通信英文文献翻译-英语论文

1、通信英文文献翻译-英语论文通信英文文献翻译切换电路的连接设置的结果是保留了从发送端到接收端的包所通过的线路带宽。其它的特性这一就是所有的包都沿同一个线路意味着如果包传输超过了时序那它将不能到达接收端.由于没有路径用来进行包交换，所以要想不同的包沿不同的路径传送就是能依靠网络的本文源自六维论文网条件了。这样包即便是超过了时序它们也有可能到达的。包交换技术比线路交换技术更能容错。事实上，这也是它被发明的理由！当一个交换开头被拉下，则所有使用它的线路将被切断并且其余的包将不能再被传送。此时如果在进行包交换，包将在已选择的路径上停止交换。预先建立一条路径也开创预先保留带宽的可能性。如果带宽被保留，那么

2、当一个包到达时，通过保留的带宽它可以被立即发送出去。假如没有带宽被保留用于包交换，那么所有的包将不得不等待着被转发。预先保留带宽意条线路那么当这个用户没有数据传送时，则这些保留下来的带宽将被浪费而不能用于其它用户的传输。而包交换则不会浪费带宽，因此它在系统宽带方面的应用更有前景。理解电路交换和包交换的这种商业差别是极其重要的。这种商业差别是在质保服务和浪费资源相对不质保服务和不浪费资源的。包交换是利用存储器并向前发送的。包是先存储在路由器的内存里然后再发送到下一个路由器的。和电路交换一样信息位也是连续不断地通过导线传输的。存储转发技术增加了延迟时间。另一个不同是电路交换是完全透明的。发送者和接

3、收者可以使用他们想要的任一比特速率，格式和帧同步.这通信公司不知道也不关心这的。而在包交换中载体决定了基本的参数的。用一个简单比喻它们就像一个是公路一个是铁路。在以前，用户决定介质的大小，速度和特性。而后来这些都由通信公司做了的。这些都是透明的它允许声音，数据和 共同存在于 系统里。电路交换和包交换的最后一个不同点是计费方式。电路交换的计费是按照传统的距离和时间的。比如移动 除打国际 外距离是不列入计算的，而时间也只是列入一个简单的计算(例:拔打2000分钟 比1000分钟 花费的多并且晚上或周末也比平时来得便宜)。时间对于包交换来说是没有问题的，但是传输流量却是个问题。ISPs为家庭用户的计

4、费方式基于按一个月的流量的一半的，这是因为它们用得相对较少并且他们的顾客也很好理解这种计费方式。但是主干网络中心的收费是基于它们的流量的。这些不同列在图2-40里。项目 电路交换 包交换呼叫设置 需要 不需要专用物理线路 是 否每个包沿相同路径 是 否包的到达中有规则的 是 否是否会致命的崩溃 是 否带宽利用 固定 动态分配可能的时间堵塞 在设置时 在每个包时可能浪费带宽 是 否以存储转发进行发送 否

5、60;是透明性 是 否收费 按分钟 按包图240：比较电路交换网络和包交换网络的不同电路交换和包交换是两者都是非常重要的，我们将立即在详细的资料里提及并描述它们之间各种不同的技术。2.6 移动 系统    传统的 的系统将不能满足用户的大量增长（即有一天实现了终端到终端的光纤连接）。人类在进步，人们期望能在飞机上，汽车里，船上，足球场甚至在公园里跑步的时候也能打 。在近几年人们更是期望能在更多的地方发电子邮件和上网冲浪。因此有很多的人关心无线 技术。在下面的章节我们将用详细的资料来学习这些主题。  

6、60; 无线 可以分为基本的两种：无绳 和移动 （有时也叫蜂窝 ）。无绳 设备是由痤机和听筒组成它们被安装在用户的家里。它们从不被应用于网络，所以我们没有办法做进一步的检测。取而代之，我们将把焦点放在在声音和数据通信上有广泛应用的移动系统上。    移动 经历了三个不同的发展阶段，每个阶段都有不同的技术：1 模拟声音2 数字声音3 数字声音和数据（互联网，电子邮件，etc.）虽然我们主要讨论这系统技术，但是我们也有必要注意政府和市场对它的巨大影响。第一个移动 系统是在美国由受美国国家联邦通讯委员会(FCC)委托的美国 电报公司(AT&

7、amp;T)设计的。结果在整个美国就有了一个简单的(模拟)系统并且移动 开始在加利福尼亚上市同时也在纽约得到应用。相反的，当移动 来到欧洲时，由于每个国家都设计了自己的系统，所以结局很惨败。当数字技术到来时，欧洲从前面的失败中吸收教训，政府和邮政制定了简单系统(GSM)的统一标准，因此移动 在欧洲的任何一个地方都是通用的。此时，美国政府决定不把标准商业化因而它失去了数字市场。这个决定导致不同的设备生产商生产出不同的移动 。结果现在美国有两不可兼容的数字移动 在使用(加强了一家就会削弱另一家)。即使美国拥有移动 的所有权，但是在应用上欧洲要远远超过美国的。原因之一就是整个欧洲拥有相同的简单系统。

8、然而更多的是由于美国和欧洲的基本 数量不同。在美国移动 和固定 是共用的，因此对于用户没有办法区分(212)234-5678到底是固定 (拨打是便宜的或者是免费的)还是移动 (拨打是昂贵的)。为了使人们从使用 中获益， 公司决定为引入呼叫而花自己的钱制造移动 。结果导致，很多人怕由于呼叫而收到一大笔帐单而不愿意买移动 。在欧洲移动 有特殊的区号(如800,900之类的数字)，所以它很快就得到了认可。因而在一般的“叫方付费”的原则在欧洲也被应用于移动 了(除国际 分开收费外)。需要很多手续。谁付费谁使用，他们预先存入，比如20或50 euro并且在快用完的时候可以用PIN码进行再充值。结果在欧洲

9、几乎所有的青年和小孩都有一个(预付费的)移动 ，这样他们的父母就可以准确的定位他们而不用去担心孩子们会用掉一大笔帐的。如果仅在偶而使用移动 ，那么它的使用是基本免费的因为这里有月租或者预先支付呼叫。2.6.1 第一代移动 ：模拟声音    移动 充分体现了政治和市场的特征。让我们来看一下它的技术吧。它是从简单的系统开始的。在20世纪的初，移动无线 被偶而应用于海上或军事通信中。在1946年，第一个基站被安置在。这个系统被安置在高层建筑上，它使用一个简单的大的发送机并且有了用于发送和接收的简单的通道。像这样的系统称做被动式系统，从1950开始它就被安装在很多的城市了

10、。可较验的收音机，出租车，警车还有电视也广泛采用这种技术。在1960年，IMTS（改良的移动 系统）被发明。它仍然使用高达200瓦的发送器，发送器被安置在山上，不过此时已经有了两各不同的频率了，一种用来发送，另一种用来接收，所以这个呼叫按钮就可不要了。相比信号出站移动 的信号入站是通过不同的通道的，因此移动用户不会互相听到(不像应用在出租车上被叫系统)。    IMTS支持从150 MHz到 450 MHz的23条通道。由于通道太少，用户经常要等很长时间才能听到拔号音。也由于小山上的发送器的功率太大，为了不互相干扰邻近的系统不得不与它相距几百千米。总而言之，这有限

11、的容量使用这系统有点不合实际应用。1360通信英文文献翻译The result of the connection setup with circuit switching is the reservation of bandwidth all the way from the sender to the receiver. All packets follow this path. Among other properties, having all packets follow the same path means that they cannot arrive out of order

12、. With packet switching there is no path , so different packets can follow different paths, depending on network conditions at the time they are sent. They may arrive out of order.Packet switching is more fault tolerant than circuit switching. In fact, that is why it was invented. If a switch goes d

13、own, all of the circuits using it are terminated and no more traffic can be sent on any of them. With Packet switching, packets can be routed may have to wait their turn to be forwarded.Having bandwidth reserved in advance means that no congestion can occur when a packet shows up (unless more packet

14、s show up than expected).On the other hand, when an attempt is made to establish a circuit, the attempt can fail due to congestion. Thus, congestion can occur at different times with circuit switching(at setup time) and packet switching(when packets are sent).If a circuit has been reserved for a par

15、ticular user and there is no traffic to send, the bandwidth of that circuit is wasted. It cannot be used for other traffic. Packet switching does not waste bandwidth and thus is more efficient form a system-wide perspective. Understanding this trade-off is crucial for comprehending the difference be

16、tween circuit switching and packet switching. The trade-off is between guaranteed service and wasting resources versus not guaranteeing service an本文源自六维论文网d not wasting resources.Packet switching uses store-and-forward transmission. A packet is accumulated. in a routers memory, then sent on to the n

17、ext router. With circuit switching, the bits just flow through the wire continuously. The store-and-forward technique adds delay.Another difference is that circuit switching is completely transparent.The sender and receiver can use any bit rate, format, or framing method they want to. The carrier do

18、es not know or care. With packet switching, the carrier determines the basic parameters. A rough analogy is a road versus a railroad. In the former, the user determines the size, speed, and nature of the vehicle; in the latter, the carrier does. It is this transparency that allows voice, data, and f

19、ax to coexist within the phone system.A final difference between circuit and packet switching is the charging algorithm. With circuit switching, charging has historically been based on distance and time. For mobile phones, distance usually does not play a role, except for international calls, and ti

20、me plays only a minor role(e.g., a calling plan with 2000 free minutes costs more than one with 1000 free minutes and sometimes night or weekend calls are cheaper than normal).With packet switching, connect time is not an issue, but the volume of traffic sometimes is. For home users, ISPs usually ch

21、arge a flat monthly rate because it is less work for them and their customers can understand this model easily, but backbone carriers charge regional networks based on the volume of their traffic. The differences are summarized in Fig.2-40.通信英文文献翻译Item Circuit Switched Packet SwitchedCall

22、setup Required Not neededDedicated physical path Yes NoEach packet follows the same route Yes NoPackets arrive in order Yes NoIs a switch crash fatal Yes NoBandwidth available Fixed DynamicTime of possible congestion At setup time 

23、;On every packetPotentially wasted bandwidth Yes NoStore-and-forward transmission No Yesused in detail.2.6 THE MOBILE TELEPHONE SYSTEMThe traditional telephone system(even if it some day gets multi-gigabit end-to-end fiber) will still not be able to satisfy a growing group of use

24、rs :people on the go .People now expect to make phone calls from airplanes ,cars ,swimming pools ,and while jogging in the park .Within a few years they will also expect to send e-mail and surf the Web from all these locations and more consequently ,there is a tremendous amount of interest in wirele

25、ss telephony .In the following sections we will study this topic in some detail.Wireless telephones come in two basic varieties :cordless phones and mobile phones(sometimes called cell phones),Cordless phones are devices consisting of a base station and a handset sold as a set for use within the hom

26、e. These are never used for networking ,so we will not examine them further .Instead we will concentrate on the mobile system ,which is used for wide area voice and data communication.Mobile phones have gone through three distinct generations ,with different technologies:1. Analog voice2. 

27、Digital voice3. Digital voice and data(Internet ,e-mail ,etc.).Although most of our discussion will be about the technology of these systems ,it is interesting to note how political and tiny marketing decisions can have a huge impact .The first mobile system was devised in the U.S. by AT&T

28、and mandated for the whole country by the FCC .As a result ,the entire U.S. had a single(analog) system and a mobile phone purchased in California also worked in New York .In contrast ,when mobile came to Europe ,every country devised its own syste本文源自六维论文网m ,which resulted in a fiasco.Europe learne

29、d from its mistake and when digital came around, the government-run PTTs got together and standardized on a single system(GSM),so any European mobile phone will word anywhere in Europe .By then ,the U.S. had decided that government should not be in the standardization business ,so it left digital to

30、 the marketplace .This decision resulted in different equipment manufacturers producing different kinds of mobile phones .As a consequence ,the U.S. now has two major incompatible digital mobile phone systems in operation(plus one minor one).Despite an initial lead by the U.S. , mobile phone ownersh

31、ip and usage in Europe is now far greater than in the U.S. Having a single system for all of Europe is part of the reason ,but there is more .A second area where the U.S. and Europe differed is in the humble matter of phone numbers .In the U.S. mobile phones are mixed in with regular (fixed) telepho

32、nes .Thus ,there is no way for a caller to see if ,say ,(212)234-5678 is a fixed telephone (cheap or free call) or a mobile phone (expensive call).To keep people from getting nervous about using the telephone ,the telephone companies decided to make the mobile phone owner 通信英文文献翻译pay for incoming ca

33、lls .As a consequence ,many people hesitated to buy a mobile phone for fear of running up a big bill by just receiving calls .In Europe ,mobile phones have a special area code (analogous to 800 and 900 numbers) so they are instantly recognizable .Consequently , the usual rule of “caller pays” also a

34、pplies to mobile phones in Europe (except for international calls where costs are split).A third issue that has had a large impact on adoption is the widespread use of prepaid mobile phones in Europe (up to 75% in some areas ).These can be purchased in many stores with no more formality than buying

35、a radio .You pay and you go .They are preloaded with ,for example ,20 or 50 euro and can be recharged (using a secret PIN code ) when the balance drops to zero . As a consequence , practically every teenager and many small children in Europe have (usually prepaid ) mobile phones so their parents can

36、 locate them ,without the danger of the child running up a huge bill .If the mobile phone is used only occasionally ,its use is essentially free since there is no monthly charge or charge for incoming calls .2.6.1 First-Generation Mobile Phones :Analog Voice    Enough about the politics and marketing aspects of mobile phones .Now let us look at the technology ,starting with the earliest system .Mobile radiotelephones were used s