Text 21Data Transmission

Data may be transmitted between two points in two different ways.Lets consider sending28 bits of digital data(1 byte).These bits may be sent all at once3(in parallel), or one after the other(serial).

To transfer data on a parallel link, a parallel transmission may convey eight bits at a time through eight parallel channels.Although the transfer rate is eight times faster than in bit-serial transmission, eight channels are needed, and the cost may be as much as eight times higher to transmit the message.When distances are short, it may nonetheless be both feasible and economic to use parallel channels in return for high data rates.The popular Centronics printer interface is a case where parallel transmission is used.As another example, it is common practice to use4a 16-bit-wide data bus to transfer data between a microprocessor and memory chips; this provides the equivalent of 16 parallel channels.On the other hand, when communicating with a5timesharing system over a modem, only a single channel is available, and serial transmission is required.

Most digital messages are vastly longer than just a few bits.Because it is neither practical nor economic to transfer all bits of a long message simultaneously, the message is broken into smaller parts and transmitted sequentially.Bit-serial transmission conveys a message one bit at a time through a channel.Each bit represents a part of the message.The individual bits are then reassembled at the destination to compose the message.In general, one channel will pass only one bit at a time.Thus, bit-serial transmission is necessary in data communications if only a single channel is available.Bit-serial transmission is normally just called serial transmission and is the chosen communications method in many computer peripherals.

Serialized data is not generally sent at a uniform rate through a channel.Instead,6there is usually a burst of regularly spaced binary data bits followed by a pause, after which the data flow resumes.Packets of binary data are sent in this manner, possibly with7variable-length pauses between packets, until the message has been fully transmitted.In order for the receiving end to know the proper moment to read individual binary bits from the channel, it must know exactly when a packet begins and how much time elapses between bits.When this timing information is known, the receiver is said to be synchronized with the transmitter, and accurate data transfer becomes possible.Failure to remain synchronized throughout a transmission will cause data to be corrupted or lost.

Two basic techniques are employed to ensure correct synchronization.In synchronous systems, separate channels are used to transmit data and timing information.The timing channel transmits clock pulses to the receiver.Upon receipt of a clock pulse, the receiver reads the data channel and latches the bit value found on the channel at that moment.The data channel is not read again until the next clock pulse arrives.Because the transmitter originates both the data and the timing pulses, the receiver will read the data channel only when told to do so by the transmitter(via the clock pulse), and synchronization is guaranteed.

Techniques exist to merge the timing signal with the data so that only a single channel is required.This is especially useful when synchronous transmissions are to be sent through a modem.Two methods in which a data signal is self-timed are8nonreturn-to-zero and biphase Manchester coding.These both refer to methods for encoding a data stream into an electrical waveform for transmission.

In asynchronous systems, a separate timing channel is not used.The transmitter and receiver must be preset9in advance to an agreed-upon baud rate.A very accurate local oscillator within the receiver will then generate an internal clock signal that is equal to the transmitter’s within a fraction of a percent.For the most common serial protocol, data is sent in small packets of 10 or 11 bits, eight of which constitute message information.When the channel is idle, the signal voltage corresponds to a continuous logic“1”.A data packet always begins with a logic“0”(the start bit)to signal the receiver that a transmission is starting.The start bit triggers an internal timer in the receiver that generates the needed clock pulses.Following the start bit, eight bits of message data are sent bit by bit at the agreed upon baud rate.The packet is concluded with a parity bit and stop bit.One complete packet is illustrated by Figure 2.

The packet length is short in asynchronous systems to minimize the risk that the local oscillators in the receiver and transmitter will drift apart.When high-quality crystal oscillators are used, synchronization can be guaranteed over an 11-bit period.Every time a new packet is sent, the start bit resets the synchronization, so the pause between packets can be10arbitrarily long.Note that the EIA232 standard defines electrical, timing, and mechanical characteristics of a serial interface.

NEW WORDS AND PHRASES

digital[´didʒitl]adj.数字的，数位的，手指的 n.数字，数字式

serial[´siəriəl]adj.连续的，串行的

parallel[´pærəlel]adj.平行的，相同的 n.平行线，相似物 v.相应，平行

convey[kən´vei]vt.搬运，传达，转让

feasible[´fi:zəbl]adj.可行的，切实可行的

equivalent[i´kwivələnt]adj.相等的，相当的，同意义的 n.等价物，相等物

vastly[´va:stli]adv.广大地，许多，巨额地

individual[:indi´vidjuəl]n.个人，个体 adj.个别的，单独的

peripheral[pə´rifərəl]adj.外围的 n.外围设备

uniform[´ju:nifɔ:m]adj.统一的，相同的 n.制服 vt.使成一样，使穿制服

synchronize[´siŋkrənaiz]v.同步

synchronous[´siŋkrənəs]adj.同时的，[物]同步的

asynchronous[ei´siŋkrənəs]adj.不同时的，[电]异步的

pulse[pʌls]n.脉搏，脉冲

originate[ə´ridʒineit]vt.引起，发明，发起 vi.起源，发生

biphase[bai´feiz]双相（的）

nonreturn-to-zero 不归零的

self-time 自定时

fraction[´frækʃən]n.小部分，片段，分数

constitute[´kɔnstitju:t]vt.制定（法律），建立（政府），组成，任命

idle[´aidl]adj.空闲的，懒惰的 vi.不做事，游手好闲 vt.虚度，使空闲

corresponds to 相对应的

trigger[´trigə]vt.引发，引起，触发 n.扳机

voltage[´vəultidʒ]n.[电工]电压，伏特数

preset[´pri:´set]vt.事先调整 n.边框形式

waveform[´weivfɔ:m]波形

packet[´pækit]n.小包裹，小捆 v.包装 信息包

characterization[:kæriktərai´zeiʃən]n.描述，人物之创造

arbitrarily adv.武断地，任意地，专横地

guarantee[:gærən´ti:]n.保证，保证书，担保，抵押品 vt.保证，担保

crystal[´kristl]adj.结晶状的 n.水晶，水晶饰品，结晶，晶体

oscillator[´ɔsileitə]n.振荡器

drift[drift]n.冲洗，漂流物，漂流 v.（使）漂流

mechanical[mi´kænikl]adj.机械的，机械制的，机械似的，呆板的

generate[´dʒenə:reit]vt.产生，发生

encode[in´kəud]vt.把（电文、情报等）译成电码（或密码）

NOTES

1.Data Transmission 本文介绍数据传输的基本内容，主要包括串行通信与并行通信的特点和异步通信和同步通信的特点。

2.“8 bits of digital data”翻译为“8比特的数字数据”。

3.“(in parallel), or one after the other(serial)”句中的in parallel指并行通信，而serial指串行的，one after the other指发送的方式是逐个传输。

4.a 16-bit-wide data bus 16位宽的数据总线。

5.timesharing system时分系统。

6.there is usually a burst of regularly spaced binary data bits followed by a pause句中“burst”突发性的脉冲，翻译为“突发性地发送一串固定长度的二进制数据脉冲，然后紧跟就是一段时间的暂停”。

7.variable-length pauses（时间）可变长度的暂停。

8.nonreturn-to-zero and biphase Manchester coding不归零的编码和差分曼彻斯特编码。

9.in advance预先。

10.arbitrarily long任意的长度。

EXERCISES

一、请将下列词组译成英文

1.异步传输　　　　　　　　　　　 2.同步传输

3.串行通信　　　　　　　　　　　 4.并行通信

5.分组格式　　　　　　　　　　　 6.触发本地时钟

7.空闲状态　　　　　　　　　　　 8.脉冲的起始位

9.脉冲下降沿　　　　　　　　　　 10.信息流

二、请将下列词组译成中文

1.computer peripherals

2.a uniform rate

3.timing information

4.transfer data on a parallel link

5.bit-serial transmission

6.a microprocessor and memory chips

7.a clock pulse

8.Packets of binary data

9.a timesharing system over a modem

10.variable-length pauses

三、选择题

1.The popular Centronics printer interface is a case where______is used.

A.serial transmission　　　　　　　　　 B.parallel transmission

C.simplex communication　　　　　　　 D.half-duplex communication

2.Bit-serial transmission is normally just called______and is the chosen communications method in many computer peripherals.

A.serial transmission　　　　　　　　　 B.parallel transmission

C.simplex communication　　　　　　　 D.half-duplex communication

3.In synchronous systems,separate channels are used to transmit data and______.

A.timing information　　　　　　　　　 B.control information

C.error information　　　　　　　　　　D.signaling

4.When the channel is idle, the signal voltage corresponds to.A data packet always begins with(the start bit)to signal the receiver that a transmission is starting.

A.a continuous logic“0”, a logic“1”

B.a continuous logic“0”, a logic“1”

C.a continuous logic“1”, a logic“0”

D.a continuous logic“1”, a logic“1”

5.Two methods in which a data signal is self-timed are nonreturn-to-zero and biphase Manchester coding.These both refer to methods for encoding a data stream into______for transmission.

A.binary digit　　　　　　　　　　　　B.light signal

C.decimal value　　　　　　　　　　　D.an electrical waveform

四、请将下列句子译成中文

1.Although the transfer rate is eight times faster than in bit-serial transmission, eight channels are needed, and the cost may be as much as eight times higher to transmit the message.

2.Most digital messages are vastly longer than just a few bits.Because it is neither practical nor economic to transfer all bits of a long message simultaneously, the message is broken into smaller parts and transmitted sequentially.

3.Serialized data is not generally sent at a uniform rate through a channel.Instead, there is usually a burst of regularly spaced binary data bits followed by a pause, after which the data flow resumes.

4.Two basic techniques are employed to ensure correct synchronization.In synchronous systems, separate channels are used to transmit data and timing information.

5.In asynchronous systems, a separate timing channel is not used.The transmitter and receiver must be preset in advance to an agreed-upon baud rate.

参考译文

数据传输

数据在两点之间可以按照不同的方式传输。我们考虑发送8位的数据（1个字节），这8位可以立刻一起发送（并行）出去，也可以逐个被发送（串行）出去。

在并行链路上传输数据，它可以在8个并行的通道上一次同时传送8位数据。尽管传输的速率将是比特串行传输的8倍，但同时也需要8条信道，这样发送信息的成本也将提高8倍。如果距离短，从提高速率的角度来说，这是可行和经济的。现流行的打印机的接口就是一个并行传输的例子。另外一个例子就是在微处理器和存储芯片之间采用 16 位宽的数据总线来传输数据，这个等同于提供了 16 条并行的通道。另外，在一个使用时分复用方式的调制解调通信中，只有一个通道能使用，这就要求使用串行通信了。

大多数的数字信息的长度要远超过几个比特，由于同时传递一个比较长信息的所有比特是不现实和不经济的，因此一个信息经常被分成几个小的部分，然后依次传输。比特串行传输方式中，在信道中一次只传输一个比特，每个比特都是信息的一部分，在目的地端，各个比特再组合成原来的信息。总之，就是一个信道一次只允许一个比特通过。因此，在只有一个信道的情况下采用比特串行传输是非常必要的。比特串行传输通常被称为串行传输，它也是很多计算机外围设备通信中主要选择的方式。

通常，串行方式的数据在信道中传输的时候，其速率并不是统一的。相反，固定空间长度的二进制数据突发后，会有一个暂停，然后再发送后面的数据。二进制数据的分组就是以这种方式来发送的。在数据的发送中，数据分组之间暂停的长度可能是变化的，为了能够在接收端进行正确的定时，保证在信道中正确地读出每个二进制的比特，就必须知道每个分组的起始和比特之间的时间间隔。一旦能够知道定时的信息，接收机和发射机之间就完成了同步，准确传输数据就很有可能了。不能达到这一点，就会导致数据的破坏和丢失。

目前主要使用两种基本的技术来确保正确的同步。在同步系统中，分开的信道用于分别传输信息和定时信息。发送定时信息的信道会发送一个定时的脉冲给接收机。根据收到的时钟脉冲，接收机读取数据信道中的数据并在正确时刻锁定比特值，下一个时钟脉冲来的时候再读取后面的数据。由于是发射机产生数据和时钟脉冲，接收机只能按发射机的指示工作（通过时钟脉冲），这样就可以确保同步了。

在只有一个单独信道的情况下，将定时信号和数据合并在一起的技术也很重要。特别在调制解调的同步中，这个技术非常的有用。自己定时的数据信号经常使用归零编码和差分曼彻斯特编码。这两种方式都是将数据流编码转换成电信号的波形进行传输。

在异步通信系统中，不需要使用独立的定时信道。发射机和接收机使用预先设定的波特率来发送数据。接收机内一个非常准确的本地振荡器产生与发射机内完全相同的时钟信号。对于大多数的通用串行协议，数据一般以10或11位小的分组为单位，其中的8位为数据信息。当信道空闲的时候，信号将是一个持续的逻辑“1”的电平。每一个数据的分组都用逻辑“0”电平做起始位，这个起始位会触发接收机的内部时钟产生所需要的定时脉冲。跟随起始位，8 个信息数据将以约定的波特率逐个传输，然后数据分组后面有一个校验位和结束位。图2表示了一个完整的数据分组。

异步系统中的分组长度比较小的原因是为了减少发射机和接收机中的本地振荡器产生漂移的风险。在异步传输中，使用高质量的晶体振荡器，可以确保超过 11 位周期长度的数据的同步。每次发送新的分组的时候，起始位就重新设定同步。因此分组之间的暂停长度可以是任意的。在EIA232标准中规定了一种串行接口中的电平、定时和机械特性。