Integrated Services Digital Network (ISDN)

ISDN has been referred to as the first "digital subscriber loop" (DSL) technology, but whereas the other DSLs are "always on", ISDN requires circuit-switching connection establishment.

ISDN defines two types of channels:

  • B, or Bearer channels — As the name implies, B channels actually carry the user voice and data.
  • D, or delta channels — The D channel is used for call setup and other signalling. The current Public Switched Telephone Network (PSTN) uses the same bandwidth you use to talk for call setup and other functions. This is called in-band signalling. ISDN uses the D channel for these functions, which is called out-of-band signalling

ISDN Service Packages
ISDN has two primary "packages", the Basic Rate Interface (BRI) and the Primary Rate Interface (PRI). In some areas, the D channel can also be used as an "always-on," slower-speed packet service.:

BRI: Defines two 64 Kbps Bearer (B) channels and one 16 Kbps D channel for signaling. Typically, the two B channels can be "bonded" or aggregated to function as a single 128 Kbps channel, often with automatic setup and teardown of the second channel, as needed.

PRI: Defines 23 B channels and 1 64 Kbps D channel in the T1 environment or 30 B channels and a D channels in the E1 environment. Additionally, with E1, there is an additional 64 Kbps channel that is used for synchronization. PRI lines are most often used at the access server end of a connection.

Always On/Dynamic ISDN (AO/DI): Additionally, the D channel can be used for an X.25 packet session. As its name implies, there is no circuit setup in this case: the channel is always available. Though its proponents promise "free" bandwidth, it is more likely that the telephone companies will charge for this usage, though at a reduced price. AO/DI automatically establishes full 64 Kbps channel connections when data rates exceed the 9.6 Kbps maximum of the D channel. The spiky nature of much remote access means that B-channel connections will be brought up frequently. Although the telephone companies must equip their central office switches with X.25 network access to offer AO/DI, many current ISDN adapters will be able to support it with just a downloaded software update. See the DSL section for information about ISDN-DSL (IDSL).

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