Question: What does a local dial-up number do? Richard Wanderman <richard@ldresources.com> writes about accessing Boston-based TIAC (The Internet Access Company) through a local dial-up phone number in Torrington, Connecticut. He notes that he used to use a number in Hartford, Connecticut, and that often the Torrington number is busy and Hartford's isn't. Richard asks: how this is possible?

Answer: When you call a local dial-up number, you're generally calling a secondary physical location (often just a single room) called a "point of presence" (or POP, not to be confused with the email Post Office Protocol, which is also abbreviated POP) where a bunch of modems are linked to devices called "terminal adapters," which in turn link to routers that connect to high-speed digital phone lines back to the main facility.

When you connect to a modem at a POP, the modem answers the call, turns to a terminal adapter for login authentication - usually a matter of consulting a remote database of usernames and passwords - and establishes the necessary protocol, usually PPP. Once those actions have taken place, the connection is set up and you can access Internet resources.

POPs typically rely on high-speed connections ranging from 56 Kbps to T1 (1.544 Mbps) to link to the central ISP facility, where the ISP connects to the rest of the Internet and maintains the bulk of their bandwidth and equipment.

In essence then, each POP is a little network hub in itself - an arm of the Internet - connected to the rest of the Internet via the ISP. Some large ISPs, like PSI, UUNET, SprintLink, and CompuServe, run hundreds of thousands of modems in thousands of POPs across the United States. They lease access to these POPs to other providers (like EarthLink, for instance) so that users can dial in from most locales without incurring long-distance charges.

To answer Richard's specific question, the two POPs might have different numbers of modems, or perhaps more people know about the phone number of the first one. Thus, it would be more likely that all the modems in the first POP would be in use, whereas at least some modems in the second POP would be free. [GF]