UMTS

    For effective, efficient communications, standardization is critical, and nowhere is this more evident than in the areas of mobile computing and cellular telephony.

    If you need data access or e-mail through your cell phone, you’re likely to be using one of two different technologies: Code Division Multiple Access (CDMA) or Global Service of Mobile Communication (GSM).

    Both CDMA and GSM are second-generation (2G) technologies, and they have co-existed for several years. Each technology has its supporters. CDMA phones are engineered specifically for an individual carrier, whereas GSM phones make use of a removable memory card called the Subscriber Identity Module (SIM). A SIM card contains all the key information required to activate a phone, including the user’s telephone number, personal identification number, address book and encoded network identification details. A user can easily move a SIM from one phone to another.

    Though GSM phones are interoperable with one another, different countries use different parts of the frequency spectrum, so “world phones” typically must be capable of using several frequencies.

    Today, the fastest-growing use of cellular networks is for the transmission of all kinds of data and rich media, including Web sites, video, music, images, and maps and driving directions. The older 2G networks simply couldn’t handle that volume of traffic, and they couldn’t offer the speed needed for transmitting large files. The answer was to make the services faster and build out the networks to deal with more traffic.

    Here, too, the CDMA and GSM paths continued their separate but parallel development. CDMA brought us CDMA2000 and 1xRTT networks. The most recent developments are 1x Evolution Data Optimized, or EV-DO, and 1x Evolution Data/Voice, or EV-DV.

    Similarly, GSM begat General Packet Radio Service, or GPRS, which begat enhanced data rates for GSM evolution, or EDGE. EDGE was developed to enable the transmission of large amounts of data at a high speed, 384Kbit/sec. The latest generation is called Wideband Code Division Multiple Access (WCDMA).

    And this finally brings us to Universal Mobile Telecommunications System.

    The International Telecommunication Union (ITU), a specialized agency of the United Nations, has attempted to coordinate these competing technologies to improve throughput and increase interoperability. The International Mobile Telecommunications 2000 standard is a third-generation digital communications specification from the ITU. And the European (i.e., GSM-based) implementation of IMT-2000 is UMTS, which is based on WCDMA. Previous cellular telephone data systems were mostly circuit-switched, requiring a dedicated connection. WCDMA is packet-switched, using the Internet Protocol. The first commercial WCDMA network was launched in Japan in 2001.

    UMTS has been specified as an integrated application for mobile voice and data systems with wide-area coverage. aUsing globally harmonized spectrum in paired and unpaired bands, early implementations of UMTS offer theoretical bit rates of up to 384Kbit/sec. in situations where the mobile device is actually moving. The current goal is to achieve 2Mbit/sec. when both ends of the connection are (at least temporarily) stationary.

    UMTS operates on radio frequencies identified by the ITU IMT-2000 specification document and licensed to operators, using a 5-MHz-wide channel that simplifies deployment for network providers that have been granted large, contiguous blocks of spectrum. Most UMTS systems use frequencies between 1,885 and 2,025 MHz.

    UMTS assigns separate carrier frequencies to incoming and outbound signals, a process called frequency division duplexing (FDD). For symmetric traffic, such as two-way videophones, FDD is highly efficient, allowing uplink and download data rates to be equal, in contrast to technologies such as Asymmetric Digital Subscriber Line service, which typically offers upload rates that are much slower than its download rates. FDD reduces interference and wastes no bandwidth in switching from transmitting to receiving.

    Ongoing work within the 3rd Generation Partnership Project promises increased throughput speeds over the WCDMA Radio Access Network. High-Speed Downlink Packet Access and High-Speed Uplink Packet Access technologies are already standardized, and commercial operators in Asia and North America are putting them through network trials. With theoretical download speeds as high as 14.4Mbit/sec. and uplink speeds of up to 5.8Mbit/sec., these technologies will make it possible for UMTS to offer data transmission speeds comparable to those of hard-wired Ethernet-based networks.

    通用移动通信系统（UMTS）

    为了有效地进行高效的通信，标准化是至关重要的，在移动计算和蜂窝网电话领域中这一点更为明显。

    如果你需要通过手机访问数据或收发电子邮件，那么你就要使用下列两种不同技术中的一种：码分多址（CDMA）或全球通（GSM）。

    CDMA和GSM都是第二代技术（2G），它们已共存多年,每种技术都有支持者。CDMA手机电话是专为一家营运商设计的，而GSM手机电话使用了可抽取的存储卡，叫做用户身份识别模块（SIM卡）。SIM卡包含了打电话所需的所有关键信息，如用户的电话号码、个人身份识别号码、地址簿以及编了码的网络识别细节等。用户很容易将SIM卡从一部电话插入另一部电话。

    虽然GSM电话相互间能互操作，但不同的国家使用不同的频段，所以典型的“世界电话”必须能使用多个频率。

    今天，蜂窝网发展最快的用途是传送各种各样的数据和富媒体，包括网站、视频、音乐、以及地图与驾车指示等。较陈旧的2G网络不能处理这么大的流量，它们也不能提供传送大文件所需的速度。解决办法就是使这些服务的速度更快和构建能处理更大流量的网络。

    对此，CDMA和GSM各自继续向前发展。CDMA带给我们的是CDMA2000和1xRTT网。最新的进展是EV-DO（1x演进数据优化）和EV-DV（1x演进数据/话音）。

    同样，GSM也产生了GPRS（通用包无线电业务），为GSM的演进提高了数据速率，即EDGE。开发EDGE是为了能在384Kb/秒的速度下实现传送大容量的数据。最新的一代叫宽带码分多址（WCDMA）。

    最后，给我们带来了通用移动通信系统（UMTS）。

    国际电信联盟（ITU）是联合国下属的一个专门机构，它试图协调这些相竞争的技术，以提高吞吐量和增强互操作性。国际移动通信2000（IMT-2000）是ITU的第三代数字通信规范。IMT2000的欧洲（基于GSM的）实现就是UMTS，它基于WCDMA。以前的蜂窝网电话数据系统多数是电路交换的，需要专门的连接。而WCDMA是包交换的，采用因特网协议。第一个商用WCDMA于2001年在日本开通。

    UMTS已经被指定为大范围覆盖的移动话音和数据系统的综合应用。利用全球协调的成对和不成对频段，早期的UMTS实现了对实际移动着的移动设备提供384Kb/秒的理论速率。当前的目标是当连接的两端是固定时（至少暂时）要达到2Mb/秒。

    UMTS工作在ITU IMT-2000规范文档指定并发放给营运商的无线电频率上，使用5兆赫频宽的信道简化了那些被授予连续的大频段的网络提供商的部署。大多数UMTS系统使用1885 至2025 兆赫之间的频率。

    UMTS分配给营运商不同的频率，用于输入和输出信号，这个过程叫频分双工（FDD）。对于对称流量，如双向视频会议，FDD非常高效，使上行链和下载的速率相等，刚好与非对称数据用户线（ADSL）技术相反，后者通常提供的上载速度远远低于下载速度。FDD也减少了从发射转换到接收时的干扰和带宽浪费。

    第三代合作伙伴计划中正在进行的工作，有望在WCDMA无线电接入网上提高吞吐速度。高速下行链包接入和高速上行链包接入已经实现了标准化，亚洲和北美的商业营运商正在网络试验中采用。理论的下载速度可达14.4Mb/秒、上行链速度达5.8Mb/秒，这些技术将使UMTS能够提供与硬布线以太网相比拟的数据传输速度。