At 10:07 AM 6/2/01, E Joseph wrote:
>Priscilla,
> What would a the resulting jam look like on a
>sniffer trace??
If the jam signal is all ones, it would look like 0xFFs on a Sniffer. The
jam doesn't have to be all ones, though. On old bus coax networks, it was
actually rare to see the jam because by the time the jam happened the
clocking was so messed up that the Sniffer had already stopped capturing
and just reported a runt, CRC error, collision.
On a network with repeaters (hubs), when the repeater detects a collision
it sends a 96-bit jam composed of alternating ones and zeros, which would
look like 0xAAs or 0x55s. That's easier to see.
A lot of Sniffers don't capture bad frames or runts, so you wouldn't see
any of these jams in those cases.
Someone else may have some additional information. It's always been a
question of mine also whether you can really see jams or not.
Of course in networks where full-duplex switch ports have replaced hub
ports, this is no longer relevant.
Please send messages to the group, not to me. Thanks,
Priscilla
> Thank You,
> Ed
>
>
>
>--- Priscilla Oppenheimer wrote:
> > When a transmitter detects a collision, the
> > transmitter continues to send
> > the preamble, (if the preamble has not completed),
> > and also sends 32
> > additional bits, which are called a jam signal. The
> > jam signal extends the
> > duration of the collision event to ensure that all
> > stations hear the
> > collision. The contents of the jam can be any
> > pattern that is not
> > intentionally designed to be the 32-bit CRC value
> > corresponding to the
> > (partial) frame already transmitted. Most
> > implementations send all ones.
> >
> > Completely sending the preamble and transmitting a
> > jam signal guarantees
> > that a signal stays on the media long enough for all
> > transmitting stations
> > involved in the collision to recognize the collision
> > and react accordingly.
> >
> > Priscilla
> >
> >
> > >On Thu, 24 May 2001, [EMAIL PROTECTED] wrote:
> > >
> > > > What is an "ethernet a jam signal"
> >
> >
> > ________________________
> >
> > Priscilla Oppenheimer
> > http://www.priscilla.com
>[EMAIL PROTECTED]
>
>
>__________________________________________________
>Do You Yahoo!?
>Get personalized email addresses from Yahoo! Mail - only $35
>a year! http://personal.mail.yahoo.com/
________________________
Priscilla Oppenheimer
http://www.priscilla.com
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=6914&t=5796
--------------------------------------------------
FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
