9.4 Multimedia Priority Service (MPS) Support

29.1633GPPInterworking between the IP Multimedia (IM) Core Network (CN) subsystem and Circuit Switched (CS) networksTS

9.4.1 General

The Multimedia Priority Service (MPS) is specified in 3GPP TS 22.153 [136]. The MGCF/IM-MGW may support the priority treatment of a call/session identified as an MPS call/session. If MPS is supported then upon receipt of the MPS priority information in the call control signalling:

– The MGCF shall recognise the call/session as having priority.

– The MGCF shall send the Priority information for a context to the IM-MGW to enable the priority treatment described below related to the IM-MGW.

– The MGCF shall apply priority handling to H.248 transactions related to priority calls/sessions when network resources are congested, e.g., preferential treatment in any queues or buffers.

– If the H.248 control association utilises a transport with the possibility for prioritisation, the MGCF may apply priority using the appropriate prioritisation procedures.

– If the MPS Priority service requires a specific MPS DSCP setting, the MGCF shall configure the IM-MGW to apply a specific MPS DSCP marking to the user data transport packets to indicate that the packets are of a higher priority than those for normal calls.

– If the IM-MGW receives an indication to apply a specific MPS DSCP marking to the user data transport packets, it shall apply this DSCP marking to the IP headers.

NOTE 1: Support of Diffserv procedures by the IMS-MGW assumes an operator uses Diffserv for prioritising user plane traffic related to an MPS call/session.

– When the MGCF marks a Context with Priority information, the IM-MGW may use the Priority information for selecting resources for the media and signalling transport with priority. The following actions may be taken by the IM-MGW if it has reached a congested state:

i) seize priority reserved resources; or

ii) if resources are completely congested, indicate that in a Command Response error code.

NOTE 2: The Priority information can be used to derive Layer 2 QoS marking and trigger priority identification and priority treatment for other QoS technologies than Diffserv.

This clause describes the Mn signalling procedures and their interactions with SIP signalling in the control plane and with user plane procedures to support the requirements for MPS. These Mn signalling procedures may or may not apply depending on the network configuration (e.g. whether the IM-MGW is shared by multiple MGCFs or whether the MGCF controls multiple IM-MGWs for a given route – Media Gateway Group).

The MGCF can receive a SIP INVITE with MPS priority information (see 3GPP TS 23.228 [12], clause 5.21).

9.4.2 IM-MGW Resource Congestion in ADD response, request is queued

If the MGCF requests a resource via the Reserve IMS Connection Point and configure remote resources procedure or Reserve IMS Connection Point procedure and receives an error indicating that the requested resource could not be seized (e.g. H.248 error code #510 "insufficient resources") and the MGCF does not an have alternative IM-MGW through which it can route the call/session, the MGCF queues the priority call/session and gives it priority over any further Reserve IMS Connection Point and configure remote resources or Reserve IMS Connection Point procedures for lower priority calls/sessions towards this IM-MGW until the requested resource for this queued call/session is successfully seized. The example sequence is shown in Figure 9.4.2.1.

Figure 9.4.2.1: Request to Reserve MPS priority call resources when IM-MGW is congested

9.4.3 IM-MGW Resource Congestion in ADD response, MGCF seizes new IM-MGW

If the MGCF requests a resource via the Reserve IMS Connection Point and configure remote resources procedure or Reserve IMS Connection Point procedure and receives an error indicating that the requested resources could not be seized due to congestion (e.g. H.248 error code #510 "insufficient resources") and Media Gateway Groups are implemented the MGCF seizes a new IM-MGW from the same Media Gateway Group before resorting to any queuing of the priority call/session (as described in clause 9.4.2) to enable the MPS call/session to proceed as early as possible.

9.4.4 IM-MGW Priority Resource Allocation

If the IM-MGW supports the Priority information (e.g. determined through provisioning or package profile), the MGCF requests a resource via the Reserve IMS Connection Point and configure remote resources procedure or Reserve IMS Connection Point procedure and includes the Priority information. The IM-MGW may then provide priority allocation of resources once a congestion threshold is reached. The example sequence is shown in Figure 9.4.4.1. If the IM-MGW is completely congested it shall indicate this to the MGCF as described in clause 9.4.2.

Figure 9.4.4.1: Request to reserve MPS priority call resources when IM-MGW is congested, priority resources are allocated

The IM-MGW may also provide priority allocation for resources requested via a subsequent Configure IMS resources procedure not including Priority information if the related context has been marked with priority information during the Reserve IMS Connection Point procedure or Reserve IMS Connection point and configure remote resources procedure.

9.4.5 IM-MGW Priority User Data marking

The MGCF may request the streams associated to an MPS call/session to be marked with a certain priority code point as described in clause 8.4. The IM-MGW shall then mark each IP packet header accordingly. The example sequence is shown in Figure 9.4.5.1.

Figure 9.4.5.1: Request to reserve IMS resources and apply DSCP marking for MPS

Annex A (informative):
Void

Annex B (normative):
Codec Negotiation between a BICC CS network and the IM CN subsystem