# Primary Rate Interface

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

The '''Primary Rate Interface''' ('''PRI''') is a [telecommunications](/source/telecommunications) interface standard used on an [Integrated Services Digital Network](/source/Integrated_Services_Digital_Network) (ISDN) for carrying multiple [DS0](/source/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](/source/T-carrier) (T1) transmission in the US, Canada, and Japan, while the [E-carrier](/source/E-carrier) (E1) is common in Europe and Australia. The T1 line consists of 23 [bearer (B) channels](/source/B_channel) and one [data (D) channel](/source/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&nbsp;Mbit/s. The E1 carrier provides 30 B- and one D-channel for a bandwidth of 2.048&nbsp;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 [synchronization](/source/Frame_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](/source/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](/source/Basic_Rate_Interface) (BRI): one 16-kbit/s [D channel](/source/D_channel) with two 64-kbit/s [B channel](/source/B_channel)s, intended for small enterprises and residential service.
* Primary Rate Interface (PRI): one 64-kbit/s D channel with 23 (1.544&nbsp;Mbit/s [T1](/source/T-carrier), a.k.a. "23B + D") or 30 (2.048&nbsp;Mbit/s [E1](/source/E-carrier), a.k.a. "30B + D") 64-kbit/s [B channel](/source/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](/source/private_branch_exchange) (PBX) telephone systems to provide digital access to the [public switched telephone network](/source/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](/source/direct_inward_dialing) are also common as a means of delivering inbound calls to [voice over IP](/source/voice_over_IP) gateways from the PSTN.

==See also==
* [Non-Facility Associated Signalling](/source/Non-Facility_Associated_Signalling)
* [H channel](/source/H_channel)
*[I.431](/source/I.431)
* [Caller ID spoofing](/source/Caller_ID_spoofing)

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

Category:Integrated Services Digital Network

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Adapted from the Wikipedia article [Primary Rate Interface](https://en.wikipedia.org/wiki/Primary_Rate_Interface) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Primary_Rate_Interface?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.