Telecommunication Modern Operation
In 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 electronically when the caller dials the number. Once the connection is made, the caller’s voice is transformed to an electrical signal using a small microphone in the callefs handset. This electrical signal is then sent through the network to the user at the other end where it transformed back into sound by a small speaker in that person’s 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 speaker’s voice directly determines the signal’s voltage. Although short-distance calls may be handled from end-to-end as analogue signals, increasingly telephone service providers are transparently converting the signals to digital fbr transmission before converting them back to analogue for reception. The advantage of this is that digitized voice data can travel side-by-side with 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 markets. 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 new 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.
There 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 capacity. TAT-8 itself was able to carry 10 times as many telephone calls as the last copper cable laid at that time and toda/s 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 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 across many modem 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 fbr public telephone networks because it establishes a pathway for data through the network 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 important because telephone calls can negotiate a contract so as to guarantee themselves a constant bit rate, something that will ensure a callefs voice is not delayed in parts or cut-off completely. There are competitors to ATM, such as Multiprotocol Label Switching (MPLS), that perfbnn a similar task and are expected to supplant ATM in the future.
Radio and television
In 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 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 information) 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 fioin analogue to digital broadcasts. This move is made possible by the production of cheaper, faster and more capable integrated 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 transmission, 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 binary 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 the 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 lo incomprehensible output and hence a breakdown of the transmission.
In digital television broadcasting, there are three competing standards that are likely to be adopted worldwide. These arc 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 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 arc much more unified with practically all countries 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 information to “piggyback” on nonnal 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 31, 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 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 Internet is a worldwide network of computers and computer networks 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. Hcncc, any computer on the Internet can send a message to any other computer using its IP address. These messages carry with them the originating computer’s 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 percentage 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 communicate 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 the 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 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 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 nehvork situation. In practice, most intercontinental communication will use the Asynchronous Transfer Mode (ATM) protocol (or 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 fbr logical addressing. For the world wide web, these “IP addresses5‘ are derived from the human readable form using the Domain Name System (e.g. 126.96.36.199 is derived from www.google.com). 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 layer, 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 arc retransmitted if they arc 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 use 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 (SSL) 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 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 networks
Despite the growth of the Internet, the characteristics of local area networks (computer networks that run at most a lew 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-eflective and efficient without them.
In the mid-1980s, several protocol suites emerged to fill the gap between the data link 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 govemment and research facilities. 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 arc common. The move to TCP/IP was helped by technologies such as DHCP that allowed TCP/IP clients to discover their own network address — a functionality that came standard with the ApplcTalk/IPX/NctBIOS protocol suites.
It is at the data link layer though that most modem local area networks diverge from the Internet. Whereas Asynchronous Transfer Mode (ATM) or Multiprotocol Label Switching (MPLS) 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 prevention. Both of these differences allow fbr more economic set-ups.
Despite the modest popularity of Token Ring in the 80’s and 90’s, 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 the 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 1 O-gigabit Ethernet implementations. Where optic fibre is used, 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 perfbnnance).
在一个模拟电话网络，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。虽然距离矩，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c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，而旦可 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（L02亿）.在从1999年之后的五年时间内新增用户来看，非洲已以58.2 % 的增长超过了其他地区的市场。手机逐渐采用如GSM或W-CDMA这些可以数字 化传输语音信号的系统,从而使AMPS这样的模拟系统衰落。
电话通信也隐约地有了戏剧性的变化。开始运作的TAT-8（跨大西洋传输电 缆）始于1988年，20世纪90年代见证了基于光纤系统的普及。光纤传输的优势在 于其所提供的数据容量的急剧增加。TAT-8可以传输相当于同轴电缆电话10倍的 数据,而现在的光纤能传输25倍于TAT-8的数据。数据能力的增加是由于几个因 素:第一，光纤体积远小于其他竞争技术。第二，他们不受到串扰这意味着数百条 光纤可以很容易地捆绑在一个单一的电缆内。最后，攵用技术的改善导致了单条 光纤数据容量的指数增长。
基于现代光纤网络的通信是一项称为异步传输模式（ATM ）的协议。如第二 段所说,ATM协议允许为并排的数据传输。它适用于公共电话网络,因为它建立了 通过网络数据通道并以此进行通信。传输协议基本上是一个用户与网络之间的协 议，它规定了网络如何来处理数据;如果网络不能满足条件的传输协议，它不接受 连接。这很重要网为电话可以通过协议，保证自己的恒定比特率,这将确保来电 者的声音，不是延迟的部分或完全切断。ATM的竞争对手，如多标签交换（MPLS）, 执行类似的任务，并可望在未来取代ATM.
在一个广播系统，中央高功率广播塔传输高频率的电磁波，到众多的低功率 接收器上。由广播塔发送的高频率波由信号调制且该信号载有视频或音频信息。 接收天线稍作调整，以提取高频率波,解调器用来恢复我有视力或音频信息的信 号。广播信号可以是模拟（信号多种多样,载有信息且连续）或数字（信息作为一套 离散值，可以编码）。
广播媒体业正处于发展中一个关键的转折点，许多国家都从模拟发展到数字 广播。此举是可使生产更经济，更快且更能够集成电路。与传统的模拟广播相比, 数字广播最大的优势是,他们防止了一些投诉。对电视来说，这包括消除问题，如雪 花屏，重影和其他失真。这些发生原因，是因为模拟传输的性质，这意味着噪声干 扰会明显影响坡后的输出。数字传输，克服了这个问题，因为接收时数字信号变为 离散值，这样小扰动不影响最终输出。举一个简单的例子，一个二进制信息1011,已 与信号的振幅［1.0 0.0 1.0 1.0 ］调制，并收到信号的振幅［0.9 0.2 1.1 0.9 ］它将仍然 解码为二进制信息1011 —一个完美原码再现。从这个例子可以看出,数字传输也由 一个问题，如果噪音足够大，它可以大大改变解码信息。使用前向错误校正接收器 可以在最终结果中纠正少数比特错误，但太多的噪音将导致难以理解的输出，因此, 传输失败。
在数字电视广播中，有3个相互竞争的标准，很可能是全世界公认的。它们是 ATSC标准,DVB标准和1SDB标准;通过这些标准，到目前为止，应用于标题地图。 所有这三个标准，使用MPEG – 2视频压缩。ATSC标准采用杜比数字AC- 3音 频压缩JSDB利用先进音频编码（MPEG – 2的第7部分），而DVB没有普频压缩 标准，但通常使用MPEG・1第3部分第2层。不同标准所用的调制方式也有所不 同。在数字音频广播中，标准更为统一,儿乎所有国家都选择采用数字音频广播的 标准（也称为作为尤里卡147标准）。也有例外，美国已选择采用高清广播。帝清广播, 不同于尤里卡147，它是基于称为在带内通道传输的传输方法，这使数字化信息，进 行“背驮式” AM或FM模拟传输。
然而，尽管数字化迫在眉睫,模拟接收机仍然普遍应用。模拟电视仍然传送儿 乎所有国家。美国希望于2006年12月31日之前结束模拟广播;不过，最近又推到 2009年2月17日。对于模拟电视而三个标准在使用中。它们是PAL制式,NTSC 制式和SECAM制式。模拟电台,切换到数字变得更加困难，因为模拟接收器只 占数字接收机的一小部分成本。模拟电台调制方式通常采用AM(幅度调制)或 FM(频率调制)。为实现立体声播放，振幅调制副载波用于立体声调频.
互联网是一个全球计算机组成的网络，也是一种用IP联系在一起的计算机网 络。在互联网上的任何一台计算机都有一个唯一的1P地址,其他计算机可以用其 进行路由选择。因此，在互联网上，任何一台电脑可以通过IP地址传送讯息给任何 其他的计算机。这些带有计算机IP地址的信息，允许计算机之间双向沟通.这样 一来,互联网可以被看作是一个计算机之间信息的交换。
据估计」6.9 %的世界人口已经进入互联网且具有最高访问率(以人口百分比 衡量)，它们在北美地区(69.7 %)，大洋洲/澳大利亚(53.5%)和欧洲(38.9%)。在宽带接 入方面，冰岛(26.7%),韩国(25.4%)和荷兰(25.3 %)世界领先。
互联网的成功，部分是因为协议管理计算机和路由器如何互相沟通。计算机 网络通信本身的性质而.助于分层实现，此时，协议栈中的各个独立协议或多或少 独立于其他协议。这使得低级别的协议适应网络的情况，而不影响高层协议的实 现。一个实际的例子可以说明它的重要性，因为它允许一个互联网浏览器上运行 相同的代码，不管运行的计算机连接到互联网是通过以太网还是通过Wi – Fi连 接。协议经常以其在OSI参考模型中的位置命名,1983年为第一步,也是一次不成 功的尝试，它试图建立一个普遍采用的网络协议套件。
对于互联网来说，物理介质和数据链路层协议可以不同的数倍包遍历全球。 这是因为互联网对所用的物理介质或数据链路协议没有限制。这导致媒体和协议 的应用，它们最适合本地网络的情况。在实践中，多数洲际通讯将使用异步转移模 式(ATM )协议(或一个现代的替代物)并辅以光纤。这是因为，对于大多数的洲 际通信来说,互联网与公共交换式电话网络一样拥有相同的基础设施。
在网络层，适用于逻辑寻址的IP开始标准化。在万维网上，这些“IP地址”来 自通过域名系统处理的人类可读格式(例如188.8.131.52是来自www.google.com) 中。目前，使用最广泛的版本的互联网协议是版本4 ,但向版本六过渡已是迫在眉
在传输层大部分通信采用的是传输控制协议（TCP）或用户数据报协议（UDP）。 TCP是基本协议，每条来自其他计算机的消息均需采用TCP,而UDP只有在有利 时才会被采用。有了 TCP,数据包若在它们置于更高层次前丢失或乱序，它们会被 重发。有了 UDP,数据包丢失时会乱序，也不会重发。TCP和UDP数据包携带端口 以便指出数据包应交由哪些应用程序或进程。因为某些应用级协议使用某些端 口，网络管理员可以通过阻断某一特定端口为目的端口的传输限制上网。
在传输层之上，有一些协议会用到并适当应用于会话层和表示层，最显着的是 安全套接层（SSL）和传输层安全（TLS）协设。这些协议，确保双方之间传输的数据 仍然完全保密并且一方或另一方在使用时，挂锁出现于Web浏览器的底部。最后, 在应用层，力.很多的协议为互联网用户所熟悉，如HTTP（ Web浏览），的POP3 （电 子邮件）,FTP （档案传输）,IRC （网上聊天）,BitTorrent（文件共享）和OSCAR（即时通 讯）。
不看互联网的发展，仅局域网的特点（运行于儿公里内的计算机网络）仍然 明显。这是因为这种规模的网络并不需要所有与较大的网络有关的功能,因此往 往更具成本效益和高效率。
在二十世纪八十年代中期，几个协议套件的出现,填补了 OSI参考模型中数据 链路层和应用层之间的空隙。如AppleTalk,IPX和NetBios与20世纪90年代初 占主导地位，因MS-DOS而广受欢迎的协议套件IPX。而TCP / IP,在这一点上，通 常只用于大型政府和研究设施。随着互联网的受欢迎程度的增长以及较大的流量 与互联网逐渐相关，局域网逐步走向TCP / IP。今天的网络大多用于TCP /IP流量 是常见的/句TCP / IP的转变由如允许的TCP / IP客户发现自己的网络地址的 DHCP的技术支撑，而这与A ppleTalkJPX/和N etBIOS协议套件以其成为标准。
在数据链路层，最现代的局域网偏离互联网。而异步转移模式（ATM）或多协议 标签转换（MPLS）技术是典型的数据链路协议，适用于较大的网络。以太网和令牌 环网是典型的局域网数据链路协议。这些协议不同于前协议，因为它们更简单（例 如,它们省略了服务质量保证等功能），并提供碰撞预防。双方的这些差异,是基于 经济成本的考虑。
尽管令牌环在80年代和90年代了一定的普及，但是现在儿乎所有的局域 网使用方线或无线以太1%在物理层,大多数有线以太网实现使用铜双绞线电缆 （包括常用的10 Base-T的网络，然而，一些早期的实现使用同轴电缆，而最近的一 些实现（特别是超高速的）使用光纤。光纤也可能在即将到来的10千兆以太网的实 现中有着出色的表现。用光纤时，必须对多模光纤和单模光纤加以区分。对于制造 商来说，多模光纤可以被认为是便宜的厚光纤,但只力.较少可用的带宽和更大的衰 减（即较差的长途性能）。