Re: some newbie DC-net questions

[pierre@shell.portal.com (Pierre Uszynski)]

> From: Jim McCoy <mccoy@ccwf.cc.utexas.edu>
> 
> > 3) What happens when two members of a "table" attempt to transmit at the
> > same time?  How is this case handled?
> 
> They will get a collision.  If an even number of members transmit at once
> then the bit will be the opposite of what each expects to see, if an odd
> number then it will be an undetected collision.  There are fairly standard
> protocols for backdown on distributed broadcast networks.
> 

Actually, it seems to me undetected collisions are not as likely as this
makes it appear: Every person trying to transmit is monitoring
at the same time to make sure the message they transmit does appear in
the "sum of differences". If you tried to transmit and any bit comes out
wrong, it's that there was a collision. Only an odd number of completely
identical messages transmitted at the same time would appear as one
un-collided message. Of course, if you are only considering very short
messages (like 1 bit answers to questions) you are looking for trouble...
but if you are sending around longer messages similar to email, then
un-noticed collisions are unlikely because messages include signatures
and such.

When you detected that your message collided, typically, you stop
transmitting. You then decide on a random time delay, wait for that
duration and try again if the way is clear (all zeroes carrier).

Pierre.
pierre@shell.portal.com