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My understanding of the cut-through definition in Sanjay's
example is
1. Pass-through packet is allowed to
transmit before it is completely received.
2. There is only one transit buffer
(regardless of class).
3. Scheduling Algorithm always give
pass-through traffic (regardlesss of class)
preference over
add-on traffic.
which somewhat contradicts with his first statement. Thus the
interesting results.
The debate should be based on a solid definition of
cut-through transmission, otherwise
there will be no convergence at the end of the
discussion.
I fully agree Sanjay's first statement, but want to add that
each class should have its
own transit buffer, (personally I prefer having 3 classes
supported as RPR MAC services).
Whether each transit buffer should reside in MAC layer or
systemlayer is up to further
discussion. Under this context, Circuit Emulation (or some may
prefer to call it
Synchronous) class will benefit from the cut-through transit.
Ideally it could further
benefit from preemptive transmission (yet another definition
to be solidly defined).
William Dai
----- Original Message -----
Sent: Thursday, March 22, 2001 11:15
AM
Subject: RE: [RPRWG] MAC Question
Hi Ajay,
Latency and jitter requirements depend on the class of
traffic. For some type (class) of services it is critical for others it is
not.
Counter intuitive as it is, actually, store and for forward is
less end-to-end latency than cut through.
In cut through approach, high add priority traffic waits while
pass low priority upstream traffic passes through. It takes two RTT to shut up
the low priority traffic through BCN. Thus high priority waits 2RTT because of
the low priority stream. In this case low priority pass streams impose 2RTT
latency or jitter to add high priority stream. For 200km ring that is 2ms. For
200km it is 20ms.
Total end to end latency = add latency + N*pass latency
In cut through end to end latency = 2RTT + N* packet delay at
link speed
In the store and forward approach, if pass traffic is low
priority it waits in the buffer while pass high priority and local high
pririty get to go in that order. Thus, max jitter or latency imposed on high
priority traffic is at worst imposed by high priority stream. Since high
priority traffic streams are committed services, they never over subscribe the
link only low priority streams do.
In store and forward end to end latency = pass hi priority
burst + N* packet delay at link speed.
Pass hi priority burst = At 10gig speeds depending on the hi
prority provisioning levels.
typically in the order
of microseconds
store and forward gives clear class based seperation. It
provides no latency panelties on committed high priority streams (typically
voice and video) due to overcommitted low priority streams (typically data).
There is no RTT dependence here which can be .1msec at 20km to
10msec at 2000km
-Sanjay K. Agrawal
Luminous
networks
> -----Original Message-----
>
From: owner-stds-802-17@xxxxxxxx [mailto:owner-stds-802-17@xxxxxxxx]On
> Behalf Of Ajay Sahai
> Sent:
Thursday, March 22, 2001 6:34 AM
> To: Ray
Zeisz
> Cc: stds-802-17@xxxxxxxx
> Subject: Re: [RPRWG] MAC Question
>
>
>
Ray:
>
> I guess the
answer is that the group is still debating this issue. Some
> vendors prefer to have a largish transit buffer where transit
frames
> are stored. Others are proposing "cut
through" transit functionality.
>
> I personally feel that latency will be larger in the
first approach.
>
> On
another note I do not believe that the similarity with 802.5 is
> on the lines of claiming a token etc. etc. The MAC
mechanism
> is going to be different.
>
> Hope this helps.
>
> Ajay Sahai
>
> Ray Zeisz wrote:
>
> > I am following the .17 group
from afar, but I have a question:
> >
> > Is it acceptable for each node in the ring to
buffer up an entire packet
> > before forwarding
it to its neighbor? Would the latency be to
>
great if this
> > were done? Or is the .17
direction more along the lines of
> 802.5 where
only
> > a few bits in each ring node are
buffered...just enough to
> detect a token
> > and set a bit to claim it.
> >
> > Ray
> >
> > Ray Zeisz
> > Technology Advisor
> > LVL7
Systems
> > http://www.LVL7.com
> > (919) 865-2735
>
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