in a few thousand meters of one something other than numeric in the first four characters, another. It can consist of subnetworks linked together in it would consider that an error. If the sending program Node C Node A Node B Node D Node E Figure 3.1-3 Sample of broadcast channel example. CHAPTER 3.1 Data and voice traffic Figure 3.1-4 Computer protocol example. stopped sending before all of the characters were different. So layers of processes have corresponding received, that would also be considered an error. layers of protocols. Early data communications programs were This concept of layering gives rise, in its simplest form monolithic-that is, a single large program provided many of two layers of services and two layers of protocols, to services and communicated with other programs with the service/protocol model shown in Figure 3.1-6. a single "low-level" protocol (see Figure 3.1-5). This did not Learning this basic model is essential to understanding work well. The programs were hard to change and the the remainder of the material in this course. protocols would not work well as new technology came along. These problems led to the development of layering Layered communications systems concepts. The simplest service/protocol model has two layers, but A layer is simply a program or set of programs that the ideas it illustrates can be extended to an arbitrary provides services to the next higher layer and uses number of layers. In fact, real data communications services of the next lower layer. A program that resides at systems typically involve from three to seven layers of the highest layer will typically provide many sophisti- services and protocols. cated services to the user, but most of these services are actually implemented, directly and indirectly, by the Peer communications lower layers. Because a program provides services only to the layer When two peer programs happen to be running on the above it and uses services only of the layer below it, same machine, communication will be through the a chang