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1、Telecommunication Modern OperationTelephoneIn an analogue telephone network, the caller is connected to the person he wants to talk to by switches at various telephone exchanges. The switches form an electrical connection between the two users and the setting of these switches is determined electron

2、ically when the caller dials the number. Once the connection is made, the callers voice is transformed to an electrical signal using a small microphone in the callers handset. This electrical signal is then sent through the network to the user at the other end where it transformed back into sound by

3、 a small speaker in that persons handset. There is a separate electrical connection that works in reverse, allowing the users to converse. The fixed-line telephones in most residential homes are analogue that is, the speakers voice directly determines the signals voltage. Although short-distance cal

4、ls may be handled from end-to-end as analogue signals, increasingly telephone service providers are transparently converting the signals to digital for transmission before converting them back to analogue for reception. The advantage of this is that digitized voice data can travel side-by-side with

5、data from the Internet and can be perfectly reproduced in long distance communication (as opposed to analogue signals that are inevitably impacted by noise).Mobile phones have had a significant impact on telephone networks. Mobile phone subscriptions now outnumber fixed-line subscriptions in many ma

6、rkets. Sales of mobile phones in 2005 totalled 816.6 million with that figure being almost equally shared amongst the markets of Asia/Pacific (204 m), Western Europe (164 m), CEMEA (Central Europe, the Middle East and Africa) (153.5 m), North America (148 m) and Latin America (102 m). In terms of ne

7、w subscriptions over the five years from 1999, Africa has outpaced other markets with 58.2% growth. Increasingly these phones are being serviced by systems where the voice content is transmitted digitally such as GSM or W-CDMA with many markets choosing to depreciate analogue systems such as AMPS. T

8、here have also been dramatic changes in telephone communication behind the scenes. Starting with the operation of TAT-8 in 1988, the 1990s saw the widespread adoption of systems based on optic fibres. The benefit of communicating with optic fibres is that they offer a drastic increase in data capaci

9、ty. TAT-8 itself was able to carry 10 times as many telephone calls as the last copper cable laid at that time and todays optic fibre cables are able to carry 25 times as many telephone calls as TAT-8. This increase in data capacity is due to several factors: First, optic fibres are physically much

10、smaller than competing technologies. Second, they do not suffer from crosstalk which means several hundred of them can be easily bundled together in a single cable. Lastly, improvements in multiplexing have led to an exponential growth in the data capacity of a single fibre. Assisting communication

11、across many modern optic fibre networks is a protocol known as Asynchronous Transfer Mode (ATM). The ATM protocol allows for the side-by-side data transmission mentioned in the second paragraph. It is suitable for public telephone networks because it establishes a pathway for data through the networ

12、k and associates a traffic contract with that pathway. The traffic contract is essentially an agreement between the client and the network about how the network is to handle the data; if the network cannot meet the conditions of the traffic contract it does not accept the connection. This is importa

13、nt because telephone calls can negotiate a contract so as to guarantee themselves a constant bit rate, something that will ensure a callers voice is not delayed in parts or cut-off completely. There are competitors to ATM, such as Multiprotocol Label Switching (MPLS), that perform a similar task and

14、 are expected to supplant ATM in the future. Radio and televisionIn a broadcast system, a central high-powered broadcast tower transmits a high-frequency electromagnetic wave to numerous low-powered receivers. The high-frequency wave sent by the tower is modulated with a signal containing visual or

15、audio information. The antenna of the receiver is then tuned so as to pick up the high-frequency wave and a demodulator is used to retrieve the signal containing the visual or audio information. The broadcast signal can be either analogue (signal is varied continuously with respect to the informatio

16、n) or digital (information is encoded as a set of discrete values). The broadcast media industry is at a critical turning point in its development, with many countries moving from analogue to digital broadcasts. This move is made possible by the production of cheaper, faster and more capable integra

17、ted circuits. The chief advantage of digital broadcasts is that they prevent a number of complaints with traditional analogue broadcasts. For television, this includes the elimination of problems such as snowy pictures, ghosting and other distortion. These occur because of the nature of analogue tra

18、nsmission, which means that perturbations due to noise will be evident in the final output. Digital transmission overcomes this problem because digital signals are reduced to discrete values upon reception and hence small perturbations do not affect the final output. In a simplified example, if a bi

19、nary message 1011 was transmitted with signal amplitudes 1.0 0.0 1.0 1.0 and received with signal amplitudes 0.9 0.2 1.1 0.9 it would still decode to the binary message 1011 a perfect reproduction of what was sent. From this example, a problem with digital transmissions can also be seen in that if t

20、he noise is great enough it can significantly alter the decoded message. Using forward error correction a receiver can correct a handful of bit errors in the resulting message but too much noise will lead to incomprehensible output and hence a breakdown of the transmission. In digital television bro

21、adcasting, there are three competing standards that are likely to be adopted worldwide. These are the ATSC, DVB and ISDB standards; the adoption of these standards thus far is presented in the captioned map. All three standards use MPEG-2 for video compression. ATSC uses Dolby Digital AC-3 for audio

22、 compression, ISDB uses Advanced Audio Coding (MPEG-2 Part 7) and DVB has no standard for audio compression but typically uses MPEG-1 Part 3 Layer 2. The choice of modulation also varies between the schemes. In digital audio broadcasting, standards are much more unified with practically all countrie

23、s choosing to adopt the Digital Audio Broadcasting standard (also known as the Eureka 147 standard). The exception being the United States which has chosen to adopt HD Radio. HD Radio, unlike Eureka 147, is based upon a transmission method known as in-band on-channel transmission that allows digital

24、 information to piggyback on normal AM or FM analogue transmissions. However, despite the pending switch to digital, analogue receivers still remain widespread. Analogue television is still transmitted in practically all countries. The United States had hoped to end analogue broadcasts on December 3

25、1, 2006; however, this was recently pushed back to February 17, 2009. For analogue television, there are three standards in use. These are known as PAL, NTSC and SECAM. For analogue radio, the switch to digital is made more difficult by the fact that analogue receivers are a fraction of the cost of

26、digital receivers. The choice of modulation for analogue radio is typically between amplitude modulation (AM) or frequency modulation (FM). To achieve stereo playback, an amplitude modulated subcarrier is used for stereo FM.The InternetThe Internet is a worldwide network of computers and computer ne

27、tworks that can communicate with each other using the Internet Protocol. Any computer on the Internet has a unique IP address that can be used by other computers to route information to it. Hence, any computer on the Internet can send a message to any other computer using its IP address. These messa

28、ges carry with them the originating computers IP address allowing for two-way communication. In this way, the Internet can be seen as an exchange of messages between computers. An estimated 16.9% of the world population has access to the Internet with the highest access rates (measured as a percenta

29、ge of the population) in North America (69.7%), Oceania/Australia (53.5%) and Europe (38.9%).In terms of broadband access, Iceland (26.7%), South Korea (25.4%) and the Netherlands (25.3%) lead the world. The Internet works in part because of protocols that govern how the computers and routers commun

30、icate with each other. The nature of computer network communication lends itself to a layered approach where individual protocols in the protocol stack run more-or-less independently of other protocols. This allows lower-level protocols to be customized for the network situation while not changing t

31、he way higher-level protocols operate. A practical example of why this is important is because it allows an Internet browser to run the same code regardless of whether the computer it is running on is connected to the Internet through an Ethernet or Wi-Fi connection. Protocols are often talked about

32、 in terms of their place in the OSI reference model, which emerged in 1983 as the first step in an unsuccessful attempt to build a universally adopted networking protocol suite. For the Internet, the physical medium and data link protocol can vary several times as packets traverse the globe. This is

33、 because the Internet places no constraints on what physical medium or data link protocol is used. This leads to the adoption of media and protocols that best suit the local network situation. In practice, most intercontinental communication will use the Asynchronous Transfer Mode (ATM) protocol (or

34、 a modern equivalent) on top of optic fibre. This is because for most intercontinental communication the Internet shares the same infrastructure as the public switched telephone network.At the network layer, things become standardized with the Internet Protocol (IP) being adopted for logical address

35、ing. For the world wide web, these “IP addresses” are derived from the human readable form using the Domain Name System (e.g. 72.14.207.99 is derived from ). At the moment, the most widely used version of the Internet Protocol is version four but a move to version six is imminent. At the transport l

36、ayer, most communication adopts either the Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP). TCP is used when it is essential every message sent is received by the other computer where as UDP is used when it is merely desirable. With TCP, packets are retransmitted if they are

37、lost and placed in order before they are presented to higher layers. With UDP, packets are not ordered or retransmitted if lost. Both TCP and UDP packets carry port numbers with them to specify what application or process the packet should be handled by. Because certain application-level protocols u

38、se certain ports, network administrators can restrict Internet access by blocking the traffic destined for a particular port.Above the transport layer, there are certain protocols that are sometimes used and loosely fit in the session and presentation layers, most notably the Secure Sockets Layer (S

39、SL) and Transport Layer Security (TLS) protocols. These protocols ensure that the data transferred between two parties remains completely confidential and one or the other is in use when a padlock appears at the bottom of your web browser. Finally, at the application layer, are many of the protocols

40、 Internet users would be familiar with such as HTTP (web browsing), POP3 (e-mail), FTP (file transfer), IRC (Internet chat), BitTorrent (file sharing) and OSCAR (instant messaging).Local area networksDespite the growth of the Internet, the characteristics of local area networks (computer networks th

41、at run at most a few kilometres) remain distinct. This is because networks on this scale do not require all the features associated with larger networks and are often more cost-effective and efficient without them.In the mid-1980s, several protocol suites emerged to fill the gap between the data lin

42、k and applications layer of the OSI reference model. These were Appletalk, IPX and NetBIOS with the dominant protocol suite during the early 1990s being IPX due to its popularity with MS-DOS users. TCP/IP existed at this point but was typically only used by large government and research facilities.

43、As the Internet grew in popularity and a larger percentage of traffic became Internet-related, local area networks gradually moved towards TCP/IP and today networks mostly dedicated to TCP/IP traffic are common. The move to TCP/IP was helped by technologies such as DHCP that allowed TCP/IP clients t

44、o discover their own network address a functionality that came standard with the AppleTalk/IPX/NetBIOS protocol suites. It is at the data link layer though that most modern local area networks diverge from the Internet. Whereas Asynchronous Transfer Mode (ATM) or Multiprotocol Label Switching (MPLS)

45、 are typical data link protocols for larger networks, Ethernet and Token Ring are typical data link protocols for local area networks. These protocols differ from the former protocols in that they are simpler (e.g. they omit features such as Quality of Service guarantees) and offer collision prevent

46、ion. Both of these differences allow for more economic set-ups. Despite the modest popularity of Token Ring in the 80s and 90s, virtually all local area networks now use wired or wireless Ethernet. At the physical layer, most wired Ethernet implementations use copper twisted-pair cables (including t

47、he common 10BASE-T networks). However, some early implementations used coaxial cables and some recent implementations (especially high-speed ones) use optic fibres. Optic fibres are also likely to feature prominently in the forthcoming 10-gigabit Ethernet implementations. Where optic fibre is used,

48、the distinction must be made between multi-mode fibre and single-mode fibre. Multi-mode fibre can be thought of as thicker optical fibre that is cheaper to manufacture but that suffers from less usable bandwidth and greater attenuation (i.e. poor long-distance performance). - 7 -外文翻译（译文）电信现代运营电话 在一个

49、模拟电话网络, 来电者通过交换机与对方进行不同的电话交流开关在两用户间形成一个电气连接,其参数是由来电者按键时的电气特性决定的 一旦连接,来电者的声音通过来电端处的电话听筒转化为电信号然后电信号通过网络发送到另一端的用户,并通过小型扬声器将信号转化为声音有一个单独的电气连接用于进行转换,以使用户交谈 固定电话 ,在多数居民区是模拟电话,那就是,发言者的声音,直接决定着信号的电压 虽然距离短,来电可能会被作为模拟信号的端到端信号处理,越来越多电话服务供应商是适度的在传输前将模拟信号数字化以便传输,之后转为模拟信号以便接收它的优势是,数字化语音数据可以从互联网上以数字形式传输,而且可以完全转载于远程通信(对比来看,模拟信号无可避免会受到噪声影响)手机已对电话网络产生了重大影响 移动电话用户现在在许多市场超过了固定线路用户 手机销量在2005年总额为8.166亿,被一下数字平分,其中亚洲/太平洋(2.04亿),西欧(1.64亿),cemea(中欧,中东和非洲)(1.535亿),北美(1.48亿)和拉丁美洲(1.02亿)在从1999年之后的五年时间内新增用