"Bowen, Shawn" wrote:
>
> I believe we are saying mostly the same thing. Your "* Extended carrier to
> indicate busy (assert carrier beyond the length of the packet)." Is an
> Ethernet JAM signal.
> And I also guess I wanted to point out that the Cisco documentation is
> not "always" 100% accurate in the real world.
I must respectfully disagree again. The "jam" signal is asserted after
a collision detection and most commonly simply continues to transmit
the data in the collided packet for another 64 bits. The flow control
method of "assert carrier beyond the length of the packet" is entirely
different -- it is sending the 802.3 preamble of 0x05 continuously
without the trailing 8th byte 0xD5 (I'm pulling this from my somewhat
foggy memory on the values, but the preamble is 7 bytes of "something"
followed by a byte signalling the start-of-packet, so don't shoot me
if my values aren't correct).
As for Cisco documentation, and some others, they refer to this method
of flow control as "back pressure". It has the advantage of not
propagating a collision (as in the intentional collision method of
flow control) as it will not interfere with any other hub/switch/NIC
on the network other than causing a transmit deferral. But as I had
said earlier, 10Mb NICs have logic to detect carrier (or preamble)
being asserted longer than the MTU plus propagation delay and consider
this to be "jabber". This was not carried forward to 100Mb or 1Gb NICs.
But I will concur on your statement:
> The only reason I took it to any depth was the fact that other than duplex
> mismatches a lot of people getting into this field (reading these posts)
> haven't ever been exposed to such nuances.
I value real world experience much more than some CC** acronym after
your signature. This is the stuff you need to know that no boot camp
will teach you, and I value this "outside the mainstream" information.
I don't read this list for the obvious topics, it's the fringe areas
that you can learn about that makes it worth it.
Jeff Kell <[EMAIL PROTECTED]>
Systems/Network Administrator
University of Tennessee at Chattanooga
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