机械毕业翻译输电线路和开关元件.docx

1、机械毕业翻译输电线路和开关元件原文部分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

2、 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

3、 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

4、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 hes

5、itation 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

6、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

7、 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

8、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

9、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 substant

10、ial 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 f

11、astest-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 , the

12、re 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

13、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,

14、 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 sit

15、es (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

16、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 comput

17、er 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 train for technical problems . Here a long cord can trail along behind (vacuum cleaner model) . On the other hand, some wireless computer

18、s are not portable. An important example here is a company that owns an older building that does not have network cabling installed and wants to connect its computers . Installing a wireless LAN may require little more than buying a small box with some electronics and setting up some antennas . This

19、 solution may be cheaper than wiring the building. Although wireless LANs are easy to install, they also have some disadvantages. Typically they have a capacity of 1 to 2 Mbps , which is much slower than wired LANs. The error rates are often much higher, too , and the transmissions from different co

20、mputers can interfere with one another. But of course, there are also the true mobile, wireless applications, ranging from the portable office to people walking around a store with a PDA doing inventory. At many busy airports, car rental return clerks work out in the parking lot with wireless portab

21、le computers . They type in a built-in printer, calls the main computer, gets the rental information, and prints out the bill on the spot. Wireless networks come in many forms. Some universities are already installing antennas all over campus to allow students to sit under the trees and consult the

22、librarys card catalog. Here the computers communicate directly with the wireless LAN in digital form. Another possibility is using a cellular ( i. e. portable ) telephone with a traditional analog modem. Direct digital cellar service , called CDPD (Cellular Digital Packet Data ) is becoming avail ab

23、le in many cities . Finally, it is possible to have different combinations of wired and wireless networking . For example, in fig. 9. 8(a) , we depict an airplane with a number of people using modems and seat-back telephones to call the office. Each call is independent of the other ones. A much more

24、 efficient option, however, is the flying LAN of Fig.9.8(b) . Here each seat comes equipped with an Ethernet connector into which passengers can plug their computers. A single router on the aircraft maintains s radio link with some router on the ground, changing routers as it flies along . This conf

25、iguration is just a traditional LAN, except that its connection to the outside world happens to be a radio link instead of a hardwired line. While many people believe that wireless portable computers are the wave of the future, at least one dissenting voice has been heard. Bob Metcalfe , the invento

26、r of Ethernet, has written: “Mobile wireless computers are like mobile pipeless bathroomsportapotties. They will be common on vehicles, and at construction sites, and rock concerts . My advice is to wire up your home and stay there.” Will most people follow Metcalfes advice? Time will tell.9. 2. 5 I

27、nter networks Many networks exist in the world , often with different hardware and software. People connected to one network often want to communicate with people attached to a different one . This desire requires connecting together different , and frequently incompatible networks, sometimes by usi

28、ng machines called gateways to make the connection and provide the necessary translation, both in terms of hardware and software. A collection of interconnected networks is called an internetwork or just internet. A common form of internet is a collection of LANs connected by a WAN. In fact, if we w

29、ere to replace the label “subnet” in Fig.9.5 by “WAN”, nothing else in the figure would have to change. The only real distinction between a subnet and a WAN in this case is whether or not hosts are present. If it contains both routers and hosts with their own users , it is a WAN. To avoid confusion,

30、 please note that the word “internet” will always be used in a generic sense. In contrast, the Internet, the Internet (note uppercase I) means a specific world wide internet that is widely used to connect universities, government offices, companies, and of late, private individuals. Subnets , networ

31、ks , and internetworks are often confused. Subnet makes the most sense in the context of a wide area network, where it refers to the collection of routers and communication lines owned by the network operator, for example, companies like America Online and CompuServe. As an analogy, the telephone sy

32、stem consists of telephone switching offices connected to each other by high-speed lines. These lines and equipment, owned and managed by the telephone company , form the subnet of the telephone system. The telephones themselves (the hosts in this analogy) are not part of the subnet. The combination

33、 of a subnet and its hosts forms a network. In the case of a LAN , the cable and the hosts form the network . There really is no subnet. An internetwork is formed when distinct networks are connected together. In our view, connecting a LAN and a WAN or connecting two LANs forms an internetwork, but there is little agreement in the industry o