英文原文：
xDSL
Analog Circuits and Signal Processing
xDSL refers to different DSL, such as ADSL, HDSL and RADSL. DSL uses ordinary telephone lines to high-bandwidth information into the home and small business enterprise technologies. Traditional telephone business through the twisted-pair families or companies connected with the telephone company. Input devices such as access to telephone voice signal (analog signal), it will be transformed into electrical signals. Because the telephone company's signal has been set to use analog signal transmission, so easily on the phone and exchange of information between the phone company. That is why the need to use a computer modem reasons, so that it will be the value of analog signal and converted to digital signals.
DSL will not assume that digital signal and then back into analog signals. Digital signal directly in the form of digital transmission to the computer, which allows phone companies to greater bandwidth to transmit information. At the same time, part of the signal is still analog signal transmission in order to use the phone at the same time. DSL including ADSL, CDSL, DSL Lite, HDSL, IDSL, RADSL, SDSL, UDSL, VDSL, x2/DSL. 1. Outline
xDSL is a DSL (Digital Subscriber Line) of collectively, that is, digital subscriber line, based on copper telephone lines for transmission of point-to-point transmission medium. DSL technology in the traditional telephone network (POTS) users loops symmetrical support and asymmetric transfer mode, often resolved in the network service providers and end-users among the \"last mile\" of transmission bottlenecks. Because of the large number of users has been laying loop, so to make full use of existing resources of copper, twisted-pair copper through the high-speed access has become the minimum cost of doing business delivered the most realistic solution for broadband access networks. DSL technology has been a large number of applications, is a very mature technology access.
System is mainly composed of xDSL central office equipment (DSLAM-Digital Subscriber Line Access Multiplexer) and customer premise equipment (CPE), from the central office DSLAM access platform, DSL central office card, voice separator, IPC (data convergence device) and so on Composition. As shown, voice separator will be on the line and high-frequency digital audio signal modulation signal separation, and audio signals into the telephone exchange, high-frequency signals into digital modulation DSL access system; DSLAM access platform can be inserted at the same time different DSL access Network cards and cards; central office line cards will be on the signal modulation for digital signal and provides data transfer interface; IPC for DSL access system to provide different WAN interfaces such as ATM, Frame Relay, T1/E1, and so on. These facilities are located in the telephone switch systems in the room. DSL Modem by the user equipment and voice component separator, DSL Modem users on the packet modulation and demodulation and provides data transfer interface.
2. DSL modulation techniques
DSL technology is the use of the phone system not being used for high-frequency signals to transmit data. DSL use of more advanced modulation techniques. At present, the widely used ADSL modulation technology 3: QAM (quadature amplitude modulation), CAP (carrierless amplitude-phase modulation), DMT (discrete multitone), which the DMT modulation technology ANSI Subcommittee T1E1.4 standardized the development of the country Standard used. However, due to the
introduction of the standards will not be long; there are still a considerable number of ADSL products using CAP or QAM modulation.
QAM modulation technology to achieve the following methods: sending data bits / symbol encoder has been divided into two (the rate for the original 1 / 2), with a pair of orthogonal modulation weight multiplied, after the summation output. Compared with other modulation, QAM coding has to make full use of bandwidth, and strong anti-noise advantages. QAM for ADSL main problem is how to adapt to different telephone lines between the performances of a larger difference. To obtain a more desirable characteristics of the work, QAM receiver and a transmitter with the same frequency and phase characteristics of the input signal for decoding, QAM receiver to use adaptive equalizer to compensate for the transmission signal distortion produced by, the use of QAM's ADSL systems from the complexity of its adaptive equalizer. CAP is based on QAM modulation technology developed, you can say it is a variant of QAM technology. Enter data into the encoder, encoder within, m-bit input has been mapped for k = 2m different complex symbols An = an + jbn, by the K different symbols constitute plural k-CAP line coding. And an encoded bn respectively, were sent to in-phase and quadrature digital shaping filter, after the summation into the D / A converter, as the final low-pass filter signal sent. CAP technology for ADSL is the main technical difficulties to overcome on the local signal crosstalk interference. General crosstalk can be offset through the use of local or proximal crosstalk equalizer to resolve this problem.
DMT modulation technology, the main principle is to band (0-1.104MHZ) divided into 256 by the instructions of orthogonal frequency channel (one for each channel bandwidth occupied by 4KHZ), after a bit of the input signal distribution and caching, will enter data into bits Block, the TCM code for 512 points after the discrete Fourier transform (IDFT) will transform into time-domain signal, then bit block will be converted to 256-QAM characters. Then each bit block add cyclic prefix (for the elimination of ISI), the data conversion module (DA) and send the signal sent to filter channel. On the contrary at the receiving end in order to carry out decoding receive. 3. DSL classification
Symmetric DSL technology as follows:
1）HDSL (high bit-rate DSL)
HDSL is xDSL technology in the most mature one, has been more widely used. The technology can be available to full-duplex twisted-pair copper T1 or E1 transmission way. It features: two pairs of twisted-pair transmission; N × 64kbps to support a variety of speeds up to E1 rates. HDSL is an alternative to T1/E1 technology, mainly used to connect digital switches, high-bandwidth video conferencing, distance learning, cellular phone base station to connect, such as the establishment of private networks. And traditional T1/E1 technology, HDSL has the following advantages: lower prices; easy to install, T1/E1 request every 0.9 to 1.8 kilometers on the installation of an amplifier, and HDSL in the 3.6 km distance transmission without amplifiers.
2）SDSL - Single-line DSL, which is one-way version of HDSL, which can provide two-way high-speed variable bit rate connection speeds ranging from 160Kbps to 2.084Mbps. It features: a one-on twisted pair; rate to support multiple T1/E1; users based on the data flow, to choose the most appropriate rate of the economy, the rate of up to E1, HDSL than saving a pair of copper wire; 0.4 mm twisted pair of the largest transmission distance of 3 km.
3）MVL-Multiple Virtual Line, its characteristics are as follows: use a pair of twisted-pair; easy to install, inexpensive; low-power, high-density can be installed; the use of ISDN technology with the same frequency, the same cable signal interference with other Very small; support the transmission of voice, no voice in the client separator; support at the same time on the same line to connect up to 8 MVL user equipment, dynamic bandwidth allocation; up / down-sharing rate of up to 768Kbps; transmission distance of up to 7 km. 3.2. Asymmetric DSL Technology Asymmetric DSL technology as follows:
1）ADSL - Asymmetric DSL (asymmetric DSL), ADSL networks to speed up the flow from the 1M and 8M down the traffic, at the same time with an online offer voice telephone services. Characterized by: a pair of twisted-pair transmission; up from the rate of up to 1M, down as much as 8M; at the same time support the transmission of data and voice.
2）RADSL - Rate Adaptive DSL (rate adaptive DSL), this technology allows service providers to adjust bandwidth xDSL connections in order to meet the actual needs and solve quality problems and long lines. Its characteristics are as follows: use a pair of twisted-pair transmission; support synchronous and asynchronous transfer mode; adaptive rate, down from a rate of 640Kbps to 12Mbps, up from the rate of 128Kbps to 1Mbps; at the same time support the transmission of data and voice.
3）VDSL - Very High Data Rate DSL, VDSL and ADSL systems, can make use of ordinary copper telephone without affecting the narrow-band voice services (POTS, ISDN), high-speed data transmission business. VDSL system and structure is very similar to ADSL. VDSL used in a wide spectrum, up to 12MHz. The scope of the spectrum can be divided into a number of downstream (DS) and upstream (US) band, the band commonly used in international division of the way (Band Plan), there are mainly two: Plan 997, Plan 998. Plan998 under the North American division of business needs, mainly business-oriented asymmetric; Plan997 and according to business needs into Europe, mainly business-oriented symmetric. ADSL as a result of the use of spectrum to only the highest 1.1MHz, comparison, VDSL can provide higher transmission speed, more flexibility for different business needs to provide different transmission capacity (of course, transmission distance generally shorter than ADSL) . VDSL application environment can be divided into three categories: (1) short-range high-speed asymmetric operations, for example, less than 300m, downlink transmission rate 26Mb / s and above can be mainly used for video transmission; (2) distance in symmetrical or asymmetrical close to the business For example, about 1km symmetry 10Mb / s; (3) asymmetric long distance business, this time as a result of high-frequency attenuation part of the larger, lower rate up. VDSL technology best suited to users in a relatively intensive, short copper areas, to provide high-speed data access business, so in the East Asian countries (such as South Korea, Japan) to promote the application of constant, in our country have been equipment manufacturers and operators The concern is to promote stage. 4. xDSL technology application 4.1. Symmetric DSL technology
Symmetric DSL technology used to replace traditional T1/E1 access technology,
and traditional T1/E1 access compared to, DSL technology has a line of low-quality, simple installation and so on. Widely used in communications, network interconnection, and other areas of campus, through the technique, can send multiple voice, video and data.
4.2. Asymmetric DSL Technology
Asymmetric DSL technology is ideal for two-way bandwidth requirements of different applications such as Web browsing, multimedia-on-demand, information dissemination, it is applied to Internet access, VOD system.
汉语翻译：
xDSL
模拟电路和信号处理
xDSL指的是不同的DSL，如ADSL、HDSL和RADSL。DSL是用普通电
话线将高带宽信息带进家庭和小型商业企业的技术。传统电话业务通过双绞线将家庭或公司与电话公司相连。输入设备如电话机获得声音信号(为模拟信号)，将它转变为电信号。因为电话公司的信号已经设置成使用模拟信号传输，所以很容易在电话和电话公司之间交换信息。这就是为什么计算机需要使用调制解调器的原因，使它能够获得模拟信号并按值转换为数字信号。
DSL假定不用将数字信号再变回模拟信号。数字信号直接以数字的形式传输到计算机，这就使得电话公司能够以更大的带宽来传输信息。同时一部分信号仍然是模拟信号传输，以便使于同时使用电话。
DSL包括ADSL、CDSL、DSL Lite、HDSL、IDSL、RADSL、SDSL、UDSL、VDSL、x2/DSL。 1. 概述
xDSL是DSL(Digital Subscriber Line)的统称，即数字用户线路，是以铜电话线为传输介质的点对点传输技术。DSL技术在传统的电话网络(POTS)的用户环路上支持对称和非对称传输模式，解决了经常发生在网络服务供应商和最终用户间的“最后一公里”的传输瓶颈问题。由于电话用户环路已经被大量铺设，因此充分利用现有的铜缆资源，通过铜质双绞线实现高速接入就成为运营商成本最小最现实的宽带接入网解决方案。DSL技术目前已经得到大量应用，是非常成熟的接入技术。
xDSL系统主要由局端设备(DSLAM—Digital Subscriber Line Access Multiplexer)和用户端设备(CPE)组成, 局端由DSLAM接入平台、DSL局端卡、语音分离器、IPC(数据汇聚设备)等组成。如图所示，语音分离器将线路上的音频信号和高频数字调制信号分离，并将音频信号送入电话交换机，高频数字调制信号送入DSL接入系统；DSLAM接入平台可以同时插入不同的DSL接入卡和网管卡等；局端卡将线路上的信号调制为数字信号，并提供数据传输接口；IPC为DSL接入系统提供不同的广域网接口，如ATM、帧中继、T1/E1等。这些设备都设在电话系统的交换机房中。用户设备由DSL Modem和语音分离器组成，DSL Modem对用户的数据包进行调制和解调，并提供数据传输接口。
2. DSL调制技术
DSL技术是利用在电话系统中没有被利用的高频信号传输数据。DSL利用了更加先进的调制技术。目前被广泛采用的ADSL调制技术有3种：QAM(quadature ampli-tude modulation)、CAP(carrierless amplitude-phase modulation) 、DMT(discrete multitone)，其中DMT调制技术被ANSI标准化小组T1E1.4制订的国家标准所采用。但由于此项标准推出时间不长，目前仍有相当数量的ADSL产品采用QAM或CAP调制技术。
QAM调制技术的实现方法如下：发送数据在比特/符号编码器内被分成两路(速率各为原来的1/2)，分别与一对正交调制分量相乘，求和后输出。与其它调制技术相比，QAM编码具有能充分利用带宽、抗噪声能力强等优点。QAM 用于ADSL的主要问题是如何适应不同电话线路之间性能较大的差异性。要取得较为理想的工作特性，QAM接收器需要一个和发送端具有相同的频谱和相位特性的输入信号用于解码，QAM接收器利用自适应均衡器来补偿传输过程中信号产生的失真，因此采用QAM的ADSL系统的复杂性主要来自于它的自适应均衡器。
CAP调制技术是以QAM调制技术为基础发展而来的,可以说它是QAM技术的一个变种。输入数据被送入编码器，在编码器内，m位输入比特被映射为ｋ＝２ｍ个不同的复数符号Ａｎ＝ａｎ＋ｊｂｎ，由K个不同的复数符号构成k-CAP线路编码。编码后ａｎ和ｂｎ被分别送入同相和正交数字整形滤波器，求和后送入Ｄ/Ａ转换器，最后经低通滤波器信号发送出去。CAP技术用于ADSL的主要技术难点是要克服近端串音对信号的干扰。一般可通过使用近端串音抵消器或近端串音均衡器来解决这一问题。
DMT调制技术的主要原理是将频带(0-1.104MHZ)分割为256个由频率指示的正交子信道（每个子信道占用4KHZ带宽），输入信号经过比特分配和缓存，
将输入数据划分为比特块，经TCM编码后再进行512点离散傅利叶反变换(IDFT)将信号变换到时域，这时比特块将转换成256个QAM子字符。随后对每个比特块加上循环前缀（用于消除码间干扰），经数据模变换(DA)和发送滤波器将信号送上信道。在接收端则按相反的次序进行接收解码。 3. DSL分类
DSL技术主要分为对称和非对称两大类。 xDSL中,“ x”代表着不同种类的数字用户线路技术。各种数字用户线路技术的不同之处，主要表现在信号的传输速率和距离，还有对称和非对称的区别上。 3.1. 对称DSL技术
对称DSL技术主要有以下几种:
1）HDSL(高比特率DSL)
HDSL是xDSL技术中最成熟的一种,已经得到了较为广泛的应用。这种技术可以通过现有的铜双绞线以全双工T1或E1方式传输。其特点是：利用两对双绞线传输；支持N×64kbps各种速率，最高可达E1速率。HDSL是T1/E1的一种替代技术，主要用于数字交换机的连接、高带宽视频会议、远程教学、蜂窝电话基站连接、专用网络建立等。与传统的T1/E1技术相比，HDSL具有以下优点：价格便宜；容易安装，T1/E1要求每隔0.9～1.8公里就安装一个放大器,而HDSL可在3.6公里的距离上传输而不用放大器。
2）SDSL--Single-line DSL，这是HDSL的单线版本，它可以提供双向高速可变比特率连接,速率范围从160Kbps到2.084Mbps。其特点是：利用单对双绞线；支持多种速率到T1/E1；用户可根据数据流量，选择最经济合适的速率，最高可达E1速率，比用HDSL节省一对铜线；在0.4mm双绞线上的最大传输距离为3公里以上。
3）MVL—Multiple Virtual Line ，其特点如下：利用一对双绞线；安装简便，价格低廉；功耗低，可以进行高密度安装；利用与ISDN技术相同的频率段，对同一电缆中的其他信号干扰非常小；支持语音传输，在用户端无需语音分离器；支持同一条线路上同时连接多至8个MVL用户设备，动态分配带宽；上/下行共享速率可达768Kbps； 传输距离可达7公里。 3.2. 非对称DSL技术
非对称DSL技术主要有以下几种:
1）ADSL--Asymmetric DSL(非对称DSL)，ADSL为网络提供速率从1M的
上行流量和8M的下行流量, 同时在同一根线上可以提供语音电话服务。其特点：利用一对双绞线传输；上行速率从最高可达1M，下行高达8M；支持同时传输数据和语音。
2）RADSL--Rate Adaptive DSL(速率自适应DSL)，这种技术允许服务提供者调整xDSL连接的带宽以适应实际需要并且解决线长和质量问题。其特点如下：利用一对双绞线传输；支持同步和非同步传输方式；速率自适应，下行速率从640Kbps到12Mbps，上行速率从128Kbps到1Mbps；支持同时传输数据和语音。
3）VDSL--Very High Data Rate DSL(甚高速数字用户线) ，VDSL系统与ADSL一样，可利用普通电话铜缆在不影响窄带话音业务（POTS、ISDN）的情况下，传送高速数据业务。VDSL的系统结构也与ADSL很相似。VDSL使用的频谱较宽，最高可达12MHz。这一频谱范围可被分割为若干下行（DS）和上行（US）频段，国际上常用的频段划分方式（Band Plan）主要有两种：Plan 997、Plan 998。Plan998根据北美的业务需求划分，主要面向非对称业务；而Plan997根据欧洲的业务需求划分，主要面向对称业务。由于ADSL使用的频谱最高只能到1.1MHz，相比而言，VDSL能够提供更高的传输速率，能够更灵活地为不同的业务需求提供不同的传送能力（当然，传输距离一般比ADSL短）。VDSL的应用环境主要可分为三类：(1)短距离高速非对称业务，例如300m以内，下行传输速率26Mb/s以上，可主要用于视频传输；(2)中距离对称或接近对称业务，例如1km左右对称10Mb/s；(3)较长距离非对称业务，这时因高频部分衰减较大，上行速率较低。VDSL技术最适合在用户相对密集、铜缆距离较短的地区，提供高速数据接入业务，因此在东亚国家（如韩国、日本）的应用不断推广，在我国也得到了设备制造商和运营商的关注，正在大力推广阶段。 4. xDSL技术的应用范围 4.1. 对称DSL技术
对称DSL技术主要用于替代传统的T1/E1接入技术，与传统的T1/E1接入相比, DSL技术具有对线路质量要求低、安装调试简单等特点。广泛地应用于通信、校园网互连等领域,通过复用技术,可以同时传送多路语音、视频和数据。 4.2. 非对称DSL技术
非对称DSL技术非常适用于对双向带宽要求不一样的应用，如Web浏览、多媒体点播、信息发布等,因此适用于Internet接入、VOD系统等。
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