Newsgroups: comp.dcom.lans.fddi,comp.dcom.cell-relay
From: mart@csri.toronto.edu (Mart Molle)
Subject: Re: >>>>Future of IP routers
Date: 2 Sep 92 15:10:25 GMT

>KUMQUAT%SMCVAX.BITNET@mitvma.mit.edu (Gary C. Kessler, +1 802-879-3375) writes:

>> If a station on an FDDI ring releases the token because its own TRT has
>> expired, it does NOT necessarily follow that anyone else who follows that
>> station will also see a "late" token.  Since we all have the same target
>> token rotation time (many feel that 8 ms. is an ideal value) and independent
>> TRTs, it really depends upon the amount of synchronous bandwidth that is
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>> allocated and used.  Thus, it is possible that I will see an "early" token,
   ^^^^^^^^^^^^^^^^^^
>> use up my entire TRT time (i.e., the entire amount of the earlyness), pass
>> it on to you, where you will also see an "early" token.
>> 
>> Yeah, this is hard to do in just prose...

Agreed!

vjs@rhyolite.wpd.sgi.com (Vernon Schryver) writes:

>I don't see how any synchronous bandwidth allocation can make the
>token appear to arrive earlier with respect to the local TRT than
>otherwise.

I think the explanation is the funny way that synchronous data is
treated in the FDDI timed token protocol.  In effect, the synchronous 
allocation for each station looks like additional propagation delay
in its *outgoing* link.  (To see this, note that the Token Holding
Timer [THT] is initialized to the value of the Token Rotation Timer
[TRT] when the token arrives, but DOES NOT START RUNNING until any
synchronous traffic has been sent -- at which point the given station
can transmit asynchronous traffic for the difference TTRT-THT.)

I assume everybody understands the dynamics about how a heavily loaded
N-station ring carrying only asynchronous traffic will degenerate into
"pessimally fair" sharing, where a given station gets the full bandwidth
(TTRT-propagation time) once every N+1 rotations of the token.  The
reason the same thing doesn't necessarily happen in the presence of
synchronous traffic is the equivalence between synchronous allocation
and propagation delay.

Suppose in the previous "cycle" station K transmitted no asynchronous
data.  That means the TRT for stations K and K+1 differ by exactly the
sum of the propagation delay from K to K+1 *and* the synchronous
traffic sent by station K in that "cycle".  Suppose, further, that on
the current "cycle" station K greedily uses up all of the available
asynchronous bandwidth (i.e., it did not release the token until
THT = TTRT).  In the absence of synchronous traffic, this would mean
that station K+1 would be unable to send any asynchronous traffic in
the current "cycle", since the token would not arrive until TRT = TTRT.
However, if station K sent *less* synchronous traffic in the current
"cycle" than the previous one, station K+1 would still have this time
difference available for asynchronous traffic.

>Some of us do not think much of an 8 ms.

It certainly does not seem to make sense when an FDDI ring is used
as a "faster Ethernet replacement"!  So the question is, how many
people actually make use of the ability to carry synchronous data?

Mart L. Molle
Computer Systems Research Institute
University of Toronto
Toronto, Canada M5S 1A4
(416)978-4928
Newsgroups: comp.dcom.lans.fddi