5.16.5 Multimedia Priority Services

TS 22.153 [24] specifies the service requirements for Multimedia Priority Service (MPS). MPS allows Service Users (as per TS 22.153 [24]) 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.

MPS is supported for Service Users using UEs connecting via 3GPP access. MPS is also supported for Service Users using UEs that connect via Trusted or Untrusted non-3GPP access for MPS. N3IWF selection is according to clause 6.3.6 for PLMN access.

A Service User may use an MPS-subscribed UE or any other UE to obtain MPS. An MPS-subscribed UE obtains priority access to the Radio Access Network by using the Unified Access Control mechanism according to TS 22.261 [2]. This mechanism provides preferential access to UEs based on its assigned Access Identity. If an MPS-subscribed UE belongs to the special Access Identity as defined in TS 22.261 [2], the UE has preferential access to the network compared to ordinary UEs in periods of congestion.

In the RRC Connection Establishment procedure and the RRC Connection Resume procedure, an MPS subscribed UE connected to NG-RAN shall use a high priority access Establishment or Resume Cause with values according to TS 24.501 [47], TS 36.331 [51] and TS 38.331 [28].

MPS subscription allows users to receive priority services, if the network supports MPS. The same MPS subscription applies to access via 3GPP access and Trusted or Untrusted non-3GPP access. MPS subscription entitles a USIM with special Access Identity. MPS subscription includes indication for support of priority PDU connectivity service including MPS for Data Transport Service, IMS priority service support and MPS for Messaging support for the end user. Priority Level regarding QoS Flows and IMS are also part of the MPS subscription information. The usage of Priority Level is defined in TS 22.153 [24], TS 23.503 [45] and TS 23.228 [15].

The UE determines whether to apply MPS exceptions for Non-Allowed Areas based on MPS subscription i.e. USIM with special Access Identity or MPS priority received from the AMF as defined in TS 23.502 [3] over 3GPP or Trusted or Untrusted non-3GPP access.

NOTE 2: The same MPS subscription in the UDM and/or on the USIM is used for priority treatment of 3GPP procedures when the access is Trusted or Untrusted non-3GPP.

NOTE 3: The term "Priority PDU connectivity services" is used to refer to 5G System functionality that corresponds to the functionality as provided by LTE/EPC Priority EPS bearer services in clause 4.3.18.3 of TS 23.401 [26].

MPS includes signalling priority and media priority. All MPS-subscribed UEs get priority for QoS Flows (e.g. used for IMS signalling) when established to the DN that is configured to have priority for a given Service User by setting MPS-appropriate values in the QoS profile in the UDM.

Service Users are treated as On Demand MPS subscribers or not, based on regional/national regulatory requirements. On Demand service is based on Service User invocation/revocation explicitly and applied to the media QoS Flows being established. When not On Demand MPS service does not require invocation and provides priority treatment for all QoS Flows only to the DN that is configured to have priority for a given Service User after attachment to the 5G network.

MPS for Data Transport Service is an on-demand service that may be invoked/revoked by an authorized 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 the PCF according to clause 6.1.3.11 of TS 23.503 [45]. Upon successful authorization, the PCF performs the necessary actions to achieve appropriate ARP and 5QI settings for the QoS Flows (see clause 6.1.3.11 of TS 23.503 [45]).

MPS for Data Transport Service enables the prioritization of all traffic on the QoS Flow associated with the default QoS rule and other QoS Flows upon AF request. The QoS modification to the QoS Flow associated with the default QoS rule and other QoS Flows is done based on operator policy and regulatory rules by means of local PCF configuration.

NOTE 4: According to regional/national regulatory requirements and operator policy, On-Demand MPS (including MPS for Data Transport Service) Service Users can be assigned the highest priority.

NOTE 5: If no configuration is provided, MPS for Data Transport Service applies only to the QoS Flow associated with the default QoS rule.

NOTE 6: MPS for DTS controls the priority of traffic on QoS Flows independent of the application(s) being used. Other mechanisms (e.g. Priority PDU connectivity service) can be used to control the priority of traffic on other QoS Flows under the control of specific data application(s), based on operator policy.

NOTE 7: MPS for Data Transport Service can be applied to any DNN other than the well-known DNN for IMS.

For MPS for Data Transport Service, the AF may also create an SDF for priority signalling between the UE and the AF (see clause 6.1.3.11 of TS 23.503 [45]).

Priority treatment is applicable to IMS based multimedia services and Priority PDU connectivity service including MPS for Data Transport Service.

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

Priority treatment for MPS session requires appropriate ARP and 5QI (plus 5G QoS characteristics) setting for QoS Flows according to the operator's policy.

NOTE 8: Use of QoS Flows for MPS with QoS characteristics signalled as part of QoS profile enables the flexible assignment of 5G QoS characteristics (e.g. Priority Level) for MPS.

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

• QoS Flows employed in an MPS session shall be assigned ARP value settings appropriate for the priority of the Service User.
• Setting ARP pre-emption capability and vulnerability for MPS QoS Flows, 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.

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).

MPS priority mechanisms can be classified as subscription-related, invocation-related and those applied to existing QoS Flows. Subscription related mechanisms, as described in clause 5.22.2, are further divided into two groups: those which are always applied and those which are conditionally applied. Invocation-related mechanisms, as described in clause 5.22.3, are further divided into three groups: those that apply for mobile originated SIP call/sessions, those that apply for mobile terminated SIP call/sessions and those that apply for the Priority PDU connectivity services including MPS for Data Transport Service. Methods applied to existing QoS Flows focus on handover and congestion control and are described in clause 5.22.4.

NOTE 9: 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 interactions with the applicable PCF. Thus, there can be no NAT of the UE IP address between the UPF and the AF supporting MPS for Data Transport Service.

For WLAN access, the UE may notify the TNAN/N3IWF of its MPS subscription before the NAS Registration Request. Based on operator policy, the TNAN/N3IWF may use this indication to provide this UE with priority treatment in the case of congestion/overload before receipt of the NAS Registration Request with an MPS priority establishment cause.

An MPS subscription supports setting (enabling)/clearing (disabling) priority treatment of the Messaging service (i.e. SMS over NAS, SMS over IP and messaging over IMS) for an MPS-subscribed UE. The usage of the MPS for Messaging indication in the subscription data is further specified in clause 5.22.