user to user signalling service (ISDN) (UUS)

User to User Signalling Service (UUS) is an information service feature of digital telephone networks that allows to activate and control service features during call setup, transmission or disconnection.

There are several UUS variants: UUS Service 1 Implicit: In this mode, UUS information is transmitted without request from the far end during call setup or disconnection and is not acknowledged by the receiving end.

UUS Service1 Explicit In this operating mode, the UUS information is transmitted on request and explicitly confirmed by the receiving side

UUSService 2: The UUS information is transported in separate messages during the connection setup

UUSService 3: The UUS information is transmitted in separate messages during the connection phase.

Subscriber-to-subscriber signalling takes place in both directions via the D channel, i.e. free of charge. The message length is limited to 32 bytes. Since this transmission option has been misappropriated, this feature is no longer included in new contracts.