The IMS shall support conveyance of voice calls and multimedia services between IMS users and users in CS domain (within the limits set by the CS domain).
If more than one IMS party is involved in a communication with a PSTN party/parties, the communication between the IMS parties shall not be adversely impacted by the presence of a PSTN party.
The boundary interworking shall be able to convey the information associated with the services listed below:
Also due to regulatory reasons the subscriber identity may be required to be conveyed via the IMS-CS/PSTN boundary to enable calling line identification services on both sides.
Interworking between IMS and PSTN/ISDN networks shall be supported. PSTN/ISDN networks in this context refer to legacy PSTN/ISDN and TISPAN NGNs supporting PSTN/ISDN Emulation.
As a network option and depending on inter-operator policies, the IMS shall support transit of traffic between PSTN/ISDN networks.
Support for overlap signalling in the IMS is an option limited to the interworking with PSTN/ISDN networks that use overlap signalling. Support of this option shall be based on inter-operator policies at the interconnection point. The IMS of operators not supporting overlap signalling shall not be affected.
In the absence of such inter-operator policies, networks interworking with IMS shall select appropriate signalling mechanisms to complete the call without any impact on IMS networks not supporting overlap signalling..
In the cases where a PSTN/ISDN network, based on inter-operator policies, provides overlap signalling into the IMS, the following requirements shall be taken into account:
For calls terminating in the IMS, conversion of overlap signalling to en-bloc shall take place within the IMS domain.
Impact on the IMS shall be minimized.
The service experience for the end-user shall be similar to the PSTN/ISDN.
When the IMS network supports transit of traffic, transit of overlap signalling may be supported towards destination networks, if the policy permits overlap signalling towards those.
Where public telecommunication numbers are used, the IMS may support the Subaddress (SUB) service that allows the called (served) user to expand his addressing capacity beyond the one given by the public telecommunication number.
Subaddressing is required when at least one of the users services are provided from an ISDN.
The IMS may support the interoperability of Subaddressing with the PSTN/ISDN networks and vice versa.
The IMS may support the User-to-User Signalling (UUS) service that enables a calling party to send and/or receive a limited amount of User-to-User-Information (UUI) to/from a called party in association with a communication.
UUS is required when at least one of the users services are provided from an ISDN.
The IMS may support the interoperability of User-to-User Signalling services with the PSTN/ISDN networks and vice versa.
Only UUS service 1 with an implicit request is supported .
Interworking between IMS-based Telepresence and other non-3GPP network provided Telepresence shall be supported (e.g. enterprise Telepresence).
A UE using CS voice shall be able to participate in a Telepresence session.
The interconnection between an operator and Enterprise should be via a QoS-enabled IP connection.