Communication Networks NETW 501 Lecture 9 Data Link

Selective Repeat ARQ Basics ØWhat is the problem of Go-Back N? – Go Back-N

Error Recovery in Selective Repeat ARQ Sender Receiver Fr_0 Fr_1 ACK_1 Fr_2 ACK_2 Fr_3

Operation of Selective Repeat WS=WR=2 , m=2 Sender Receiver Fr_0 Sending Window Fr_1 NAK_0

Send and Receive Window Size m=2 WS=WR=3 Sender Receiver Fr_0 WS=WR=2 Sender Receiver Fr_0

Efficiency Plots Frame Size 1024, R=1. 5 Mbps, tprop=5 ms, Window Size = 4

References • NETW 501 Lectures slides by Assoc. Prof. Tallal El-Shabrawy • “Communication Networks

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Communication Networks NETW 501 Lecture 9 Data Link Layer: Error Control & Flow Control Course Instructor: Dr. -Ing. Maggie Mashaly maggie. ezzat@guc. edu. eg C 3. 220 1

Selective Repeat ARQ Protocol 2

Selective Repeat ARQ Basics ØWhat is the problem of Go-Back N? – Go Back-N retransmits the erroneous frame and all subsequent frames – This reduces efficiency especially for High Error Channels ØSelective Repeat ARQ – Introduce a receive window to store CORRECT out of sequence frames – Retransmit individual frames ØTCP uses a form of selective repeat 3

Error Recovery in Selective Repeat ARQ Sender Receiver Fr_0 Fr_1 ACK_1 Fr_2 ACK_2 Fr_3 Fr_4 NAK_2 ACK_2 Fr_5 ACK_6 4

Operation of Selective Repeat WS=WR=2 , m=2 Sender Receiver Fr_0 Sending Window Fr_1 NAK_0 Fr_1 Receiving Window Fr_0 ACK_2 Fr_0 Fr_1 Fr_2 Fr_3 ACK_0 Frame Sequences that are allowed to be sent Frame Sequences that are allowed to be received Outstanding Frame Sequences that are received correctly and buffered 5

Operation of Selective Repeat WS=WR=2 , m=2 Sender Receiver Fr_0 Sending Window Fr_1 NAK_0 Fr_1 Receiving Window Fr_0 ACK_2 Fr_3 Fr_2 Fr_3 ACK_0 Frame Sequences that are allowed to be sent Frame Sequences that are allowed to be received Outstanding Frame Sequences that are received correctly and buffered 10

Operation of Selective Repeat WS=WR=2 , m=2 Sender Receiver Fr_0 Sending Window Fr_1 NAK_0 Fr_2 Fr_3 Receiving Window Fr_0 ACK_2 Fr_3 Fr_2 Fr_3 ACK_0 Frame Sequences that are allowed to be sent Frame Sequences that are allowed to be received Outstanding Frame Sequences that are received correctly and buffered 11

Operation of Selective Repeat WS=WR=2 , m=2 Sender Receiver Fr_0 Sending Window Fr_1 NAK_0 Fr_2 Fr_3 Receiving Window Fr_0 ACK_2 Fr_3 Fr_0 Fr_2 Fr_3 ACK_0 Frame Sequences that are allowed to be sent Frame Sequences that are allowed to be received Outstanding Frame Sequences that are received correctly and buffered 13

Send and Receive Window Size m=2 WS=WR=3 Sender Receiver Fr_0 WS=WR=2 Sender Receiver Fr_0 Fr_1 ACK_1 Fr_2 ACK_2 Fr_0 ACK_0 Fr_0 ACK_3 Receive Window {3, 0, 1} GENERAL RULE: WS+ WR ≤ 2 m Fr_0 rejected Receive Window {2, 3} 15

Efficiency Plots Frame Size 1024, R=1. 5 Mbps, tprop=5 ms, Window Size = 4 16

References • NETW 501 Lectures slides by Assoc. Prof. Tallal El-Shabrawy • “Communication Networks 2 nd Edition”, A. Leon-Garcia and I. Widjaja, Mc. Graw Hill, 2013 • “Computer Networks 4 th Edition”, A. S. Tanenbaum, Pearson International 17