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Re: Cogent service

  • From: Petri Helenius
  • Date: Fri Sep 20 16:38:15 2002

(apologies for the previous email being HTML)

>Yes, but only once. With a layer 3 network (or non-ATM layer 2 network)
>you get this at every hop.

About 40% all packets are minimum size. Depending on your encapsulation
these are usually less than 53 bytes on a POS link. So you suffer only the few
microseconds of switching latency.

>Ok, if you push your line to 99% utilization your average queue size is
>100 packets. Assuming those are 1500 bytes this adds up to 1200000 bits or
>some 480 microseconds delay...

Remember that in order to generate a queue you have to receive packets faster
than you can send. So you have to figure out how to get packets into the box
faster than your OC48 can sink them. The math gets complicated but
even strict real world applications tolerate ~10ms delay variance. So for 10 or
20 hops you can easily afford 500 microseconds on a hop. And if you really
care about latency you can drop the frames on the packet train to give better
service to other people.

>Well, I don't have an OC48 but I have seen slower lines that aren't
>considered congested by any definition still experience tail drops from
>time to time, so bursts happen. But that could be just 0.1% of the time,

You make my point. Bursts happen mostly on slow lines. Thatīs because
there is a fast line somewhere which can burst your slow link.

>The problem with running IP over ATM is that both protocols need/use very
>different ways handle congestion and those ways tend to conflict. So then
>you have to cripple either one or the other. (Or buy more bandwidth.)

That discussion is very old news, we would be done with ATM unless the
stupid and ignorant DSL equipment vendors wouldnīt be forcing it back on
us once again.
(though I like ATM over MPLS or Frame Relay)


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