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T1: A Survival Guide by Matthew Gast

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ISDN PRI

The Integrated Services Digital Network, or ISDN, simply extends digital telephone service all the way to the customer. Instead of analog drops from the CO to the customer, it is a complete digital path.

At a high level, ISDN is a composite pipe with subchannels. Subchannels come in two types. Bearer channels, or B channels, are 64-kbps channels that move user data. User data might be a telephone conversation that is digitized at 64 kbps, videoconferencing, or Internet access. B channels are 64-kbps raw digital pipes that can be directed over the telephone network. To connect bearer channels to endpoints across the telephone network, a second subchannel type, called the D channel, provides signaling information. The D channel carries call setup and teardown messages.

ISDN comes in two flavors. Basic rate service is composed of two bearer channels plus a 16-kbps D channel. This package, also known as the ISDN Basic Rate Interface (BRI), is called 2B+D. It provides 128 kbps of user bandwidth in two 64-kbps increments. Each B channel can be used for a digitized telephone call or a raw data pipe. ISDN BRI can be used as two telephone lines, a telephone line plus a 64-kbps data pipe, or a 128-kbps data pipe with no telephone access. Part of ISDN’s appeal is its flexibility. It establishes data calls as circuits through the telephone network, so the division between data and telephony can be dynamic. You may start a large download and fire up both B channels for Internet ...

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