计算机专业英语第九章部分翻译(2)

的类型的连接服务,拨号服务相当差,很多用户发现它不足以满足其通信需求。

For years, large corporations have been leasing special high-speed lines from telephone companies. These lines-known as T1, T2, T3, and T4 lines-support all digital communications, do not require conventional modems, and provide very high capacity. Unfortunately, this type of connection is very expensive.For example,T1 lines provide a speed of 1.5 mbps (over 26 times as fast as a conventional modem) and cost several thousand dollars.

多年以来,大型公司已经租赁特殊的高速线路从电话公司。这些线已知为T1、T2、T3、T4线支持所有数字通信,不需要传统的调制解调器,并提供非常高的能力。不幸的是,这种类型的连接是非常昂贵的。例如,T1线路提供的速度为1.5 mbps(超过26倍常规调制解调器)和成本数千美元。

While the special high-speed lines are too costly for most individuals,there are affordable connections that provide significantly higher capacity than standard dial-up service. These include DSL, cable modems, satellite,and cellular. For a comparison of typical user connection costs and speeds,see figure 9-13

而特殊的高速线太过昂贵,大多数个人,有负担得起的连接,提供能力显著高于比标准的拨号服务。这些包括DSL、电缆调制解调器、卫星,和细胞。对于一个比较典型的用户连接的成本和速度,见图9 - 13

? Digital subscriber line (DSL) uses existing telephone lines to provide

high-speed connections. ADSL (asymmetric digital subscriber line) is one of the most widely used types of DSL. This technology is widely available in most areas. 数字用户线(DSL)使用现有电话线提供高速连接。ADSL(不对称数字用户线)是一种最广泛使用的类型的DSL。这个技术是广泛使用在大多数地区。

? Cable modems use existing television cables to provide high-speed connections

as fast as a T1 or DSL connection, at a lower cost. Although cable connections reach 90 percent of the homes in America, all cable companies do not support cable modems. Industry observers, however. Predict 100 percent availability within the next few years. 电缆调制解调器使用现有的电视电缆提供高速互联网连接速度T1或DSL连接,以较低的成本。虽然电缆连接达到90%的家庭在美国,所有的电缆公司不支持电缆调制解调器。行业观察人士,然而。100%的可用性预测在未来几年。

? Satellite/air connection services use satellites and the air to download(downlink)

or send data to users at a rate seven times faster than dial-up connections. While older satellite services could not upload (uplink)or send data to satellites and had to rely on slow dial-up connections,newer two-way satellite connections are now available. While slower than DSL and cable modems, satellite/air connections are available almost anywhere that a satellite-receiving disk can be aimed at the southern skies. 卫星/空气连接服务使用卫星和空中下载(下行),或将数据发送给用户速率比拨号连接快7倍。而年长的卫星服务不能上传(上行)或将数据发送给卫星,不得不依靠缓慢的拨号连接,更新的双向卫星连接现在可用。虽然低于DSL和电缆调制解调器、卫星/空气连接可用的几乎任何地方,卫星接收磁盘可以针对南方的天空。

? Cellular services offer an alternative for mobile devices and laptops. Using 3G

cellular networks, devices such as cell phones and appropriately equipped laptop computers can download data from the Internet at 400 to 700 kbps. Uploading data, however, is a much slower 50 kbps. At present,the availability of this service is limited to specific metropolitan areas. In the future, however, this service is expected to be more widespread and to operate at speeds comparable to DSL. 蜂窝服务替代移动设备和笔记本电脑。使用3 g蜂窝网络,设备,如手机和笔记本电脑可以适当装备从网上下载数据在400至700 kbps。上传数据,然而,是一个非常慢的50 kbps。目前,可用性的服务仅限于特定的大都市地区。在未来,然而,这种服务将成为更广泛和运行时速与DSL。

DATA TRANSMISSION

Several factors affect how data is transmitted. These factors include bandwidth and protocols.

几个因素影响数据传输。这些因素包括带宽和协议。

BANDWIDTH

Bandwidth is a measurement of the width or capacity of the communication channel. Effectively, it means how much information can move across the communication channel in a given amount of time. For example, to transmit text documents, a slow bandwidth would be acceptable. However, to effectively transmit video and audio, a wider bandwidth is required. There are three categories of bandwidth.

带宽是一个测量的宽度或容量的通信通道。实际上,这意味着多少信息可以移动整个通信信道在给定的时间。例如,传输文本文档,一个缓慢的带宽将是可以接受的。然而,有效地传输视频和音频,一个更广泛的带宽是必需的。有三个类别的带宽。

? Voiceband, also known as low bandwidth, is used for standard telephone

communication. Microcomputers with standard modems and dial-up service use this bandwidth. While effective for transmitting text documents, it is too slow for many types of transmission, including high-quality audio and video. Typical speeds are 56 to 96 kbps. 音频带宽,也称为低带宽,是用于标准的电话沟通。微型计算机与标准调制解调器和拨号服务使用这个带宽。而有效的传输文本文档,它太慢了许多类型的传输,包括高质量的音频和视频。典型的速度是56至96 kbps

? Medium band is the bandwidth used in special leased lines to connect

minicomputers and mainframes as well as to transmit data over long dis-tances. Unlike voiceband and broadband, medium band is not typically used by individuals. 介质带的带宽用于特殊租用线路连接小型机、大型机以及传输数据长dis自然界。与音频带宽和宽带、中带通常不是由个人使用。

? Broadband is the bandwidth used for high-capacity transmissions.

Microcomputers with DSL, cable, and satellite connections as well as other more specialized high-speed devices use this bandwidth. It is capable of effectively meeting most of today's communication needs, including transmitting high-quality audio and video. Speeds are typically 1.5 mbps, although much higher speeds are possible. 宽带是用于高容量的传输带宽。微型计算机与DSL,有线电视和卫星连接以及其他更专门的高速设备使用这个带宽。它能够有效的会议今天大多数的通信需求,包括传输高质量的音频和视频。速度通常是1.5 mbps,尽管更高的速度是可能的。 PROTOCOLS

For data transmission to be successful, sending and receiving devices must follow a set of communication rules for the exchange of information. These rules for exchanging data between computers are known as protocols.

对于数据传输成功,发送和接收设备必须遵循一套通信规则的信息交流。这些规则之间交换数据的计算机被称为协议。

The standard protocol for the Internet is TCP/IP (transmission control protocol/Internet protocol). The essential features of this protocol involve (1) identifying sending and receiving devices and (2) reformatting information for transmission across the Internet.

互联网的标准协议是TCP / IP(传输控制协议/互联网协议)。本议定书的基本特点包括:(1)确定发送和接收设备和(2)重新格式化信息在互联网上传播。

? Identification: Every computer on the Internet has a unique numeric address

called an IP address (Internet protocol address). Similar to the way a postal service uses addresses to deliver mail, the Internet uses IP addresses to deliver e-mail and to locate Web sites. Because these numeric addresses are difficult for people to remember and use, a system was developed to automatically convert text-based addresses to numeric IP addresses. This system uses a domain name server (DNS) that converts text-based addresses to IP addresses. For example, whenever you enter a URL, say www.computing2008.com, a DNS converts this to an IP address before a connection can be made. (See Figure 9-14.) 标识:每台电脑在互联网上有一个独特的数字地址称为一个IP地址(互联网协议地址)。类似于一个邮政服务使用地址送邮件,互联网使用IP地址提供电子邮件和网站定位。因为这些数字地址是困难的对于人们记住和使用,系统的开发目的自动转换文本地址到IP地址。该系统使用一个域名服务器(DNS),将基于文本的地址的IP地址。例如,当你输入一个URL www.computing2008.com,一个域名,说到IP地址转换这个连接可以之前。

? Reformatting: Information sent or transmitted across the Internet usually travels

through numerous interconnected networks. Before the message is sent, it is reformatted or broken down into small parts called packets. Each packet is then sent separately over the Internet, possibly traveling different routes to one common destination. At the receiving end, the packets are reassembled into the correct order. 重新格式化:信息发送或通过互联网通常穿过众多的互联网络。在消息被发送,这是重新格式化或分解成小的部分称为数据包。每个包然后在互联网上单独发送,可能旅行不同的路线到一个共同的目标。在接收端,包成正确的顺序重新组装。

NETWORK ARCHITECTURE

Network architecture describes how a network is arranged and how resources are coordinated and shared.It encompasses a variety of different network specifics, including network configurations and strategies.Network configurations describe the

physical arrangement of the network. Network strategies define how information and resources are shared.

网络体系结构描述了一个网络安排和资源协调和共享。它包含了各种不同的网络细节,包括网络配置和策略。网络配置描述物理安排网络。网络策略定义信息和资源共享。

CONFIGURATIONS

A network can be arranged or configured in several different ways. This arrangement is called the network's topology.The four principal network topologies are star, bus, ring, and hierarchical.

一个网络可以安排或配置在几种不同的方式。这样的安排被称为网络的拓扑结构。四项主要的网络拓扑是明星,巴士,戒指,和层次。

In a star network, a number of small computers or peripheral devices are linked to a central unit.(See Figure 9-18.) The central unit is the network hub and is typically a host computer or file server.

在一个星形网络,一些小型计算机或外围设备被连接到一个中央单位。中央单位是网络中心,通常是一个主机或文件服务器。

All communications pass through this central unit. Control is maintained by polling. That is, each connecting device is asked (\Each device is then in turn allowed to send its message

所有通信通过这个中央单位。控制是由轮询。即,每个连接装置是问(“调查”)是否有一个消息发送。每个装置,然后反过来允许发送的信息

One particular advantage of the star topology is that it can be used to support a time-sharing system. That is,several users can share resources (time) on a central computer. The star is a common topology for linking microcomputers to a mainframe that allows access to an organization's database.

一个特定的优势是,它星型拓朴结构可以用于支持分时系统。那是,几个用户可以共享资源(时间)在一个中央计算机。星是一种常见的拓扑结构为连接微机到主机,允许访问一个组织的数据库。

In a bus network each device in the network handles its own communications control. There is no host computer. All communications travel along a common connecting cable called a bus or backbone. (See Figure 9-19.) As the information

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