The MCPTT service supports communication between several users (i.e. group call), where each user has the ability to gain access to the permission to talk in an arbitrated manner. The MCPTT service also supports private calls between two users.
The MCPTT architecture utilises the common functional architecture to support mission critical services over LTE defined in TS 23.280 and aspects of the IMS architecture defined in TS 23.228, the Proximity-based Services (ProSe) architecture defined in TS 23.303, the Group Communication System Enablers for LTE (GCSE_LTE) architecture defined in TS 23.468 and the PS-PS access transfer procedures defined in TS 23.237 to enable support of the MCPTT service.
The MCPTT UE primarily obtains access to the MCPTT service via E-UTRAN, using the EPS architecture defined in TS 23.401. Certain application functions of MCPTT service such as dispatch and administrative functions can be supported using either MCPTT UEs in E-UTRAN or using MCPTT UEs via non-3GPP access networks.
MCPTT UEs that use non-3GPP access can only support a subset of the functionality specified in this specification that is supported by the non-3GPP access network.
The MCPTT system provides the function to support interworking with LMR systems defined in TS 23.283.
The voice media flow for a private call shall be routed according to one of the following two options:
Through the primary MCPTT system if both users in the call belong to the same organisation; or
Through the primary MCPTT system of both users, if the users in the call do not belong to the same organisation.
Option 2: The voice media flow may be routed locally, under the control of the primary MCPTT system, through an entity allowing the duplication of the media flow to the primary MCPTT system of each user.
The voice media flow for a group call shall be routed to the group home MCPTT system.
The routing of media flow shall be end-to-end from transmitter to receiver(s), except for the MCPTT control function.
To allow for confidentiality of user identities in various cases of business relationship as defined in clause 6, the MCPTT application may provide public user identities to the MCPTT UE, to be used by MCPTT UE for MCPTT services.
For an MCPTT call session request, resources shall be requested utilising interaction with dynamic PCC. The MCPTT system shall request resources over Rx to a PCRF. The dedicated bearer for voice and MCPTT‑4 reference point messaging shall utilise the QCI value of 65 (as specified in TS 23.203). The request of resources over Rx shall include an application identifier for MCPTT in order for the PCRF to evaluate the correct QCI.
The UE is required to support at minimum one UM bearer which is used for MCPTT voice (see annex A in TS 36.331).
Depending on operator policy:
the MCPTT system may be able to request modification of the priority (ARP) of an existing bearer without the need to initiate a new dedicated GBR bearer; or
the EPS bearers for MCPTT call may enable pre-emption of lower priority EPS bearers if the maximum number of UM bearers has been reached in favour of MCPTT initiated EPS bearer, if the EPS bearer used for MCPTT call has higher priority level (ARP) than the UM bearer(s) used for other application(s) and if the bearers for non MCPTT application are pre-emptable. In this case, the EPS bearer for MCPTT call pre-empts one of the existing EPS bearers when the maximum number of bearers is established for other applications.
The EPS bearer for MCPTT emergency call shall have highest priority level among MCPTT call types. The EPS bearer for MCPTT imminent peril call shall have higher priority level than one for MCPTT call.
To ensure that the MCPTT service always has access to a dedicated bearer for MCPTT media, a pre-established session may be setup that includes a request for resources at the first MCPTT group affiliation.
This means that the PCC may multiplex MCPTT media streams from multiple concurrent MCPTT calls into one EPS bearer on one shared network priority regardless of MCPTT call priority.
The architecture for interconnect between MCPTT systems is specified, allowing the affiliation of MCPTT users from an MCPTT system with MCPTT groups defined in another MCPTT system. When both MCPTT systems are served by different networks, interconnect of signalling and media is achieved using the interfaces defined for interconnect between PLMNs.