lec8 - UCI University of California Irvine EECS 161...

EECS 161 - Introduction to Computer Networking
Scott Jordan
Local Area Networks / Ethernet
UCI
University of California, Irvine
CSMA-CD protocol:
(1) Wait until channel is idle.
(2) Transmit & listen.
(3) If collision, stop transmitting packet, transmit "jam signal", wait random delay, and
go to step (1).

EECS 161 - Introduction to Computer Networking
Scott Jordan
Local Area Networks / Ethernet
UCI
University of California, Irvine
CSMA-CD protocol:
(1) Wait until channel is idle.
(2) Transmit & listen.
(3) If collision, stop transmitting packet, transmit "jam signal", wait random delay, and
go to step (1).
Random delay:
Let 1 "slot" = 512 bit times
Upon 1
st
collision, randomly choose among {0,1} slot delay.
Upon 2
nd
collision, randomly choose among {0,1,2,3} slot delay.
...
Upon m
th
collision, randomly choose among {0,1,...,2
m
-1} slot delay.
...
Upon 10
th
-15
th
collision, randomly choose among {0,1,...,1023} slot delay.
Gives up at 16
th
collision.
"binary Exponential backoff"
This slows down the retransmission rate as congestion increases, and tries to avoid the
undesirable equilibrium point in the ALOHA throughput curve.