4.3.18 Multimedia Priority Service

23.4013GPPGeneral Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) accessRelease 18TS

4.3.18.1 General

Multimedia Priority Service (MPS) allows certain subscribers (i.e. Service Users as per TS 22.153 [68]) priority access to system resources in situations such as during congestion, creating the ability to deliver or complete sessions of a high priority nature. Service Users are government-authorized personnel, emergency management officials and/or other authorized users. MPS supports priority sessions on an "end-to-end" priority basis.

MPS is based on the ability to invoke, modify, maintain and release sessions with priority, and deliver the priority media packets under network congestion conditions. MPS is supported in a roaming environment when roaming agreements are in place and where regulatory requirements apply.

NOTE 1: If a session terminates on a server in the Internet (e.g. web-based service), then the remote end and the Internet transport are out of scope for this specification.

A Service User obtains priority access to the Radio Access Network by using the Access Class Barring mechanism according to TS 36.331 [37] and TS 22.011 [67]. This mechanism provides preferential access to UEs based on its assigned Access Class. If a Service User belongs to one of the special access-classes as defined in TS 22.011 [67], the UE has preferential access to the network compared to ordinary users in periods of congestion.

MPS subscription allows users to receive priority services, if the network supports MPS. MPS subscription entitles a USIM with special Access Class(es). MPS subscription includes indication for support of Priority EPS Bearer Service, IMS priority service and CS Fallback priority service support for the end user. Priority level regarding Priority EPS Bearer Service and IMS are also part of the MPS subscription information. The usage of priority level is defined in TS 23.203 [6] and TS 23.228 [52].

NOTE 2: The above statements for the Priority EPS Bearer Service are also applicable for the MPS for Data Transport Service.

An MPS Service User is treated as an On Demand MPS subscriber or not, based on regional/national regulatory requirements. On Demand service is based on Service User invocation/revocation explicitly and applied to the PDN connections for an APN. When not On Demand, MPS service does not require invocation, and provides priority treatment for all EPS bearers for a given Service User after attachment to the EPS network.

NOTE 3: According to regional/national regulatory requirements and operator policy, On-Demand MPS Service Users can be assigned the highest priority.

For this release of the specification, MPS is supported for E-UTRAN access only in the case of 3GPP accesses.

Since the Service User has an access class within the range for priority services, the Establishment Cause in RRC connection request is set to highPriorityAccess. When the eNodeB receives mobile initiated signalling with establishment cause set to highPriorityAccess, the eNodeB handles the RRC connection request with priority. When the MME receives and verifies mobile initiated signalling with establishment cause set to highPriorityAccess, the MME establishes the S1 bearer with priority.

The terminating network identifies the priority of the MPS session and applies priority treatment, including paging with priority, to ensure that the MPS session can be established with priority to the terminating user (either a Service User or normal user).

Priority treatment for MPS includes priority message handling, including priority treatment during authentication, security, and location management procedures.

Priority treatment for MPS session requires appropriate ARP and QCI (where necessary for non-GBR bearers) setting for bearers according to the operator’s policy.

When an MPS session is requested by a Service User, the following bearer management principles apply in the network:

– EPS bearers (including default bearer) employed in an MPS session shall be assigned ARP value settings appropriate for the priority level of the Service User.

– Setting ARP pre-emption capability and vulnerability for MPS bearers, subject to operator policies and depending on national/regional regulatory requirements.

– Pre-emption of non-Service Users over Service Users during network congestion situation, subject to operator policy and national/regional regulations.

Priority treatment is applicable to IMS based multimedia services, priority EPS bearer services (PS data without IMS interaction) and CS Fallback.

For Multimedia Priority services any EPC functions, procedures and capabilities are provided according to this clause’s specification except when specified differently in the following clauses.

4.3.18.2 IMS-based Multimedia Priority Services

4.3.18.2.1 Originating IMS-based MPS Session

IMS based MPS sessions are permitted to be originated from any UE, in addition to MPS-subscribed UEs.

The MPS-subscribed UE, based on the MPS IMS subscription information, operator’s policy and national/regional regulations, may be given priority treatment for the default bearer and the EPS bearer carrying IMS signalling in the EPS prior to and during IMS-based MPS invocation. Further, priority treatment in the EPS for signalling and media bearers may be modified/established via dynamic PCC based on the session authorization information received from the AF.

As the IMS media bearer is established after the IMS session of the MPS service has been established, it can be assigned with correct ARP value when it is established. However IMS signalling related EPS bearer needs to be upgraded if it has not been assigned with an appropriate ARP setting for the MPS service when the IMS session of the MPS service has been initiated.

Also to avoid cases where the default bearer may not be allocated resources in the handover case, due to low ARP priority for the PDN connection, it is necessary to assure that the default bearer has an ARP setting appropriate for the MPS service.

4.3.18.2.2 Terminating IMS-based MPS Session

The terminating network identifies the priority of the IMS-based MPS session and applies priority treatment to ensure that the call is delivered with priority to the terminating user (either a Service User or normal user).

If the existing ARP of the default or dedicated EPS bearer that is used to transport IMS signalling are not appropriate for the MPS service, then PCRF updates to the appropriate settings.

S-GW triggers a new priority paging towards MME if the ongoing paging is lower priority than the incoming data received in the S-GW for IMS terminating session.

4.3.18.3 Priority EPS Bearer Services

The Service User receives on demand priority treatment according to its MPS profile, i.e. On-Demand. If the Service User is not authorized to use on-demand priority request, the Service User receives priority treatment (i.e. appropriate ARP and QCI ) at initial attach for all bearers, based on user profile data stored in the HSS/SPR and authorized by the PCRF (see TS 23.203 [6], clause 7.2).

An On-Demand Service User requires explicit invocation/revocation via SPR MPS user profile update (see TS 23.203 [6], clause 7.5). Since MPS user profile are part of inputs for PCC rules, the update will trigger PCC rules modification to achieve appropriate ARP and QCI settings for bearers (see TS 23.203 [6], clause 7.4.2).

When the eNodeB receives mobile initiated signalling with establishment cause set to highPriorityAccess, the eNodeB handles the RRC connection request with priority. When the MME receives and verifies mobile initiated signalling with establishment cause set to highPriorityAccess, the MME establishes the S1 bearer with priority. Based on MPS EPS priority subscription, MME can verify whether the UE is permitted to handle the request preferentially comparing to other UEs not prioritized.

An AF for MPS Priority Service is used to provide Priority EPS Bearer Services using network-initiated resource allocation procedures (via interaction with PCC) for originating accesses.

NOTE: Use of 3rd party AF for MPS services for Service Users is outside the scope of 3GPP specification.

4.3.18.3a MPS for Data Transport Service

MPS for Data Transport Service is an on-demand service that may be invoked/revoked by an authorized MPS Service User using a UE with a subscription for MPS (i.e. according to its MPS profile), or using a UE that does not have a subscription for MPS (using methods not in scope of this specification).

MPS for Data Transport Service requires explicit invocation. The Service User invokes the service by communicating with an AF. The authorization of an MPS for Data Transport Service request is done by the AF or by the PCRF according to clause 6.1.11.5 of TS 23.203 [6]. Upon successful authorization the PCRF performs the necessary actions to achieve appropriate ARP and QCI settings for the bearers (see clause 6.1.11.5 of TS 23.203 [6]).

NOTE 1: MPS for Data Transport Service can be applied to any APN other than the well-known APN for IMS.

MPS for Data Transport Service enables the prioritization of all traffic on the default bearer and other bearers upon AF request. The QoS modification to the default bearer and other bearers is done based on operator policy and regulatory rules by means of local PCRF configuration.

NOTE 2: If no configuration is provided, MPS for Data Transport Service applies only to the default bearer.

NOTE 3: MPS for Data Transport Service controls the priority of traffic on bearers independent of the application(s) being used. Other mechanisms (e.g., Priority EPS Bearer Service) can be used to control the priority of traffic on other bearers not under control by MPS for Data Transport Service, based on operator policy.

For MPS for Data Transport Service, the AF may also create an SDF for signalling priority between the UE and the AF (see clause 6.1.11.5 of TS 23.203 [6]).

NOTE 4: The network can hide its topology from the AF supporting MPS for Data Transport Service. At the same time, the UE needs to provide its locally known IP address to the AF supporting MPS for Data Transport Service to support Diameter routing to the applicable PCRF. Thus, there can be no NAT of the UE IP address between the PDN-GW and the AF supporting MPS for Data Transport Service.

4.3.18.4 CS fallback

CS Fallback allows users to fallback to GERAN/UTRAN/1x RTT while in E-UTRAN access thus allowing the network to transfer the call towards GERAN/UTRAN CS domain. In order to ensure that a priority CSFB call to/from a service user is given proper priority treatment in the EPS, MPS subscription indicates the user’s CS priority status, i.e. MPS CS Priority, which is provided to MME with user’s subscription information. When the eNodeB receives mobile initiated signalling with establishment cause set to highPriorityAccess, the eNodeB handles the RRC connection request with priority. When the MME receives and verifies mobile initiated signalling with establishment cause set to highPriorityAccess, the MME establishes the S1 bearer with priority.

Details on the priority treatment of CSFB, see TS 23.272 [58].

4.3.18.5 Network Congestion Controls for MPS

Based on regional/national requirements and network operator policy, MPS shall be exempted from network congestion controls up to the point where further exemptions cause network instability. The MME should not apply NAS level congestion control for mobile initiated signalling with establishment cause set to highPriorityAccess. The MME should not apply congestion control for termination requests related with an ARP associated with MPS.

4.3.18.6 Load Re-balancing between MMEs for MPS

When a UE is in ECM-CONNECTED mode with a bearer having an ARP associated with MPS, the MME should not release the UE for load re-balancing, except under critical conditions such as the need to perform an MME node restart. The MME should wait until the UE initiates a TAU or becomes ECM-IDLE before initiating load re-balancing.