[工學(xué)]07- datalinkcontrol-全文預(yù)覽
【正文】 en parity) or odd (odd parity) number of oneszEven number of bit errors goes undetected3Cyclic Redundancy Check(循環(huán)冗余 ) P179zFor a block of k bits message, transmitter generates n bit sequence as a frame check sequence (FCS)幀檢驗(yàn)序列zTransmit k+n bits which is exactly divisible by some numberzReceiver divides frame by that numberyIf no remainder, assume no error4Modulo 2 Arithmetic （模 2運(yùn)算）zUses binary addition with no carries (進(jìn)位 ), just the exclusive OR (異或 ) operation. 做加減運(yùn)算時(shí)：相同為 0，相異為 15CyclicRedundancy CheckRemainderAt the receiver:Transmitted Frame T=1010001101011108T= n bit frame to be transmitted 發(fā)送出去的幀（ n bits）D= k bit message, the first k bits of T 數(shù)據(jù)（ k bits）F= nk bit FCS, the last nk bits of T 校驗(yàn)碼（ nk） bitsP=pattern of nk+1 bits。（ 6） 模 2運(yùn)算，相同相加為 0。z也就是從發(fā)送數(shù)據(jù)幀的第一個(gè)比特算起，到該幀的最后一個(gè)比特發(fā)送完畢所需的時(shí)間。完全空閑的信道的利用率是零。 鏈路的利用率 U:A BDATADATAACK傳播時(shí)延 tp處理時(shí)間 tpr傳播時(shí)延 tp處理時(shí)間 tprtT時(shí)間tftoutAppendix 7A Link utilization of stop and wait flow control 鏈路利用率的計(jì)算 p233zUtilization (efficiency)利用率忽略處理時(shí)間，一個(gè)往返周期的總時(shí)間 tT: tT=2tP+tF19Appendix 7A Link utilization of stop and wait flow control p233定義一個(gè)參數(shù) a：其中： d鏈路的傳輸距離 v信號的傳播速度 L信息的長度 R數(shù)據(jù)的發(fā)送速率所以：20Link utilizationzWith the use of multiple frames ,the stop and wait procedure is inadequate. (Inefficient line utilization)yOnly one frame at a time can be in transitzTransmission time is normalized to onezPropagation time is azIf a1, the sender pletes transmission before the leading bits arrive at the receiver.zIf a1, the frame is long, the first bits have arrived before the source pleted the transmission21Stop and Wait Link Utilization22Sliding Windows Flow Control P212zAllow multiple frames to be in transitzReceiver has buffer W longzTransmitter can send up to W frames without ACKzACK includes the number of next frame expectedzSequence number bounded by size of field (k)yFrames are numbered modulo 2kzFor a
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