OK, if we really want to get nitty-gritty:
The jam signal is 32 bits.
The preamble is 10101010 etc, or AA in HEX. The last byte is 10101011, or
AB. You can actually see AAs sometimes on a Sniffer when there's a
collision and the Sniffer captures someone else's preamble on the end of a
frame.
But all else you say is right on as far as I know.
Priscilla
At 12:04 AM 12/28/00, Jeff Kell wrote:
>"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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Priscilla Oppenheimer
http://www.priscilla.com
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