The Primary Rate Interface (PRI) is a telecommunications interface standard used on an Integrated Services Digital Network (ISDN) for carrying multiple DS0 voice and data transmissions between the network and a user.
It is based on T-carrier (T1) transmission in the US, Canada, and Japan, while the E-carrier (E1) is common in Europe and Australia.
When configuring multiple T1's as ISDN-PRI's, it's possible to use NFAS (non-facility associated signaling) to enable one or two D-channels to support additional B-channels on separate T1 circuits.
The Primary Rate Interface channels are typically used by medium to large enterprises with digital private branch exchange (PBX) telephone systems to provide digital access to the public switched telephone network (PSTN).
PRI channels and direct inward dialing are also common as a means of delivering inbound calls to voice over IP gateways from the PSTN.