These are conceptual reference points to define the relationships between non ISDN and ISDN terminating equipment with the ISDN user interface. The Reference Points may be defined in the following way:

R: a conceptual interface between non ISDN terminals , e.g. X or V series terminals, called TE 2 terminals, and TAs. Different types of TA are required for interworking with different TE2s.

TA: This is equipment which allows TE2 equipment connect to the S reference point.

S: a conceptual reference point between an ISDN terminal, TE1, and an NT2 or NT1.

NT2: Is a devices such as an ISDN PABX (ISPBX) which support ISDN protocols for layers 1 to 3 and connects to the NT1 via the T reference point.

T: a conceptual interface which is positioned between the NT2 and NT1. If an NT2 is not present then the S and T reference points are coincident.

NT1: connects a TE1 and TA via an NT2, if present, to the U interface.

U: the conceptual interface between the NT1 and the ET in the LE.

Legend:

TE: Terminating Equipment.
NT: Network Terminating Unit.
TA: Terminal Adapter.
ET: Exchange Termination.
LE: Local Exchange.