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英文翻译：PARTⅠ各种光纤接入技术Optical Fiber Technology With Various Access1 光网络主流1.1 光纤技术光纤生产技术已经成熟，现在大批量生产，广泛应用于今天的零色散波长λ0=1.3μm的单模光纤，而零色散波长λ0=1.55μm的单模光纤已开发并已进入实用阶段，这是非常小的1.55μm的波长衰减，约0.22dB/km，它更适合长距离大容量传输，是首选的长途骨干传输介质。
目前，为了适应不同的线路和局域网的发展要求，已经制定了一个非分散纤维，低色散斜率光纤，大有效面积光纤，水峰光纤等新型光纤。
长波光学研究人员研究认为，传输距离可以达到数千公里的理论，可以实现无中继传输距离，但它仍然是阶段理论。
1.2 光纤放大器1550nm波长掺铒（ER）的光纤放大器（EDFA），掺铒数字，模拟和相干光通信中继器可以以不同的速率传输光纤放大器，也可以发送特定波长的光信号。
在从模拟信号转换成数字信号、从低到高比特率比特率的光纤网络升级中，系统采用光复用技术的扩大，他们都不必改变掺铒放大器电路和设备。
掺铒放大器可作为光接收机前置放大器，后置放大器的光发射机和放大器的补偿光源装置。
1.3 宽带接入不同的环境中企业和住宅客户提供了多种宽带接入解决方案。
接入系统主要完成三大功能：高速传输，复用/路由，网络的扩展。
目前，接入系统的主流技术，ADSL 技术可以双绞铜线传输经济每秒几兆比特的信息，即支持传统的语音服务，而且还支持面向数据的因特网接入位，理事会结束的ADSL多路复用访问的数据流量，路由的分组网络，语音流量将传送到PSTN，ISDN或其它分组网络。
电缆调制解调器在HFC网络提供高速数据通信，将带宽分为上行和下行信道同轴电缆渠道，它可以提供挥发性有机化合物的在线娱乐，互联网接入等服务，同时还提供PSTN业务。
固定无线接入系统如智能天线和接收机的无线接入系统使用了许多高新技术，是一个以创新的方式接入的技术，作为目前仍滞留在今后进一步探索实践的方式最不确定的接入技术。
8种宽带网络接入技术解析宽带网络接入技术是指能够提供高带宽、高速率、高质量的网络接入服务的技术。
我们将介绍以下8种常见的宽带网络接入技术。
1. 数字用户线路（DSL）：DSL利用电话线传输数据信号，分为ADSL（非对称数字用户线路）和VDSL（Very high bit-rate DSL）。
ADSL适用于家庭用户，具有较高的下行速率和较低的上行速率；而VDSL适用于企业用户，具有更高的上下行速率。
2. 电缆网络：电缆网络利用有线电视网络传输数据信号，广泛应用于家庭和企业用户。
其特点是速度较快，使用方便，但由于带宽是共享的，可能会受到网络拥堵的影响。
3. 光纤到户（FTTH）：光纤到户是将光纤网络直接连接到用户家庭或企业的终端设备上。
它具有高带宽、低延迟、稳定性好的特点，适用于需要大量数据传输和高速互联网接入的用户。
4. 卫星网络：卫星网络通过卫星传输数据信号，适用于地理条件复杂或无法铺设光纤的区域。
它能够提供全球范围内的覆盖，但由于信号传输存在一定的延迟，对于实时性要求较高的应用有一定影响。
5. 无线局域网（WLAN）：无线局域网是通过无线信号传输数据的网络，适用于办公室、家庭和公共场所。
它具有灵活性高、便于移动的特点，但受到信号覆盖范围和干扰的限制。
6. 移动网络：移动网络是通过移动通信基站进行数据传输的网络，适用于移动设备和移动用户。
它能够提供移动性强、覆盖面广的特点，但速率可能受到网络拥塞和信号强弱的影响。
8. 光纤到线（FTTC）：光纤到线是将光纤网络延伸至距离用户较近的地方，然后通过铜线将信号传输到用户终端。
它兼顾了光纤的高速率和铜线的成本效益，适用于一些需要高速率但离光纤接入点较远的用户。
教材（IP专业）WLAN接入技术目录第1章宽带无线接入技术概述 (4)1.1宽带无线接入技术简介 (4)1.2宽带无线接入的关键技术 (5)1.2.1 调制技术 (5)1.2.2 天线技术 (6)1.2.3 动态带宽分配技术 (6)1.3典型宽带接入技术介绍 (7)1.3.1 MMDS (7)1.3.2 SDHIP (7)1.3.3 LMDS (8)1.3.4 WiMAX技术 (8)1.3.5 Clearwire明线技术 (9)1.3.6 McWiLL技术 (10)1.3.7 WLAN技术 (11)1.3.8 蓝牙技术 (12)第2章WLAN技术原理 (13)2.1WLAN基本概念 (13)2.1.1 IEEE 802.11协议简述 (13)2.1.2 802.11b的物理层 (13)2.1.3 WLAN数据链路层 (14)2.1.4 802.11g-最新WLAN主流标准 (15)2.1.5 802.11a (18)2.1.6 802.11n (18)2.2WLAN互联结构 (18)2.3WLAN的操作 (21)2.4WLAN安全性 (22)2.4.1 WEP（Wired Equivalent Privacy有线等效加密） (22)2.4.2 RADIUS认证 (23)2.4.3 地址过滤 (23)第3章WLAN组网方式与典型应用 (25)3.1IP网络结构 (25)3.2数据网络的分层结构－组网模型 (25)3.3宽带接入组网结构 (26)3.4WLAN典型组网应用 (26)3.4.1 无线接入点（AP）组网方式 (26)3.4.2 接入控制器基本应用方式 (27)3.4.3 家庭和办公室WLAN接入 (28)3.4.4 大中型办公室WLAN解决方案 (28)3.4.5 无线局域网在Hotspot的应用 (29)3.4.6 可运营WLAN整体解决方案 (29)参考资料 (30)第1章宽带无线接入技术概述1.1 宽带无线接入技术简介宽带无线接入(BroadbandWirelessAccess，简BWA)技术目前还没有通用的定义，一般是指把高效率的无线技术应用于宽带接入网络中，以无线方式向用户提供宽带接入的技术。
列举六种接入技术一、ADSL接入技术ADSL（Asymmetric Digital Subscriber Line）是一种对称数字用户线路技术，其特点是在普通电话线路上实现宽带数据传输。
ADSL 接入技术允许用户在使用互联网的同时仍能保持电话线路的正常通话功能。
ADSL接入技术的上行速度较低，下行速度较高，适合家庭用户和小型企业使用。
二、光纤接入技术光纤接入技术是一种基于光纤传输的高速互联网接入技术。
光纤接入技术的主要特点是传输速度快、带宽大、信号稳定，并且能够支持长距离传输。
光纤接入技术广泛应用于大型企业、学校、医院等场所，能够满足大量用户同时高速上网的需求。
三、无线局域网接入技术无线局域网接入技术是基于无线通信技术实现的局域网接入方式。
无线局域网接入技术可以通过Wi-Fi热点实现用户的无线上网需求，使用户可以在无线覆盖范围内随时随地连接到互联网。
无线局域网接入技术适用于家庭、办公室、咖啡厅等场所，用户可以通过移动设备实现高速上网。
四、移动网络接入技术移动网络接入技术是一种基于移动通信网络的互联网接入方式。
移动网络接入技术可以通过移动网络运营商提供的数据业务，使用户通过手机、平板电脑等移动设备连接到互联网。
移动网络接入技术的优势在于用户的移动性强，可以随时随地进行上网，适用于个人用户和移动办公需求。
五、卫星接入技术卫星接入技术是一种通过卫星通信实现的互联网接入方式。
卫星接入技术主要应用于偏远地区、海洋、航空等无法通过有线网络覆盖的场所，能够实现全球范围内的互联网接入。
卫星接入技术的特点是覆盖范围广，但信号传输延迟较高，适用于特定场景的互联网接入需求。
六、电力线接入技术电力线接入技术是一种利用电力线路实现的互联网接入方式。
电力线接入技术通过在家庭或办公室内的电力线路上传输数据信号，实现用户的互联网接入需求。
电力线接入技术的优势在于无需铺设新的网络线路，节省了成本，适用于对网络速度要求不高的用户。
宽带接入概述宽带接入是指通过宽带网络来实现互联网的接入。
宽带网络通常指的是传输速度较快、带宽较宽的网络。
宽带接入已经成为现代人们生活中必不可少的一部分，无论是家庭用户还是企业用户，都需要通过宽带接入来访问互联网。
宽带接入的类型宽带接入的类型多种多样，常见的有以下几种：1. ADSLADSL（Asymmetric Digital Subscriber Line）是一种使用普通电话线作为传输介质的宽带接入技术。
ADSL的特点是上行和下行速度不对称，下行速度比上行速度要快。
这种技术在家庭用户中广泛应用，可以满足普通上网、视频点播等需求。
2. 光纤宽带光纤宽带是使用光纤作为传输介质的宽带接入技术。
由于光纤具有传输速度快、带宽大的特点，因此光纤宽带可以提供更高速的上网体验。
光纤宽带在企业用户中较为常用，能够满足大规模数据传输和高速互联网接入的需求。
3. 无线宽带无线宽带是通过无线信号来实现互联网接入的技术。
无线宽带的优势是无需使用传统的有线接入方式，使用更加灵活方便。
无线宽带常见的形式有Wi-Fi、4G/5G移动网络等，适用于家庭用户和移动办公等场景。
宽带接入的优势宽带接入相比于传统的拨号接入方式具有以下优势：1. 传输速度快宽带接入使用高速网络传输技术，可以提供更快的传输速度。
这使得用户可以更快地下载和上传文件、观看高清视频、进行在线游戏等，大大提高了互联网使用体验。
2. 带宽大宽带接入具有较大的带宽，能够同时支持多个用户进行互联网访问。
这使得家庭用户可以在多个设备上同时使用互联网，同时也满足了企业用户大规模数据传输的需求。
3. 稳定可靠宽带接入采用了成熟的传输技术和设备，具有较高的稳定性和可靠性。
相比于拨号接入方式，宽带接入更不容易断线，用户可以更稳定地访问互联网。
4. 全天候在线宽带接入提供全天候在线的互联网连接，用户可以随时随地访问互联网。
无论是工作、学习还是娱乐，用户都可以通过宽带接入来满足各种需求。
中文1866字附 录一、英文原文Optical fiber access network technologyDuring the early 1980s, analog cellular telephone systems were experiencing rapid growth in Europe, particularly in Scandinavia and the United Kingdom, but also in France and Germany. Each country developed its own system, which was incompatible.The entire telecommunication network according to network function is divided into three sections: switching, transmission,and access. Access to telecommunications business transparent to users, transmit specific, access to local switch and the user that the connection between the parts, usually include subscriber line transmission system, reuse equipment, crossing connection device or user/network terminal equipment. And actually pick for business entity is business nodes.1. Access network development process1.1 Access Network （AN: Access by business） refers to a node interface （SNI） and related users Network interface （UNI） between a series of transmit entity （such as line facilities and transmission facilities） for transmission consisting of telecommunication business provide the required transmit load capacity, implementation system configurations percentile management interface via and management. Access can be seen as with business and application irrelevant transmission network, mainly finish crossing connection, reuse and transmission function, it is transparent to users of signaling.Based on modern of telecommunication network access network is integrated service access, simple local voice access will be increasingly cannot meet the requirements, the voice, data and image comprehensive access is becoming access network development trend. After years of development, access network define though does not change, but its connotation and forms has changed significantly. Superior traditional access network integrated optical fiber access network will be users access the future developing trend.Optical fiber access from SNI to UNI refers to all or part of the access network by using optical fiber as medium. ITU - T about access network recommendations indicate that fiber, user access system is composed of three parts: optical line terminal（OLT）, optical distribution network （ODN） and optical network unit （ONU）. 1.2According to the requirement of system access framework and the important characteristics, access network can be summarized as the following:1.2.1 Access to access business provides bearing capacity, achieve business transparent transfer.1.2.2. The access is transparent to users signaling, except for some users signaling format conversion outside, signaling and business processing function is still in business node.1.2.3 The introduction of should not be restricted access all kinds of existing access types and business, access network should through the limited standardization interface and businessnode connected.1.2.4Access network have independent from the business node in the network management system, this system through the standardized interfaces connected TMN, access network operation for implementation TMN, maintenance and management.2 .fiber structure2.1general linear structure. Refer to the public bus, fiber as each user terminals through direct connect with bus coupler network structure. Its characteristic is sharing backbone fiber, save lines to remove node investment, the demand is higher, dynamic range, interfere with each other effect is small. Defect is loss of accumulation, the backbone fiber user acceptance dependence is strong.2.2 ring structure. Refers to all share a fiber optic link node, fiber optic link its end to end the closed loop network structure. Characteristic is self-healing, namely can be realized without intervention, the network can be in a relatively short period of time to recover from failure have preached business, high reliability. Defect is hanged by the number of users of monocylic limited, polycyclic interchange is relatively complex, not suitable for CATV, etc FenPeiXing business.2.3star structure. This structure is actually point-to-point way, each user terminals in the central node through with control and switch functions of the astral couplers exchange information. Characteristics of simple structure, maintenance is convenient and easy to upgrade and expansion between, each user relatively independent, good secrecy, business adaptability. Defect is higher, networking required fiber cost of poor flexibility, high reliability requirements of central node.2.4 tree structure. Similar to the branches, a hierarchical structure, shape in transfer boxes and FenXianHe place adopts multiple optical distributor, will signal filtering down top end innings allocation, have strong control coordination. Characteristic is suitable for broadcasting business. Defect is power loss is bigger, two-way communication difficulty bigger.3．The status of optical access network in the modern communicationOptical fiber in realizing national information modernization access plays a more and more important role, is an indispensable part of modern communications network. Modern communications network basic realized based on fiber backbone transmission and exchange, and access section is still restricted modern telecommunications further development and improvement of the "bottleneck", therefore, to construction of national information infrastructure （NII）, access network is the key. Access network technology development, will result in great changes of telecommunications and information network, namely, voice, data, video, etc. Various kinds of information transmission, comprehensive business together for implementing the resources sharing, and gradually optimized communication network, greatly improving network benefits.Two optical fiber optical transmission technology access with the combination of the access technology3.1 Light transmission technology development and evolutionSince 1979, the human use of optical fiber as communication mode, optical transmission technology experience since from analog to digital, from PDH to sdh-based WDM, from passive and active to a series of MSTP from SDH to the development and evolution process, transmission capacity and reliability, the transmission distance such indicators have reached a very high level.The current mainstream of optical transmission technology is still SDH, STM - 1/4/16/64/256 series synchronous transfer module has been achieved, including STM - 256 framerate as high as 40Gbps. Based on SDH, fusion broadband data business multi-task transmitting platform （the MSTP） has also become the best choice for today's construction intracity networks, moreover elastic grouping ring （RPR） technology is also gradually become the direction of the development of a light transmission. The future will be optical transmission network to optical transmission network （otns） in the direction of development, many exchange, network otns choose road and other intelligent function will be implemented on light layer. Otns the intelligent network development degree can realize ASON intelligent optical network.3.2 Light transmission technology and access technology constitute fiber access networkThe needs of the user, the diversification and broadband access technology from initial made of pure narrowband access to broadband access, size, and with integrated access to transmission platform requirement light more and more is also high.Optical transmission technology and access technology together constitute fiber access network system. Optical fiber access network in different stages of development of need to resort to the corresponding optical transmission means to achieve the operational load and transfer. With the light network will be dispersed access devices （OLT ONU） together with organic whole, form a unified fiber access network system.二、英文翻译光纤接入网技术整个电信网按网络功能分为三个部分：整个电信网按网络功能分为三个部分：传输网、传输网、传输网、交换网和接入网。
宽带接入技术1. 简介宽带接入技术是指通过一系列网络设备和协议，将高速的互联网接入服务提供给用户的一种技术。
与传统的拨号上网方式相比，宽带接入技术具有更高的传输速率和稳定性，能够满足现代人对于互联网的高带宽需求。
本文将对几种常见的宽带接入技术进行介绍。
2. ADSLADSL（Asymmetric Digital Subscriber Line）是一种采用普通电话线进行宽带接入的技术。
ADSL利用了电话线在高频段的传输能力，将数据和语音分成不同的频段进行传输，以保证语音通信的质量。
ADSL的特点是上行传输速率较低，通常为几百kbps，下行传输速率较高，可以达到几Mbps。
ADSL在家庭和小型企业用户中应用广泛，它的成本相对较低，安装和维护也比较方便。
3. 光纤接入光纤接入是一种将光纤作为传输介质的宽带接入技术。
相比传统的金属电缆，光纤具有更高的传输速率和更远的传输距离。
光纤接入可以分为FTTH（Fiber To The Home）和FTTB （Fiber To The Building）两种方式。
FTTH是将光纤直接延伸到用户家中，提供高速、稳定的宽带接入服务，可以满足用户对高清视频、在线游戏等大带宽应用的需求。
FTTB则是将光纤延伸到楼宇或小区，在楼内通过局域网等方式将宽带信号分发给每个用户。
光纤接入具有传输速率高、抗干扰性强等优点，但安装和维护成本相对较高。
4. 电力线通信电力线通信（Power Line Communication）是一种利用电力线路提供宽带接入的技术。
它通过在电力线上叠加高频信号，并利用电力线路的传输能力，将宽带信号传输到用户家中。
电力线通信的优点是无需额外布设网络线路，可以利用现有的电力线路实现宽带接入，节省了布线成本。
然而，电力线通信在实际应用中还面临着干扰、传输速率较低等问题，目前主要应用于一些特定场景，如远程抄表和智能电网等领域。
5. 无线接入无线接入是一种利用无线技术提供宽带接入的方式。
宽带技术解读ADSL与VDSL 宽带技术的不断发展与进步，为人们的网络使用提供了更加便捷和高效的方式。
其中，ADSL（Asymmetric Digital Subscriber Line，不对称数字用户线路）和VDSL（Very High Bitrate Digital Subscriber Line，超高比特率数字用户线路）是两种常见的宽带接入技术。
本文将从原理、特点和应用方面对ADSL和VDSL进行解读。
一、ADSL技术解析ADSL技术是目前最为广泛应用的宽带接入技术之一。
它使用了天线的高频部分，将电话线从传统的模拟通信转变为数字通信，以提供更快速的上网服务。
ADSL技术的原理是利用了电话线的高频部分传输数据。
它采用了一种称为OFDM（Orthogonal Frequency Division Multiplexing，正交频分复用）的调制技术，将数据划分成多个小频段进行传输。
由于ADSL采用了不对称的传输方式，即下载速度和上传速度不同，因此也得名不对称数字用户线路。
ADSL技术的特点在于其高速的下载速度和低成本的接入方式。
通过ADSL，用户可在拥有电话线的基础上，实现较高的下载速度，从而满足了日益增长的网络需求。
此外，ADSL的接入方式也十分简便，无需额外的设备和复杂的布线，只需一个电话线和ADSL调制解调器即可接入宽带网络。
二、VDSL技术解析VDSL技术是一种新一代的宽带接入技术，与ADSL相比，在带宽和传输距离方面有所突破。
VDSL采用的是高频段的传输，通过光纤到达用户所在的小区节点，再通过铜线进一步传输数据，以提供更高速的网络连接。
VDSL技术的原理是通过将光纤信号转换为电信号，然后在铜线上进行传输。
相较于ADSL，VDSL的带宽更高、传输距离更远，能够满足更多用户同时高速上网的需求，因此被称为超高比特率数字用户线路。
VDSL技术的特点包括高速、稳定和灵活多样。
VDSL的下载速度可达几十兆甚至更高，用户可以迅速下载大型文件和高清视频，享受更加流畅的网络体验。
中文3856字附录3：英文资料Basic Concepts of WCDMA Radio Access Network1. BackgroundThere has been a tremendous growth in wireless communication technology over the past decade. The significant increase in subscribers and traffic, new bandwidth consuming applications such as gaming, music down loading and video streaming will place new demands on capacity. The answer to the capacity demand is the provision of new spectrum and the development of a new technology - Wideband CDMA or hereinafter referred to as WCDMA. WCDMA was developed in order to create a global standard for real time multimedia services that ensured international roaming. With the support of ITU (International Telecommunication Union) a specific spectrum was allocated - 20Hz for 3G telecom systems. The work was later taken over by the 3GPP (3rd Generation Partnership Project), which is now the WCDMA specification body with delegates from all over the world.2. WCDMA a development from GSM and CDMANaturally there are a lot of differences between WCDMA and GSM systems, hut there are many similarities as well.The GSM Base Station Subsystem (BSS) and the WCDMA Radio Access Network (RAN) are both connected to the GSM core network for providing a radio connection to the handset. Hence, the technologies can share the same core network. Furthermore, both GSM BSS and WCDMA RAN systems are based on the principles of a cellular radio system. The GSM Base Station Controller (ESC) corresponds to the WCDMA Radio Network Controller (RNC). The GSM Radio Base Station (RBS) corresponds to the WCDMA RES, and the A -interface of GSM was the basis of the development of the Iu-interface of WCDMA, which mainly differs in the inclusion of the new services offered by WCDMA. The significant differences, apart from the lack of interface between the GSM BSCs and GSM Abis-interface to provide multi-vendoroperability, are more of a systemic matter. The GSM system uses TDMA (Time Division Multiple Access) technology with a lot of radio functionality based on managing the timeslots. The WCDMA system on the other hand uses CDMA, which means that both the hardware and the control functions are different. Examples of WCDMA-specific functions are fast power control and soft handover.Code Division Multiple Access and WCDMACode Division Multiple Access (CDMA) is a multiple access technology where the users are separated by unique codes, which means that all users can use the same frequency and transmit at the same time. With the fast development in signal processing,' it has become feasible to use the technology for wireless communication, also referred to as WCDMA and CDMA2000.In CDMA One and CDMA2000, a 1.25MHz wide radio signal is multiplied by a spreading signal (which is a pseudo-noise code sequence) with a higher rate than the data rate of the message. The resultant signal appears as seemingly random, but if the intended recipient has the right code, this process is reversed and the original signal is extracted. Using unique codes means that the same frequency is repeated in all cells. which is commonly referred to as a frequency re-use of 1.WCDMA is a step further in the CDMA technology. It uses a 5MHz wide radio signal and a chip rate of 3.84Mcps, which is about three times higher than the chip rate of CDMA2000. The main benefits of a wideband carrier with a higher chip rate are:●Support for higher bit rates●Higher spectrum efficiency thanks to improved trunking efficiency (i.e. a better statistical averaging)●Higher QoSFurther, experience from second-generation systems like GSM and CDMA One has enabled improvements to be incorporated in WCDMA. Focus has also been put on ensuring that as much as possible of WCDMA operators' investments in GSM equipment can be reused. Examples are the re-use and evolution of the core network, the focus on co-siting and the support of GSM handover. In order to use GSMhandover, the' subscribers need dual mode handsets.3. Radio Network FunctionalityFor optimal operation of a complete wireless system i.e. from handset to radio access network (RAN) several functions are needed to control the radio network and the many handsets using it. All functions described in this section, except' for Handover to GSM,' are essential and therefore necessary for a WCDMA system.3.1 Power controlThe power control regulates the transmit power of the terminal and base station, which results in less interference and allows more users on the same carrier. Transmit power regulation thus provides more capacity in the network. With a frequency re-use of 1, it is very important to have efficient power control in order to keep the interference at a minimum. For each subscriber service the aim is that the base station shall receive the same power level from all handsets in the cell regardless of distance from the base station. If the power level from one handset is higher than needed, the quality will be excessive, taking a disproportionate share of the resources and generating unnecessary interference with the other subscribers in the network.. On the other hand, if power levels are too low this will result in poor quality. In order to keep the received power at a suitable level, WCDMA has a fast power control that updates power levels 1500 times every second. By doing that the rapid change in the radio channel is handled. To ensure good performance, power control is implemented in both the up-link and the down-link, which means that both the output powers of the hanpset and the base station are frequently updated.Power control also gives rise to a phenomenon called "cell breathing". This is the trade-off between coverage and capacity, which means that the size of the cell varies depending on the traffic load. When the number of subscribers in the cell is low (low load), good quality can be achieved even at a long distance from the base station. On the other hand, when the number of users in the cell is high, the large number of subscribers generates a high interference level and subscribers have to get closer to the base station to achieve good quality.3.2 Soft and softer handoverWith soft and softer handover functionality the handset can communicate simultaneously with two or more cells in two or more base stations. This flexibility in keeping the connection open to more than one base station results in fewer lost calls, which is very important to the operator.To achieve good system performance with a frequency re-use of 1 and power control, soft and softer handover is required. Soft and softer handover enables the handset to maintain the continuity and quality of the connection while moving from. one cell to another.During soft handover, the handset will momentarily adjust its power to the base station that requires the smallest amount of transmit power and the preferred cell may change very rapidly. The difference between soft and softer handover is that during soft handover, the handset is connected to multiple cells at different base stations, while during softer handover, the handset is connected to multiple cells at the same base station. A drawback with soft handover is that it requires additional hardware resources on the network side, as the handset has multiple connections. In a well-designed radio network, 30%-40% of the users will be in soft or softer handover. As an example of soft or softer handover.3.3 Handover to GSM (inter-system handover)When WCDMA was standardized a key aspect was to ensure that existing investments could be re-used as much as possible. One example is handover between the new (WCDMA) network and the existing (GSM) network, which can be triggered by coverage, capacity or service requirements. Handover from WCDMA to GSM, for coverage reasons, is initially expected to be very important since operators are expected to deploy WCDMA gradually within their existing GSM network . When a subscriber moves out of the WCDMA coverage area, a handover to GSM has to be conducted in order to keep the connection.Handover between GSM and WCDMA can also have a positive effect on capacity through the possibility of load sharing. If for example the numbers of subscribers in the GSM network is close to the capacity limit in one area, handover ofsome subscribers to the WCDMA network can be performed.Another function that is related to inter-system handover is the compressed mode. When performing handover to GSM, measurements have to be made in order to identify the GSM cell to which the handover will be made. The compressed mode is used to create the measurement periods for the handset to make the required measurements.3.4 Inter-frequency handover (intra-system handover)The need for inter-frequency handover occurs in high capacity areas where multiple 5MHz WCDMA carriers are deployed. Inter-frequency handover, which is handover between WCDMA carriers on different frequencies, has many similarities with GSM handover, for example the compressed mode functionality.3.5 Channel type switchingIn WCDMA there are different types of channels that can be used to carry data in order to maximize the total traffic throughput. The two most basic ones are common channels and dedicated channels. Channel type switching functionality is used to move subscribers between the common and the dedicated channel, depending on how much information the subscriber needs to transmit. The dedicated channel is used when there is much information to transmit, such as a voice conversation or downloading a web page. It utilizes the radio resources efficiently as it supports both power control and soft handover.3.6 Admission controlAs there is a very clear trade-off between coverage and capacity in WCDMA systems, the admission control functionality is used to avoid system overload and to provide the planned coverage. When a new subscriber seeks access to the network, admission control estimates the network load and based on the new expected load, the subscriber is either admitted or blocked out. By this the operator can maximize the network usage within a set of network quality levels, i.e. levels depending on what kind of service/information the subscriber wants to use.3.7 Congestion controlEven though an efficient admission control is used, overload may still occur,which is mainly caused by subscribers moving from one area to another area. If overload occurs, four 'different actions can be taken. First, congestion control is activated and reduces the bit rate of non real-time applications, to resolve the overload. Second, if the reduced bit rate activity is not sufficient, the congestion control triggers the inter- or intra-frequency handover, which moves some subscribers to less loaded frequencies. Third, handover of some subscribers to GSM and fourth action is to discontinue connections, and thus protect the quality of the remaining connections. 3.8 SynchronizationOne of the basic requirements when WCDMA was standardized was to avoid dependence on external systems for accurate synchronization of base stations. This has been achieved by a mechanism, where the handset, when needed, measures the synchronization offset between the cells and reports this to the network. In addition, there is also an option to use an external source, such as GPS, for synchronizing the nodes, i.e. to always provide the best solution both asynchronous and synchronous nodes are supported.4. Basic architecture concepts/ System overview4.1 Radio Access Network (RAN) ArchitectureThe main purpose of the WCDMA Radio Access Network is to provide a connection between the handset and the core network and to isolate all the radio issues from the core network. The advantage is one core network supporting multiple access technologies.The WCDMA Radio Access Network consists of two types of nodes:Radio Base Station (Node B)The Radio Base Station handles the radio transmission and reception to/from the handset over the radio interface (Iu). It is controlled from the Radio Network Controller via the Iub interface. One Radio Base Station can handle one or more cells.Radio Network Controller (RNC)The Radio Network Controller is the node that controls all WCDMA Radio Access Network functions. It connects the WCDMA Radio Access Network to thecore network via the Iu interface. There are two distinct roles for the RNC, to serve and to control. The Serving RNC has overall control of the handset that is connected to WCDMA Radio Access Network. It controls the connection on the Iu interface for the handset and it terminates several protocols in the contact between the handset and the WCDMA Radio Access Network.The Controlling RNC has the overall control of a particular set of cells, and their associated base stations. When a handset must use resources in a cell not controlled by its Serving RNC, the Serving RNC must ask the Controlling RNC for those resources. This request is made via the Iur interface, which connects the RNCs with each other. In this case, the Controlling RNC is also said to be a Drift RNC for this particular handset. This kind of operation is primarily needed to be able to provide soft handover throughout the network.Radio Access Bearers (RAB)The main service offered by WCDMA RAN is the Radio Access Bearer (RAB).A RAB is needed to establish a call connection between the handset and the base station. Its characteristics ate determined by certain Quality of Service (QoS) parameters, such as bit rate and delay, and are different depending on what kind of service/information to be transported.The RAB carries the subscriber data between the handset and the core network. It is composed of one or more Radio Access Bearers between the handset and the Serving RNC, and one Iu bearer between the Serving RNC and the core network. 3GPP has defined four different quality classes of Radio Access Bearers:●Conversational (used for e.g. voice telephony) - low delay, strict ordering●Streaming (used for e.g. watching a video) - moderate delay, strict ordering●Interactive (used for e.g. web surfing) - moderate delay●Background (used for e.g. me transfer) - no delay4.2 Transport in WCDMA Radio Access NetworkThe WCDMA Radio Access Network nodes communicate with each other over a transport network. The 3GPP specification provides a very clear, split between radio related (WCDMA)functionality and the transport technology, meaning that there is noparticular bias to any technology. The transport network is initially based on ATM, but IP will soon be included as an option.附录4：英文资料译文宽带CDMA无线接入网的基本概念一、背景过去十年，无线通信技术有了巨大发展，用户量的显著增加，使得诸如游戏、音乐下载和视频流量有了新的容量需求。
附录1 外文文献译文宽带接入技术宽带接入技术是在电信和视频网络行业的催生影响下产生出来的，其应用主要体现在物理层。
相反，数据网络行业的关注点主要集中在网络层和运输层上（例如IP电话）。
形成这种应用方法上的区别的理由是，一旦传输的内容被数字化，就能作为数据进行处理。
换句话说，从网络的观点来看，网络只是以数字的形式来传输数据。
在应用层上，这些数据位不论实际代表话音、数据，还是代表视频流都没有区别。
因此，数据网络行业在让数据流适应跨网传输这方面并没有做什么。
另外一方面，电信和视频网络行业必须解决利用现有接入基础设施将内容以分组数据传输的方法进行传输。
这成为在开发下述几类宽带接入技术中的重要催化剂。
铜回路接入技术，又称为数字用户线（DSL）技术。
DSL技术又统称XDSL。
XDSL是由电信行业开发的，利用了世界范围内的几百万英里长的现有铜回路通信设施。
光纤/同轴电缆的电缆接入技术。
电缆接入是由视频网络行业中的有线电视部分开发的，利用了已经连接全世界几百万个（大多数是常驻的）用户的视频信道电缆基础设施。
在线卫星接入技术。
卫星接入是由视频网络行业中的无线部门开发的，通过使用卫星基础设施，经视频信道接到有线电视中心终端局并传送到世界范围内的几百用户（大多数是常驻的）。
非对称数字用户线（ADSL）20世纪80年代，ADSL作为电信行业为应对电缆行业支持视频点播需求而提供的一种解决方案，首先被开发了出来。
然而直到20世纪90年代中期才认识到，它也可以作为能访问像因特网这样的高速业务的技术。
ADSL提供非对称传输，典型的下传速率可达到9Mb/s (从中心局到设备），上传速率为16Kb/s到640Kb/s （从设备到中心局）。
像所有的铜线传输系统一样，速率越高，传输范围越短。
混合光纤同轴网技术（HFC）混合光纤同轴网技术是基于现有的有线电视（有线TV或CATV）技术的。
最初，有线电视是基于从多系统运作员（MSO）到客户设备的同轴电缆设施的，并采用树形拓扑。
这些系统大多数已升级到HFC。
在这种系统中，信号通过一对光缆被传送到一个光缆节点上，然后再经过同轴电缆分发给客户。
在中心终端局上，各种源信号，如传统的卫星业务、使用广域网的模拟和数字业务，以及使用专用主干网的因特网服务提供商（ISP）的业务被复用，并从电信号射频转换成为光信号。
光缆上的通信是单向的：从中心终端局到光缆节点的每一对光缆，都以不同的方向承载单向业务。
光信号在光缆节点处转换成射频信号并以双工方式沿同轴电缆传送。
从中心终端局到客户设备的信号称为下传信号或者前向通路信号。
从客户设备到中心终端局的信号称为上传信号或者反向通路信号。
光纤通信介绍80年代一项最最重要的技术发展是光纤通信成为一个主要的国际性产业。
用光纤敷设总长度可以表明其发展程序。
据估计，到1987年底仅美国的光纤敷设总长将达320万公里，其中90% 以上是在1982-1987年间敷设并开通的，而长度干线占主导地位，数量约为光纤总长的95% 。
虽然现在人们对于纤维光学的兴趣主要在于通信，但早期发展纤维光学的目的并不在此。
50年代初研究人员制造出第一根具有包层玻璃光纤时，并不想用于通信而是想用它们传送内窥镜需要的成像光束。
1966年Kao和Hockham发表了那篇著名的论文，建议将低损耗光纤用于通信，此时纤维光学已发展为一项很实用的技术了。
1970年10月，第一根低损耗（20dB/km）石英光纤问世了。
有时将这一日期作为光纤通信时代的开端。
虽然这一成果当时在研究领域确实引起了极大的关注，但这种光纤距离通信所要求的条件还相差甚远：每千米20dB的损耗对于长途通信系统仍然是太大了；光纤易断裂，必须寻找保护方法；没有合适的光源。
研究人员不知道光缆的终端和接头是否会发展到实用阶段，至于生产这些器件是否在经济上可行，从而使之在市场上占有重要地位，他们更是存有严重的疑虑。
虽然技术障碍好像不可逾越，但经济潜力却非常明显。
正因为如此，在70年代早期研究和开发工作发展迅速，一些重要问题得以解决。
70年代中后期，由于发展重点由研究领域转入工程实用，因而加速发展了适应市场的产品。
在实验室研制的光纤损减值接近瑞利极限值：0.8µm波长处为2dB/km，1.3µm波长处为0.3dB/km和1.55µm波长处为0.15dB/km。
通过改进光纤外涂层方法和成缆技术，克服了微弯损耗。
生产了加强型光和多纤连接器用于室外作业。
工作在0.8-0.85µm波长区的商用镓铝砷激光器的室温阈值电流减少到20-30mA范围。
据称，激光器和发光管的设计寿命达10万到100万小时。
开发了工作于1.3µm波长附近的光源和改进的光电检测器，从而可以利用光纤在长波长区的低损耗和低色散特性。
这一时期进行的室外实验较重要的有AT&T于1976年在亚特兰大的实验，1977年在芝加哥的实验和1977年日本的用户通路项目。
到了80年代，光纤器件在性能、价格和可靠性方面的改善使众多电话公司受益匪浅。
光纤很快成为长途干线的首选传输媒质。
一些早期敷设的光缆线路采用0.8µm光源和渐变折射率多模光纤，但到1983年，城市间线路的设计者们就考虑使用1.3µm单模光纤系统了。
单模光纤与1.3µm激光器相连，可以提供宽带特性，增加了高速率系统的中继距离。
最近敷设的光纤系统的数据速率已移至每秒几比特范围。
这种系统采用光谱纯的分布反馈激光器，将光纤色散效应减至最小。
在1.55µm波长上设计的低色散光纤，相应地具有低损耗特性，目前广泛用于长途通信。
为进一步增加光纤的信息容量，逐渐广泛采用波分复用方法。
人们对于光纤在其他领域的潜力刚刚开始认识。
用于计算机系统和办公室的光纤网络逐渐变得更加重要。
在电话系统中，光纤在主要城市地区中心交换局间互联和低级交换中的使用继续迅速增加。
入户光缆已经有了示范工程。
许多观察家相信，全国电话系统将使用光纤传输视频宽带信号而逐渐升级。
这些宽带用户环路系统将为可视电话、视频娱乐节目等业务提供通路。
宽带业务广泛使用光纤将会变得经济可行。
异步转移模式ATM（异步转移模式）既是复用技术又是交换技术。
最初，人们是想用ATM 来处理高比特率的数字信号，事实却证明它是一种通用技术，可以用来传输和交换任何类型并具有各种比特率的数字化信息。
无论传输的信息是什么，ATM都以称作“ 信元”的短的分组采传送信息。
信元是由固定的48字节加上5字节的信头组成。
信元寻找路由是基于带有双重识别的逻辑信道。
ATM既与电路方式有关又与分组方式有关。
由于使用简单的协议，信元至网络节点的转移可完全由硬件处理完成，这就缩短了转送时间，提高了传输路径的速率，使比特速率甚至可以达到每秒几百兆比特。
另一方面，ATM保留了分组方式所有的灵活性：只传送所需要的信息，提供简单、独特的复用方法而不管不同信息流的比特率，并且允许比特流的变化。
ATM开始于80年代初，那时人们试图找到一种更适合的技术用于交换超过100Mbit/s的高比特率的信道。
1988年，ITU批准了I .121建议，该建议选择ATM 作为用于各种类型信息宽带网络的目标传送模式，其中包括如话音的低比特率的信息。
1991年一些运营公司和厂商建立了ATM论坛以加速ATM标准化工作。
现在ATM论坛的成员已超过600个，它对A TM 的标准化和规范化有着重要影响。
第一批ATM产品1992年面世，主要用于局域网，其设计主要是用来解决计算机终端间随计算机数量和功能不断增加所带来的共享同一承载电路的问题。
ATM网络可以近似在看作是由三个覆盖功能层组成：业务和应用层、ATM 网络层和传输层。
应用层提供端到端的业务。
应用层使用ATM网络层的逻辑连接，当信元通过由逻辑连接（称作虚连接）共享的传输链路时，ATM网络层依次对信息流复用并寻找信息流的逻辑路由。
传输层提供物理链路并处理信元的实际物理传输。
ATM网络能够传输和交换话音、数据和视频业务，从接入的角度看这些业务使用传统的数字接口并具有同样的服务质量。
这就意味着任何两个终端间的物理连接都可由等效的逻辑连接代替，逻辑连接可在共用的传输。
链路中与其他的逻辑连接复用。
资源可在所有连接中动态共享与同步时分复用技术相比，同步复用技术僵硬地将业务与传输资源相连，而异步技术的优势是根据其确切的需要来占用传输链路。
ATM技术将网络传输的应用和业务与所使用的传输资源完全分开。
构成虚网络的能力意味着物理网络可以由许多用户动态实时地共享，因而使网络结构得到低价高效的使用，对高比特率业务也一样。
对所有网络层的投资都是适应未来需要的，因为不同的应用在出现新的需求时可及时在同一网络结构中进行重新分配。
ATM提供一种独特的方式将传输不同业务的网络协调成单一的物理网络。
以上所描述的ATM优点说明了为什么人们对ATM充满热情。
随着数字化和图像编码技术的进步，交互视频业务和更通常的多媒体业务开始出现。
这些业务将会对网络产生很大的影响。
今天，ATM是唯一能够提供这些业务所需的高比特率和灵活性的传输的技术。
ATM，远比任何其他电信技术更能满足运营公司和用户对当前和未来业务的需求。
与其他有可能在某些应用领域与ATM竞争的技术相比，ATM（主要是由于其通用性，无论是比特率还是传输的信息类型）都具有特殊的优点。
ATM 对所有比特率的信号都可提供交换功能，这一点特别适合于高比特率和可变化比特率信号。
ATM的独特性将使它成为卓越的多媒体业务的自然载体，特别是对于可变比特率的视频，并且使它成为能够提供如视频点播新业务的未来信息高速公路必不可少的一部分。
在很短的时间内，运营者对ATM产生了很大的兴趣，主要是由于将连接的概念与实际资源分开所引入网络的灵活性和虚拟性。
这就简化了网络的管理功能并能最佳地使用网络资源，特别是通过统计复用和建立虚拟专用网络。
当然，在ATM技术普遍使用之前仍会有很长的路要走，但是这场正在进行的技术革命将会深刻在影响数据处理和视频处理，影响电信世界。
这一革命所产生的影响无疑会比在模拟网络中出现数字技术的影响要大得多。
附录2 外文文献原文BROADBAND ACCESS TECHNOLOGIES Broadband access technologies have been spawned by the efforts of the telecommunications and video networking industries to move toward convergence at the physical layer. By contrast, the efforts of the data networking industry toward convergence have generally focused on the networking and transport layers (e.g., IP telephony). The reason for this difference in approach is that, once content has been digitized, it is treated as data. In other words，from a networking perspective, the network merely transports data in digital format. It makes no difference whether the bits actually represent voice，data，or video steams at the application level. Consequently, the data networking industry did not have to do anything different to adapt a data stream for transport across a network. On the other hand, the telecommunications and video networking industries had to figure out way to transport their content as packetized data cost-effectively by working with existing access infrastructure. This was an important catalyst in the development of the following classes of broadband access technologies.Copper-loop access technologies, also known as digital subscriber line (DSL) technologies. DSL technologies are collectively referred to as XDSL.XDSL was developed by the telecommunications industry to make use of the several million miles of existing copper loop infrastructure around the world.Cable access technologies over fiber/coaxial cable. Cable access was developed by the cable TV portion of the video networking industry to take advantage of the cable infrastructure that feeds video channels to several million (mostly residential) subscribers around the world.Satellite access technologies over wireless medium. Satellite access was developed by the wireless portion of the video networking industry to make use of the satellite infrastructure that feeds video channels to cable TV head end offices and to several hundred(mostly residential) subscribers around the world.ADSLADSL was first developed in the 1980s as the telecommunications industry's answer to the cable industry' request to support video on demand. In the mid 1990s, however, it was quickly recognized as a viable technology to enable access tohigh-speed services such as the Internet. ADSL delivers asymmetric transmission rates typically up to 9 Mbps downstream (from the CO to the premises) and 16 Kbps to 640 Kbps upstream (from the premises to the CO). Like all copper transmission systems, the higher the bit rate, the shorter the range.HFC TechnologyHFC technology is based on existing cable television (cable TV or CATV) technology. Originally, cable TV systems were based on coaxial cable facilities from an MSO to a customer premises and used a tree topology. Most of these systems have been upgraded to HFC, by which the signal is brought to a fiber node via a pair of optical fibers and then distributed via coaxial cable to customers. At the head end, signals from various sources, such traditional satellite services, analog and digital services using WAN, and Internet service provider (ISP) services using private backbone network, are multiplexed and converted up from an electrical (radio frequency) signal to an optical signal. Communication is one way on the optical fiber; each of a pair of optical fibers from the head end to the fiber node carries one-way traffic in opposite directions. The optical signal is converted down to RF at the fiber node and travel over the coaxial cable in duplex mode. The signal going from the head to the customer premises is called a downstream signal or a forward path signal. The signal going from the customer premises to the head end is called an upstream signal or a reverse path signal.Introduce to Optical Fiber CommunicationOne of the most important technological developments during the 1980s has been the emergence of optical fiber communication as a major international industry. One indication of the extent of this development is the total length of installed fiber, which was estimated to be 3.2 million kilometers in the U.S. alone by the end of 1987. Over 90% of this fiber was placed in service during the time period of 1982~1987. Long-haul trunk installations have been dominated, accounting for about 95% of the fiber in the U.S.Although telecommunication is the rationale for most of the current interest in fiber optics, this was not the case during the early days of the technology. The researchers who produced the first clad glass optical fibers in the early 1950s were not thinking of using them for communications; they wanted to make imaging bundles for endoscopy. Fiber optics was already a well-established commercial technology when the famous paper by Kao and Hockham, suggesting the use of low-loss optical fibersfor communication, appeared in 1966.The first low-loss (20dB/km) silica fiber was described in a publication which appeared in October of 1970. The date of this publication is sometimes cited as the beginning of the era of fiber communication. Although this development did receive considerable attention in the research community at the time, it was from inevitable that a major industry would evolve. The 20dB/km loss figure was still too high for long-hall telecommunication systems. The fibers were fragile, and a way to protect them would have to be found. There were no suitable light sources. Researchers did not know whether field termination and splicing of optical cables would ever be practical. Finally, there were serous doubts as to whether these components could ever be produced economically enough for the technology to play a major role in the marketplace.Although the technological barriers appeared formidable, the economic potential was very significant. As a consequence, research and development activity expanded rapidly, and a number of important issues were resolved during the early 1970s.During the middle and late 1970s, the rate of progress toward marketable products accelerated as the emphasis shifted from research to engineering. Fibers with losses approaching the Rayleigh limit of 2dB/km at a wavelength of 0.8um, 0.3dB/km at 1.3um, and 0.15dB/km at 1.55um, were produced in the laboratory .Microbend loss problems were overcome through the use of improved fiber coatings and cabling techniques . Rugged cables and multiform connectors were produced for field installation. Rom temperature threshold currents for commercial gallium aluminum arsenide lasers operating in the 0.8 to 0.85um spectral region were reduced to the 20 to 30 mA range, and projected lifetimes in the 100,000 to 1,000,000 hour range were claimed for both lasers and LEDs. Light sources and improved photodetectors which operated near 1.3um, were developed to take advantage of the low fiber loss and dispersion in this “longer wavelength region”. Several major field trials were undertaken during this period, including AT&T，s Atlanta experiment and Chicago installation, and Japan，s subscriber access project.Improvements in component performance, cost, and reliability by 1980 led to major commitments on the part of telephone companies. Fiber soon became the preferred transmission medium for long-haul trunks. Some early installations used 1.8um light sources and graded-index multimode fiber, but by 1983, designers of intercity links were thinking in terms of 1.3um, single-mode systems. Thesingle-mode fiber, used in conjunction with a 1.3um laser, provides a bandwidth advantage which translates into increased repeater spacings for high data rate systems.Data rates for installed fiber optic systems have recently moved into gigabit per second range. Such systems use the spectrally pure distributed-feedback lasers to minimize fiber dispersion effects. Fibers designed for low dispersion at 1.55um wavelength, which corresponds to minimum fiber lass, are now commonly used in long distance transmission. The use of wavelength multiplexing to further increase the fiber information capacity is becoming more widespread.The potential of fiber optics in other areas is only beginning to be realized. Fiber optic networks for computer systems offices are becoming more prominent. In the telephone system, the use of fiber optics for interconnecting central offices within a metropolitan area and for lower levels in the switching hierarchy is still increasing rapidly. Fiber links to the home have been used in demonstration projects. Many observers believe that national telephone systems will eventually be upgraded to handle video bandwidths by using fiber optics. These wideband subscriber loop systems would provide access to services such as picturephone, video entertainment. Widespread installation of these broadband services will become economically feasible.ATMATM (Asynchronous Transfer Mode) is both a multiplexing and switching technique. It was initially intended to handle high bit rates, but it has in fact proved to be a universal technique for transporting and switching any type of digitized information at a wide variety of bit rates.ATM transfers information in short packets called “cells” with a fixed length of 48 bytes plus five header bytes, irrespective of the underlying type of transmission. Cell routing is based on the principle of logical channels with dual identification: the cell header contains the identifier of the basic connection to which the cell belongs-called a virtual circuit (VC) and the identifier of the group of VCs to which the connection belongs-called a virtual path (VP) .ATM is related to both circuit and packet modes. Because of the simplicity of the protocol used, the transfer of cells to the network nodes can be handled entirely by hardware, which leads to very short transit time and high usage of transmission paths, even at bit rates of several hundred megabits a second. On the other hand, ATM retains all the flexibility of the packet mode, enabling only required information to beconveyed, offering a simple, unique multiplexing method irrespective of the bit rates of the different information flows, and allowing these bit rates to be varied.ATM dated from the beginning of the 1980s: at the time , people were trying to find the most suitable technique for switching high bit rate channel at more than 1000 Mbit/s . In 1988, the ITU approved recommendation 1.121 which ratified the choice of ATM as the target transfer mode for broadband networks for all types of information, including low bit rate information such as voice. In 1991 several operators and manufactures founded the ATM Forum to expedite standardization. The ATM Forum now has more than 600 members and has a significant influence on ATM standards and specifications. The first ATM products appeared on the market in 1992: they were for local area networks and were designed to solve the problems of sharing the same bearer circuit between computer terminals as they continue to increase in numbers and power.An ATM network can be considered, in a first approximation, as being three overlaid functional levels: a service and applications level, an ATM network level and a transmission level. The applications provide an end-to-end service. They use the logical connections of ATM network level which in turn multiplexes and logically routes the information flow as ATM cells go through the transmission links shared by logical connections called virtual connections.The transmission level provides these physical links and handles the actual physical transport of the cells.An ATM network can transport and switch voice, data and video which, seen from the access, use traditional digital interfaces with the same quality of service. This means that a physical connection between any two terminals can be replaced with an equivalent logical connection which is multiplexed with others in a common transmission link. The resource is shared dynamically between all the connections.Compared with the synchronous time division multiplexing techniques which rigidly link service to resource, the asynchronous technique has the advantage of occupying the transmission link only in proportion to the exact requirement.The ATM technique completely separates the applications and services transported over a network from the transmission resources used. The ability to construct virtual networks means that the physical network can be shared by many users dynamically and in real time , thereby achieving cost-effective use of infrastructure, for high bit rate services too. Investments at all levels are also future-proofed, because of thedifferent applications can be reallocated in time over the same network infrastructure as requirements arise. ATM offers a unique way of coordinating different networks carrying different services into a single physical network .The advantages described above have explained the enthusiasm for ATM.As digitization and image encoding progress, interactive video services, and more generally multimedia services, are starting to emerge. Their impact on the network will be considerable. Today, ATM is the only transfer technique to offer the high bit rates and flexibility required by these services.ATM, much more than any other telecommunications technique, is able to meet the current and the future requirements of both operators and users. Compared with other techniques that may compete in certain applications, ATM is special mainly due to its universal nature, both in terms of bit rate and type of information transferred. ATM offers a switching function for all bit rates and this is particularly suitable for high and variable bit rates.ATM’s specific features will make it the preeminent nature vehicle for multime dia services, and especially for varying bit rate video, and will make it one of the essential components of future information superhighways offering new services such as video on demand. In the short term, ATM is also proving of great interests to the operators, because of the flexibility and virtuality that it can introduce into networks, by separating the concept of connection from that of physical resources. This simplifies network management functions and makes optimum use of resources, particularly through statistical multiplexing and the creation of virtual private networks.Of course, there is still a long way to go before the ATM techniques is in general use , but a revolution is underway which will deeply affect the worlds of telecommunications, data processing and video. The impact of this upheaval will without any doubt be greater than the advent of digital techniques in analogue networks.。
