原文部分In most wide area networks, the subnet consists of two distinct components: transmission lines and switching elements. Transmission lines (also called circuits, channels, or trunks) move bits between machines. The switching elements are specialized computers used to connect two or more transmission lined. When data arrive on an incoming line , the switching element must choose an outgoing line to forward them on. Unfortunately, there is no standard terminology used to name these computers. They are variously called packet switching exchanges, among other things. As a generic term for the switching computers, will use exists here. In this model, shown in fig .9 .5, each host is generally connected to a LAN on a router. The collection of communication lines and routers (but not the hosts) form the subnet. An aside about the term “subnet” is worth making. Originally, its only meaning was the collection of routers and communication lines that moved packets from the source host to the destination host. However, network addressing. Hence the term has a certain ambiguity about it. Unfortunately, no widely-used alternative exists for its initial meaning, so with some hesitation we will use it in both senses. From the context, it will always be clear which is meant. In most WANs, the network contains numerous cables or telephone lines, each one connecting a pair of routers. If two routers that do not share a cable nevertheless wish to communicate, they must do this indirectly, via other routers. When a packet is sent from one router to another via one or more intermediate routers, the packet is received at each intermediate router in its entirety, stored there until the required output line is free, and then forwarded. A subnet using this principle is called a point-to-point, store-and-forward, or packet-switched subnet. Nearly all wide area networks (except those using satellites) have store-and-forward subnets. When the packets are small and all the same size, they are often called cells. When a point-to-point subnet is used, an important design issue is what the router interconnection topology should look like. Fig.9.6 shows several possible topologies. Local networks that were designed as such usually have a symmetric topology. In contrast, wide area networks typically have irregular topologies. A second possibility for a WAN is a satellite or ground radio system. Each router has an antenna through which it can send and receive. All routers can hear the output from the satellite, and in some cases they can also hear the upward transmissions of their fellow routers to the satellite as well. Sometimes the routers are connected to a substantial point-to-point subnet, with only some of them having a satellite antenna. Satellite networks are inherently broadcast and are most useful when the broadcast property is important. 9.2.4 Wireless Networks Mobile computers, such as notebook computers and personal digital assistants(PDA), are the fastest-growing segment of the computer industry .Many of the owners of these computers have desktop machines on LANs and WANs back at the office and want to be connected to their home base even when away from home or a route . Since having a wired connection is impossible in cars and airplanes , there is a lot of interest in wireless networks .On this section we will briefly introduce this topic. Actually, digital wireless communication is not a new idea. As early as 1901, the Italian physicist Guglielmo Marconi demonstrated a ship-to-shore wireless telegraph using Morse Code (dots and dashes re binary, after all ). Modem digital wireless systems have better performance , but the basic idea is the same. Wireless networks have many uses. A common one is the portable office. People on the road often want to use their portable electronic equipment to send and receive telephone calls, faxes, and electronic mail, read remote files , login on remote machines, and so on, and do this from anywhere on land ,sea, or air. Wireless networks are lf great value to fleets of trucks, taxis, buses, and repairpersons for keeping in contact with home. Another use is for rescue workers at disaster sites (fires, floods, earthquakes, etc.)where the telephone system has been destroyed. Computers there can send messages, keep records, and so on. Finally, wireless networks are important to the military. If you have to be able fight a war anywhere on earth on short notice, counting on using the local net working infrastructure is probably not a good idea. It is better to bring your own . Although wireless networking and mobile computing are often related, they are not identical, as fig. 9. 7 shows. Portable computers are sometimes wired. For example, if a traveler plugs a plugs a portable computer into the telephone jack in a hotel, we have mobility without a wireless network. Another example is someone carrying a portable computer along as he inspects a tra