1 Comnet 2006 Communication Networks Recitation 8 ARQ Protocols & Performance

2 Comnet 2006 Error Control Automatic Repeat Request (ARQ) Automatic Repeat Request (ARQ) – Error detection – Acknowledgement (ACK) – Retransmission after timeout – Negative acknowledgement (NACK) DATA ACK NACK t tran t prop

3 Comnet 2006 Pipelining Acknowldegement arrives every msec Packet arrives every msec Network performance 100%

4 Comnet 2006 Sliding Window Flow Control Allow multiple frames to be in transitAllow multiple frames to be in transit Receiver may have a buffer W longReceiver may have a buffer W long Transmitter can send up to W frames without ACKTransmitter can send up to W frames without ACK Each frame is numberedEach frame is numbered ACK includes number of frameACK includes number of frame Sequence number bounded by k bitsSequence number bounded by k bits –frames are numbered modulo 2 k

5 Comnet 2006 Sliding Window …. Sender Perspective Window of frames that may be transmitted Frames already acknowledged Frame sequence numbers Last frame acknowledged Window shrinks from trailing edge as frames are acknowledged Window expands from leading edge as ACKs received Last frame acknowledged Window shrinks from trailing edge as frames are received Window expands from leading edge as ACKs are sent Window of frames that may be transmitted …. Receiver Perspective Frames already received

6 Comnet 2006 Sliding Window: Example … F …. F0 F1 F … ACK 2 F4 F5 F … ACK ….

7 Comnet 2006 Sliding Window Enhancements Receiver can acknowledge frames without permitting further transmissionReceiver can acknowledge frames without permitting further transmission If duplex, use piggybackingIf duplex, use piggybacking –If no data to send, use ack frame –If data but no acknowledgement to send, send last acknowledgement number again, or have ACK valid flag (TCP)

8 Comnet 2006 Go Back N Based on sliding windowBased on sliding window If no error, ACK as usual with frame sequence numberIf no error, ACK as usual with frame sequence number Use window to control number of outstanding framesUse window to control number of outstanding frames If error, reply with rejectionIf error, reply with rejection –Discard that frame and all future frames until error frame received correctly –Transmitter must go back and retransmit that frame and all subsequent frames

9 Comnet 2006 Go-Back-N: Example Time 01 EDDDDDD Timeout Interval Frames discarded by data link layer Error ACK 0 ACK 2 ACK 3 ACK 4ACK 5 ACK 6 ACK 7 ACK 1

10 Comnet 2006 Selective Repeat Only rejected frames are retransmittedOnly rejected frames are retransmitted Subsequent frames are accepted by the receiver and bufferedSubsequent frames are accepted by the receiver and buffered Minimizes retransmissionMinimizes retransmission Receiver must maintain large enough bufferReceiver must maintain large enough buffer More complex login in senderMore complex login in sender

11 Comnet 2006 Selective Repeat Example E 3452 Transmitter (Primary) Receiver (Secondary) ACK 0 ACK 1 ACK 3 ACK 4 ACK 5ACK 2 Buffered by receiver Error Frames 2-5 released

12 Comnet 2006 Selective Repeat: Window Size Sequence Number space >= 2 Window Size Sequence Number space >= 2 Window Size TIMEOUT ACK

13 Comnet 2006 Performance: Notation TRANS = Frame transmission time PROP = Link propagation delay a = PROP/TRANS Probability of Frame error in Sender-Receiver direction is P<<1 Probability of Frame error in Receiver-Sender direction is 0 W = Number of frames per window Nr: average number of transmissions of an original frame

14 Comnet 2006 TRANS = Frame transmission time PROP = Link propagation delay a = PROP/TRANS Probability of Frame error in Sender-Receiver direction is P<<1 Probability of Frame error in Receiver-Sender direction is 0 Number of frames per window = W. Simplifying assumption: Use NACKs Performance: Notation

15 Comnet 2006 Performance: No Errors Frames transmitted = 2a + 1 if W > 2a + 1Frames transmitted = 2a + 1 if W > 2a + 1 = W otherwise = W otherwise W >= 2a + 1W >= 2a + 1 Link is always utilized (U = 1)Link is always utilized (U = 1) W < 2a + 1W < 2a + 1 U = W/(2a + 1)U = W/(2a + 1)

16 Comnet 2006 Performance: Unreliable Link Selective Repeat Number of retransmissions:Number of retransmissions: N r =  i P i-1 (1-P)=1/(1-P) U = (1 – P) if (W > 1 +2a) W (1 – P) / (1 + 2a) if (W <= 1 +2a)

17 Comnet 2006 Performance: Unreliable Link G-Back-N Number of frames retransmitted:Number of frames retransmitted: 1+2a If (W > 1 +2a) W if (W <= 1 +2a) U = (1 – P)/(1 + 2aP)if (W > 1 +2a) W (1 – P) / [(1 + 2a) (1-P+WP)]if (W <= 1 +2a)