30B + 2D

PRA (Primary Rate Access) Telecom Definition
The user interface to an integrated services digital network (ISDN) intended for large business applications in European countries and elsewhere outside of North America and Japan. Also known as 30B+D, PRA offers 30 bearer (B) channels for user payload, plus one data (D) 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.