BRI delivers two bi-directional 64 kbps B channels and one bi-directional 16 kbps D channel over standard two-wire telephone lines. Basic rate ISDN service typically is used for residential and small office, home office (SOHO) applications. Each B channel can transmit speech or data; the D channel transmits the signaling or call control messages.

The configuration and reference points for BRI are specified in Figure 3-8.

Figure 3-8. ISDN BRI Reference Points

Figure 3-8. ISDN BRI Reference Points

The reference configuration for ISDN is defined in the ITU specification I.411. The reference points specify the transmission medium, interface, and connectors (if used).

• U Reference Point—The U reference point specifies the transmission characteristics of the local loop. For BRI, this two-wire interface operates at 160 kbps (2B + D + 16 kbps for overhead) over standard copper-twisted wires.

• S/T Reference Point—For basic rate access, this interface provides a four-wire connection to ISDN-compatible terminals or terminal adapters. The interface operates at 144 kbps (2B + D) between the ISDN device and the network termination device. You can connect up to eight ISDN devices to the S/T interface.

• R Reference Point—The R reference point provides connection for non-ISDN devices. Such devices connect to the terminal adapter using interfaces such as RS-232 and V.35.

This reference configuration also specifies the set of functions required to access ISDN networks:

o Network Termination 1 (NT1)—Outside the United States, NT1 is on the network side of the defined user-network interface and is considered part of the service provider network. NT1s terminate the two-wire local loop and provide four-wire S/T bus for ISDN terminal equipment (TE).

o TE1—TE1s are ISDN-compatible devices that connect directly to the S/T connector on the NT1.

o TE2—TE2s are non-ISDN compatible devices that require terminal adapter (TA) interconnection.

o TA—TAs provide an ISDN-compliant interface to NT1s and standard interfaces for TE2s. These standard interfaces include RS-232, V.35, RS-449, and X.21.

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