ISDN terminals come in two types, Type 1 or Type 2.

Specialized ISDN terminals are known as Terminal Equipment type 1 (TE1).

Non-ISDN terminals such as Data Terminal Equipment (DTE) that predate the ISDN standards are referred to as Terminal Equipment type 2 (TE2).

TE1s connect to the ISDN network through a four-wire, twisted-pair digital link.

TE2s connect to the ISDN network through a TA. The ISDN TA can be either a standalone device or a board inside the TE2.

If the TE2 is implemented as a standalone device, it connects to the TA via a standard physical-layer interface.