# ISDN functional devices and reference points

ISDN equipment is classified into a number of categories by international and United States domestic standards as described in the following table:

Category | Description
TE1 | ISDN end-user terminating equipment, class 1, which terminates a single ISDN trunk.
NT2 | Network terminating equipment, class 2. NT2 equipment can support more than one primary rate trunk, switch channels between different trunks, and take primary synchronization from the T1 or E1 network.
NT1 | Physically terminates the local loop. In Europe, the NT1 equipment is the network in I.411-compliant installations. In the USA, NT1 is often referred to as the CSU, a separate device on the customer premises.
LE | Local exchange equipment.

Note: The International Telecommunications Union (ITU) document I.411 defines ISDN user-network interface reference configurations. In the USA, the definition is provided by a number of documents, including AT&T TR-41449 ISDN Primary Rate Interface Specification.

The interface between each category is called a point:

- The interface between TE1 and NT2 equipment is the S point.

- The interface between NT2 and NT1 equipment is the T point.

- The interface between NT1 and LE equipment is the U point.

The following illustration shows the component categories and associated reference points: