5.16.6 Mission Critical Services

According to TS 22.280 [37], a Mission Critical Service (MCX Service) is a communication service reflecting enabling capabilities Mission Critical Applications and provided to end users from Mission Critical Organizations and mission critical applications for other businesses and organizations (e.g. utilities, railways). An MCX Service is either Mission Critical Push To Talk (MCPTT) as defined in TS 23.379 [38], Mission Critical Video (MCVideo) as defined in TS 23.281 [39], or Mission Critical Data (MCData) as defined in TS 23.282 [40] and represents a shared underlying set of requirements between two or more MCX Service types. MCX Services are not restricted only to the ones defined in this clause and such services can also have priority treatment, if defined via operator's policy and/or local regulation.

MCX Services are based on the ability to invoke, modify, maintain and release sessions with priority and deliver the priority media packets under network congestion conditions. As specified in clause 6.8 of TS 22.261 [2], MCX Users require 5GS functionality that allows for real-time, dynamic, secure and limited interaction with the QoS and policy framework for modification of the QoS and policy framework by authorized users. The limited interaction is based on operator policy and provides specific limitations on what aspects of the QoS and policy framework an authorized MCX User can modify. MCX Services are supported in a roaming environment when roaming agreements are in place and where regulatory requirements apply.

An MCX-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 MCX-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. MCX subscription allows users to receive priority services, if the network supports MCX. MCX subscription entitles a USIM with special Access Identity.

NOTE 1: For Mission Critical Services that require low latency and zero packet loss even for the first downlink packet(s), periodic keep-alive packets during interruptions of media transmission (e.g. Floor Idle as specified in TS 23.379 [38], referenced by TS 23.289 [184]), which is sent over user plane of PDU session, can be used so that the UE is kept in RRC_CONNECTED state without being paged when unicast transmission is used. It is up to the implementation, the periodicity of the keep-alive packets configured in the AF can consider NG-RAN's configuration.

NOTE 2: For support of Mission Critical Services that require low latency and zero packet loss when using MBS, see TS 23.247 [121].

MCX Services leverage the foundation of the 5G QoS Model as defined in clause 5.7 and 5G Policy Control as defined in clause 5.14. It requires that the necessary subscriptions are in place for both the 5G QoS Profile and the necessary Policies. In addition, MCX Services leverage priority mechanism as defined in clause 5.22.

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

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

Priority treatment for MCX Service sessions require appropriate ARP and 5QI (plus 5G QoS characteristics) setting for QoS Flows according to the operator's policy.

NOTE 3: Use of QoS Flows for MCX Service sessions with non-standardized 5QI values enables the flexible assignment of 5G QoS characteristics (e.g. Priority Level).

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

• QoS Flows employed in a MCX Service session shall be assigned ARP value settings appropriate for the priority of the MCX User.
• Setting ARP pre-emption capability and vulnerability of QoS Flows related to a MCX Service session, subject to operator policies and depending on national/regional regulatory requirements.
• Pre-emption of non-MCX Users over MCX Users during network congestion situations, subject to operator policy and national/regional regulations.

Priority treatment is applicable to IMS based multimedia services and priority PDU connectivity services.

Relative PDU priority decisions for MCX Service sessions are based on real-time data of the state of the network and/or based on modification of the QoS and policy framework by authorized users as described in clause 6.8 of TS 22.261 [2].