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: