Editing ISDN (section)

=== Primary Rate Interface ===

The other ISDN access available is the [[Primary Rate Interface]] (PRI), which is carried over [[T-carrier]] (T1) with 24 time slots (channels) in North America, and over [[E-carrier]] (E1) with 32 channels in most other countries. Each channel provides transmission at a {{nowrap|64 kbit/s}} data rate.

With the E1 carrier, the available channels are divided into 30 bearer (''B'') channels, one data (''D'') channel, and one timing and alarm channel. This scheme is often referred to as 30B+2D.<ref>{{cite book | first = John C. | last = Bellamy | title = Digital Telephony | url = https://archive.org/details/digitaltelephony00bell_133 | url-access = limited | edition = 3 | publisher = Wiley Interscience | date = 2000 | isbn = 978-0-471-34571-8| page = [https://archive.org/details/digitaltelephony00bell_133/page/n521 496]}}</ref>
 
In North America, PRI service is delivered via T1 carriers with only one data channel, often referred to as 23B+D, and a total data rate of {{nowrap|1544 kbit/s}}. [[Non-Facility Associated Signalling]] (NFAS) allows two or more PRI circuits to be controlled by a single [[D channel]], which is sometimes called ''23B+D + n*24B''. D-channel backup allows for a second D channel in case the primary fails. NFAS is commonly used on a [[Digital Signal 3]] (DS3/T3).

PRI-ISDN is popular throughout the world, especially for connecting [[private branch exchange]]s to the [[public switched telephone network]] (PSTN).

Even though many network professionals use the term ''ISDN'' to refer to the lower-bandwidth BRI circuit, in North America BRI is relatively uncommon whilst PRI circuits serving PBXs are commonplace.