ymqi06數(shù)字數(shù)據(jù)通信技術

1、通信工程學院通信工程學院主主講講教教師師：祁祁永永敏敏2021-10-101Ch6 Digital Data Communications Techniques 數(shù)字數(shù)據(jù)通信技術數(shù)字數(shù)據(jù)通信技術PRESENTED BY YONGMIN QI (祁永敏)辦公室：辦公室：1#428Email: 主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-102OverviewuAsynchronous Transmission異步傳輸異步傳輸uSynchronous Transmission同步傳輸同步傳輸uTypes of ErrorsuError Detection檢錯檢錯uErro

2、r Correction糾錯糾錯主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-103數(shù)據(jù)傳輸數(shù)據(jù)傳輸 同步同步 異步異步數(shù)據(jù)傳輸數(shù)據(jù)傳輸主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-104并行傳輸并行傳輸主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-105串行傳輸串行傳輸主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-106uTiming定時定時 problems require a mechanism to synchronize the transmitter and receiver接收器在比特

3、中央對信息流采樣接收器在比特中央對信息流采樣如果時鐘排列不規(guī)則或者漂移，那么足夠多的比如果時鐘排列不規(guī)則或者漂移，那么足夠多的比特發(fā)送將導致在錯誤時間采樣的后果。特發(fā)送將導致在錯誤時間采樣的后果。uTwo solutionsAsynchronousSynchronousAsynchronous and Synchronous Transmission主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-107AsynchronousuData transmitted on character字符字符 at a time5 to 8 bitsuTiming only needs

4、maintaining within each characteruResynchronize with each character異步傳輸異步傳輸發(fā)送定時器比特寬度發(fā)送定時器比特寬度100接收器定時接收器定時=141-47=94主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-109實際的起始位和終止位實際的起始位和終止位主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-1010Asynchronous - BehavioruIn a steady stream, interval between characters is uniform 均勻分布

5、均勻分布uIn idle state, receiver looks for transition跳跳變變1 to 0uThen samples next seven intervals (char length)uThen looks for next 1 to 0 for next char主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-1011Pros and ConsuSimpleuCheapuOverhead of 2 or 3 bits per char (20%)uGood for sending data in small blocks主主講講教教師師：祁

6、祁永永敏敏通信工程學院通信工程學院2021-10-1012uBlock of data transmitted without start or stop bitsuClocks must be synchronizeduCan use separate clock lineGood over short distancesSubject to impairments for long distancesuEmbed嵌入clock signal in dataManchester encodingSynchronous - Bit Level主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學

7、院2021-10-1013Synchronous - Block Level數(shù)據(jù)塊級數(shù)據(jù)塊級uNeed to indicate start and end of blockuUse preamble and postamble前前/后同步碼后同步碼uMore efficient (lower overhead) than asynchronousExample: HDLC高級數(shù)據(jù)鏈路控制高級數(shù)據(jù)鏈路控制48 bits control information、preamble and postambleEach data blocks 1000 charactersoverhead=48/(8x

8、1000+48)x100%=0.6%同步幀的格式同步幀的格式主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-1015Types of ErrorsuSingle bit errors單比特差錯單比特差錯One bit alteredAdjacent bits not affectedWhite noise由白噪聲引起由白噪聲引起uBurst errors突發(fā)性差錯突發(fā)性差錯Length BContiguous sequence of B bits in which first last and any number of intermediate bits in erro

9、rImpulse noise由沖激噪聲引起由沖激噪聲引起Fading in wirelessEffect greater at higher data ratesError Detection Process差錯檢驗碼差錯檢驗碼差錯檢驗碼函數(shù)差錯檢驗碼函數(shù)主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-1017Error Detection差錯檢測差錯檢測uAdditional bits added by transmitter for error detection code差錯檢驗碼差錯檢驗碼uParity奇偶校驗奇偶校驗even parity偶校驗：使整個字符中偶校

10、驗：使整個字符中1的個數(shù)為的個數(shù)為偶數(shù)偶數(shù)odd parity奇校驗奇校驗：使整個字符中使整個字符中1的個數(shù)為的個數(shù)為奇數(shù)奇數(shù)Even number of bit errors goes undetected主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-1018例子例子uIf 發(fā)送器發(fā)送字符發(fā)送器發(fā)送字符G(1110001) 并且使用奇校驗并且使用奇校驗, 就附加就附加 1 ，然后發(fā)送，然后發(fā)送 11110001。u接收器檢測接受的字符，如果接收器檢測接受的字符，如果 1的個數(shù)是的個數(shù)是 odd, 則則認為沒錯誤發(fā)生。認為沒錯誤發(fā)生。If 1 bit(or any od

11、d number of bits) 在傳輸在傳輸過程錯誤翻轉過程錯誤翻轉 (for example 11100001) ,那么那么接收器就能檢測到一個錯誤。接收器就能檢測到一個錯誤。Note that, if 2 bits (or any even number) 由于錯誤由于錯誤, an undetected error occurs 主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-1019Cyclic Redundancy Check（CRC）循環(huán)冗余校驗）循環(huán)冗余校驗u Modulo 2 arithmetic模模2運算運算u使用無進位的二進制加法，即使用無進位的二進

12、制加法，即異或異或（exclusive-OR，XOR）操作操作模模2加（加（XOR）：）：ABABAB主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-1020Cyclic Redundancy Check（CRC）循環(huán)冗余校驗）循環(huán)冗余校驗uFor a block of k bits transmitter generates n-k bit sequence（FCS）uTransmit n bits which is exactly divisible整整除除 by some numberuReceive divides frame by that numberIf n

13、o remainder余數(shù)余數(shù), assume no error主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-1021CRCu給定一個給定一個 k bits 數(shù)據(jù)塊，發(fā)送器生成一個數(shù)據(jù)塊，發(fā)送器生成一個（n-k）位比特序列）位比特序列 (FCS)u必須使最后得到的含有必須使最后得到的含有n個比特的幀可以被個比特的幀可以被預先設定的整數(shù)（除數(shù)）整除整除u接收器用接收器用預先設定的整數(shù)把幀區(qū)分把幀區(qū)分 沒有余數(shù)就說明沒有錯誤。沒有余數(shù)就說明沒有錯誤。主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-1022符號定義符號定義T=要發(fā)送的要發(fā)送的n位幀位幀M

14、（D）=k bit 數(shù)據(jù)塊，數(shù)據(jù)塊，T中的前中的前k位位F=n-k位位bit FCS，T中后中后n-k位位P =n-k+1 位的特定比特序列，是預定的位的特定比特序列，是預定的除數(shù)。除數(shù)。2n kMRQPP2nkTMRPPRRQQP2n kTMR2n kMPRTn-kn-k+12n-k模模 2乘乘 M ，這相當于在，這相當于在 M 后面添加后面添加 n-k 個個 0。R+R=0二進制除法二進制除法例子例子:M=110011 M(X)=X5+ X4 +X+1P=11001 P(X)=X4+ X3 +1Polynomial and DivisorGeneral CRC Division Circu

15、it: register，exclusive-OR gateC2C1C0Input000001101111111011011110110110000100100010FCSExample:M=111010, P=1101P(X)=X3+X2+1a3 a21， a100210212CInputCCCCCCInput 111010000C0C1C2主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-1028Error CorrectionuCorrection of detected errors usually requires data block to be retrans

16、mitted uNot appropriate for wireless applicationsBit error rate is highLots of retransmissionsPropagation delay can be long (satellite) compared with frame transmission time導致系統(tǒng)效率很低導致系統(tǒng)效率很低uNeed to correct errors on basis of bits receivedError Correction Process Diagram差錯糾正：通過傳輸報文附加冗余信息完成差錯糾正：通過傳輸報文

17、附加冗余信息完成Error Correction Process1.Each k bit block mapped to an n bit block (nk)Codeword碼字碼字Forward error correction (FEC前向糾錯前向糾錯) encoder2.Codeword sent3.Received bit string similar to transmitted but may contain errors主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-1031Error Correction Process4.Received code w

18、ord passed to FEC decoderIf no errors, original data block outputSome error patterns can be detected and correctedSome error patterns can be detected but not correctedSome (rare) error patterns are not detectedResults in incorrect data output from FEC主主講講教教師師：祁祁永永敏敏通信工程學院通信工程學院2021-10-1032傳輸線路上數(shù)據(jù)交換的分類傳輸線路上數(shù)據(jù)交換的分類u單工傳輸：一個站點發(fā)送，另一個站點接收。單工傳輸：一個站點發(fā)送，另一個站點接收。uHalf duplex半雙工傳輸半雙工傳輸Only one station may transmit at a time Re