Path: utzoo!attcan!uunet!mcsun!unido!fauern!tumuc!guug!rup
From: rup@guug.guug.de
Newsgroups: comp.dcom.lans
Subject: Re: CSMA/CD algorithm/code needed
Message-ID: <116@guug.guug.de>
Date: 4 Jul 90 11:57:45 GMT
References: <1990Jun25.182126.12942@brutus.cs.uiuc.edu> <1990Jun25.190243.13144@brutus.cs.uiuc.edu>
Sender: rup@guug.guug.de
Reply-To: rup@guug.UUCP (Rupert Grafendorfer)
Distribution: comp
Organization: GUUG e.V., Munich, W. Germany
Lines: 52

In article <1990Jun25.190243.13144@brutus.cs.uiuc.edu> cattanac@casca.cs.uiuc.edu (Scott Cattanach) writes:
>cattanac@clitus.cs.uiuc.edu (Scott Cattanach) writes:
>
>
>I forgot to be clearer, I am really looking for the way to determine
>how long to wait to retrans after a collision.

IEEE 802.3 states: 

Collision Backoff and Retransmission

When a transmission attempt has terminated due to a collision, it is
retried by the transmitting CSMA/CD MAC sublayer until either it is 
successful or a maximum number of attempts (attemptLimit) have been made
and all have terminated due to collisions. Note that all attempts to 
transmit a given frame are completed before any subsequent outgoing frames
are transmitted. The scheduling of the retransmission is determined by a
controlled randomization process called "truncated binary exponential backoff".
At the end of enforcing a collision (jamming), the CSMA/CD sublayer delays
before attempting to retransmit the frame. The delay is an integer multiple
of slotTime. The number of slot times to delay before the nth retransmission
attempt is chosen as a uniformly distributed random integer r in the range

0 <= r < 2**k

where

k= min (n, 10)

If all attemptLimit attempts fail, this event is reported as an error.
Algorithms used to generate the integer r should be designed to
minimize the correlation between the numbers generated by any two stations 
at any given time. Note that the values given above define the most
aggressive behavior that a station may exhibit in attempting to retransmit
after a collision. In the course of implementing the retransmission scheduling
procedure, a station may introduce extra delays which will degrade its own
throughput, but in no case may a station's retransmission scheduling result
in a lower average delay between retransmission attempts than the procedure 
defined above.

(Quoted from IEEE 802.3)
Parameters:

slotTime		512 bit times
attemptLimit		16
backoffLimit		10
...

If you want to have the algorithms in pseudo-code purchase the IEEE 802.3 specs.

Rup Grafendorfer
BICC Data Networks