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什么是DSL?英文版

作者:stephen    文章来源:方向标教育网 www.59edu.com    点击数:    更新时间:2007-11-8 【我来说两句

什么是DSL?英文版

Digital Subscriber Line

Digital Subscriber Line, or DSL, is a family of technologies that provide digital data transmission over the wires used in the "last mile" of a local telephone network. Typically, the download speed of DSL ranges from 128 kilobits per second (Kbps) to 24,000 Kbps depending on DSL technology and service level implemented. Upload speed is lower than download speed for ADSL and symmetrical for SDSL.

History

The origin of Digital Subscriber Line technology dates back to 1988, when engineers at Bellcore (now Telcordia Technologies) devised a way to carry a digital signal over the unused frequency spectrum available on the twisted pair cables running between the telephone company's central office and the customer premises. Implementation of DSL could permit an ordinary telephone line to provide digital communication without interfering with voice services. However, incumbent local exchange carriers (ILEC) were not enthusiastic about DSL, since it was not as profitable as installing a second phone line for consumers who preferred simultaneous dial-up internet and voice connections, and the broadband data connection would cannibalize existing ISDN customers. This changed in the late 1990s when cable television companies began marketing broadband Internet access. Realizing that most consumers would prefer broadband Internet to dial-up Internet, ILECs rushed out the DSL technology, which they had delayed implementing, as an attempt to win market share from the broadband Internet access offered by cable television operators.

DSL is the principal competition of cable modems for providing high speed Internet access to home consumers in Europe and North America. Older ADSL standards can deliver 8 Mbit/s over about 1? miles (2 km) of unshielded twisted pair copper wire. The latest standard, ADSL2+, can deliver up to 24 Mbit/s, depending on the distance from the DSLAM.Some customers, however, are located farther than 1? miles (2 km) from the central office, which significantly reduces the amount of bandwidth available (thereby reducing the data rate) on the wires.

Operation

The local loop of the Public Switched Telephone Network was initially designed to carry POTS voice communication and signaling, since the concept of data communications as we know it today did not exist. For reasons of economy, the phone system nominally passes audio between 300 and 3,400 Hz, which is regarded as the range required for human speech to be clearly intelligible. This is known as commercial bandwidth. Dial-up services using modems are constrained by the Shannon capacity of the POTS channel.

At the local telephone exchange (UK terminology) or central office (US terminology) the speech is generally digitized into a 64 kbit/s data stream in the form of an 8 bit signal using a sampling rate of 8,000 Hz, therefore – according to the Nyquist theorem – any signal above 4,000 Hz is not passed by the phone network (and has to be blocked by a filter to prevent aliasing effects).

The local loop connecting the telephone exchange to most subscribers is capable of carrying frequencies well beyond the 3.4 kHz upper limit of POTS. Depending on the length and quality of the loop, the upper limit can be tens of megahertz. DSL takes advantage of this unused bandwidth of the local loop by creating 4312.5 Hz wide channels starting between 10 and 100 kHz, depending on how the system is configured. Allocation of channels continues at higher and higher frequencies (up to 1.1 MHz for ADSL) until new channels are deemed unusable. Each channel is evaluated for usability in much the same way an analog modem would on a POTS connection. More usable channels equates to more available bandwidth, which is why distance and line quality are a factor. The pool of usable channels is then split into two groups for upstream and downstream traffic based on a preconfigured ratio. Once the channel groups have been established, the individual channels are bonded into a pair of virtual circuits, one in each direction. Like analog modems, DSL transceivers constantly monitor the quality of each channel and will add or remove them from service depending on whether or not they are usable.

The commercial success of DSL and similar technologies largely reflects the fact that in recent decades, while electronics have been getting faster and cheaper, the cost of digging trenches in the ground for new cables (copper or fiber) remains expensive. All flavors of DSL employ highly complex digital signal processing algorithms to overcome the inherent limitations of the existing twisted pair wires. Not long ago, the cost of such signal processing would have been prohibitive but because of VLSI technology, the cost of installing DSL on an existing local loop, with a DSLAM at one end and a DSL modem at the other end is orders of magnitude less than would be the cost of installing a new, high-bandwidth fiber-optic cable over the same route and distance.

Most residential and small-office DSL implementations reserve low frequencies for POTS service, so that with suitable filters and/or splitters the existing voice service continues to operate independent of the DSL service. Thus POTS-based communications, including fax machines and analog modems, can share the wires with DSL. Only one DSL modem can use the subscriber line at a time. The standard way to let multiple computers share a DSL connection is to use a router that establishes a connection between the DSL modem and a local Ethernet or Wi-Fi network on the customer's premises.

Once upstream and downstream channels are established, they are used to connect the subscriber to a service such as an Internet service provider.

Equipment

The subscriber end of the connection consists of a DSL modem. This converts data from the digital signals used by computers into a voltage signal of a suitable frequency range which is then applied to the phone line.

In the early days of DSL, installation required a technician to visit the premises. A "splitter" was installed near the demarcation point, from which a dedicated data line was installed. Today, many DSL vendors offer a self-install option, in which they ship equipment and instructions to the customer. In this case, since no changes are made to the cable plant on the customer premises, all the phone wires are carrying both POTS and DSL signal frequencies; therefore the customer generally needs to plug a DSL filter into each telephone outlet. However, this can sometimes cause degradation of the DSL signal (especially if more than 5 analogue devices are connected to the line) because the DSL signal is present on all telephone wiring in the building. A way to circumvent this is to install one filter upstream from all telephone jacks in the building, except for the jack to which the DSL modem will be connected. Since this requires wiring changes by the customer and may not work on some (poorly designed) household telephone wiring, it is rarely done. It is usually much easier to install filters at each telephone jack that is in use.

At the exchange a digital subscriber line access multiplexer (DSLAM) terminates the DSL circuits and aggregates them, where they are handed off onto other networking transports. It also separates out the voice component.

Protocols and configurations

Many DSL technologies implement an ATM layer over the low-level bitstream layer to enable the adaptation of a number of different technologies over the same link.

DSL implementations may create bridged or routed networks. In a bridged configuration, the group of subscriber computers effectively connect into a single subnet. The earliest implementations used DHCP to provide network details such as the IP address to the subscriber equipment, with authentication via MAC address or an assigned host name. Later implementations often use PPP over Ethernet or ATM (PPPoE or PPPoA), while authenticating with a userid and password and using PPP mechanisms to provide network details.

DSL also has contention ratios which need to be taken into consideration when deciding between broadband technologies

DSL technologies

The line length limitations from telephone exchange to subscriber are more restrictive for higher data transmission rates. Technologies such as VDSL provide very high speed, short-range links as a method of delivering "triple play" services (typically implemented in fiber to the curb network architectures).

Example DSL technologies (sometimes called xDSL) include:

  • High-bit-rate Digital Subscriber Line (HDSL), covered in this article
  • Symmetric Digital Subscriber Line (SDSL), a standardised version of HDSL
  • Asymmetric Digital Subscriber Line (ADSL), a version of DSL with a slower upload speed
  • Rate-Adaptive Digital Subscriber Line (RADSL)
  • Very-high-bit-rate Digital Subscriber Line (VDSL)
  • Very-high-bit-rate Digital Subscriber Line 2 (VDSL2), an improved version of VDSL
  • G. Symmetric High-speed Digital Subscriber Line (G.SHDSL), a standardised replacement for early proprietary SDSL by the International Telecommunication Union Telecommunication Standardization Sector
  • Powerline Digital Subscriber Line (PDSL), a high speed powerline communications solution which modulates high speed data onto existing electricity distribution infrastructure

Transmission methods

Transmission methods vary by market, region, carrier, and equipment.

  • CAP: Carrierless Amplitude Phase Modulation - deprecated in 1996
  • DMT: discrete multitone modulation, otherwise known as OFDM
  • OFDM: Orthogonal frequency-division multiplexing

数字用户线路Digital Subscriber Line或简称DSL),是通过铜线或者本地电话网提供数字连接的一种技术。它的历史要追溯到1988年,贝尔实验室一位工程师设计了一种方法可以让数字信号加载到电话线路未使用频段,这就实现了不影响话音服务的前提下在普通电话线上提供数据通信。但是贝尔的管理层对这个并不热心, 因为如果用户安装两条线路会带来更多的利润。这一状况直到1990年代晚期有线电视公司开始推销宽带互联网访问时才得到改善。当意识到大多数用户绝对会放弃安装两条电话线访问互联网,贝尔公司才搬出他们已经讨论了10年的DSL技术,来争夺有线电视网络公司的宽带市场份额。

到2005年, DSL技术是线缆调制解调器向欧洲和北美的家庭用户的提供宽带接入主要竞争对手。

工作原理

电话系统设计之初,主要用来传送话音呼叫,出于经济的考虑,电话系统设计传送频率范围在300Hz到3.4kHz范围的信号(尽管人的话音可以到15kHz,但是这个范围内还是很容易辨别对方的)。

然而本地电话网的到最终用户的铜缆实际上可以提供更高的带宽,至少从最低频率到200-800kHz不等,这取决于电路质量和设备的复杂度(一般认为到最终用户分线器之间接头越少越有利于提高带宽,线路传输路过的环境,电子干扰越小越有益于提高线路带宽)。

DSL服务通过利用电话线的附加频段成功克服了在话音频带上传送大量数据的难题(参看香农定理)。

DSL服务通常保留0.3-4kHz这个范围的频段给话音服务,也就是所谓的普通老式电话业务({lang|en|POTS}})使用的频段, 使用这个范围以外的频率传送数据。

DSL连接在用户设备DSL调制解调器和电话交换机之间建立, 然后交换机通过一些其他的协议与用户真正要连接的(典型的)ISP建立连接。这不同于普通的公共电话网与用户端到端的电话连接 。如果用户到交换机距离超过5.5公里,服务质量会因为干扰急剧下降。

设备

用户终端设备是DSL调制解调器。它转换二进制数据到数字电脉冲,使得信号在数字音频流的频段内传输。

另外如果用户早同一根线路上使用老式电话,还需要加装一个被动电子滤波器(很多叫法,"滤波器","微分器"或者"分路器")(可能还有助于改善DSL终端信号抑止回声信号)。这样就能保证DSL调制解调器和电话只接受他们设计使用的信号。如果使用"wires-only"服务,用户可以把滤波器插入一个现有的电话插槽,或者DSL运营商可能安装它。

在交换局端使用数字用户线路访问复用器(DSLAM)将DSL电路上的数据汇聚然后转发到其他的网络。它还能分离出语音部分。

协议和配置

很多DSL技术在低水平比特流的ATM层实现,以保证不同的技术能够在相同的链路上实现。

DSL设备可以创建网桥或者路由网络,在网桥模式,一组用户的计算机可以方便的连接到一个子网。早期的设备使用DHCP服务来分配提供一些配置细节例如网卡的IP地址,基于MAC地址的认证或者分配主机名。 后来的的设备一般使用PPPoE(以太网上的PPP)或者PPPoAATM网上的点到点协议), 验证的时候使用用户名和密码,然后使用PPP原理去分配网络配置(IP地址,子网掩码,网关,DNS等)

DSL技术

可达范围(从电话交换中心到用户的线路长度)与数据速率成反比, 象VDSL这样的技术只能提供短距离链路(典型的就是FTTC-光线到路边)。

使用DSL技术的技术(有时也叫 xDSL)有:

  • ADSL(非对称用户数字线)
  • HDSL(高速用户数字线)
  • RADSL(速率自适应数字用户线路)
  • SDSL(对称数字用户线路, 标准版HDSL
  • VDSL(超高速用户数字线)
  • G.SHDSLITU-T标准替换早期SDSL

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