【正文】 ical work is just beginning, and is advancing very swiftly towards a future online world in which bandwidth is essentially unlimited, reliable and lowcost. Circuit Switching and Packet Switching There are tow basic types of switching techniques: circuit switching and message switching. In circuit switching, a total path of connected made, and the path remains allocated to the sourcedestination pair (whether used or not) until it is released by the municating parties2. The switches, called circuit switches (or office exchange in telephone jargon), have no capability of storing or manipulating user?s data on their sage that finds its way through the work, seizing channels in the path as it proceeds4. Once the path is established, a return signal informs the source to begin transmission.. Direct transmission of the part of the sub. In message switching, the transmission unit is a welldefined block of data called a message. In addition to the text to be transmitted5, a message prises a header and a checksum. The header contains information regarding the source and destination addresses as well as other control information6。 the checksum is used for error control purposes. The switching element is a puter referred to as a message processor7, with processing and storage capabilities. Messages travel independently and asynchronously, finding their own way from source to destination8. First the message is transmitted from the bost to the message processor to which it is attached9. Once the message is entirely received, the message processor examines its header, and accordingly decides on the next outgoing channel on which to transmit it. If this selected channel is busy, the message waits in a queue until the channel bees free, at which time transmission begins. At the next message processor, the message is again received, stored, examined, and transmitted on some outgoing channel, and the same process continues until the message is delivered to its destination. This transmission technique is also referred to as the storeandforward transmission technipue. A variation of message switching is packet switching. Here the message is 90broken up into several pieces of a given maximum length, called packets. As with message switching, each packet contains a header and a checksum. Packets are transmitted independently in a storeandforward manner. With circuit switching, there is always an initial connection cost incurred in setting up the circuit. It is costeffective only in those situations where once the circuit is set up there is a guaranteed steady flow of information transfer to amortize the initial cost. This is certainly the case with voice munication in the traditional way, and indeed circuit switching is the technique used in the telephone system. Communication among puters, however, is characterized as bursty. Burstiness is a result of the high degree of randomness encountered in the messagegeneration process and the message size, and of the low delay constraint required by the user. The users and devices require the munication resources relatively infrequently。 but when they do, they require a relatively rapid response. If a fixed dedicated end –to –end circuit were to be set up connecting the end users ,then one must assign enough transmission bandwidth to the circuit in order to meet the delay constraint with the consequence that the resulting channel utilization is low .If the circuit of high bandwidth were set up and released at each message transmission resulting again in low channel utilization .therefore ,for bursty users(which can also be characterized by high peakto –average data rate requirements ) ,store –and –forward transmission techparticular munications link only for the duration of its transmission on that link 。the rest of the time it is stored at some intermediate message switch and the link is available for other transmissions . of storeandforward transmission techniques offer a more costeffective solution, since a message occupies a particular munications link only for the duration of its transmission on that link 。the rest of the time it is stored at some intermediate message vantage of store –and –forward transmission over circuit switching is that the done on the link is available for other transmissions. Thus the main advantage of storeand –forward transmission over circuit switching is that the munication bandwidth is dynamically allocated ,and the allocation is done on the fine basis of a particular link in the work and a particular link in the work and a particular message ( for a particular sourcedestination pair ).. Packet switching achieves the benefits discussed so far and offers added features .It provides the full advantage of the dynamic allocation of the bandwidth ,even when messages are long .Indeed, with packet switching ,many packets of the same message may be in transmission simulsl over consecutive links of a path from source to destination. 移動(dòng)通信 蜂窩電話系統(tǒng) 一個(gè)蜂窩電話系統(tǒng)可以為在該系統(tǒng)無線電收發(fā)范圍內(nèi)的任何用戶提供一個(gè)到 PSTN 的無線電話連接。在一定的頻譜內(nèi)，蜂窩系統(tǒng)可以在一個(gè)大區(qū)域中擁有相當(dāng)多的用戶。蜂窩無線電系統(tǒng)常?？梢蕴峁┍扔芯€電話系統(tǒng)更高質(zhì)量的服務(wù)。通過限制從每一個(gè)基站發(fā)射機(jī)到一個(gè)叫做蜂窩小區(qū)的范圍來過得高容量，同一條無線電通道可以被其他遠(yuǎn)距離基站再次利用。當(dāng)一個(gè)用戶從某歌嘯傲去轉(zhuǎn)移到另一個(gè)小區(qū)時(shí)，采用一種復(fù)雜的交換技術(shù)就可以使摘機(jī)狀態(tài)的手機(jī)連續(xù)地進(jìn)行呼叫處理。 一個(gè)基本的蜂窩系統(tǒng)由移動(dòng)站、基站和一個(gè)移動(dòng)交換中心（ MSC）組 成。由于這歌移動(dòng)交換中心負(fù)責(zé)一個(gè)蜂窩系統(tǒng)所有移動(dòng)電話到 PSTN 的連接，因而有時(shí)又被稱為移動(dòng)電話交換局（ MTSO）。每一次通信都要借助某個(gè)基站的無線電波傳送信號(hào)，并且在整個(gè)呼叫過程中可以摘機(jī)，撥打其他任何基站的電話號(hào)碼。一個(gè)移動(dòng)包括無線電收發(fā)機(jī)、天線和控制電路，這種控制電路可以安裝在傳輸系統(tǒng)或手機(jī)里。基站包括幾臺(tái)發(fā)射機(jī)和接收機(jī)，能同時(shí)進(jìn)行全雙工通信，通常還有用以支持收、發(fā)無線電的天線塔。基站就像建立在一個(gè)蜂窩小區(qū)內(nèi)所有用戶和那些通過電話線或 I 微波鏈接到 MSC 的集群呼叫連接之間的以一座橋梁。 MSC 負(fù)責(zé)統(tǒng)一所有基 站的操作及整個(gè)蜂窩系統(tǒng)到 PSTN 的連接。一個(gè)典型的 MSC 一次可以處理100000 個(gè)蜂窩通信用戶，同時(shí)進(jìn)行 5000 個(gè)通話，并且完成所有計(jì)費(fèi)和系統(tǒng)維護(hù)功能。在大城市中，一個(gè)獨(dú)立的載波可以服務(wù)于若干個(gè) MSC. 無繩電話系統(tǒng) 無繩電話系統(tǒng)使全雙工通信系統(tǒng)。它通過無線電將手持機(jī)與一專用基站連接起來，而后再連接到 PSTN 上的某條電話線上。在第一代無繩電話系統(tǒng)（制造于20 世紀(jì) 80 年代）中，手持設(shè)備僅能與一個(gè)專用的基本設(shè)備相連，而