scope:

The present document describes a minimum IMS profile of the Gm reference point for SIP core implementation to guide interoperable implementation of MCPTT solutions. In order to support procedures over the SIP-1 reference point defined in 3GPP TS 23.179 [2] and 3GPP TS 24.379 [3], and which exists between the MCPTT UE Signaling User Agent and the SIP core for obtaining access to MCPTT service and supporting MCPTT sessions, the Gm reference point is profiled to meet the minimum requirements to support MCPTT.

The Gm reference point contains many options which are not necessarily applicable for support of MCPTT or options which are alternative mechanisms (e.g. P-CSCF discovery) where if the same option is not supported by both the MCPTT UE and the network interoperability cannot be achieved.

The present document describes the following:

- Where options exist in 3GPP TS 24.229 [4] related to provision of IMS based services over the Gm reference point, those options which need to be supported to ensure interoperable MCPTT service will be identified.

- Where options exist in 3GPP TS 24.301 [5] and stage 2 functionality that are needed to provide MCPTT service over IMS, those options which need to be supported to ensure interoperable MCPTT service will be identified.

- Where optional features of MCPTT are defined that require additional options on the Gm reference point to be supported, these options will also be identified as recommended to be supported to ensure interoperability of the optional feature.

The present document will only cover on-network unicast operation.

The present document covers EPC aspects that are required by MCPTT to establish MCPTT sessions.