Due to the increasing volume of data traffic exchanged by mobile users and the rapid decrease of roaming rates that is being imposed by the regulator, mobile operators will most likely have to revisit their roaming agreements, moving towards a more extensive usage of local breakout. This will allow to reduce the cost per bit of data traffic exchanged by roaming customers, since at least part of it will be handled directly by the visited operator, with no need to waste bandwidth on the international links between home and visited networks; moreover, local breakout would allow to offer better performance to the customers. Based on the analysis of the requirements on local breakout and on the increasing number of the services controlled by IMS that operators are expected to face, it has been proposed that system enhancements are needed to enable extensive usage of IMS services in local breakout. Furthermore, international communications and terminal roaming introduce a number of scenarios where sessions may traverse multiple IMS networks. The use of Border Control Function makes both the signalling and bearer path traverse through the same networks path and could make the media path not optimized. In order to ensure Quality of Service (QoS) and, in certain cases, minimal routeing costs, there is a need to enable the routeing of media traffic via an optimal path between those networks, without necessarily being linked to the path that the signalling flow needs to take. The optimal media path between two endpoints may involve IP transit networks, which in normal circumstances are not included in the SIP signalling path. Current QoS reservation is negotiated based on the SIP pre-conditions model, and hence the lack of SIP signalling in the transit network presents a problem for the negotiation of QoS between the end-points.

This study intends to investigate the general problem of system enhancements for the use of IMS services in local breakout and optimal routeing of media. In particular the above issues will be addressed identifying

• solutions for the home operator to control
  • whether the IMS user may connect to a PDN in the visited network, and
  • whether connections to PDNs provided from the home and visited network may exist in parallel;
• solutions to enable the IMS network to be aware of whether local breakout can be invoked or not;
• solutions to allow the home operator to determine which of the IMS sessions (for a given UE) can be handled in local breakout and which in home routed mode, and what information (e.g. operator's policies, customer's subscription profile, UE connectivity, and location of the remote end terminal/service) is needed for the decision;
• solutions to allow the UE to concurrently use IMS services through local breakout and other IMS services through home routeing;
• the feasibility of having the local breakout option in IMS service nodes:
  • is there a need for a P-CSCF at both PDN accesses?
  • if one P-CSCF is enough, what requirements are there for connectivity between the PDNs?
• if methods are necessary to discover an additional P-CSCF in the visited network after the UE has moved to the visited network, even if the network-layer mobility mechanisms can sustain IP connectivity to the previously discovered P-CSCF in the home network;
• the exact location of the decision point in the home network whether to use local breakout (application or delegated to IP-CAN);
• solutions for SIP/SDP signalling related to the use of IMS services through local breakout.
• interactions with network entities such as NAT (as specified in TS 23.228) when providing IMS services through local breakout;
• interactions with and support of PCC to provide IMS services through local breakout;
• security implications if there is need for multiple P-CSCFs per UE.

Moreover:

• describing a set of scenarios where the selection of an alternative media path (i.e., different to the signalling path) provides benefits to IMS operators by reducing the number of network entities in the media path;
• providing requirements for suitable mechanisms to achieve optimal media routeing;
• analysing the potential solution(s) to solve those scenarios in line with IMS procedures, while taking into account any impact of extensions required to existing functions/procedures (e.g., NAT, transcoding, Security, PCC, BCF, LI, etc.);
• reducing the number of options for solving the same requirement and agree on a preferred solution.

In the end this study will provide conclusions with respects to what further specification work is required in order to fulfil the requirements for the use of IMS services through local breakout and achieve optimal routeing of media.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.

The overall requirements to provide IMS services through local breakout are defined in TS 23.401.

The following general architecture principles shall be used when developing solutions for LBO and OMR:

• Radio impacts/Access network procedures like IDLE mobility should not be affected;
• S-CSCF is the service control entity for IMS, as per current IMS core principle; even though media may be routed according to LBO or OMR procedures; (except for emergency case where E-CSCF is used as described in TS 23.167);
• Backward compatibility with Rel-7 (e.g. Rel-8 Terminal shall be able to connect to a Rel-7 IMS system and Rel-7 terminal shall be able to connect to Rel-8 IMS) shall be maintained and impacts from the development within Rel-8 IMS system shall be addressed before reaching final conclusion;
• UE battery consumption and complexity/cost of impacts must be considered;
• An UE shall not use the same IP address simultaneously across multiple accesses;
• Any solution(s) developed should work in single PLMN scenario as well as roaming scenarios.

In addition to the general requirements above, the following requirements shall be used when developing solutions for Optimal Media Routeing (OMR):

• OMR shall apply to IMS systems that use IBCF and TrGW for interconnection.
• OMR shall establish an optimal media path for each of the media streams of an IMS session, subject to the Home operator's policy, system constraints (such as transcoding function location) and the information available within SIP signalling;
• All media components of a session that traverse the same un-optimized sequence of IBCFs/TrGWs and networks shall, subject to Home operators' policy, traverse the same optimized sequence of IBCFs/TrGWs and networks. This ensures similar end to end path delay characteristics for media components that may be synchronized;
• Where end points are located within the same residence or enterprise network, OMR should be able to support the routeing of the media path such that it does not egress that network;
• OMR should be capable of optimizing the media paths for a session where the same interconnect network is used for multiple legs of the un-optimized media path;
• OMR should be capable of optimizing the media paths for a session between two UEs where the same serving network is used by the UEs;
• Home operators (of both calling and called UE) may be informed, upon session establishment/modification, of the successful enforcement or removal of OMR for that session;
• On session establishment, OMR shall establish an optimal media path separately for each remote endpoint of the session;
• OMR should re-establish an optimal media path for a session in the event of session modification (SDP offer/answer exchange);
• Impacts on IMS shall be minimized. Solutions should be based on existing IMS functional entities, use existing message flows and avoid the addition of new protocols or new messages between network elements;
• A single bandwidth reservation mechanism shall be used (for both roaming and non-roaming cases and for optimized and non-optimized sessions);
• Entities in one network shall not need to be aware of the internal structure of other networks;
• The routeing of media within a network shall not be constrained by OMR;
• OMR shall not be dependant on the direct peering of policies across a network boundary. No peering of Policy and Charging Rules Function (PCRF) [6] across network boundaries shall be required (since this does not represent a likely commercially acceptable solution);
• The service disruption of an on-going session when establishing OMR should be minimised;
• The impacts of OMR on the transport layers shall be minimised;
• The effects of introducing OMR on EPS shall be minimised;
• OMR shall provide mechanisms supporting media optimizations across multiple transit networks supporting homogenous interconnect agreements (e.g. IPX [7]);
• The originating or terminating networks shall be able to apply OMR on a session by session basis (e.g. for lawful intercept reasons, for media streams that need transcoding, or for services that require announcements to be played from the home network);