軟考計算機基礎之影響數據傳送的主要因素英語
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#
譯文：
影響數據傳送的主要因素
有幾個影響數據傳送的因素,包括速度或帶寬、串行或并行傳送數據流的方向、數據傳送的方式及協議。
帶寬 不同的通信信道有不同的數據傳送速度。信道每秒鐘能傳送的能力被稱為帶寬。帶寬可以是三種類型：話音頻帶，中速帶寬和寬帶。話音頻帶是標準電話線的帶寬，通常被用于微型機的傳送，其每秒傳送的位數是300到期9600。中帶帶寬主要是由小型機和大型機使用的特殊連接線的帶寬，其每秒鐘傳送的位數是56000到264百萬位。寬帶包括微波、衛星、同軸電纜和光纜的信道的帶寬。它主要用于處理器之間直接進行交流的高速計算機中，其每秒鐘傳送的位數在56000到300億位。
串行或并行傳輸 數據以兩種方式傳送：串行和并行。在串行數據傳輸中，數據位的流動是一串或連續的流量，就像汽車通過只有一個車道的橋。每一位在自己的通信線路上流動。串行傳輸是絕大多數數據通過電話線的方式。因此，構成微型計算機的調制解調器的串行連接器的插入板通常被稱為是串行口。對于串行口，其技術的名稱是RS-232C連接器和異步通信端口。對于并行數據傳輸，數據位是通過分離的線路同時流動的。換句話說，它們類似于汽車一起以同樣的速度通過多道快車道。并行傳送主要局限于短距離的傳送，并不通過電話線。然而，它都是以計算機的CPU到打印機的數據傳送的標準方式。
數據傳輸的方向性 在數據通信系統中有三種方向式方式的數據流：單工通信、半雙工通信和全雙工通信。單工通信類似于汽車在單向的街上行駛。數據僅以一個方向傳送。它并不經常用于現在的數據通信系統中。它被使用的一個例子是數據僅僅被輸入的零售點終端。在半雙工通信中，數據是以兩個方向流動，但不是同時。也就是說，數據在任何時候都只以一個方向流動，這類似于只有一個車道的橋上的交通。半雙工很普遍，經常被用于電話線連接的微型機之間以及和小型機及大型機之間的通信。因此，當你通過微機撥入到電子公告牌，你也許會使用半雙工通信。在全雙工通信中，數據同時來回地傳送，像雙向街上的交通。很顯然它是雙向通信中最快最有效的形式。然而它需要特殊的設備并且主要用于大型機的通信。其應用的一個例子是超市或區域辦公室每周將其銷售的數據發送給在另一個地方的公司總部。
數據傳送方式 數據能以異步或同步方式傳送。在異步會傳送方式下，一次只能發送和接收一個字節，是微型機經常使用的方法。異步傳送也常用于速度慢的終端設備。其優點是發送方一方便就可以傳送數據。缺點是相對較慢的數據傳送速率。同步傳送用于一次發送幾個字節數據塊的大量的信息傳送。為了實現數據傳送，發送方和接收方的字節塊必須以精確的時間間隔出現。因此，系統需要一個同步時鐘。其優點是數據可以被很快地地傳送，缺點是需要花費所需的設備。
協議 為了成功的傳送數據，發送方和接收方必須遵循用以交換信息的一套通信規則。這些用于計算機之間交換數據的規則被稱為是線路協議。為了和其他微機連接，像Crosstalk這樣的通信軟件包就幫助定義協議，諸如速度和方式。TCP/IP(傳輸控制協議和因特網協議)就是Internet上的用于通信的兩個標準協議。
TCP/IP是Internet的“語言”，它是由美國政府防御高級研究所項目機構在20世紀70年代開發的網絡技術。它最常用于提供訪問Internet，但也被許多人用于建立與Internet相聯或不聯的局域網。在許許多多方面，PCP/IP是客戶服務器類型的局域網，但許多TCP/IP軟件的生產商具有允許“客戶機”提供文件甚至應用的應用軟件。PCP/IP是一個真正的開放系統協議。這就意味著沒有一個生產商生產這個產品——任何運行PCP/IP軟件的計算機都可以和另一個具有PCP/IP軟件的任一機器相聯(假設該用戶具有賬號和安全口令)，而不管是誰制定的這個指定版本的軟件。
當不同類型的微機在網絡中相聯時，協議是非常復雜的。顯然，為了實現通信，這些網絡協議必須堅持一定的標準。第一套商業可用標準是IBM公司的系統網絡結構，該標準只用于IBM自己的設備，不過，其他的機器不需要與它們交流。國際標準化組織已經制定了一套被稱為開放系統互聯的通信協議。開放系統互聯模式的目的是判別由任一網絡提供的功能。它把每個網絡的功能分成七層協議，即通信規則。當每個網絡系統通信時，它們對應層可以交換數據。這種情況是假設每一個網絡中的微機和其他的設備完成了同樣的功能和具有同樣的界面。
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