4 General

23.0873GPPRelease 17Stage 2TSUser-to-User Signalling (UUS) supplementary service

The UUS supplementary service allows the transfer of UUI to a remote subscriber over the signalling channel in association with a call to this subscriber. The UUS supplementary service is divided in Service 1, Service 2 and Service 3.

The UUS Service 1 allows the transfer of UUI embedded within Call Control messages.

The UUS Service 2 allows the transfer of UUI with a specific User-to-user message after the served subscriber has received an address complete indication and prior to the answer indication from the B-side.

The UUS Service 3 allows the transfer of UUI with a specific User-to-user message during an active call.

For the use of the UUS supplementary service in GSM PLMNs the support of UUS capabilities within the inter‑exchange signalling system (e.g. ISUP) is necessary. However this is out of scope of the present document.

4.1 Activation of UUS components

If any of the UUS Services 1, 2 or 3 shall be used within a call, the UUS capabilities have to be activated by the served subscriber either when initiating this outgoing call (Service 1, 2 and 3) or during an established call (Service 3).

The UUS capabilities for a call can be requested by means of an implicit request or an explicit request. UUS Service 1 can be requested either implicitly or explicitly. Service 2 and 3 can only be requested explicitly.

4.1.1 Implicit request

The UUS1 service is activated implicitly by the presence of UUI in the set-up request from the mobile station. The network shall transfer the received UUI transparently to the B-side. For the purpose of UUS service 1 implicitly requested, no explicit check for the availability of UUS capabilities at the destination network has to be performed by the originating network.

If the calling subscriber has UUS 1 provisioned and implicitly requested, the network shall transfer all UUI contained in call control messages.

If the calling subscriber has UUS1 not provisioned, the contained UUI shall be discarded by the serving MSC. The call shall be established without further restrictions.

The served subscriber shall not be informed whether the implicit request was successful or not.

4.1.2 Explicit request

Any UUS Service can be explicitly activated by the served subscriber within the set-up request initiating a mobile originated call. In addition UUS Service 3 can be activated during an established call with a Facility message. A UUS Service 1 request can be accompanied by appropriate UUI.

The network shall check for the availability of UUS capabilities for the call by passing the UUS request and the eventually accompanied UUI to the remote side. If a UUS Service is available for the call an appropriate "UUS provided" indication for this UUS Service shall be sent within the first backward message from the remote side.

A UUS Service shall not be activated for the call if a "UUS not provided" indication or no indication about the availability of this UUS Service is received from the remote side.

Service 3 can be explicitly activated during an active call by both parties. This may lead to a collision of activation requests. The collision of activation requests occurs when there is an outstanding request for service 3 and a subsequent request is received from the remote user. The entity (user or network) that observes the collision shall reject the second request with Facility message. This leads to the rejection of both requests.

4.1.3 UUS required option

As an option at call set-up the served subscriber can specify whether the requested UUS Service is required or not required for the call. If service 1 is implicitly requested or if service 3 is requested during the call, it cannot be requested as required.

If the served subscriber has specified that one of the UUS services is required for the call and this UUS service can not be activated by the network, the call attempt of the served subscriber shall be cleared.

If the served subscriber has specified that one of the UUS services is not required for the call and this UUS service can not be activated by the network, the call establishment shall be continued.