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  A simplified block diagram of a data communication link is shown in Fig.8-1. Although only one secondary is shown[1], it represents a typical secondary in a multipoint system. One of the functions of the host computer is to store the applications programs for the various secondaries.

  The end equipment which either generates the digital information for transmission or uses the received digital data can be computers, printers, keyboards, CRTs, and so on. This equipment generally manipulates digital information internally in word units – the number of bits that make up a word in a particular piece of equipment are transferred in parallel. Digital data, when transmitted, are in serial form. Parallel transmission of an 8-bit word would require eight pairs of transmission lines – not at all cost-effective[3]. Data terminal equipment (DTE) is a general phrase encompassing all of the circuitry necessary to perform parallel-to-serial and serial-to-parallel conversions for transmission and reception respectively and for data link management. The UART (Universal Asynchronous Receiver/Transmitter) and USART(Universal Synchronous/Asynchronous Receiver/Transmitter) are the devices that perform the parallel-to-serial and serial-to-parallel conversions. The primary DTE includes a line control unit (LCU or LinCo) which controls the flow of information in a multipoint data link system. A station controller (STACO) is the corresponding unit at the secondaries. If there is software associated with the LCU, it is then called a front-end processor (FEP). At one time, the DTE was the last piece of equipment that belonged to the subscriber in a data link system. Between the DTEs, starting with the modems, was communications equipment owned and maintained by Telco[4]. Recent judgments have removed modems from the realm of exclusive Telco property.

  Data communications equipment (DCE)[5] accepts the serial data stream from the DTE and converts it to some form of analog signal suitable for transmission on voice-grade lines. At the receive end, the DCE performs the reverse function of converting the received analog signal to a serial digital data stream. The simplest form of DCE is a modem (modulator/demodulator) or data set. At the transmit end, the modem can be considered a form of digital-to-analog converter, while at the receive end, it can be considered a form of analog-to-digital converter. The most common of modulation by modems are frequency shift keying (FSK), phase shift keying (PSK), and quadrature amplitude modulation (QAM). This is a typically data transmission mode using the analog telephone lines, see the bottom line between the two DCEs in Fig.8-1. If you transmit data by digital channel (sometimes it is called "Digital T-carrier[6]"), a Pulse Code Modulation (PCM) equipment must be used, see the middle part of Fig.8-1. A microwave transmission system can also be used for the data communication. Finally, you can use the satellite communication system for data transmission, non-shown in Fig.8-1.

  If the cables and signal levels used to interconnect the DTE and DCE were left unregulated, the variations generated would probably be proportional to the number of manufacturers. Electronics industries Association (EIA), an organization of manufacturers concerned with establishing industry standards, have agreed on the RS-232C as the standard interface between the DTE and the modem. This is a 25-pin cable[7] whose pins have designated functions and specified signal levels. The RS-232C is anticipated to be replaced by an updated standard.


  [1] 从设备是指负责处理I/O和通信任务的设备。在大型通信系统中一般是指前端处理机FEP、线路控制器LCU;在微机组成的网络中采用双CPU的服务器,其中一个CPU可作为从设备的控制部件。

  [2] 图中ROP-Read-Only Printers,早期计算机用的只作输出的打印机,因为当时还有一种键盘打印机,既可作为输出又可作为输入。

  [3] 此句最后省略which is,即应为which is not at all cost-effective.

  [4] Telco是指美国所有进入公共电话网(PTN)的电话公司。

  [5] DCE也称为数据电路终接设备Data Circuit-terminating Equipment.

  [6] T-carrier源于美国贝尔公司的数字通信系统,将从音频频带上来的信号转换成数字脉冲,沿金属线传送称为T-carrier.

  [7] cable:电缆,此处系指RS-232C标准使用的25针插座。





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