Terminal endpoint identifier
The Terminal Endpoint Identifier ( TEI ) is in the ISDN - signaling protocol DSS1 an identifier for identifying the terminal . Together with the Service Access Point Identifier (SAPI), the TEI in the security layer (layer 2) of the D-channel forms a unique address for a specific terminal. TEI and SAPI are standardized in ITU-T guideline Q.921 and ETSI ETS 300402-2.
There is no connection between the terminal identity and the telephone number of the connection. This is why a unique identifier is required for each end device in order to identify end devices. The unique identifier is required in particular for an ISDN multi-device connection (point-to-multipoint), where several end devices are connected to the same S 0 bus . As a rule, each terminal device is automatically assigned a TEI value by the switching device ( telephone system , local exchange ) when it is connected to the S 0 bus . The protocol standard provides that the TEI can also be permanently set in the terminal. Flexible TEI assignment is not required on an ISDN system connection (point-to-point), as only one terminal device can be connected. A fixed TEI value is therefore used for system connections.
| value | use |
|---|---|
| 0 | System connection (point-to-point) |
| 1 - 63 | Manual assignment in the end device |
| 64 - 126 | Automatic assignment by switching facility |
| 127 | Group address ("Global TEI") for all connected end devices |