【正文】 ransmitted in two direction 187。 Sequence number Ch. 3 Data Link Layer 31 Sliding Windows Protocols (1) ? A One Bit Sliding Window Protocol ? A Protocol Using Go Back n ? A Protocol Using Selective Repeat Ch. 3 Data Link Layer 32 Sliding Windows Protocols (2) ? Data transmission (according to data flow direction) – Simplex (單工 ) 187。 Timer 187。Data in one direction ? Errorfree (無雜訊 ) channel ?The sender sends infinite number of data with infinite transmission speed ? The receiver receives infinite number of data and processes with infinite transmission speed – Frames may be corrupted or lost pletely ? Error Control – ARQ: 187。Data in one direction ? Errorfree (無雜訊 ) channel ? The sender sends infinite number of data with infinite transmission speed ? The receiver receives infinite number of data and processes with infinite transmission speed Ch. 3 Data Link Layer 29 A Simplex StopandWait Protocol ? Protocol 2 (StopandWait) ? Simplex (單工 ) 187。x16 + x15 + x2 + 1 – CRCCCITT 187。 else ERROR. Ch. 3 Data Link Layer 23 錯(cuò)誤偵測碼 (3) At the Sender M(x) = 1101011011 G(x) = 10011 ? 011 ? ? Ch. 3 Data Link Layer 24 錯(cuò)誤偵測碼 (4) At the receiver 1 1 1 0 1 1 1 0 0 0 0 1 0 0 0 0 = 0 (Correct) ? ? ? Received Ch. 3 Data Link Layer 25 錯(cuò)誤偵測碼 (5) At the receiver 1 1 1 0 1 1 1 0 0 0 0 1 0 0 0 1 ≠ 0 (Error) 0 0 0 1 1 1 1 1 0 0 1 1 1 ? Received ? ? Ch. 3 Data Link Layer 26 ErrorDetecting Codes (6) ? International Standards – CRC12 187。 G(x) ? With the quotient Q(x) and the remainder R(x) (3) Transmit xr M(x) – R(x) ? Append R(x) to the message bit string – At the receiver (1) Receive xr M(x) – R(x) (2) xr M(x) 247。Message bit string M(x) 187。For example: 10011 as x4 + x + 1 – Polynomial arithmetic is done modulo 2 (模數(shù) 2), . no carry, no borrow ? ExclusiveOR (⊕ ，互斥或 ) 187。If b = 0, then no error。Counter b = 0 187。To detect d errors ? d+1 codewords Ch. 3 Data Link Layer 18 ErrorCorrecting Codes (2) ? Hamming Code (漢明碼 ) – For m message bits and r redundant bits ( m + r + 1 ) ≦ 2r 187。Example 010111000 010001001 oooxxooox ∴ Hamming distance = 3 ? Error Handling vs. Hamming Code – Error Correcting Code 187。Need for ACKretransmission mechanism 187。The sender generates only enough redundancy to allow the receiver to detect an error, but not which error, and requests for a retransmission 187。No need for ACKretransmission mechanism 187。The sender includes enough redundant information along with each block of data to enable the receiver not only to detect an error but also to deduce the correct data 187。? Less accurate ? No increased traffic 187。? More accurate ? Increased traffic – Without feedback (無回饋 ), implicit (隱含的 ) 187。 NACK (1) D Data (1) Data (2) S ? NACK (1) Data (3) ? Data (1) Ch. 3 Data Link Layer 14 Flow Control ? Goal – To throttle the sender into sending no faster than the receiver can handle the traffic ? Mechanisms – With feedback (有回饋 ), explicit (明確的 ) 187。[Problem] What if the NACK is lost? 187。 If the data is not correct, then the receiver ACKs. 187。 PACK Timer out ? ReTx Data (1) PACK (1) Data (1) ? Timer PACK (1) Data (2) Ch. 3 Data Link Layer 13 Error Control (4) (b) Negative ACK (負(fù)回覆 , NACK ) 187。[Problem 2] What if the frame is not lost but the PACK is lost? ? The data frame is correctly received. ? The sender times out and retransmit. ? The frame is duplicated. 187。[Problem 1] What if the frame is not correct or the frame is lost? ? No PACK is sent at all. ? The sender does not receive PACK and suspends? 187。 If the data is not correct, then the receiver does not ACK. 187。 – And the receiver checks and sends back an acknowledgement (ACK, 回覆 ) (a) Positive ACK (正回覆 , PACK) * 187。 Manchester Encoding (used in ) ◆ Transition in the middle of every bit ◆ In the middle of a bit: 0 – Ascending edge 1 – Descending edge 187。IBM BISYNC SYNC SYNC SYNC Frame Contents (in character) Ch. 3 Data Link Layer 8 Framing (4) – (3) Starting and ending flags, with bit stuffing 187。 2: Software in Data Link Layer 187。Ch. 3 Data Link Layer 1 Chapter 3 Data Link Layer ? Data Link Layer Design Issues ? Error Detection and Correction ? Elementary Data Link Protocols ? Sliding Windows Protocols ? Protocol Sp