ISDN supports a BRI service with 2 B channels, and a PRI service with either 23 (T1) or 30 (E1) B channels. Signaling for call setup and teardown occurs over an out-of-band D channel.
After a circuit has been established over a B channel, ISDN provides a Layer 1 service, delivering a serial bit stream between the two endpoints of the circuit.
ISDN's advantages include the capability to support voice calls concurrently with a data call. Also, ISDN can be used over the local telco loop, with no significant distance limitations. And it provides more bandwidth than do modems, particularly with both B channels dialed to the same remote site.
ISDN does have a few disadvantages, with the biggest disadvantage being the lower speeds than DSL or cable.
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