Integrated Services Digital Network Primary Rate Interface (ISDN PRI) is a complex voice and data service that provides high-volume access to the public switched telephone network (PSTN). ISDN PRI accommodates the transmission of voice, data, image, and video over one 1.544 Mbps transport lines. ISDN PRI provides 23 bearer (B) Channels and one (1) delta (D) Channel that carries signaling information, which controls the B Channel connections.
ISDN is based on a number of fundamental building blocks.
B-channel
The Bearer ("B") channel is a 64 kbps channel, which can be used for voice, video, data, or multimedia calls. B-channels can be aggregated together for even higher bandwidth applications.
D-channel
The Delta ("D") channel can be either a 16 kbps or 64 kbps channel used primarily for communications (or "signaling") between switching equipment in the ISDN network and the ISDN equipment at your site.
These ISDN channels are delivered to the user in one of two pre-defined configurations:
Basic Rate Interface (BRI)
BRI is the ISDN service most people use to connect to the Internet. An ISDN BRI connection supports two 64 kbps B-channels and one 16 kbps D-channel over a standard phone line. BRI is often called "2B+D" referring to its two B-channels and one D-channel.
Primary Rate Interface (PRI)
ISDN PRI service is used primarily by large organizations with intensive communications needs. An ISDN PRI connection supports 23 64 kbps B-channels and one 64 kbps D-channel (or 23B+D) over a high speed DS1 (or T-1) circuit. The European PRI configuration is slightly different, supporting 30B+D.
