whoa, After all that, I knew IP/ATM had to show up somewhere! ;-) Marc On Wed, 3 Apr 1996, Wolfgang Henke wrote:

bob_metcalfe@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.

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