Marc E. Hidalgo mhidalgo at
Wed Apr 3 20:15:52 UTC 1996


After all that, I knew IP/ATM had to show up somewhere! ;-)


On Wed, 3 Apr 1996, Wolfgang Henke wrote:

>      bob_metcalfe at (Bob Metcalfe) wrote:
>      Perhaps I am confusing terms here.  How can it be a fact that
>      "store-and-forward delays are a mere fraction of wire propagation delays?"
>      I don't think so.  Check me on this:
>      Packets travel over wires at large fractions of the speed of light, but
>      then sadly at each hop they must be received, checked, routed, and then
>      queued for forwarding.  Do I have that right?
>      Forget checking, routing, and queueing (ha!), and you get, I think, that
>      store and forward delay is roughly proportional to the number of hops times
>      packet length divided by circuit speed (N*P/C).
>      For 10 hops of a thousand bit packet at Ethernet speed, that would be 1 ms,
>      or a couple hundred miles of prop delay.  Check me on this, one of us might
>      be off by several orders of magnitude.
> Hmm... 
> Using a real in use backbone of one of the mayor service providers,
> I find that a DS3 between silicon valley to Chicago has a 44 msec 
> latency going through 4 hops. That's about the speed of light in
> fiber for the 5000 mile roundtrip ICMP ping packets. 
> Using ATM will reduce the router latency. I estimate that with TCP/IP
> over ATM over SONET OC-3c the latency will be reduced from 44 msec
> to 40 msec, only a rather small improvement. The bandwidth used on the
> fiber wont matter much. With OC-12c I would still expect 40 msec or so
> since the speed of light in fiber is the limiting factor.
> Wolfgang

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