What is ISDN?

ISDN stands for Integrated Services Digital Network. It is a phone company standard for what was supposed to replace the plain old telephone line (POTS).

The standard is 20 years old so you see how quickly the phone companies move.

Over a single copper pair, ISDN provides two "B" channels and one "D" channel -- all digital. The "B" channels are 56kbps or 64kbps each. The "D" channel is generally reserved for signalling and is 16kbps (?). This normal setup is called a BRI (Basic Rate Interface). There's also something called a PRI (Primary Rate Interface) that has 23 "B" channels.

ISDN can be difficult to order. In addition to having lots of configuration options, there are quite a few different ISDN services from Pacific Bell:

• Residential ISDN
  Costs about $24/month + $0.01/minute/B-channel from 8AM to 5PM. Unmetered ISDN service is also available from PacBell for about $80/mo.
• Business ISDN
  Costs about $35/month + $0.01/minute/B-channel.
• Centrex ISDN
  Costs about $35/month. Calls within a Centrex group are free, but centrex groups can only exist within a single phone company switch. There are substantial setup charges for Centrex ISDN. NOTE: Idiom does not offer connectivity via Centrex ISDN.

ISDN lines can use the "B" channels independently. They could be used to call different places simultaneously. Or one could carry data while the other carries voice. The important thing is that it is possible to log in twice (once for each "B" channel) and then split your packet traffic across both links. The protocol for doing this is called MLPPP (Multi-Link PPP).