Main Factors Affecting Data Transmission
There are several factors that affect data transmission. They include speed or bandwidth, serial or parallel transmission, direction of data flow, modes of transmission data, and protocols.
Bandwidth The different communications channels have different data transmission speeds. This bit-per-second transmission capability of a channel is called its bandwidth. Bandwidth may be of three types: voiceband, medium band, and broadband. Voiceband is the bandwidth of a standard telephone line and used often for microcomputer transmission, the bps is 300-9600. Medium band is the bandwidth of special liased lines used mainly with minicomputers and mainframe computers, the bps is 56,000/264 million. Broadband is the bandwidth that includes microwave, satellite, coaxial cable, and fiber-optic channels. It is used for very high-speed computers whose processors communicate directly with each other. It is in the range of 56,000/30 billion bps.
Serial or Parallel Transmission Data travels in two ways: serially and in parallel. In serial data transmission, bits flow in a serial or continuous stream, like cars crossing a one-lane bridge. Each bit travels on its own communications line. Serial transmission is the way most data is sent over telephones lines. Thus, the plug-in board making up the serial connector in a microcomputer's modem is usually called a serial port. More technical names for the serial port are RS-232C connector and asynchronous communications port. With parallel data transmission, bits flow through separate lines simultaneously. In other words, they resemble cars moving together at the same speed on a multilane freeway. Parallel transmission is typically limited to communications over short distances and is not used over telephone lines. It is, however, a standard methods of sending data from a computer's CPU to a printer.
Direction of Data Transmission There are three directions or modes of data flow in a data communications system: simplex communication, half-duplex communication, and full-duplex communication. Simplex communication resembles the movement of cars on a one-way street. Data travels in one direction only. It is not frequently used in data communication systems today. One instance in which it is used may be in point-of sale (POS) terminals in which data is being entered only. In half-duplex communication, data flows in both directions, but not simultaneously. That is, data flows in only one direction at any one time. This resembles traffic on a one-lane bridge. Half-duplex is very common and is frequently used for linking microcomputers by telephone lines to other microcomputers, minicomputers, and mainframes. Thus, when you dial into an electronic bulletin board through your microcomputer, you may well be using half-duplex communication. In full-duplex communication, data is transmitted back and forth at the same time, like traffic on a two-way street. It is clearly the fastest and most efficient form of two-way communication. However, it requires special equipment and is used primarily for mainframe communications. An example is the weekly sales figures that a supermarket or regional office sends to its corporate headquarters in another place.
Modes of Transmitting Data Data may be sent by asynchronous or synchronous transmission. In asynchronous transmission, the method frequently used with microcomputers, data is sent and received one byte a time. Asynchronous transmission is often used for terminals with slow speeds. Its advantage is that the data can be transmitted whenever convenient for the sender. Its disadvantage is a relatively slow rate of data transfer. Synchronous transmission is used to transfer great quantities of information by sending several bytes or a block at a time. For the data transmission to occur, the sending and receiving of the blocks of bytes must occur at carefully timed intervals. Thus, the system requires a synchronized clock. Its advantage is that data can be sent very quickly. Its disadvantage is the cost of the required equipment.
Protocols For data transmission to be successful, sender and receiver must follows a set of communication rules for the exchange of information. These rules for exchanging data between computers are known as the line protocol. A communication software package like Crosstalk helps define the protocol, such as speeds and modes, for connecting with another microcomputer. TCP/IP ( Transmission Control Protocol and Internet Protocol) are the two standard protocols for communications on the Internet.
TCP/IP is the "language" of the Internet. It is a networking technology developed by the United States Government Defense Advanced Research Project Agency (DARP) in the 1970s. It is most commonly employed to provide access to the Internet but can be and is used by many people to create a LAN that may or may not connect to the Internet. In many aspects TCP/IP is a client/server-type LAN, but many manufacturers of TCP/IP software have applications that allow the "clients" to serve files or even applications. TCP/IP is truly an open systems protocol. This means that no one manufacturer creates the product—any computer running TCP/IP software can connect to anyone else who has TCP/IP software (provided the user has an account and security permissions), regardless of who made the particular version of software.
When different types of microcomputers are connected in a network, the protocols can become very complex. Obviously, for the connections to work, these network protocols must adhere to certain standards. The first commercially available set of standards was IBM's Systems Network Architecture (SNA). This works for IBM's own equipment, but other machines won't necessarily communicate with them. The International Standards Organization has defined a set of communications protocols called the Open Systems Interconnection (OSI). The purpose of the OSI model is to identify functions provided by any network. It separates each network's functions into seven "layers" of protocols, or communication rules. When two network systems communicate, their corresponding layers may exchange data. This assumes that the microcomputers and other equipment on each network have implemented the same functions and interfaces.#p#
译文：
影响数据传送的主要因素
有几个影响数据传送的因素,包括速度或带宽、串行或并行传送数据流的方向、数据传送的方式及协议。
带宽 不同的通信信道有不同的数据传送速度。信道每秒钟能传送的能力被称为带宽。带宽可以是三种类型：话音频带，中速带宽和宽带。话音频带是标准电话线的带宽，通常被用于微型机的传送，其每秒传送的位数是300到期9600。中带带宽主要是由小型机和大型机使用的特殊连接线的带宽，其每秒钟传送的位数是56000到264百万位。宽带包括微波、卫星、同轴电缆和光缆的信道的带宽。它主要用于处理器之间直接进行交流的高速计算机中，其每秒钟传送的位数在56000到300亿位。
串行或并行传输 数据以两种方式传送：串行和并行。在串行数据传输中，数据位的流动是一串或连续的流量，就像汽车通过只有一个车道的桥。每一位在自己的通信线路上流动。串行传输是绝大多数数据通过电话线的方式。因此，构成微型计算机的调制解调器的串行连接器的插入板通常被称为是串行口。对于串行口，其技术的名称是RS-232C连接器和异步通信端口。对于并行数据传输，数据位是通过分离的线路同时流动的。换句话说，它们类似于汽车一起以同样的速度通过多道快车道。并行传送主要局限于短距离的传送，并不通过电话线。然而，它都是以计算机的CPU到打印机的数据传送的标准方式。
数据传输的方向性 在数据通信系统中有三种方向式方式的数据流：单工通信、半双工通信和全双工通信。单工通信类似于汽车在单向的街上行驶。数据仅以一个方向传送。它并不经常用于现在的数据通信系统中。它被使用的一个例子是数据仅仅被输入的零售点终端。在半双工通信中，数据是以两个方向流动，但不是同时。也就是说，数据在任何时候都只以一个方向流动，这类似于只有一个车道的桥上的交通。半双工很普遍，经常被用于电话线连接的微型机之间以及和小型机及大型机之间的通信。因此，当你通过微机拨入到电子公告牌，你也许会使用半双工通信。在全双工通信中，数据同时来回地传送，像双向街上的交通。很显然它是双向通信中最快最有效的形式。然而它需要特殊的设备并且主要用于大型机的通信。其应用的一个例子是超市或区域办公室每周将其销售的数据发送给在另一个地方的公司总部。
数据传送方式 数据能以异步或同步方式传送。在异步会传送方式下，一次只能发送和接收一个字节，是微型机经常使用的方法。异步传送也常用于速度慢的终端设备。其优点是发送方一方便就可以传送数据。缺点是相对较慢的数据传送速率。同步传送用于一次发送几个字节数据块的大量的信息传送。为了实现数据传送，发送方和接收方的字节块必须以精确的时间间隔出现。因此，系统需要一个同步时钟。其优点是数据可以被很快地地传送，缺点是需要花费所需的设备。
协议 为了成功的传送数据，发送方和接收方必须遵循用以交换信息的一套通信规则。这些用于计算机之间交换数据的规则被称为是线路协议。为了和其他微机连接，像Crosstalk这样的通信软件包就帮助定义协议，诸如速度和方式。TCP/IP(传输控制协议和因特网协议)就是Internet上的用于通信的两个标准协议。
TCP/IP是Internet的“语言”，它是由美国政府防御高级研究所项目机构在20世纪70年代开发的网络技术。它最常用于提供访问Internet，但也被许多人用于建立与Internet相联或不联的局域网。在许许多多方面，PCP/IP是客户服务器类型的局域网，但许多TCP/IP软件的生产商具有允许“客户机”提供文件甚至应用的应用软件。PCP/IP是一个真正的开放系统协议。这就意味着没有一个生产商生产这个产品——任何运行PCP/IP软件的计算机都可以和另一个具有PCP/IP软件的任一机器相联(假设该用户具有账号和安全口令)，而不管是谁制定的这个指定版本的软件。
当不同类型的微机在网络中相联时，协议是非常复杂的。显然，为了实现通信，这些网络协议必须坚持一定的标准。第一套商业可用标准是IBM公司的系统网络结构，该标准只用于IBM自己的设备，不过，其他的机器不需要与它们交流。国际标准化组织已经制定了一套被称为开放系统互联的通信协议。开放系统互联模式的目的是判别由任一网络提供的功能。它把每个网络的功能分成七层协议，即通信规则。当每个网络系统通信时，它们对应层可以交换数据。这种情况是假设每一个网络中的微机和其他的设备完成了同样的功能和具有同样的界面。
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