This can be particularly important in large metropolitan areas, which have a complex web of local calling charges, and in rural areas, where there are not enough customers in a single area to support an ISP.
Basically, the setup looks something like this:
USR Total Control Rack (The gold-plated option; see below for more cost-effective solutions) | | Router | CSU/DSU | Remote POP side above this line -------------------- | Leased Line | -------------------- Your main setup below | CSU/DSU | Router | (the rest of your existing network)You can use the same router you use to connect to the Internet, if it has a spare port. Most Cisco 2500 series routers have two. However, for more than one remote POP, you'll need a more advanced (and expensive!) router such as a Cisco 4000.
Why the USR Total Control modem rack, a formidibly expensive gadget? Because you can automatically control the modems remotely, so if there's a problem you don't need to drive all the way to your remote POP to fix it.
If you're on a budget, you can get a Livingston Portmaster or similar terminal server and modems of your choice. If you go with this option, be sure to get a "call forward/no answer" service from the phone company. That way, if a modem has trouble, the phone system will automatically route around it. You can request this service from your local phone company, normally as part of a Centrex package.
The most cost-effective package for a remote POP is probably Livingston's combo router/terminal server unit. I've heard poor reviews of it as your main router, but it apparently works well in this application, since the routing required is very simple. Using this unit would mean that you'd just need modems and a CSU/DSU.
You can probably support up to ten modems on a 56k dedicated line between your remote POP and the system. For more lines, you'll need to get a T1. I would recommend fractional T1 service to connect, since that will give you the best flexibility in expansion.
The most cost-effective way of doing this is likely to be Frame Relay, if your phone company has a frame relay service connecting the two areas. Frame relay lets you connect from your system to a network provided by the phone company; you just connect both ends to the network and you're all set. Traditional leased lines, requiring the running of a fixed wire between your site and the remote POP, are likely to be too expensive. Watch for the Committed Information Rate (CIR); the higher it is, the better. A low CIR means that they're only guaranteeing that much bandwidth, so don't accept it unless you know you can live with that figure during peak load times.