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From: pat@hprnd.HP.COM (Pat Thaler)
Newsgroups: comp.dcom.lans
Subject: Re: Do multiport repeaters buffer packets?
Message-ID: <2230107@hprnd.HP.COM>
Date: 27 Aug 90 19:28:55 GMT
References: <65056@yarra.oz.au>
Organization: HP Roseville Networks Division
Lines: 30

> Assume an asymmetrical configuration in the sense that there is
> a large server on one segment and a large number of PCs talking to it.
> The PCs are distributed among the other ports hanging off the multiport
> repeater.

What happens in this case is roughly the same thing as would happen if
all the stations were connected to the same coax segment.  An 802.3 repeater's
job is to restore the signal quality in terms of amplitude and jitter
without interfering with operation of the CSMA/CD access method.

> Just consider the following example:
> Assume that N-1 PCs each on different Ethernet segment generates a packet
> directed to the server at the same time. Even if the server's Ethernet
> segment is quiet only one of those packets can be delivered at a time.
> What happens with the others? 
> 
None of the packets gets delivered.  A collision occurs.

What an 802.3 repeater does when it receives multiple transmissions 
concurrently from several ports is transmit to all ports.  This 
causes a collision on any segment where another transmitter is active.  
All transmitting stations detect collision and the CSMA/CD algorithm 
is used to resolve the collision.  This is the same thing that would
happen if they were all on the same segment except that the worst case
delay is longer.  The parameters of the Ethernet/802.3 CSMA/CD access
method such as minimum frame size are set large enough to allow for
the delay of 4 repeaters and 5 segments with at most 3 of those being 
coax.  See section 10.7.1 of IEEE 802.3 for details on allowed topologies.

Pat Thaler