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RE: MAC control Pause clarification


That practice follows a long-standing tradition with 802.3.  For instance,
on the Tx side, the MAC outputs a preamble which is exactly 7 octets.
However, on the Rx side, the preamble is allowed to be 0 or more octets.  

The Tx is constrained, the Rx is forgiving (read that: flexible for future

It was thought that sometime down the road, other Control frames or other
opcodes might use unicast addresses...

Kevin Daines

-----Original Message-----
From: FEffenberger@xxxxxxxxxxxxxxxxx
Sent: Wednesday, June 06, 2001 8:03 PM
Subject: MAC control Pause clarification

I got the impression from the discussion at the last meeting that the pause 
command was not defined for use with unicast MAC addresses.  

I checked in Annex 31B, and I found conflicting information: 

In section 31B.3.1, Transmit operation, it says in the first line item (a) 
that "The destinationParam ... is currently restricted to the value
specified in 31B.1"  
That value is the globally assigned multicast address for pause. 

In section 31B.3.3, Receive operation, it says in the second paragraph that 
"Upon receipt of a valid MAC Control frame with ... the destination address
either: (1) the reserved multicast address specified in 31B.1 or (2) the
unique physical 
address associated with this station,"  

So, it seems that the standard allows for the reception of unicast pause,
but not 
the transmission of same.  Could any of the IEEE alumni explain why this is

Dr. Frank J. Effenberger
Director, System Engineering
Quantum Bridge Communications
(978) 983-2532