George H. Clapp <clapp@bellcore.com> wrote:

The reasons for co-location that have been mentioned so far include...

- zero-mile circuits

That means expansion of that "circuit" is free.

- a hardened location for equipment

That is hardly relevant, as all large players have facilities just as good.

- to establish a local presence rather than building out one's own space.

Same.

- skepticism about the value of the fast packet services; the skepticism has several flavors: - reduced bandwidth due to protocol overhead

That is not "scepticism" but a sad truth. Getting 40% less bang (on a typical IP traffic mix which includes *lots* of packets barely exceeding one cell) is rather silly.

- less reliability than a shared FDDI

Shared FDDI is simply a way to offer lower performance for lower price. ATM port costs are not variable.

- performance of ATM switches compared to FDDI switches

FDDI just works. It worked five years ago.

'Zero-mile circuit' isn't clear to me because, regardless of the technology used for the NAP, it's still necessary to purchase a circuit from your site to the NAP.

The difference is between the clearline circuit and circuit with 40% protocol overhead. Some big players are carriers themselves and want to use _own_ circuits whereever possible. The reason for collocated NAPs you completely missed is called "scalability". Having all equipment in one room provides a lot of flexibility in regard to interconnections. For example, if A and B found that they have lots of mutual traffic they may want to plug one more FDDI board in their boxes and drag a private FDDI wire, to offload the traffic from shared infrastructure. When new whiz-bang LAN technology appears you can easily upgrade connection by plugging a new board into the collocated router, leaving the old interface as a backup. Which also leads to the reason #2 which you also missed -- redundancy. When ATM switch is fried or went banana you're dead. FIXes and their progeny had backup connections from the day one. --vadim