ervices from more than one process on the lower level and thus from more than one physical Data networks link. Note that the two lowest-level protocols do not need to be identical protocols; in fact, often they will not A data network connects nodes, some of which are hosts be. Because higher levels can pass more complex mes- to which terminal nodes attach, in two different ways: sages, they can include routing information, making it point-to-point and broadcast. Point-to-point networks possible for data to flow across more than one commu- fall into two classes: circuit-switched networks, in which nication link, and making large networks possible. So, an a connection is formed between the nodes, as in important service provided by lower-level protocols in a telephone network; and packet-switched or connec- any of the networking systems we will study (which are tionless networks, in which packets of data or datagrams all layered) is to handle the routing of messages between are passed from node to node until they reach their computers that are not connected by a physical link. destination, like telegrams. All of the nodes in a broad- In a system with a layered architecture, programs at cast network share the same channel, and the network a high level deal with more complicated data but don't protocol controls access to the channel and avoids or have to deal with implementation details and routine recovers from the collisions that occur when more than tasks. For example, in a network the routing of messages one node tries to use the channel at the same time. is done by lower levels, SO upper-level programs need not LANs extend across a single site and consist of one or be aware of it. The data flow only through the highest more subnets, which are usually, but not necessarily, level that is concerned with handling them. So when homogeneous. MANs, the metropolitan area networks CHAPTER 3.1 Data and voice traffic now evolving, will be developed by data carriers to facilities of the long-distance carriers to construct 