value of co-location

> I'm also curious about the value of co-location. Using a fast packet
> service (Frame Relay, SMDS, or ATM) allows your on-site router to
> communicate directly with a router of another ISP. There's no need to
> purchase another router to place at the co-location site. Why incur the
> additional cost?

To avoid having to use any of the fast packet services you mentioned?
Or to allow you to use routers, which you know about, instead of being
dependent on the random characteristics of switches the telephone
company bought?

Not using either of the last two fast packet services, in particular, will
also yield 30% more useful bandwidth from a T3 circuit. This all by itself
may make up for the co-location costs and the cost of a router.

Actually, the overhead for IP over SMDS is more like 40%, if I remember
correctly, because of all the headers (8 byte SNAP, 32 byte SMDS) on top of
the ATM cell tax (5 out of 53 on cell headers, average 1/2 a cell wasted at
the end of the packet which occurs about ever 5 or 6 cells), figured on the
observed Internet average packet size of ~220 bytes a couple years ago.
What's the average packet size now? I think the overhead on DS-3 ATM/SMDS
is worse than on OC-3, if I recall correctly, because the DS-3 PLCP is so
wasteful as well. Someone should redo these calculations more accurately.

So, if the fast packet based access to NAPs is priced at ~40% less, *and*
the ATM switches perform as well as the FDDI switches, then it is a don't

  -- Jim

With PLCP, maximum bit rate available for ATM cells =

53 cells * 12 rows * 8bits / 125us = 40.704 mpbs.

So, 40.704 * 48/53 = 36.864 mpbs = data rate.

Now if you use AAL5, then it's another 2 bytes for SAR, and 2 bytes for
CS, so, 40.704 * (48-4)/53 = 33.792 mpbs.

Now, that's the theorectical maximum, and you go down from there.