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What about ISDN? What is a TA? What's a SPID?

Integrated Services Digital Network (ISDN) is a set of CCITT/ITU standards for digital transmission over ordinary telephone copper wire. ISDN requires adapters at both ends to handle a digital call, so users must dial into an ISDN-capable PoP. (When you dial into one of these PoPs with a handset, you will hear a two-tone short beep before the regular modem handshake). There are two levels of service: the Basic Rate Interface (BRI), intended for the home and small enterprise, and the Primary Rate Interface (PRI), for larger users. Our customers get a BRI line to connect to us. We have a PRI (or several) to the PoPs to handle the incoming calls. The BRI consists of two B (bearer) channels for carrying data at 64Kbps (56K in California). Hence, a "2B" connection is at 128K (or 112K). On top of that is one D (data or delta) channel, which carries control and signaling information at 16K. A PRI consists of 23 B channels and 1 64K D channel.

The Terminal Adapter is the network interface to the Public Switched Telephone Network, which makes a digital call to our PoP. It negotiates Bell connection protocols for speed, compression, and error control, and turns the connection over to TCP/IP. It works like a phone switch and digitally routes the call, the way a phone switch routes analog calls for a modem. SPIDs are the numbers used by the telephone company to determine the TA's node on the isdn circuit to route calls to. They're generally (but not always) the same as or similar to the customer's phonenumber, plus a circuit ID, one for each 1B channel.


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