[精选简短的英语儿歌双语]英语儿歌：北风呼呼吹The north wind doth blow,北风呼呼吹，And we shall he snow,我们将会迎来一场雪，And what will the robin do then, poor thing?知更鸟怎么办，小可怜...+阅读

A simplified block diagram of a data munication 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 puter 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 puters, 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 enpassing 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 munications equipment owned and maintained by Telco[4]. Recent judgments he removed modems from the realm of exclusive Telco property.

Data munications 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 mon 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 microwe transmission system can also be used for the data munication. Finally, you can use the satellite munication 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 anization of manufacturers concerned with establishing industry standards, he agreed on the RS-232C as the standard interface between the DTE and the modem. This is a 25-pin cable[7] whose pins he designated functions and specified signal levels. The RS-232C is anticipated to be replaced by an updated standard.

NOTES

[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。

图8-1是数据通信链路的简化框图，虽然图中只画了一个辅助设备，但它代表了多点系统中的典型辅助设备。主计算机的功能之一是存储各种辅助设备的应用程序。

端设备可以是计算机、打印机、键盘、CRT等，它们可以生成要发送的数字信息，也可以使用所接收的数字数据，这种设备通常在内部以字为单位处理数字信息——在一个实际设备中构成一个字的所有位是并行传送的。数字数据在发送时是串行的。并行发送一个8位的字需要8对传输线，这是不经济的。那些在发送、接收数据时分别完成并-串和串-并转换并进行数据链路管理的所有电路，一般称为数据终端设备（DTE）。UART（通用异步接收/发送器）和USART（通用同步/异步接收/发送器）是实现并串和串并转换的设备。基本的DTE包括用于控制多点数据链路系统中信息流的线路控制部件（LCU或LinCo）。辅助设备一方的相应设备是工作站控制器（STACO）。如果DTE有与LCU相关的软件，则称为前端处理机（FEP）。同时，DTE也是数据链路系统中属于用户端的最后一个设备。在两个DTE之间，先是调制解调器，接着是属于并由电话公司维护的通信设备。最近的调整结果是调制解调器已不属于电话公司管理的设备了。

数据通信设备（DCE）从DTE接收串行数据流，并将其转换成适合于在话音线路上发送的某种模拟信号形式。在接收端，DCE完成相反的功能，把接收到的模拟信号转换成串行数字数据流。最简单的DCE是调制解调器或数传机。在发送端，调制解调器可以视为一种数/模转换器，而在接收端则可视为一种模/数转换器。调制解调器最常用的几种调制方式是移频键控（FSK）、移相键控（PSK）和正交调幅（QAM）。这是一种典型的使用模拟电话线路传输数据的方式，见图8-1中两个DCE之间底部的传输线。如果使用数字信道（有时称为数字T载波）传输数据，则应使用脉码调制（PCM）设备，见图8-1中间部分。也可以用微波传输系统进行数据通信。当然也可以用卫星通信系统传输数据，图8-1中未画出。

如果连接DTE和DCE的电缆和信号电平都是未调整的，则所产生的偏差量就可能与生产厂家的数量成正比。电子工业协会（EIA），一个由建立行业标准的厂家组成的组织，同意将RS-232C作为DTE和调制解调器间的标准接口。这是一个25针的电缆，每一针都规定了其功能及信号电平。预计RS-232C将被修订的标准所取代

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