Marc E. Hidalgo
mhidalgo at sprint.net
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 infoworld.com (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.
> 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.
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