Basic Rate Interface
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The Basic Rate Interface (BRI) provides three time division multiplexed channels for full-duplex, user-to-network information exchange. The visual shows how the three channels are multiplexed at the S/T reference point: one bit from the D-channel is interleaved between each group of eight bits of B-channel information (the multiplexing at the U reference point has a different interleaved pattern). The two B-channels operate at 64 kbps and provide either circuit, packet, or frame service. The D-channel operates at 16 kbps and is used for ISDN signaling and user packet data.
The two B-channels can be used independently to access different network services. Each B-Channel is “owned” by a user device for the length of the call (an exception to this is when packet service is used). However, a user can use CPE to multiplex multiple data streams onto a single B-channel. In addition, CPE can be used that allows the B-channels to be multiplexed together, thus providing a single 128 kbps channel.
The D-channel on a BRI operates at 16 kbps. It is used primarily for signaling for the interface, and can also be used to send user data packets. Support exists for two forms of user data: the X.25 standard Packet Layer Protocol (PLP) and ISDN signaling messages.
Although the ITU-T Recommendations specify that the BRI will support up to two B-channels and one D-channel, provisioning options have specified different configurations. These include:
- 1B+D: specifies that only one of the B-channels is active on the interface.
- 0B+D: provides low-speed data service only.
- 2B+S: another option is forbidding the use of the D-channel for data; the D-channel data is 2B+S (S indicates that the D-channel is only used for signaling
- 2B+0: another name for IDSL, where 128 kbps of bandwidth is provided but without the signaling capabilities.
Basic Rate Interface Applications
The Basic Rate Interface is appropriate for either a single user or a small community of interest. Therefore, it competes with centrex services, measured or flat plain old telephone service (POTS) lines, and 56 kbps and slower dedicated lines.
The BRI provides higher bandwidth over the existing local loop than typical analog services, and is used where multiple channels or higher rates are required by the user. The pricing of the BRI service will be critical to end-users trying to decide between existing services and ISDN.
Additionally, two users can split a single BRI. In an office environment, a single centrex line is needed for each desk. With ISDN it is possible to split a single BRI between two desks; each desk has a single B-channel and shares the D-channel.
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