The MCPTT Service while operating in off-network mode comprises a set or collection of functions necessary to provide Mission Critical Push To Talk (MCPTT) using a ProSe direct (UE-to-UE) Communication path (ProSe direct communication path) for transport. The ProSe direct communication path does not traverse the network infrastructure. Users operating off the network are either out of network coverage (not served by a 3GPP network) (e.g., in a remote mountain area fighting a forest fire 20 miles from the nearest network) or have selected a ProSe direct communication path for MCPTT while in network coverage. MCPTT Users operating off the network need to be in ProSe direct communication range in order to communicate. MCPTT Users outside of the coverage of the fixed network might be first responders in a rural area assisting in a response to a plane crash, fire fighters in a remote mountain area fighting a forest fire or police officers inside a residence responding to a domestic issue. Off-network MCPTT communications are expected to be immediately accessible to users in the absence of the network. MCPTT Users in network coverage might be working in a confined area, such as fire fighters fighting a structure fire where direct UE-to-UE communication is more desirable and reliable. Users can communicate directly with one another without having to overcome the resistance of a building and distance to the nearest base station to communicate with other members of their team inside the building that are nearby. To operate off the network, an MCPTT UE is capable of automatically switching to a ProSe direct communication path for use of MCPTT when detecting an off-network (out of coverage) condition. In addition a mechanism is provided for an authorized user to select (manually switch to) a ProSe direct communication path for use of off-network MCPTT communications (e.g., while in network coverage). When operating off the network, the MCPTT Service is provided by the MCPTT application on the UE as compared to operations on the network, where the MCPTT Application on the UE interacts with an MCPTT server and the network to provide the MCPTT Service. The Off-Network MCPTT Service builds upon ProSe enablers to establish, maintain and terminate the signalling and communication path(s) among the off-network users. To the extent feasible, it is expected that the end user's experience is similar regardless if the MCPTT Service is used with a 3GPP network or based on the use of a ProSe direct communication path. The Off-Network MCPTT Service is intended to support communication between a group of users (a group call), where each user has the ability to gain access to the permission to talk in an arbitrated manner. However, the MCPTT Service also supports Private Calls between pairs of users. When operating off the network the MCPTT Service allows users to request the permission to talk (transmit voice/audio) and provides a deterministic mechanism to arbitrate between requests that are in contention (i.e., Floor control). The Off-Network MCPTT Service provides a means for a user with higher priority (e.g., MCPTT Emergency condition) to override (interrupt) the current talker. The Off-Network MCPTT Service also supports a mechanism to limit the time a user talks (hold the floor) thus permitting users of the same or lower priority a chance to gain the floor. The Off-Network MCPTT Service provides the means for a user to monitor activity on a number of separate calls and enables the user to switch focus to a chosen call. An Off-Network MCPTT Service user might join an already established MCPTT Group call (Late call entry). In addition the Off-Network MCPTT Service supports User IDs, aliases and user Location determination features. For operation off the network (e.g., when out of network coverage), an MCPTT UE is (pre-)provisioned by an MCPTT Administrator and/or authorized user with the following in order to use MCPTT:

1. An MCPTT User Profile associated with each of the intended MCPTT Users of the MCPTT UE that might be used for off-network operation;
   1. An alphanumeric identifier (with a minimum length of N3) (i.e., alias) for each MCPTT User;
   2. A number of off-network MCPTT Groups for use by the MCPTT User;
   3. An alphanumeric identifier (i. e., alias) for the authorized off-network MCPTT Groups;
   4. A Mission Critical Organization name if available, associated with each of the intended MCPTT Users or Administrator;
   5. A number of off-network MCPTT Users for Private Call for which the MCPTT User is authorized;
2. Authentication and end to end security keys.

An MCPTT UE operating off the network is capable of transmitting the talker Location information, User ID, alias(es), off-network MCPTT Group ID, group alias and, if available, Mission Critical Organization name of the user who is talking (i.e., whose UE is transmitting) to all other users in a call including MCPTT UEs operating off the network that are late entering a call in progress. The Off-Network MCPTT Service uses the capabilities defined in ProSe TS 22.278, including the ProSe Relay capabilities defined in ProSe TS 22.278 and GCSE_LTE TS 22.468. MCPTT Service requirements specific to off-network use are defined in clause 7. Common MCPTT service requirements defined in clause 5 apply whether the MCPTT Service is in use on the network or off the network.

[R-7.2-001] [R-7.2-002] [R-7.2-003] [R-7.2-004] [R-7.2-005]

[R-7.4-001] [R-7.4-002] [R-7.4-003] [R-7.4-004] [R-7.4-005] [R-7.4-006]

[R-7.5-001] [R-7.5-002]

[R-7.7-001] [R-7.7-002] [R-7.7-003]

[R-7.9-001]

[R-7.10-001] [R-7.10-002]