ISDN has two data channels, called "B channels," and one extra, used for a and carrier called the "D channel." Each B channel gives you 64K bits per second, so that a one channel connect is 64K and a two is 128K.
When you dial in, you're connecting to a Point of Presence, or PoP. At the PoP are big banks of routers. Each router has 42 modems and a pool of IP addresses that it can give to incoming calls. The routers can only "talk" to the IPs from their own pool. This last is important.
When you have your ISDN connect, the first B channel picks up, dials and is connected to a modem, connected to a router at the PoP. It logs on and receives an IP address. You have a one channel, 64K connection.
Then, your second B channel dials, is connected to a modem, connected to a router at the PoP. It logs on and tells the router, "You don't have to give me an IP address, I already have one. Here it is."
If the second modem is on the same router as the first, no problem; you have a two channel, 128K connection. If it's on a different router, the new router sees that the IP isn't from its own pool and it replies, "Go away, kid, you bother me!" and hangs up.
Alas, there's no way to force both channels onto the same router under current technology. Some ISDN adapters come with test numbers, to make sure they're working. You always get two channels with them. Why? Because the number only has one router, so there's no chance of failure. Besides, nobody ever stays connected to them for more than a few seconds, so there's always room. In some areas, there are so many ISDN accounts, compared to the room for them at the PoPs, that there's little chance of getting both channels simply because of crowding. Also, there are a number of different ways that the incoming calls can be distributed to the routers. One of them is a straight round-robin. In this case, each incoming call goes to the next router. Unless you're very lucky, there's no way to get a two channel connect, so these locations are generally listed as offering one channel only.
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