Sarah:
Funny you should ask this question about legacy Internet access, since I used to be part of a team of UUNET implementation engineers who used to build out AOL's dial-in infrastructure around the country.
To answer your question, AOL issues its users a phone number that their modem should dial to access the AOL network. That number is part of a hunt-group, in which calls rollover from modem to modem as the calls come in.
When the modem on the AOL side answers the call, a session is negotiated between the two modems, taking into consideration the quality of the dial-up POTS line. Good quality means high data rate of exchange, with data rate decreasing as quality decreases. This is called the "handshake."
These modems were typically deployed in proprietary configurations in a self-contained cabinet of equipment that included high-density modem modules HDMs (24-modems per HDM at that time, with each shelf containing 15 HDMs); T1 D4/AMI or ISDN PRI circuits terminating on the HDMs; a UNIX-based "server" (shelf-mounted, too) that bridged the data traffic from the DTE side of the modem to the LAN interface of a Cisco 3640 access router.
The router would prvide the IP connection to the AOL FEP (front-end processor) in Virginia.
HTH
Victor
