1 Netcomm 2005 Communication Networks Recitation 8

2 Netcomm 2005 Important השיעור (הרצאה) הבא ( ) יערך בבניין נפתלי, חדר 004, בקומת הכניסה, ימינה בסוף המסדרוןהשיעור (הרצאה) הבא ( ) יערך בבניין נפתלי, חדר 004, בקומת הכניסה, ימינה בסוף המסדרון בתרגול הבא ( ) אני לא אהיהבתרגול הבא ( ) אני לא אהיה ישי יעביר תרגול חלופיישי יעביר תרגול חלופי

3 Netcomm 2005 ARQ Protocols & Performance

4 Netcomm 2005 Pipelining Acknowldegement arrives every msec Packet arrives every msec Network performance 100%

5 Netcomm 2005 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

6 Netcomm 2005 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

7 Netcomm 2005 Sliding Window: Example … F … F0 F1 F … ACK 2 F4 F5 F … ACK 3

8 Netcomm 2005 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

9 Netcomm 2005 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)

10 Netcomm 2005 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

11 Netcomm 2005 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

12 Netcomm 2005 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

13 Netcomm 2005 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

14 Netcomm 2005 Selective Repeat: Window Size Sequence Number space >= 2 Window Size Sequence Number space >= 2 Window Size TIMEOUT ACK

15 Netcomm 2005 Frame transmission time TRANS Link propagation delay is PROP a = PROP/TRANS Probability of Frame error in Sender-Receiver direction is P<<1 Probability of Frame error in Receiver-Sender direction is 0 N r : average number of transmissions of an original frame Simplifying assumption: Use NACKs Performance: Notation

16 Netcomm 2005 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)

17 Netcomm 2005 Performance: Unreliable Link GBNGBN –U = (1 – P)/(1 + 2aP) if W > 2a + 1 = W (1 – P)/[(2a + 1)(1 – P + WP)] otherwise = W (1 – P)/[(2a + 1)(1 – P + WP)] otherwise SRSR –Nr = 1 / (1 – P) –U = (1 – P) if (W >> 1 +2a) <= W (1 – P) / (1 + 2a) if (W <= 1 +2a) <= W (1 – P) / (1 + 2a) if (W <= 1 +2a)