This has to do with the "bit budget" constraint in the CSMA/CD protocol.
The collision detection part of the CSMA/CD protocol only works while
ethernet devices are transmitting data. If any collision happens after
transmission then that collision is termed a "late collision" and will not
cause ethernet itself to recover. Higher layer protocols will have to
retransmit. So, in order to detect collisions the smallest ethernet packet
(64 bytes) factored in with the speed and distance of the link determine the
maximum cable distance allowable for hosts to still effectively detect
collisions while transmitting and CSMA/CD to operate correctly. This
information also addresses a recent post by Dan West regarding CSMA/CD.
When using full duplex ethernet, transmitting and recieving takes place on
seperate physical wires. This means that the CSMA/CD protocol is not needed
or used on full duplex connections. There was a reply to that post stating
that CSMA/CD is still required when using full duplex ethernet due to
multiple incoming frames from a switch to pc. This is incorrect. If a
switch has multiple frames destined for one physical port it will transmit
one at a time based on buffering mechanisms explained in detail by Howard
Berkowitz in past posts to the list. However these buffering mechanisms are
internal to the switch and are not in any way related to CSMA/CD. Full
duplex ethernet is a collision free environment with no CSMA/CD protocol
which is why the distance limitation is much greater than half duplex which
must conform to CSMA/CD restraints.
-Mike Cohen
-----Original Message-----
From: Tan Choh Koon [mailto:[EMAIL PROTECTED]]
Sent: Wednesday, May 10, 2000 8:31 AM
To: CiscoGroupStudy
Subject: Fibre Optic - 100FX
Hi all,
Do anyone know the theory why 100Mbps Hub with Multimode fiber link, the
distance is 412 m for Half Duplex and 2000 meter for Full Duplex mode ??
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