MIT16_36s09_lec18

MIT16_36s09_lec18 - MIT OpenCourseWare http:/ocw.mit.edu...

Info iconThis preview shows pages 1–7. Sign up to view the full content.

View Full Document Right Arrow Icon
MIT OpenCourseWare http://ocw.mit.edu 16.36 Communication Systems Engineering Spring 2009 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
16.36: Communication Systems Engineering ARQ Protocols: Go Back N and SRP Eytan Modiano
Background image of page 2
Go Back N ARQ (Sliding Window) Stop and Wait is inefficient when propagation delay is larger than the packet transmission time Can only send one packet per round-trip time Go Back N allows the transmission of new packets before earlier ones are acknowledged Go back N uses a window mechanism where the sender can send packets that are within a “window” (range) of packets The window advances as acknowledgements for earlier packets are received ACK-0 ACK-1 ACK-2 ACK-3 ACK-4 ACK-5 ACK-6 ACK-7 ACK-8 PKT-0 PKT-1 PKT-2 PKT-3 PKT-9 PKT-8 PKT-7 PKT-6 PKT-5 PKT-4 WINDOW WINDOW WINDOW WINDOW
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Features of Go Back N Window size = N – Sender cannot send packet i+N until it has received the ACK for packet i Receiver operates just like in Stop and Wait Receive packets in order Receiver cannot accept packet out of sequence Send RN = i + 1 => ACK for all packets up to and including i Use of piggybacking When traffic is bi-directional RN s are piggybacked on packets going in the other direction Each packet contains a SN field indicating that packet s sequence number and a RN field acknowledging packets in the other direction <--Frame Header ---------> SN RN Packet CRC
Background image of page 4
Go Back N ARQ The transmitter has a "window" of N packets that can be sent without acknowledgements This window ranges from the last value of RN obtained from the receiver (denoted SN min ) to SN min +N-1 When the transmitter reaches the end of its window, or times out, it goes back and retransmits packet SN min Let SN min be the smallest number packet not yet ACKed Let SN max be the number of the next packet to be accepted from the higher layer (I.e., the next new packet to be transmitted)
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Go Back N Sender Rules SN min = 0; SN max = 0 Repeat If SN max < SN min + N (entire window not yet sent) Send packet SN max ; SN max = SN max + 1; If packet arrives from receiver with RN > SN min SN min = RN; If SN min < SN max (there are still some unacknowledged packets) and sender cannot send any new packets Choose some packet between SN min and SN max and re-send it The last rule says that when you cannot send any new packets you should re-send an old (not yet ACKed) packet – There may be two reasons for not being able to send a new packet
Background image of page 6
Image of page 7
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 21

MIT16_36s09_lec18 - MIT OpenCourseWare http:/ocw.mit.edu...

This preview shows document pages 1 - 7. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online