Bilingual course Network Communications 网络通信 (For Master Students in the Department of Electronic Engineering) Chapter 6 The Data Communications Interface nstructor: Dr. Tianshuang Qiu School of the Electronic and Information Engineering Fall of 2004
Bilingual Course Network Communications 网络通信 (For Master Students in the Department of Electronic Engineering) Chapter 6 The Data Communications Interface Instructor: Dr. Tianshuang Qiu School of the Electronic and Information Engineering Fall of 2004
86-1 Asynchronous and Synchronous Transmission 说明 本节主要讨论信号的串行传输 假定每个信号元素表示一个bit ·两个主要问题 噪声等传输损伤对接收数据的影响,可能导致出错; 时序问题,若发/收不精确一致,也可能出错,且这种时间上 的不一致会积累 网络通信
网络通信 2 §6-1 Asynchronous and Synchronous Transmission • 说明 – 本节主要讨论信号的串行传输 – 假定每个信号元素表示一个bit • 两个主要问题 – 噪声等传输损伤对接收数据的影响，可能导致出错; – 时序问题，若发/收不精确一致，也可能出错，且这种时间上 的不一致会积累
1.异步和同步传输 (1)异步传输 1)概念 In asynchronous transmission, each character of data is treated independently. Each character begins with a start bit that alerts the receiver that a character is arriving. The receiver samples each bit in the character and then looks for the beginning of the next character 2)方法 The strategy with this scheme is to avoid the timing problem by not sending long, uninterrupted streams of bits. Instead, data are transmitted one character at a time where each character is five to eight bits in length. Timing or synchronization must only be maintained within each character; the receiver has the opportunity to resynchronize at the beginning of each new character 3)工作过程 (Please see Figure 6. 1 in Page 176, E6, Next slide) 4)优缺点 优点:仅需一条信道,且传输距离较远 缺点:额外开销较大(约20侧络通信
网络通信 3 1. 异步和同步传输 （1）异步传输 1) 概念 In asynchronous transmission, each character of data is treated independently. Each character begins with a start bit that alerts the receiver that a character is arriving. The receiver samples each bit in the character and then looks for the beginning of the next character. 2) 方法 The strategy with this scheme is to avoid the timing problem by not sending long, uninterrupted streams of bits. Instead, data are transmitted one character at a time, where each character is five to eight bits in length. Timing or synchronization must only be maintained within each character; the receiver has the opportunity to resynchronize at the beginning of each new character. 3）工作过程 （Please see Figure 6.1 in Page 176, E6, Next slide） 4) 优缺点 优点：仅需一条信道，且传输距离较远； 缺点：额外开销较大（约20%）
Idle state色色 Dr knees. Odd or even w o Remain idle o lAn of line b ig ac parity or unused ai next start bit 5 to 8 data bits I to 2 bit time Start I Stop bit element bit (a) Character format Unpredicatable time interval 界积 between characters Stop Stop Start element Start bit s element ↓ 11010001 01101。10 to nL-luartib simd s a s(b)8-bit asynchronous character stream vins ni won 120m 1 50(150250350450550650750850 Transmitter timing(s) 应 ↓-↓↓↓-↓↓↓↓ Fs。 ↑↑个个↑↑↑↑↑ 471412353294235176117057 Receiver timing (us) Der: (e) Effect of timing error s006 Figure 6.1 Aynchronous Transmission
网络通信 4
Description on Figure 6.1 不传送字符时,发收之间的线路处于“空闲”状态 字符的传送从“起始位( start bit)”开始(二进制0),其后跟 着组成字符的5-8bit数据 数据字符的传送从最低位开始 在数据bit之后,通常跟有一个奇偶校验位( odd or even parity bit),校验位的值由发送器设置。其作用是使整个字符(包括 校验位本身)的二进制1的总数为偶数(称为偶校验)或奇数 (称为奇校验) 最后一个元素是“停止位( stopelement) 发送器不停地发送停止位,直到出现下一个起始位 网络通信
网络通信 5 Description on Figure 6.1 • 不传送字符时，发/收之间的线路处于“空闲”状态； • 字符的传送从“起始位（start bit）”开始（二进制0），其后跟 着组成字符的5—8bit数据； • 数据字符的传送从最低位开始； • 在数据bit之后，通常跟有一个奇偶校验位（odd or even parity bit），校验位的值由发送器设置。其作用是使整个字符（包括 校验位本身）的二进制1的总数为偶数（称为偶校验）或奇数 （称为奇校验）； • 最后一个元素是“停止位（stop element）”。 • 发送器不停地发送停止位，直到出现下一个起始位
Effect of Timing Error The timing requirements for this scheme are modest(适中的)For example, IRA (International Reference Alphabet )character are typically sent as 8-bit units, including the parity bit. If the receiver 5 percent slower or faster than the transmitter, the sampling of the eight character bit will be displaced(EJ)45 percent and still b correctly sampled. Figure 6. 1(c)shows the effects of a timing error of sufficient magnitude to cause an error in reception. In this example we assume a data rate of 10, 000 bits per second(10kbps); therefore each bit is0.1 ms. or 100us. duration. assume that the receiver is fast by 6 percent, or 6us per bit time. Thus, the receiver samples the incoming character every 94us. As can be seen, the last sample is erroneous 网络通信
网络通信 6 Effect of Timing Error • The timing requirements for this scheme are modest（适中的）. For example, IRA (International Reference Alphabet) character are typically sent as 8-bit units, including the parity bit. If the receiver is 5 percent slower or faster than the transmitter, the sampling of the eight character bit will be displaced（移动）45 percent and still be correctly sampled. Figure 6.1(c)shows the effects of a timing error of sufficient magnitude to cause an error in reception. In this example we assume a data rate of 10,000 bits per second (10kbps); therefore, each bit is 0.1 ms, or 100us, duration. Assume that the receiver is fast by 6 percent, or 6us per bit time. Thus, the receiver samples the incoming character every 94us. As can be seen, the last sample is erroneous
两种错误: The last sampled bit is incorrectly received The bit count may now be out ofalignment If bit 7 is a l and bit 8 is ao. bit 8 could be mistaken for a start bit. this condition is termed a framing error (组帧错误), as the character plus start bit and stop element are sometimes referred to as a frame. a frame error can also occur if some noise condition causes the false appearance of a start bit during the idle state 网络通信 7
网络通信 7 • 两种错误： – The last sampled bit is incorrectly received. – The bit count may now be out of alignment. If bit 7 is a 1 and bit 8 is a 0, bit 8 could be mistaken for a start bit. This condition is termed a framing error（组帧错误）, as the character plus start bit and stop element are sometimes referred to as a frame. A frame error can also occur if some noise condition causes the false appearance of a start bit during the idle state
(2)同步传输 1)概念 With synchronous transmission a block of bits is transmitted in a steady stream without start and stop codes. The block may be many bits in length. To prevent timing drift between transmitter and receiver, their clocks must somehow(某种方 式) be synchronized One possibility is to provide a separate clock line between transmitter and receiver. One side(transmitter or receiver) pulses the line regularly with one short pulse per bit time The other side uses these regular pulses as a clock. Th technique works well over short distances as the data signal but over long distances the clock pulses are subject to the same impairment (afi) as the data signal, and timing errors can occur 网络通信
网络通信 8 （2）同步传输 1）概念 With synchronous transmission, a block of bits is transmitted in a steady stream without start and stop codes. The block may be many bits in length. To prevent timing drift between transmitter and receiver, their clocks must somehow（某种方 式）be synchronized. One possibility is to provide a separate clock line between transmitter and receiver. One side (transmitter or receiver) pulses the line regularly with one short pulse per bit time. The other side uses these regular pulses as a clock. This technique works well over short distances as the data signal, but over long distances the clock pulses are subject to the same impairment （损伤）as the data signal, and timing errors can occur
The other alternative is to embed the clocking information in the signal. For digital signals, this can be accomplished with Manchester or Differential Manchester encoding For analog signals, a number of techniques can be used; for example the carrier frequency itself can be used to synchronize the receiver based on the phase of the carrier 每个数据块以一个“前同步码( preamble)”开始,用 个“后同步”码( postamble)”结束。此外,数据中的控 制信息中也有一部分是起同步作用的 方法(如何让接收器判断数据块的开始和结束) 网络通信 9
网络通信 9 The other alternative is to embed the clocking information in the signal. For digital signals, this can be accomplished with Manchester or Differential Manchester encoding. For analog signals, a number of techniques can be used; for example, the carrier frequency itself can be used to synchronize the receiver based on the phase of the carrier. 每个数据块以一个“前同步码（preamble）”开始，用一 个“后同步”码（postamble）”结束。此外，数据中的控 制信息中也有一部分是起同步作用的。 – 方法（如何让接收器判断数据块的开始和结束）
嵌入式时钟编码举例 o:1:001:1:0:0:0:11 NRZ-L NRZI a)白日 a:nisin:5 Bipolar-AN氵 Pseudoternary: Manchester 几Ln Differentiel LLLLLLL! L Figure 5.2 Digital Signal Encoding Formats 10
网络通信 10 嵌入式时钟编码举例