On Tue, 18 July 2000, Robert Cannon wrote:

I am curious how often you think that ATT telephone long distance would hand traffic off to MCI telephone long distance. My impression of telephone long distance is that it is largely an end to end service (one of the great differences from the Internet). That as far as long distance goes, there is not a great deal traffic hand off (one exception is times of network trouble where carriers have agreements to hand off traffic to maintain network reliability).

The urban legend has been MCI was AT&T's third largest customer (after General Motors and US Government). However as far as I know, there is no publicly available source of which telco uses what. Telco's swap a lot of things to reach different places. There are 1800 local telephone companies, but IXCs like Sprint or Worldcom only have a few hundred POPs around the country. So frequently an ILEC (usually the former RBOC) provides tandem service between an IXC and another LEC. Or an IXC may backhaul the call across another IXC's circuits. Look back to many of the "cream-skimming" complaints. Add in complications like Illuminet's SS7 service between multiple carriers, and a simple problem can involve a lot of different carriers. As Patrick mentioned, international calling is even more likely to involve "hidden" carriers. An inter-exchange carrier may not have its own international agreements with every carrier in every country. So if you are in PacBell land with Qwest as your long distance provider and call some place like Cuba it may pass through several other carriers or countries on its way to Cuba. And for odd requests, such as a person-to-person call to a sub-sahara country, the IXC will punt the call and tell you to call AT&T for help. I've heard of one international circuit between New York and Stockholm involving 13 different carriers. And no, the telephone carriers don't speak to each other any better. ATIS has an operations group which maintains an extremely thick manual how telco network management and control centers are supposed to work together. If you read the minutes from their meetings, it sounds remarkably like polite NANOG. The problems are the same, the words just have more syllables. Read Cliff Stoll's book The Cuckoo's Egg for a fun description of how many telephone companies may be involved in a call as he tried to trace his intruder. The National Communication System compiled a database covering the physical connectivity of the three major IXCs and dominant ILECs under an non-disclosure agreement for "national security/emergency preparedness" purposes. But even that is incomplete. And woefully inadequate for trying to predict what would happen to the Internet because most Internet circuits seem to run on the new carriers with cheap bandwidth instead of the three major IXCs. For example, UUNET uses some non-MCI/Worldcom circuits, as you can often see when Worldcom has a fiber cut isolating the Worldcom POP, but UUNET is unaffected.