6 Interworking Reference Model for control plane interworking and user plane interworking

29.1623GPPInterworking between the IM CN subsystem and IP networksRelease 17TS

6.0 Reference Model

Figure 2 details the reference architecture required to support interworking between the IM CN subsystem and IP networks for IM services. Figure 3 details the reference architecture required to support interworking between the IMS and IP SIP networks supporting IP version 4.

NOTE: Multimedia IP networks may be connected via the Mb interface to various network entities, such as an UE (via an GTP Tunnel reaching to the GGSN), an MRFP, or an application server.

Figure 2: IM CN Subsystem to IP network interworking reference Architecture without IP version interworking

Figure 3: Border Control Functions

Mm reference point: The call control protocol applied to the Mm interface between CSCF and external IP networks is SIP, IETF RFC 3261 [2], as detailed in 3GPP TS 24.229 [1]. SIP extension packages mandated by 3GPP are possibly not supported.

Mb reference point: This interface is defined in 3GPP TS 23.002 [5] and is IP based. Further information is provided in 3GPP TS 29.061 [4] and 3GPP TS 26.114 [36].

Mx reference point: The protocol applied at the Mx reference point is specified in 3GPP TS 24.229 [1].

Ix reference point: The protocol applied at the Ix reference point is specified in 3GPP TS 29.238 [25].

6.1 Interworking Functional Entities

6.1.1 IBCF

This entity provides control plane functionality to connect entities following the 3GPP profile of SIP, 3GPP TS 24.229 [1], and external SIP entities following IETF RFC 3261 [2].

6.1.2 IMS-ALG

IMS-ALG functionality resides in IBCF. An IMS-ALG provides the application level translation function for SIP and SDP in order to communicate between IPv6 and IPv4 SIP applications or, based on operator policies between different realms using the same IP version. The IBCF acts as a SIP B2BUA when IBCF performs IMS-ALG functionality.

6.1.3 TrGW

The TrGW is a NAT-PT/NAPT-PT, which uses a pool of globally unique IPv4 addresses for assignment to IPv6 nodes on a dynamic basis as sessions are initiated across the IP version boundaries. NAT-PT binds addresses in IPv6 network with addresses in IPv4 network and vice versa to provide transparent routing between the two IP domain without requiring any changes to end points. NAPT-PT provides additional translation of transport identifier (TCP, SCTP and UDP port numbers). More detailed information on the NAT-PT/NAPT-PT is given in IETF RFC 2766 [11] and IETF RFC 2663 [12].

The TrGW may provide the NAT/NAPT functionality between two disparate address realms.

6.1.4 Acces Transfer Control Function

The ATCF may reside within the IBCF to support "SRVCC enhanced with ATCF" procedures as described in 3GPP TS 23.237 [41] and 3GPP TS 24.237 [42] . In this case, the Ix reference point is used for IMS sessions that the ATCF decides to anchor at the ATGW to provide the following functions:

– reservation and configuration of ATGW resources for media anchoring during PS session origination or termination;

– reconfiguration of ATGW resources during access transfer to the CS domain;

– release of ATGW resources upon completion of the access transfer or release of the session;

– media transcoding if the media that was used prior to the access transfer is not supported by the MSC server;

– IP version interworking if different IP versions are used between the access and the remote legs; and

– indication of IP realm during allocation of transport addresses/resources (the PS and CS accesses may be reachable via different IP realms).

6.1.5 Acces Transfer GateWay

The ATGW may reside within the TrGW to support "SRVCC enhanced with ATCF" procedures as described in 3GPP TS 23.237 [41] and 3GPP TS 24.237 [42]. If the IBCF supports the ATCF functionality (see subclause 6.1.4), the IBCF shall select a TRGW which supports the ATGW functionality.