Editing Primary Rate Interface (section)

==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&nbsp;Mbit/s [[T-carrier|T1]], a.k.a. "23B + D") or 30 (2.048&nbsp;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.