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Posted by Michelot on October 17, 2007, 11:50 am
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Bonjour,
Finally, I don't know really what is collision. Could we exchange some
words about this?
Let's suppose 2 stations connected through a 100Base-TX link.
- If the 2 NIC are configured in full duplex, the collision detection
is inactive.
- If the 2 NIC are configured in half duplex, the collision detection
is active although there is no possible collision on the medium (the 2
twisted pair). At which other Ethernet level the collision can occur
if the 2 stations send a frame at the same time?
Best regards,
Michelot
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Posted by Rich Seifert on October 17, 2007, 12:16 pm
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> Bonjour,
>
> Finally, I don't know really what is collision. Could we exchange some
> words about this?
>
> Let's suppose 2 stations connected through a 100Base-TX link.
>
> - If the 2 NIC are configured in full duplex, the collision detection
> is inactive.
>
Correct.
> - If the 2 NIC are configured in half duplex, the collision detection
> is active although there is no possible collision on the medium (the 2
> twisted pair).
Not correct. A collision is a condition, on a half-duplex Ethernet,
where two or more stations are attempting to transmit a frame at the
same time. In the case of a two station, point-to-point half-duplex
link, if both stations have a frame queued for transmission at the same
time, they will both attempt to transmit it, a collision will occur, the
stations will detect the collision, and then backoff and retry.
Clearly, in the simple point-to-point configuration, the stations
*could* have been configured for full-duplex operation, in which case
the event of "I am transmitting and receiving at the same time" would
not constitute a collision, and no backoff-and-retry would be necessary.
> At which other Ethernet level the collision can occur
> if the 2 stations send a frame at the same time?
>
Collisions occur and are detected in the Physical Layer; the reaction
(backoff and retry) occurs at the MAC sublayer.
--
Rich Seifert Networks and Communications Consulting
21885 Bear Creek Way
(408) 395-5700 Los Gatos, CA 95033
(408) 228-0803 FAX
Send replies to: usenet at richseifert dot com
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Posted by Michelot on October 17, 2007, 6:02 pm
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> A collision is a condition...
The word "condition" is important, and bring certainly to my
understanding.
> A collision is a condition, on a half-duplex Ethernet,
> where two or more stations are attempting to transmit a frame at the
> same time.
And could we say that? the condition of the collision is on even if
the received signal could have been a correct frame.
Thanks for all your interresting development that helps me.
Best regards,
Michelot
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Posted by Rich Seifert on October 17, 2007, 7:25 pm
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>
> > A collision is a condition...
>
> The word "condition" is important, and bring certainly to my
> understanding.
>
> > A collision is a condition, on a half-duplex Ethernet,
> > where two or more stations are attempting to transmit a frame at the
> > same time.
>
> And could we say that? the condition of the collision is on even if
> the received signal could have been a correct frame.
>
Yes; in the situation of two half-duplex NICs connected via
10BASE-T/100BASE-TX, this is exactly the case. The received signal
carries a valid frame, but the NIC declares a collision anyway because
in half-duplex, transmitting while receiving ist verboten.
--
Rich Seifert Networks and Communications Consulting
21885 Bear Creek Way
(408) 395-5700 Los Gatos, CA 95033
(408) 228-0803 FAX
Send replies to: usenet at richseifert dot com
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Posted by Michelot on October 20, 2007, 5:16 pm
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> Yes; in the situation of two half-duplex NICs connected via
> 10BASE-T/100BASE-TX, this is exactly the case. The received signal
> carries a valid frame, but the NIC declares a collision anyway because
> in half-duplex, transmitting while receiving ist verboten.
"Bitte" in German, and "merci" in French.
And we can perhaps summarize like that:
(1) See section 4.1.1 in 802.3:2005: half duplex is required on those
media that are incapable of supporting full duplex, for example,
10BASE2 and 100BASE-T4.
(2) Our discussion: half duplex is always possible on media that are
capable of supporting full duplex, for example, 10BASE-T, 10BASE-FL,
100BASE-TX, 1000BASE-T.
(3) The half duplex violation is detected by the collision function.
Are the 2 last statements correct?
Best regards,
Michelot
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