The MCPTT client functional entity acts as the user agent for all MCPTT application transactions. The client reports the information of where the client is currently located.

The MCPTT server functional entity provides centralised support for MCPTT services. All the MCPTT clients supporting users belonging to a single group are required to use the same MCPTT server for that group. An MCPTT client supporting a user involved in multiple groups can have relationships with multiple MCPTT servers. The MCPTT server functional entity represents a specific instantiation of the GCS AS described in TS 23.468 to control multicast and unicast operations for group communications. The MCPTT server functional entity is supported by the SIP AS, HTTP client and HTTP server functional entities of the signalling control plane. By assuming the role of a GCS AS, the MCPTT server functional entity is responsible for:

• requesting the allocation of multicast resources utilizing the media distribution function;
• announcing the association of multicast resources to calls to MCPTT UEs;
• determining for each MCPTT UE involved in a given call whether to use unicast or multicast transport;
• announcing the assignment of multicast transport for specific calls to MCPTT UEs; and
• informing the media distribution function of the media streams requiring support for a given call.

The MCPTT server shall support the controlling role and the participating role. The MCPTT server may perform the controlling role for private calls and group calls. The MCPTT server performing the controlling role for a private call or group call may also perform a participating role for the same private call or group call. For each private call and group call, there shall be only one MCPTT server assuming the controlling role, while one or more MCPTT servers in participating role may be involved. The MCPTT server performing the controlling roles is responsible for:

• call control (e.g. policy enforcement for participation in the MCPTT group calls) towards all the MCPTT users of the group call and private call;
• interfacing with the group management server for group policy and affiliation status information of this MCPTT server's served affiliated users;
• enforcing functional alias priority handling;
• managing floor control entity in a group call and private call; and
• managing media handling entity in call i.e. conferencing, transcoding.

The MCPTT server performing the functional alias controlling role is responsible for:

• interfacing with the functional alias management server for functional alias policy from the functional alias configuration;
• functional alias activation, deactivation, take over and interrogation support for MCPTT user.

The controlling roles for group call, private call and functional alias are independent with each other. The MCPTT server performing the participating roles is responsible for:

• call control (e.g. authorization for participation in the MCPTT group calls) to its MCPTT users for group call and private call;
• group affiliation support for MCPTT user, including enforcement of maximum N2 number of simultaneous group affiliations by a user;
• enforcing functional alias priority handling;
• relaying the call control and floor control messages between the MCPTT client and the MCPTT server performing the controlling role; and
• media handling in call for its MCPTT users, i.e. transcoding, recording, lawful interception for both unicast and multicast media.

For group regrouping involving multiple groups from primary and partner MCPTT systems,

• the group host MCPTT server of the temporary group performs the controlling role and is responsible for the centralized floor control, and for arbitration according to the temporary group or user policies (e.g., priority);
• the group host MCPTT server of the constituent MCPTT group is responsible for providing call invitations to their group members, and for filtering between constituent group members' floor control requests according to the constituent group or user policies (e.g., priority); and
• the MCPTT server responsible for the constituent MCPTT group members performs the participating role.

The floor participant functional entity is responsible for floor requests. This functional entity is located in the UE for both on-network and off-network operations.

This functional entity provides support for centralised floor control for on-network and distributed floor control for off-network operation. It may provide arbitration between floor control requests between different users, grant the floor in response to successful requests, and provide queuing in cases of contention. For on-network operation, this functional entity is located with the MCPTT server. For off-network operation, this functional entity is located in the UE.

The media distribution function is responsible for the distribution of media to call participants. By means of information provided by the MCPTT server (e.g. IP addresses, transport layer ports), it will provide the following functionality:

• provide for the reception of uplink MCPTT UE media transmission by means of the MCPTT-7 reference point;
• replicate the media as needed for distribution to those participants using unicast transport;
• distribute downlink media to MCPTT UEs by IP unicast transmission to those participants utilizing unicast transport by means of the MCPTT-7 reference point;
• distribute downlink media to MCPTT UEs using multicast downlink transport of media for the call by means of the MCPTT-8 reference point; and
• provide a media mixing function where multiple media streams are combined (e.g. multi-talker control) into a single media stream for transmission to the MCPTT UE.

Group configuration data determines whether audio mixing for multi-talker control is applied by the media mixing function in the MCPTT server.

This functional entity exists on the UE and provides support for combining multiple media streams (e.g. multi-talker control) into one media stream through the enforcement of media policy information. Group configuration data determines whether audio mixing for multi-talker control is applied by the media mixing function in the UE.

This functional entity contains information of the MCPTT user profile associated with an MCPTT ID that is held by the MCPTT service provider at the application plane. The MCPTT user profile is determined by the mission critical organization, the MCPTT service provider, and potentially the MCPTT user.

The MC gateway server provides support for MCPTT interconnection services with a partner MCPTT system in a different trust domain whilst providing topology hiding. It acts as a proxy for one or more MCPTT servers in the partner MCPTT system without needing to expose the MCPTT servers in the primary MCPTT system outside the trusted domain of the primary MCPTT system. It may be a role of the MCPTT server described in subclause 7.4.2.3.2 of the present document. The MC gateway server is responsible for relaying call control and floor control signalling messages, and media between MCPTT servers within the MCPTT system and the interconnected MCPTT system.