数据通信的基础知识概述

数据通信的基础知识概述数据通信是通信技术和计算机技术相结合而产生的一种新的通信方式。

要在两地间传输信息必须有传输信道，根据传输媒体的不同，有有线数据通信与无线数据通信之分。

但它们都是通过传输信道将数据终端与计算机联结起来，而使不同地点的数据终端实现软、硬件和信息资源的共享。

数据通信的交换方式及适用范围1．数据通信的交换方式通常数据通信有三种交换方式：（1）电路交换电路交换是指两台计算机或终端在相互通信时，使用同一条实际的物理链路，通信中自始至终使用该链路进行信息传输，且不允许其它计算机或终端同时共亨该电路。

（2）报文交换报文交换是将用户的报文存储在交换机的存储器中（内存或外存），当所需输出电路空闲时，再将该报文发往需接收的交换机或终端。

这种存储—转发的方式可以提高中继线和电路的利用率。

（3）分组交换分组交换是将用户发来的整份报文分割成若于个定长的数据块（称为分组或打包），将这些分组以存储—转发的方式在网内传输。

第一个分组信息都连有接收地址和发送地址的标识。

在分组交换网中，不同用户的分组数据均采用动态复用的技术传送，即网络具有路由选择，同一条路由可以有不同用户的分组在传送，所以线路利用率较高。

2．各种交换方式的适用范围（1）电路交换方式通常应用于公用电话网、公用电报网及电路交换的公用数据网（cSPDN）等通信网络中。

前两种电路交换方式系传统方式；后一种方式与公用电话网基本相似，但它是用四线或二线方式连接用户，适用于较高速率的数据交换。

正由于它是专用的公用数据网，其接通率、工作速率、用户线距离、线路均衡条件等均优于公用电话网。

其优点是实时性强、延迟很小、交换成本较低；其缺点是线路利用率低。

电路交换适用于一次接续后，长报文的通信。

（2）报文交换方式适用于实现不同速率、不同协议、不同代码终端的终端间或一点对多点的同文为单位进行存储转发的数据通信。

由于这种方式，网络传输时延大，并且占用了大量的内存与外存空间，因而不适用于要求系统安全性高、网络时延较小的数据通信。

（3）分组交换是在存储—转发方式的基础上发展起来的，但它兼有电路交换及报文交换的优点。

它适用于对话式的计算机通信，如数据库检索、图文信息存取、电子邮件传递和计算机间通信等各方面，传输质量高、成本较低，并可在不同速率终端间通信。

其缺点是不适宜于实时性要求高、信息量很大的业务使用。

数据通信的分类1．有线数据通信（1）数字数据网（DDN）数字数据网由用户环路、DDN节点、数字信道和网络控制管理中心组成。

DDN是利用光纤或数字微波、卫星等数字信道和数字交叉复用设备组成的数字数据传输网。

也可以说DDN是把数据通信技术、数字通信技术、光纤通信技术以及数字交叉连接技术结合在一起的数字通信网络。

数字信道应包括用户到网络的连接线路，即用户环路的传输也应该是数字的，但实际上也有普通电缆和双绞线，但传输质量不如前。

（2）分组交换网分组交换网（PSPDN）是以ccITT Ｘ．2５建议为基础的，所以又称为Ｘ.25网。

它是采用存储—转发方式，•将用户送来的报文分成具用一定长度的数据段，并在每个数据段上加上控制信息，构成一个带有地址的分组组合群体，在网上传输。

分组交换网最突出的优点是在一条电路上同时可开放多条虚通路，为多个用户同时使用，网络具有动态路由选择功能和先进的误码检错功能，但网络性能较差。

（3）帧中继网帧中继网络通常由帧中继存取设备、帧中继交换设备和公共帧中继服务网3部分组成。

帧中继网是从分组交换技术发展起来的。

帧中继技术是把不同长度的用户数据组均包封在较大的帧中继帧内，加上寻址和控制信息后在网上传输。

2．无线数据通信无线数据通信也称移动数据通信，它是在有线数据通信的基础上发展起来的。

有线数据通信依赖于有线传输，因此只适合于固定终端与计算机或计算机之间的通信。

而移动数据通信是通过无线电波的传播来传送数据的，因而有可能实现移动状态下的移动通信。

狭义地说，移动数据通信就是计算机间或计算机与人之间的无线通信。

它通过与有线数据网互联，把有线数据网路的应用扩展到移动和便携用户。

网络及其协议1．计算机网络计算机网络（Computer Network)，就是通过光缆、双绞电话线或有、•无线信道将两台以上计算机互联的集合。

通过网络各用户可实现网络资源共享，如文档、程序、打印机和调制解调器等。

计算机网络按地理位置划分，可分为网际网、广域网、城域网和局域网四种。

Internet是世界上最大的网际网；广域网一般指连接一个国家内各个地区的网络，广域网一般分布距离在100－1000公里之间；城域网又称为都市网，它的覆盖范围一般为一个城市，方圆不超过10－100公里；局域网的地理分布则相对较小，如一栋建筑物，或一个单位、一所学校，甚至一个大房间等。

局域网是目前使用最多的计算机网络，一个单位可使用多个局域网，如财务部门使用局域网来管理财务帐目，劳动人事部门使用局域网来管理人事档案、各种人才信息，公安刑侦部门使用局域网来管理犯罪信息系统、交警部门使用局域网来管理机动车辆、驾驶员信息等等。

2．网络协议网络协议是两台计算机之间进行网络对话所使用的语言，网络协议很多，有面向字符的协议、面向比特的协议，•还有面向字节计数的协议，•但最常用的是TCP/IP协议。

它适用于由许多LAN组成的大型网络和不需要路由选择的小型网络。

TCP／IP协议的特点是具有开放体系结构，并且非常容易管理。

TCP／IP实际上是一种标准网络协议，是有关协议的集合，它包括传输控制协议和因特网协议。

TCP协议用于在应用程序之间传送数据，IP协议用于在程序与主机之间传送数据。

由于TCP／IP具有跨平台性，现已成为Internet的标准连接协议。

网络协议分为如下四层：〈1〉网络接口层：负责接收和发送物理帧；〈2〉网络层：负责相邻节点之间的通信；〈3〉传输层：负责起点到终端的通信；〈4〉应用层：提供诸如文件传输、电子邮件等应用程序要把数据以TCP／IP 协议方式从一台计算机传送到另一台计算机，数据需经过上述四层通信软件的处理才能在物理网络中传输。

目前的IP协议是由32位二进制数组成的，如202．0．96．133就表示连接到因特网上的计算机使用的IP地址，在整个因特网上IP地址是唯一的。

数据通信的应用前景1．有线数据通信的应用（1）数字数据电路（DDN）的应用范围有：①组建公用数字数据通信网；②可为公用数据交换网、各种专用网、无线寻呼系统、可视图文系统、•高速数据传输、会议电视、ISDN（2B+D信道或30B+D信道）、邮政储汇计算机网络等提供中继或数据信道；③为帧中继、虚拟专用网、LAN，以及不同类型的网络提供网间连接；④利用DDN实现大用户局域网联网；⑤提供租用线，让大用户自己组建专用数字数据传输网；⑥使用DDN作为集中操作维护的传输手段。

（2）分组交换网的应用分组交换网能提供永久虚电路（PVc）及交换虚电路（SVc）等多种业务。

利用分组交换网的通信平台，还可以开发与提供一些增值数据业务：①电子信箱业务②电子数据交换业务③传真存储转发业务④可视图文业务（2）帧中继技术的应用帧中继技术适用于对广域网进行数据访问和高速数据传输。

帧中继也是一种ISDN承载业务，主要用于局域网互联和高速主机环境下作为宽带网的数据入口，是向未来宽带ATM交换过渡的手段之一。

常用于：①组建帧中继公用网，提供帧中继业务。

②在分组交换机上安装帧中继接口，提供业务。

③为用户提供低成本的虚拟宽带业务。

④在专用网中，采用复用的物理接口可以减少局域网互联时的桥接器、•路由器和控制器所需的端口数量，并减少互连设备所需通信设施的数量。

帧中继的数字链路连接鉴别（DLcI）寻址功能可允许单个中继接入设备与上千个接入设备通信。

其本地管理接口（LMI）可大大简化帧中继网的配置和管理。

⑤局域网（LAN）与广域网（WAN）的高速连接。

⑥LAN与LAN的互联。

⑦远程计算机辅助设计/制造文件的传送、图像查询以及图像监视、会议电视等2．无线数据通信的应用无线数据通信也称为移动数据通信。

它的业务范围很广，也有广泛的应前景。

（1）移动数据通信在业务上的应用。

移动数据通信的业务，通常分为基本数据业务和专用数据业务两种：基本数据业务的应用有电子信箱、传真、信息广播、局域网（LAN）接入等。

•专用业务的应用有个人移动数据通信、计算机辅助调度、车、船、舰队管理、ＧPS汽车卫星定位、远程数据接入等。

（2）移动数据通信在工业及其它领域的应用。

移动数据通信在这些领域的应用可分为固定式应用、移动式应用和个人应用三种类型。

①固定式应用是指通过无线接入公用数据网的固定式应用系统及网络。

如边远山区的计算机入网、交警部门的交通监测与控制、收费停车场、加油站以及灾害的遥测和告警系统等。

②移动式应用是指野外勘探、施工、设计部门及交通运输部门的运输车、船队和快递公司为发布指示或记录实时事件，通过无线数据网络实现业务调度、远程数据访问、报告输入、通知联络、数据收集等均需采用移动式数据终端。

移动数据终端在公安部门的刑警、巡警、交警也开始应用。

③个人应用是指专业性很强的业务技术人员、公安外线侦察破案人员等需要在外办公时，通过无线数据终端进行远程打印、传真、访问主机、数据库查询、查证。

股票交易商也可以通过无线数据终端随时随地跟踪查询股票信息，即使度假也可以从远程参加股票交易。

Rudimentary knowledge summary of data communicationThe communication technology and computer technology that data communication is combine but a kind of new communication way that produce . Must transmit the channel while transmitting information between two places, according to transmitting the difference of medias, difference of wired data communication and wireless data communication. But they all connect data terminal and computer through transmitting the channel , and make the data terminal of different places realize the sharing of software and hardware and information resources.The pattern of exchanging of data communication and scope of application1. Usually there are three kinds of patterns of exchanging in data communication for the pattern of exchanging of data communication:(1) The circuit exchangingIt means two computers or terminal stations are while communicating each other that the circuit exchanges the circuit and exchanges, use the same real physics chain, use this chain to carry on the information transmission from start to finish in communication, allow other computer or terminal station henries of this circuit altogether at the same time.(2)Newspaper document exchangenewspaper document exchange is it store among memory of exchanger newspaper document of user to exchange (whether memory or outside deposit), as is it when circuit idle , mail exchanger or terminal station received to need to this newspaper document and then to output while being necessary. This kind of memory - the way to transmit can improve the utilization ratio of junction line and circuit .(3)Dividing into groups exchangesit make long data piece on piece to cut whole copies of newspaper document that user send apart into to exchange to divide into groups to exchange to divide into groups (is it divide into groups or bale to call), divide into groups in order to store these - way that transmit transmit in network. The first message of dividing into groups has identifications receiving the address and sending the address in succession . In the packet switching network, different users' grouping dataadopt the technology transfer that is replied and used dynamically , namely the network has routes to choose, there can be different users' grouping to convey in the same route, so the utilization ratio of the circuit is relatively high.2. Scope of application of different patterns of exchanging(1)Circuit pattern of exchanging apply public telephone network, public telegram network and public datum network(cSPDN) that circuit exchange,etc. among the communication networks usually. Department's traditional way of pattern of exchanging of first two kinds of circuit; The last way and public telephone network are similar basically, but it joins users in Line four or way of Line two, suitable for the data interchange of higher speed . Data network not public on it is to be special-purpose in it,putting through rate, speed , the user line not working of it from, circuit terms not balanced ,etc. last telephone network not public . Its advantage is that real-time character is strong, delay very small and exchanging the cost relatively low; Its shortcoming is that a utilization ratio of a circuit is low. The circuit exchanges and is suitable for once after continuing, the communication of the long message.(2)The gentle pattern of exchanging of the newspaper is suitable for realizing different speed , different agreements , terminal stations or the one point to many point one , with storing the data communication that is transmitted for the unit gentlies of different codes terminal station. Because of way this, network prolong heavy when transmitting, take a large amount of memory and outside check space, not suitable to prolong while requiring the system security high , network smaller data communication.(3)Is it store - transmit foundation of the way not to develop to exchange to divide into groups, but it has advantage that circuit exchanges and the newspaper document exchange concurrently. It is suitable for conversational compunication , for instance the database is searched , picture and text information depositting and withdraw , E-mail transmitting and such different fields as communication ,etc. among the computers, it is relatively low to transmit the cost high in quality, and in the communication among different speed terminal station. Its shortcoming is not to suitable for expecting much , business use very great in amount of information of real-time character .1. Wired data communication(1)Digital Data Net (DDN)Digital Data Net is controlled the administrative center to make up by user cycle , DDNnode , digital channel and network. DDN utilizes such digital channels as optic fibre or digital microwave , satellite ,etc. and figure to reply the digital data transmission net made up of equipment alternately. We can say too DDN connect data communication technology , digital communication technology , fiber optic communication technology and figure alternately digital communication networks that technology conbined together. The digital channel should include users to the connection circuit of the network , the transmission of the namely users cycle should be the the figure one, but there are ordinary cables and paired line in fact, but before transmitting quality and is not so good as.(2)The packet switching networkpacket switching network (PSPDN ) is with ccITT X. Based on 25 suggestions, so also called X. 25 networks . It adopts storing- transmit the way, • divides the newspaper document which users sent here into having data section which uses certain length, control information on the section in each data , form a grouping with address make the colony up, transmit on the net. Packet switching network most outstanding advantage to can open large timid thorough fares at the same time at one circuit, use at the same time for a lot of user, network have dynamic route choose function and a advanced one yard examine function by mistake by mistake, but the performance of the network is relatively bad.(3)Frame relaying networkframe relaying network deposit and withdraw by frame relaying equipment , frame relaying exchange equipment and public frame relaying service network 3 part make up usually. The frame relaying network is developed from the swapper of dividing into groups. Frame relaying technology pairs of different user data group in bigger frame relaying frame seal of length, in addition, transmit on the net after seeking the location and controlling information.2. Wireless data communicationwireless data communication call the mobile data communication too, it develops on the basis of wired data communication. Wired data communication depends on transmitting wiredly, is only suitable for the communication between regular terminal station and the computer or the computer. And the mobile data communication conveyed the data through the spread of the radiowave, therefore may realize moving mobile communication under the state . Narrow sense say mobile data communication computer computer and wireless communication of people sometimes. It passes with the wired data network interconnectedly, expands the application of thenetworking of the wired data to moving and portable users.puter network (Computer Network), entangle telephone wire or have , • wireless channel two the above-mentioned computer interconnected set through optical cable , pair. Can realize network resource-sharing through network every user, for instance file , procedure , printer and modem ,etc.. Computer network divide according to geographical position , can divide into Internet , WAN , city land network and LAN 4 kinds. Internet is the biggest Internet in the world; WAN generally means that joins the network of each area in a country, WAN is generally distributed in the distance among 100-1000 kilometers; City land net is call city net, its coverage is generally a city, do not have an area of 10-100 kilometers; The geographical distribution of the LAN is relatively small , such as a building, or a unit , a school, even a large room ,etc..The LAN is a computer network used most at present, a unit can use a lot of LANs , if financial department use LAN is it manage financial account to come, personnel department use LAN is it manage personal file , various kinds of talent message to come, public security men criminal investigation department use LAN is it manage crime information system , traffic police department use LAN is it manage motor vehicle , driver message ,etc. to come to come.2. ProcotolProcotol whether two computer carry on language that network's talking use, there are a lot of procotols, there are agreements facing agreement , facing bit of the character , the agreement that • still faces bytes and counts, but the most frequently used one is TCP/IP agreement in •. It is suitable for the large-scale network composed of a lot of LANs and small-scale network not needing the route to choose . The characteristic of TCP/IP agreement has open system structure, and very manageable.。