Primary Rate Interface
The Primary Rate Interface (PRI) is a standardized telecommunications service level within the Integrated Services Digital Network (ISDN) specification for carrying multiple DS0 voice and data transmissions between a network and a user.
PRI is the standard for providing telecommunication services to offices. It is based on the T-carrier (T1) line in the US and Canada, and the E-carrier (E1) line in Europe. The T1 line consists of 24 channels, while an E1 has 32.
T1 and E1 systems 
PRI provides a varying number of channels depending on the standards in the country of implementation. In North America and Japan it consists of 23xB (B channels (be/s). In Europe and Australia it is 30xB + 1xD on an E1 2.048 Mbit/s. One timeslot on the E1 is used for synchronization purposes and is not considered to be a B or D channel.
Fractional T1 
Fewer active B channels (also called bearer channels) can be used for a fractional T1. Bearer channels may also be known as user channels. More channels can be used with more T1s, within certain design limits.
PRI and BRI 
The Integrated Services Digital Network (ISDN) prescribes two levels of service, the Basic Rate Interface (BRI), intended for the homes and small enterprises, and the Primary Rate Interface for large organisations with 30bearer channels and 2xD (D channel (delta channel)) (23 64-kbit/s digital channels + 1 64-kbit/s signaling/control channel) on a T1 (1.544 Mbit (PRI)), for larger applications. Both rates include a number of B-channels and a D-channel. Each B-channel carries data, voice, and other services. The D-channel carries control and signaling information. The Basic Rate Interface consists of two 64-kbit/s B-channels and one 16-kbit/s D-channel.
The Primary Rate Interface (PRI) consists of 23 64-kbit/s B-channels and one 64-kbit/s D-channel using a T1 line, often referred to as "23B + D", (North American and Japanese standard) or 30 B-channels and two D-channels using an E1 line (Europe/rest of world), often referred to as "30B + 2D". A T1 Primary Rate Interface user would have access to a 1.472-Mbit/s data service. An E1 Primary Rate Interface user would have access to a 1.920 Mbit/s data service Larger connections are possible using PRI pairing. A dual 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. Normally, no more than 2 D-channels are provisioned as additional PRIs are added to the group.
The Primary Rate Interface channels are typically used by medium to large enterprises with digital PBXs to provide them digital access to the Public Switched Telephone Network (PSTN). The 23 (or 30) B-channels can be used flexibly and reassigned when necessary to meet special needs such as video conferences. The Primary Rate user is hooked up directly to the telephone company central office..
ISDN PRI variants (switch types) 
primary-ni2 National ISDN-2 Switch type for the U.S. (PRI) primary-4ess Lucent 4ESS switch type for the U.S. primary-5ess Lucent 5ESS switch type for the U.S. primary-dms100 Northern Telecom DMS-100 switch type for the U.S. primary-dpnss DPNSS switch type for Europe primary-net5 NET5 switch type for UK, Europe, Asia and Australia primary-ni National ISDN-1 Switch type for the U.S. (BRI) primary-ni2c The Cisco NAS-SC switchtype based on NI2C primary-qsig QSIG switch type primary-ts014 TS014 switch type for Australia (obsolete) primary 30 Channels