Cogent service

Fri Sep 20 18:12:22 UTC 2002

On Fri, 20 Sep 2002, David Diaz wrote:

> The hop count question is interesting.  Is the consensus that it's
> mostly a customer service issue, where latency isnt affected but
> customer perception is?  Or is it a real latency issue as more
> routers take a few CPU cycles to make a routing decision.

Under the best possible circumstances, most of the extra delay is due to
the fact that routers do "store and forward" forwarding, so you have to
wait for the last bit of the packet to come in before you can start
sending the first bit over the next link. This delay is directly
proportional to the bandwidth and the packet size. Since ATM uses very
small "packets" this isn't as much an issue there.

However, the real problem with many hops comes when there is congestion.
Then the packet suffers a queuing delay at each hop. Now everyone is going
to say "but our network isn't congested" and it probably isn't when you
look at the 5 minute average, but short term (a few ms - several seconds)
congestion happens all the time because IP is so bursty. This adds to the
jitter. It doesn't matter whether those extra hops are layer 2 or layer 3,
though: this can happen just as easily in an ethernet switch as in a
router. Because ATM generally doesn't buffer cells but discards them, this
also isn't much of an issue for ATM.

However, when an ATM network gets in trouble it's much, much worse than
some jitter or even full-blown congestion, so I'll take as many hops as I
must to avoid ATM (but preferably no more than that) any day.