press-to-talk (PTT)

channel for signaling and control, and is backward-compatible with E-1 transmission. Primary rate interface (PRI), the North American version, offers 23 bearer (B) channels, plus one data (D) channel, and is backward-compatible with T1 and J-1 transmission systems. PRI and PRA both provide a full-duplex (FDX) point-to-point connection through an NT2-type intelligent CPE switching device, such as a PBX

or router, for interfacing with the central office (CO) switch.The B and D channels are clear channels of 64 kbps, as signaling and control are out-of-band.The B channels can be used individually to connect on demand to any other ISDN device, or multiple B channels can be bonded and treated as a single fast connection for bandwidth-intensive applications such as data file transfers, videoconferencing, and any multimedia combination. Although the D channel is reserved exclusively for signaling and control, those functions are fairly light. ISDN standards, therefore, provide for non-facility associated signaling (NFAS), which allows a D channel to support up to five PRA/PRI circuits. See also B channel, bond, BRI, CO, CPE, D channel, E-1, FDX, ISDN, J-1, out-of-band, PBX, PRI, router, signaling and control, and T1.

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