Communication Networks Recitation 8 ARQ Protocols Performance Comnet

Communication Networks Recitation 8 ARQ Protocols & Performance Comnet 2006 1

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

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

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

Sliding Window Sender Perspective Window of frames that may be transmitted Frames already acknowledged …. 0 1 2 Frame sequence numbers 3 4 5 6 Last frame acknowledged 1 2 3 4 5 6 …. Window expands from leading edge as ACKs received Window of frames that may be transmitted Frames already received 0 0 Window shrinks from trailing edge as frames are acknowledged Receiver Perspective …. 7 5 6 7 0 1 2 Window shrinks from trailing edge as frames Comnet 2006 are received 3 4 5 6 …. Window expands from leading edge as ACKs 5 are sent

Sliding Window: Example …. 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 …. F 0 F 1 …. 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 …. F 2 …. 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 …. ACK …. 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 …. 2 F 3 F 4 F 5 …. 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 …. F 6 3 …. 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 …. K C A …. 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 …. Comnet 2006 6

Go Back N • Based on sliding window • If no error, ACK as usual with frame sequence number • Use window to control number of outstanding frames • If 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 Comnet 2006 8

Go-Back-N: Example Timeout Interval 2 3 4 5 1 Error 2 ACK 3 6 ACK E D D D 2 3 7 8 9 4 5 6 10 ACK 7 8 ACK 6 7 5 6 ACK 5 4 0 4 ACK 3 1 2 ACK 1 ACK 0 0 7 8 Frames discarded by data link layer Time Comnet 2006 9

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

Selective Repeat Example 0 1 2 3 CK A 0 1 Error 0 4 K C A E 5 2 1 CK A 3 3 CK A 4 Buffered by receiver Comnet 2006 6 5 Transmitter (Primary) 0 4 5 2 K K AC AC 2 6 Receiver (Secondary) Frames 2 -5 released 11

Selective Repeat: Window Size 0 1 2 3 4 5 6 7 TIMEOUT ACK 0 Sequence Number space >= 2 Window Size Comnet 2006 12

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 Comnet 2006 13

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 • Number of frames per window = W. • Simplifying assumption: Use NACKs Comnet 2006 14

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

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

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