Access Lines FIGURE 72.5 Generic computer communication network. Many researchers at this time had computing resources available under the scenario described in the first paragraph above. The idea of interconnecting the computers to extend the reach of these researchers to other computers developed. In addition, the interconnection of the computers would provide for comn among the researchers themselves. In order to investigate the feasibility of providing the interconnectivity Inticipated for the future using a new technology called packet switching, the Advanced Research Projects Agency(ARPA)of the Department of the Army sponsored a networking effort, which resulted in the computer communication network called the arpanet. The end results of the ARPA networking effort, its derivatives, and the early initiatives of many compar ch as at&T, DATAPOINT, DEC, IBM, and NCR have been far-reaching in the extreme. Any finitely delimited discussion of the accomplishments of those efforts would appear to underestimate their impact on our lives We will concentrate on the most visible product of these efforts, which is a collection of programs that allows applications running in different computers to intercommunicate. Before turning to our discussion of the software, however, we will provide a brief description of a generic computer communication network. Figure 72.5 shows a diagram of a generic computer communication network. The most visible components of the network are the terminals, the access lines, the trunks, and the switching nodes. Work is accomplished when the users of the network, the terminals, exchange messages over the network. The terminals represent the set of communication terminating equipment communicating over the network. quipment in this class includes, but is not limited to, user terminals, general-purpose computers, and database systems. This equipment, either through software or through human interaction, provides the functions equired for information exchange between pairs of application programs or between application programs and people. The functions include, but are not limited to, call set-up, session management, and message transmission control. Examples of applications include electronic mail transfer, terminal-to-computer connec tion for time sharing or other purposes, and terminal-to-database connections. Access lines provide for data transmission between the terminals and the network switching nodes. These connections may be set up on a permanent basis or they may be switched connections, and there are numerous transmission schemes and protocols available to manage these connections. The essence of these connections, however, from our point of view is a channel that provides data transmission at some number of bits per second ) called the channel capacity, C. The access line capacities may range from a few hundred bits per second to in excess of millions of bits per second, and they are usually not the same for all terminating equipments of a given network. The actual information-carrying capacity of the link depends upon the protocols employed to effect the transfer; the interested reader is referred to Bertsekas and Gallagher [1987], especially Chapter 2, for a general discussion of the issues involved in transmission of data over communication links Trunks, or internodal trunks, are the transmission facilities that provide for transmission of data between pairs of communication switches. These are analogous to access lines, and, from our point of view, they simply provide a communication path at some capacity, specified in bits per second. e 2000 by CRC Press LLC© 2000 by CRC Press LLC Many researchers at this time had computing resources available under the scenario described in the first paragraph above. The idea of interconnecting the computers to extend the reach of these researchers to other computers developed. In addition, the interconnection of the computers would provide for communication among the researchers themselves. In order to investigate the feasibility of providing the interconnectivity anticipated for the future using a new technology called packet switching, the Advanced Research Projects Agency (ARPA) of the Department of the Army sponsored a networking effort, which resulted in the computer communication network called the ARPANET. The end results of the ARPA networking effort, its derivatives, and the early initiatives of many companies such as AT&T, DATAPOINT, DEC, IBM, and NCR have been far-reaching in the extreme. Any finitely delimited discussion of the accomplishments of those efforts would appear to underestimate their impact on our lives. We will concentrate on the most visible product of these efforts, which is a collection of programs that allows applications running in different computers to intercommunicate. Before turning to our discussion of the software, however, we will provide a brief description of a generic computer communication network. Figure 72.5 shows a diagram of a generic computer communication network. The most visible components of the network are the terminals, the access lines, the trunks, and the switching nodes. Work is accomplished when the users of the network, the terminals, exchange messages over the network. The terminals represent the set of communication terminating equipment communicating over the network. Equipment in this class includes, but is not limited to, user terminals, general-purpose computers, and database systems. This equipment, either through software or through human interaction, provides the functions required for information exchange between pairs of application programs or between application programs and people. The functions include, but are not limited to, call set-up, session management, and message transmission control. Examples of applications include electronic mail transfer, terminal-to-computer connec￾tion for time sharing or other purposes, and terminal-to-database connections. Access lines provide for data transmission between the terminals and the network switching nodes. These connections may be set up on a permanent basis or they may be switched connections, and there are numerous transmission schemes and protocols available to manage these connections. The essence of these connections, however, from our point of view is a channel that provides data transmission at some number of bits per second (bps), called the channel capacity, C. The access line capacities may range from a few hundred bits per second to in excess of millions of bits per second, and they are usually not the same for all terminating equipments of a given network. The actual information-carrying capacity of the link depends upon the protocols employed to effect the transfer; the interested reader is referred to Bertsekas and Gallagher [1987], especially Chapter 2, for a general discussion of the issues involved in transmission of data over communication links. Trunks, or internodal trunks, are the transmission facilities that provide for transmission of data between pairs of communication switches. These are analogous to access lines, and, from our point of view, they simply provide a communication path at some capacity, specified in bits per second. FIGURE 72.5 Generic computer communication network