Re: CSMA/CD

[Ansgar Wiechers <bugtraq () planetcobalt net>]

On 2003-08-21 David Gillett wrote:
> Before a CSMA/CD device transmits, it listens to hear that the wire is
> clear.  (If it isn't, it waits for a while and listens again.)
> There's a potential race condition where two (or more!) devices
> listen, find that the wire is clear, and both decide to transmit. Part
> of resolving this race condition is for each frame transmission to
> start with a "pad" of known filler.

I may be wrong here, but AFAIK this is not the way padding works.

Say you have two stations located at the two furthermost ends of the
ether. If one station starts sending, the signal takes some time to
propagate (since it travels with approximately 0,6 light speed only).
Now assume the other station starts sending right before the signal
reaches its port. In the next moment it will receive the first signal,
detect a collision (this is implemented in hardware AFAIK - something
like an XOR gate between input and output - since software would not be
responsive enough to accomplish this task) and a jam signal is sent.
This station is aware of the collision now, but the first station still
is not! Only when it receives the signal the second box sent before
finishing its own transmission, it will be able to detect that a
collision occured. This is accomplished by defining the minimum frame
size (64 byte) large enough to make sure a station keeps sending at
least the time a signal needs to cover twice the maximum segment size
("there and back again").

So where does pad fit in here? Unfortunately, en empty eternet frame
(headers only) is smaller than 64 byte (just 18 byte: destination
address, source address, type, FCS), so every frame smaller than 64 byte
is filled up with zeroes (or maybe even garbage, I'm not sure) to have
the required minimum size. This fillup is called pad.

Regards
Ansgar Wiechers

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