Basic Rate Interface = basic access to the ISDN.
Standardized ISDN network connection giving a data rate of 144 kbit/s: two 64 kbit/s B channels and one 16 bit/s D channel. The B channel, switched to circuit mode, is used for the routing of all types of information. Channel D carries signaling or data in packet mode. The interface is called So or To depending on whether you are on the terminal side or the public exchange side.
Not to be confused with PRI (Primary Rate Interface = primary access to the ISDN)