The user interface to an integrated services digital network (ISDN) intended for large business applications, primarily in North America. PRI offers 23 bearer (B) channels for user payload, plus one data (D) channel for signaling and control, and is backward-compatible with T1 and J-1 transmission systems. The European version is known as primary rate access (PRA) or 30B+D. PRA offers 30 B channels, plus one D channel, and is backward-compatible with E-1 transmission. 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, and 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 PRI connections. PRI requires the extended superframe (ESF) format. Line coding is alternate mark inversion (AMI) with bipolar with eight-zeros substitution (B8ZS).This process is the same combination of techniques used in contemporary T1 circuits. See also AMI, B8ZS, B channel, bonding, CO, CPE, D channel, E-1, ESF, FDX, ISDN, J-1, out-of-band, PBX, PRA, router, signaling and control, and T1.
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