[Cisco] Local vs Remote vs Late Collisions -- calling all experts

Thu Apr 28 15:22:41 EST 2005

Hi Kevork,

Indeed you have a point. The statement is not clear at all and can lead
to various (and erroneous interpretations). And as far as I understand,
CSMA/CD is "looking" into the RX pair in order to detect the collision
and therefore would see the data in the pair (compare it with the
transmitted data) and conclude that there was a corruption in the data
stream. About the repeaters, they indeed don't repeat the over-voltage
signals, but in the other hand they repeat all the frames (in any
direction) that are present in the segment. If the intention is to
buffer those frames one should use a bridge and not a repeater. A simple
exercise is to connect a couple of hubs together and 2 hosts (one in
each hub) and check the signals in the cable connecting the 2 hubs. A
simple FTP from each computer to the other should suffice to generate
collisions.  What will be seen is that the hubs will 'repeat' in all
directions whatever signal they can make up of the collisions. And most
likely the collisions will happen in their internal bus (Hub= bus in a
box).

Back to the material, perhaps we should put a feedback on AC reporting
the text issues and suggesting a better format for it.

Best Regards,
Straube

-----Original Message-----
From: Kevork Krozian [mailto:Kroset at novell1.fhc.vic.edu.au] 
Sent: Thursday, 28 April 2005 10:34 AM
To: cisco at edulists.com.au
Subject: RE: [Cisco] Local vs Remote vs Late Collisions -- calling all
experts

Hi Straube,

  Actually I would have called yours a $2K worth ( a great deal higher
than 2 cents ! ) 

I am happy with the clarification. I guess what threw me is the
statements in Slide 6.2.6 about remote collisions :
" A repeater will not forward an over-voltage state, and cannot cause a
station to have both the TX and RX pairs active at the same time. The
station would have to be transmitting to have both pairs active, and
that would constitute a local collision. "

   That is, if station A is still transmitting for a remote collision to
occur, then how can it recognise something is coming back be it a
jamming signal or a frame fragment , if it cannot have both Rx and Tx
active ? How can it NOT have a signal come back on Rx while Tx is active
if it is to detect the problem while it is transmitting ?

   Can you see where I am coming from ?

With thanks
Kevork

>>> J.Straube at bhtafe.edu.au 04/27/05 04:29pm >>>
Hi Kevork,

Here you have my 2 cents worth of thought.

"The question is :  Is the sending machine still transmitting for a
remote collision to occur? "

Yes, the 2 machines are still sending (within the first 64 octects)

"If not, then is it not a Late collision?" 

No, because it will be detected within the 64 octets. 

"Also, how can the sending machine know there was a remote collision ?
Does it get fragments of the collision back across the repeater with the
FCS not matching ?" 

Two scenarios: 
1)Let's assume machine A and machine B are separated by a repeater.

A ------ R ----xxx-- B

In here we see that the remote collision for A (represented by xxx) is
also a local collision for B. When B detects the collision it sends a
Jamming signal (which is repeated) that can be detected by A which will
in turn also send the Jamming signal.

2) Let's now assume 3 repeaters in the logical segment (worst case
scenario).

A ------ R ------ R ---xxx--- R ------ B

In this case the collision is remote to both sources. A or B can only
detect the collision by checking the FCS (which in any case will be
incorrect) of the received frame. In this case you could say that the
collision can also be seen as 'Late' collision due to the fact that the
FCS is at the end of the frame.

"and does that happen before the 64 bytes slot time is exhausted ?"

Please refer to the previous answer.

I hope it helps to clarify a bit.

Regards,
Straube

-----Original Message-----
From: Kevork Krozian [mailto:Kroset at novell1.fhc.vic.edu.au] 
Sent: Wednesday, 27 April 2005 2:07 PM
To: cisco at edulists.com.au 
Subject: [Cisco] Local vs Remote vs Late Collisions -- calling all
experts

  Hi Folks,

        I am wondering if anyone can help me here. 
I am reading contradictory information about Collision types on
Ethernet.

 Local collisions look OK.  These happen during the first 64 bytes of
transmission ( also known as slot time ). There is an amplitude increase
on the local segment, and both Rx and Tx wires become active hence the
collision. The sending machine has to resend. No problems here.

 Remote Collsions . Happen on the other side of repeaters, which is does
not send back the amplitude increase or send back a signal on the Rx
wire. However, these must happen while the sending machine is still
transmitting the first 64 bytes which means it is still transmitting
when a remote collision occurs. See Slide 6.2.6 Semester 1. 
 The question is :  Is the sending machine still transmitting for a
remote collision to occur? If not, then is it not a Late collision ?
Also, how can the sending machine know there was a remote collision ?
Does it get fragments of the collision back across the repeater with the
FCS not matching ? and does that happen before the 64 bytes slot time is
exhausted ?

  Late Collisions :  These happen after the slot time is exhausted and
the sending machine does not know there was a collision. The higher
layers handle the error.

   I am happy to hear any and all suggestions

With thanks

Kevork Krozian
IT Manager , Forest Hill College
Mailing List(s) Creator and Administrator
http://www.edulists.com.au 
k.krozian at fhc.vic.edu.au 
Mobile: 0419 356 034

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