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{{Short description|Telecommunication standard}}
{{More citations needed|date=January 2016}}

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.

PRI is the standard for providing telecommunication services to enterprises and offices. 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. The T1 line consists of 23 [[B channel|bearer (B) channels]] and one [[D channel|data (D) channel]] for control purposes,<ref>{{cite book|url=https://books.google.com/books?id=0LyDQgAACAAJ&q=newton's+telecom+dictionary+16th+edition|title=Newton's Telecom Dictionary: The Official Dictionary of Telecommunications & the Internet|first=Harry|last=Newton|date=15 January 2019|publisher=Telecom|isbn=978-1-57820-053-5 |via=Google Books}}</ref> for a total bandwidth of 24x64-kbit/s or 1.544 Mbit/s. The E1 carrier provides 30 B- and one D-channel for a bandwidth of 2.048 Mbit/s.<ref>{{cite web|url=https://www.itu.int/rec/T-REC-I.412-198811-I/en|title=I.412:ISDN user-network interfaces - Interface structures and access capabilities|website=www.itu.int}}</ref> The first timeslot on the E1 is used for [[Frame synchronization|synchronization]] purposes and is not considered to be a B- or D-channel. The D-channel typically uses timeslot 16 on an E1, while it is timeslot 24 for a T1. Fewer active bearer channels, sometimes called user channels, may be used in fractional T1 or E1 services.

==ISDN service types==
The [[Integrated Services Digital Network]] (ISDN) prescribes two levels of service:<ref>{{cite web|title=A Technical Coverage of Primary Rate Interface (PRI)|url=http://ispfast.com/a-technical-coverage-of-primary-rate-interface-pri/|publisher=ISPFast|access-date=18 June 2015}}</ref>
* [[Basic Rate Interface]] (BRI): one 16-kbit/s [[D channel]] with two 64-kbit/s [[B channel]]s, intended for small enterprises and residential service.
* Primary Rate Interface (PRI): one 64-kbit/s D channel with 23 (1.544 Mbit/s [[T-carrier|T1]], a.k.a. "23B + D") or 30 (2.048 Mbit/s [[E-carrier|E1]], a.k.a. "30B + D") 64-kbit/s [[B channel]]s, intended for large organizations.

Each B-channel carries data, voice, and other services. The D-channel carries control and signaling information. Larger connections are possible using PRI pairing. A dual T1-PRI could have 24 + 23 = 47 B-channels and 1 D-channel (often called "47B + D"), but more commonly has 46 B-channels and 2 D-channels thus providing a backup signaling channel. The concept applies to E1s as well and both can include more than 2 PRIs. 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.

==Application==
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). The B-channels may be used flexibly and reassigned when necessary to meet special needs such as video conferences.

PRI channels and [[direct inward dialing]] are also common as a means of delivering inbound calls to [[voice over IP]] gateways from the PSTN.

==See also==
* [[Non-Facility Associated Signalling]]
* [[H channel]]
*[[I.431]]
* [[Caller ID spoofing]]

==References==
{{reflist}}
*{{FS1037C}}

[[Category:Integrated Services Digital Network]]

Primary Rate Interface - Revision history

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