8 Interworking requirements
22.2283GPPRelease 16Service requirements for the Internet Protocol (IP) multimedia core network subsystem (IMS)Stage 1TS
8.1 General
The IMS shall support the capability for interoperability with the following:
– PLMN CS domain,
– PSTN/ISDN networks,
– Packet Cable network,
– Internet.
– Other non 3GPP networks providing IP Multimedia sessions.
The scope of this interoperability may result in a limited service capability.
8.2 Interworking with PLMN CS Domain
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.
Note: That this boundary may still be subject to regulatory requirements associated with communications with the PSTN including, but not limited to, lawful interception of voice calls and multimedia services, and number portability.
The boundary interworking shall be able to convey the information associated with the services listed below:
CLIP/CLIR;
Call Forwarding.
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.
Support of:
Call barring,
Call waiting/hold,
MPTY,
on the boundary interface is for further study.
8.3 Interworking with PSTN/ISDN networks
8.3.0 General
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.
Note: TISPAN NGN supporting PSTN/ISDN Emulation provide to the end-user the same experience as a legacy PSTN/ISDN and from a interconnection perspective the TISPAN NGN behaves like an IMS network.
As a network option and depending on inter-operator policies, the IMS shall support transit of traffic between PSTN/ISDN networks.
8.3.1 Overlap Signalling
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..
Note 1: PSTN/ISDN networks that convert overlap signalling to en-bloc, are considered to be networks that do not use 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.
Note 2: Overlap signalling support should not be linked to E.164 [26] numbering schemes.
Note 3: Overlap signalling is not generated or terminated by an IMS UE it is only generated or terminated by devices within the PSTN/ISDN networks.
8.3.2 Subaddressing (SUB)
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.
Note: Subaddressing is required for interoperability with existing users.
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.
8.3.3 User-to-User Signalling (UUS)
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.
Note: UUS is required for interoperability with existing users.
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 [17].
8.4 Void
8.5 Interworking with Packet Cable
The IMS shall support the capability for interoperability with Packet Cable networks. The scope of this interoperability may result in a limited service capability.
8.6 Interworking with Internet
The IMS shall support interworking with the Internet.
8.7 Voice Interworking with Enterprise IP-PBX
Voice interworking with Enterprise IP-PBX shall be handled as specified in TS 22.519 [33].
8.8 Interworking of Telepresence
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.