Basic rate interface

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Basic Rate Interface (BRI, 2B+D, 2B1D) is an Integrated Services Digital Network (ISDN) configuration intended primarily for use in subscriber lines similar to those that have long been used for plain old telephone service. BRI stands in contrast to the Primary Rate Interface (PRI) configuration which is also used in the access network but uses more robust transmission lines to carry a higher bit rate.

BRI is defined in the physical layer standard I.430 produced by the International Telecommunication Union (ITU). The BRI configuration provides two 64 kbit/s bearer channels (B channels) and one 16 kbit/s Delta channel (D channel).[1] The B channels are used for voice or user data, and the D channel is used for any combination of data, control/signalling, and X.25 packet networking. The two B channels can be aggregated by channel bonding providing a total data rate of 128 kbit/s. The BRI ISDN service is commonly installed for residential or small business service in many countries.

The I.430 protocol defines 48-bit packets comprising 16 bits from the B1 channel, 16 bits from B2 channel, 4 bits from the D channel, and 12 bits used for synchronization purposes. These packets are sent at a rate of kHz, giving the data rates listed above for a maximum possible throughput of 144kbit/s.

BRI service is delivered via three types of physical interfaces. Signals are encoded by two modulation techniques, 2B1Q in North America, and 4B3T elsewhere.


Physical interfaces

ISDN technology standards define three main types of interfaces for Basic Rate Interface service:

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