ISDN uses the term function group to refer to a set of functions that a piece of hardware or software must
perform to support customer premises equipment (CPE). The ITU has defined several different function
groups, providing several different ISDN options for customers. These function groups include:
• Terminal Equipment 1 (TE1), which is ISDN-capable four-wire cable that understands signalling and
2B+D.
• Terminal Equipment 2 (TE2), which is equipment that does not have ISDN awareness and thus does
not understand ISDN protocols and specifications.
• Terminal Adapter (TA), which can be thought of as TE1 function group on behalf of a TE2.
• Network Termination Type 1 (NT1), which is CPE equipment in North America.
• Network Termination Type 2 (NT2), which is found both inside and outside North America. Inside
North America.
• NT1/NT2, which is a combination of NT1 and NT2 in the same device. This is relatively common inside
North America.
These function groups might be implemented by separate products from different vendors. Therefore the
ITU has explicitly defined the interfaces between the devices that perform each function. The term
reference point is used to refer to this interface between two function groups. The various reference points
are:
• A U reference point, which is used by a NT1 to connect to the telephone company.
• A S reference point, which is used by a TE1 to connect to other devices; a TA; a NT1 to connect to
other CPE; and a NT2 to connect to other CPE.
• A T reference point, which is used by a NT1 to connect to other CPE; and a NT2 to connect to the
telephone company outside North America or to a NT1 inside North America.
• A R reference point, which is used by a TE2 to connect to a TA.
Note: These function groups and reference points are only defined for BRI.
When the ITU planned for multiple implementation options, BRI was to be
installed when connecting to consumers while PRI was seen as a service for
businesses.