chronous transfer mode, which is a part of the larger broadband integrated services digital network effort. These technologies are likely to be important building blocks for the computer communication networks of the future. General Networking Concepts Data communication networks have existed since about 1950. The early networks existed primarily for the purpose of connecting users of a large computer to the computer itself, with additional capability to provide communications between computers of the same variety and having the same operating software. The lessons learned during the first twenty or so years of operation of these types of networks have been valuable in preparing the way for modern networks. For the purposes of our current discussion, however, we will think of commu￾nication networks as being networks whose purpose is to interconnect a set of applications that are implemented on hosts manufactured by possibly different vendors and managed by a variety of operating systems. Networking capability is provided by software systems that implement standardized interfaces specifically designed for the exchange of information among heterogeneous computers. During the late 1960s, many forward-looking thinkers began to recognize that significant computing resources (that is, supercomputers) would be expensive and unlikely to be affordable by many of the researchers needing this kind of computer power. In addition, they realized that significant computing resources would not be needed all of the time by those having local access. If the computing resource could be shared by a number of research sites, then the cost of the resource could be shared by its users