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