22.2283GPPService requirements for the Internet Protocol (IP) multimedia core network subsystem (IMS)Stage 1TS
IMS interconnect represents the interconnection of IMS functionality between 2 IMS networks over an underlying IP infrastructure.
A distinction can be made between:
– Direct IMS interconnect, and
– Indirect IMS interconnect.
Direct IMS interconnect is interconnection of IMS functionality between 2 IMS networks using a common IP interconnect network that is not SIP aware. In this case there is only one IMS-NNI (see Figure 10.1).
Figure 10.1 Basic IMS interconnect with single IMS-NNI
Indirect IMS interconnect is interconnection of IMS functionality between 2 IMS networks using a common IP interconnect network that is SIP aware. In this case there are two or more IMS-NNIs (see Figure 10.2). One or more of the intermediate networks may also be IMS networks providing "transit" functionality.
Figure 10.2 Indirect IMS interconnect with multiple IMS-NNIs
Note: An IMS NNI could also be applied between an IMS network and another SIP based network if that SIP based network conforms to IMS specifications on the interconnect interface.
An IMS network B delivers originating IMS services (e.g. least cost routing, service dependant routing, call barring, number translation, break-out) to a network A if the services are delivered for a session that arrives in the IMS network B over an IMS NNI from network A (Figure 10.3).
Figure 10.3: delivery of originating IMS application services across an IMS-NNI
An IMS network C delivers terminating IMS services (e.g. bulk rerouting, redundancy, profile based reachability, number translation) to a network D in case the services are delivered for a session that shall continue from the IMS network C over an IMS NNI to network D (Figure 10.4).
Figure 10.4: delivery of terminating IMS application services across an IMS-NNI
10.2 IP interconnect
The IP connection used for IMS IP interconnect shall be generic such that it can support all combinations of core network interconnection. E.g. the IP interconnection shall be shared between the IMS interconnection and the CS IP interconnection.
It shall be possible to handle the inter-connection of all services over this generic IP interface. The handling of security and charging shall also be generic for all IP inter-connect scenarios.
10.3 IMS network interconnect
The following requirements apply at the interconnection point (IMS NNI) between two different IMS networks or between an IMS network and a SIP based network that conforms to the IMS specification at the interconnect interface.
The IMS network and intermediate networks shall support the capability for service identification, when such information is available.
The IMS intermediate networks shall be able to apply operator defined policy at the interconnection point.
The IMS intermediate networks shall support the capability for control of the session resources when two different IMS networks are connected which have different IP addressing schemes.
The IMS network shall support both bilateral interconnection between two networks and multilateral interconnection (e.g. GSMA IPX ) provided by intermediate network(s).
The IMS network shall support both international interconnect and national interconnect (e.g. as specified in ETSI TR 180 003 ).
The IMS network and intermediate networks shall support service aware interconnection.
Note 1: service awareness could be based on service identification, media parameters etc.
The IMS network and intermediate networks shall be able to support multiple peering points for the interconnection to another IMS network or intermediate network.
Note 2: selection of a peering point could be based on location, load balancing, or called party PUI, etc.
An IMS network shall be able to deliver IMS originating application services and terminating application services to other networks interconnected over the IMS NNI .
An IMS network shall be able to deliver IMS originating application services and/or terminating application services to users that are registered to another network.
The IMS network and intermediate networks shall support codec negotiation across one or multiple interconnects to minimise transcoding (and preferably eliminate it) to provide the highest quality service to the user.
If two UEs, belonging to two IMS networks, do not support a common codec for voice service session, the network and/or intermediate networks shall be capable to provide transcoding functionality at the interconnection point.
In case of interconnect between an IMS network and a SIP based network (e.g. SIP-I) the following requirements apply.
The IMS network and intermediate networks shall support the capability for service interoperability by means of service interworking requirements defined in clause 8.
In case of interconnection between two networks using the same non-IMS SIP Profile (e.g. SIP-I), the intermediate network shall transparently transport the non-IMS signalling information.