多媒體網(wǎng)絡基礎-第2章 網(wǎng)絡接口層(第3部分,同步技術、差錯控制)

1、Beijing Institute of Fashion Beijing Institute of Fashion TechnologyTechnologyLiuYi數(shù)據(jù)通信的一個基本要求是接收方必須知道它所接收的每一位的數(shù)據(jù)通信的一個基本要求是接收方必須知道它所接收的每一位的開始時間和持續(xù)時間，這樣才能正確地接收發(fā)送方發(fā)來的數(shù)據(jù)。開始時間和持續(xù)時間，這樣才能正確地接收發(fā)送方發(fā)來的數(shù)據(jù)。這個過程叫同步過程。這個過程叫同步過程。 例：例： 0 0 1 1 0 0 0 0 0 0 1 1 0 0 幾個幾個1 1？v同步的方法有兩種：同步的方法有兩種： 同步通信同步通信（位同步）：同步通信要求接收端

2、時鐘頻率和發(fā)送（位同步）：同步通信要求接收端時鐘頻率和發(fā)送端時鐘頻率一致。發(fā)送端發(fā)送連續(xù)的比特流。端時鐘頻率一致。發(fā)送端發(fā)送連續(xù)的比特流。異步通信異步通信（群同步）（群同步）: :異步通信時不要求接收端時鐘和發(fā)送端異步通信時不要求接收端時鐘和發(fā)送端時鐘同步。發(fā)送端發(fā)送完一個字節(jié)后，可經過任意長的時間時鐘同步。發(fā)送端發(fā)送完一個字節(jié)后，可經過任意長的時間間隔再發(fā)送下一個字節(jié)。間隔再發(fā)送下一個字節(jié)。Using a field in the header to Using a field in the header to specify the number of specify the number

3、 of characterscharacters in the frame. When the data link layer at in the frame. When the data link layer at the destination sees the character count ,the destination sees the character count ,it knows it knows how many charactershow many characters follow and hence where the end of follow and hence

4、 where the end of the frame is.the frame is.(a) A character stream Without errors. (b) A character stream With one error.Having each frame start and end with special bytes, and getting around the problem of resynchronization after an error . It may easily happen that flag bytes bit pattern occurs in

5、 the data. The way to resolve this problem is to have the senders data link layer insert a special escape byte（ESC，轉義字節(jié)） just before each “accidental” flag byte in data.A frame delimited by flag bytes.Four examples of byte sequences before and after stuffing.A major disadvantage is that it is closel

6、y tied to the use of 8-bit character.Whenever the senders data link layer encounters Whenever the senders data link layer encounters five consecutive 1s in the data ,it automatically five consecutive 1s in the data ,it automatically stuffs a 0 bit into the outgoing bit stream. stuffs a 0 bit into th

7、e outgoing bit stream. When the receiver sees five consecutive incoming 1 When the receiver sees five consecutive incoming 1 bits followed by a 0 bit, it automatically destuffs bits followed by a 0 bit, it automatically destuffs the 0 bit.the 0 bit.The original data.The data as they appear on the li

8、ne.The data as they are stored in receivers memory after destuffing.The method is only applicable to networks in The method is only applicable to networks in which the encoding on the physical medium which the encoding on the physical medium contains some redundancy.contains some redundancy.Eg:Encod

9、e 1 bit of data by using 2 physical Eg:Encode 1 bit of data by using 2 physical bits. Manchester ,Different Manchester .bits. Manchester ,Different Manchester .vMany data link protocols use a combination of Many data link protocols use a combination of a character count with one of the other a chara

10、cter count with one of the other methods for extra safety.methods for extra safety.vBit stuffingBit stuffing and and coding violationscoding violations is more is more popular used in computer network.popular used in computer network.一、差錯產生原因一、差錯產生原因由于線路本身的電氣特性由于線路本身的電氣特性各種外界因素各種外界因素( (如：閃電，強電流磁場的變化

11、等如：閃電，強電流磁場的變化等) )v二、差錯控制技術二、差錯控制技術反饋檢測法反饋檢測法自動重發(fā)請求法自動重發(fā)請求法( (ARQ一一 Automatic Repeat Request ) )向前糾錯法（向前糾錯法（FEC-Forward Error Correction) )1 1反饋檢測法反饋檢測法又稱回送校驗法或又稱回送校驗法或“回聲回聲”法。法。檢測方法：檢測方法：反饋檢測法特點反饋檢測法特點v原理簡單，易實現(xiàn)，可靠性高。原理簡單，易實現(xiàn)，可靠性高。v每個字符均被傳輸兩次，信道利用率很低。每個字符均被傳輸兩次，信道利用率很低。 ：Sending Information：Return I

12、nformation：Data Comparision：Continue Or RepeatSR2 2自動重發(fā)請求法（自動重發(fā)請求法（ARQARQ法）法）ARQARQ法實現(xiàn)過程法實現(xiàn)過程Repeat RequestvARQARQ法特點法特點信道的利用率高信道的利用率高傳輸?shù)目煽啃暂^高傳輸?shù)目煽啃暂^高接受方需要對接受的數(shù)據(jù)進行判斷接受方需要對接受的數(shù)據(jù)進行判斷：Data detection：Sending Information：Otherwise asking repeat：If right then continueSRR3 3、向前糾錯法（、向前糾錯法（FECFEC）FECFEC實現(xiàn)的過程

13、實現(xiàn)的過程vFEC法特點法特點單向信道單向信道接受端設備復雜接受端設備復雜：data detection：Sending Information：Continue Or Error CorrectionS：Sending Information：Return Information：Data Comparision：Continue Or RepeatSRR：Data detection：Otherwise asking repeat：If right then continueS：Sending InformationRS：Sending Information：data detection

14、：Continue Or Error Correction反反饋饋檢檢測測法法向前糾錯法（向前糾錯法（FEC）自動重發(fā)請求法（自動重發(fā)請求法（ARQ法）法）v基本理念基本理念通過增加冗余信息通過增加冗余信息( (Redundant InformationRedundant Information) )使接受方對信息進行檢使接受方對信息進行檢測。測。v衡量編碼性能的兩個重要參數(shù)衡量編碼性能的兩個重要參數(shù)編碼效率編碼效率( (Coding EfficiencyCoding Efficiency) )漏檢率漏檢率( (Omission FactorOmission Factor) )v差錯控制編碼差

15、錯控制編碼奇偶校驗法（奇偶校驗法（Parity Check)Parity Check)塊校驗法（塊校驗法（Block Check Character-BBC)Block Check Character-BBC)校驗和（校驗和（Check SumsCheck Sums）循環(huán)冗余校驗（循環(huán)冗余校驗（Cyclic Redundancy ChecksCyclic Redundancy Checks）1 1、垂直奇偶校驗編碼規(guī)則：、垂直奇偶校驗編碼規(guī)則：以字符為單位。以字符為單位。奇校驗：通過添加校驗位，使字符中奇校驗：通過添加校驗位，使字符中“1”1”的個數(shù)為奇數(shù)個。的個數(shù)為奇數(shù)個。偶校驗：通過添加校

16、驗位，使字符中偶校驗：通過添加校驗位，使字符中“1”1”的個數(shù)為偶數(shù)個。的個數(shù)為偶數(shù)個。1 0 0 1 1 0 1 1采用垂直奇校驗：采用垂直奇校驗：采用垂直偶校驗：采用垂直偶校驗：1 0 0 1 1 0 1 01001101110111011干干 擾擾有錯誤有錯誤請求重發(fā)請求重發(fā)v編碼效率為：編碼效率為：R=p/(p+1)。v漏檢率高。漏檢率高。能檢測出每列中所有奇數(shù)個錯，檢測不出偶數(shù)個的錯。能檢測出每列中所有奇數(shù)個錯，檢測不出偶數(shù)個的錯。v異步傳輸。異步傳輸。1 0 0 1 1 0 1 11 0 1 0 1 1 1 0 如果傳輸中出現(xiàn)兩個錯誤：如果傳輸中出現(xiàn)兩個錯誤：？2、CRC循環(huán)冗余碼

17、校驗循環(huán)冗余碼校驗 Cyclic Redundancy Check v最常用的差錯控制技術最常用的差錯控制技術v使用使用CRC16，9600b/s傳輸傳輸3000年，才會有一個差錯查年，才會有一個差錯查不出來。不出來。CRC校驗的基本思想校驗的基本思想信息碼信息碼MK位位校驗碼校驗碼FCSn位位TransmitterReceiver發(fā)送碼發(fā)送碼TK+n位發(fā)送碼位發(fā)送碼接受碼接受碼SK+n位接收碼位接收碼以二進制數(shù)以二進制數(shù)R R為除數(shù)做模為除數(shù)做模2 2運運算除法，生成校驗碼算除法，生成校驗碼FCSFCS（Frame Check SequenceFrame Check Sequence）用用R

18、做除數(shù)做除數(shù)做模做模2運算運算+能夠能夠整除整除不能不能整除整除正確正確有錯誤有錯誤CRC-410011CRC-410011，1100111001CRC-8100000111CRC-8100000111，100110001100110001CRC-12CRC-12（1212，1111，3 3，2 2，1 1，0 0）CRC-16CRC-16，CRC-CCITTCRC-CCITT，CRC-32CRC-32例：信息碼例：信息碼110011110011，生成碼用，生成碼用CRC-411001CRC-411001，求，求FCSFCS及發(fā)送碼及發(fā)送碼1 1、信息碼左移、信息碼左移n n位；位；2 2、以生成碼、以生成碼R R為除數(shù)做模為除數(shù)做模2 2除法，余數(shù)即為除法，余數(shù)即為FCSFCS；3 3、將除法做得余數(shù)作為、將除法做得余數(shù)作為FCSFCS加在信息碼后，形成發(fā)送碼。加在信息碼后，形成發(fā)送碼。FCS：1001發(fā)送碼：發(fā)送碼：1100111001（1）、如何找到除數(shù)）、如何找到除數(shù)R：生成碼（：生成碼（CRC Polynomial