3GPP TS 23.179 defines that the SIP-1 reference point between the MCPTT UE and the SIP core uses the 3GPP Gm reference point (with necessary enhancements to support MCPTT requirements and profiled to meet the minimum requirements for support of MCPTT). 3GPP TS 24.229 and TS 24.301 contain several options or alternatives for the MCPTT UE and network for support of procedures on the Gm reference point in order to obtain IMS services. In order to ensure interoperability it is necessary to define recommendations that if adhered to by both MCPTT UE and network will ensure that the MCPTT UE can obtain access to the MCPTT service. The present document makes recommendations for the P-CSCF and S-CSCF entities defined in TS 23.228, however if the SIP core for MCPTT does not internally comply with the architecture of TS 23.228 these recommendations apply generically to whatever SIP core entity provides the equivalent functionality to the P-CCSF and S-CSCF. The MCPTT UE and SIP core adhere to the procedures on the Gm reference point defined in TS 24.229, TS 24.301 and TS 24.379. The present document cross-references and aligns with GSMA PRD IR.92 [12], which profiles Voice and SMS over LTE (VoLTE).

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 TS 23.179 and TS 24.379, 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 TS 24.229 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 TS 24.301 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.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.