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:
An MCPTT User Profile associated with each of the intended MCPTT Users of the MCPTT UE that might be used for off-network operation;
An alphanumeric identifier (with a minimum length of N3) (i.e., alias) for each MCPTT User;
A number of off-network MCPTT Groups for use by the MCPTT User;
An alphanumeric identifier (i. e., alias) for the authorized off-network MCPTT Groups;
A Mission Critical Organization name if available, associated with each of the intended MCPTT Users or Administrator;
A number of off-network MCPTT Users for Private Call for which the MCPTT User is authorized;
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.
The off-network Floor control functionality in an MCPTT Service shall determine at a point in time which off-network Participant(s) are allowed to transmit to other off-network Participants.
The off-network Floor control functionality in an MCPTT Service shall determine at a point in time which received transmission(s) from off-network Participant(s) shall be presented to the receiving off-network Participant(s).
A Participant in an off-network MCPTT Group call, with the authority to transmit, shall be able to request to transmit to the off-network MCPTT Group.
The off-network Floor control functionality shall have a mechanism for resolving simultaneous requests for permission to transmit within the same call.
Following an MCPTT Request for permission to transmit, the Affiliated MCPTT Group Member that is allowed to transmit shall be given an indication that the member is allowed to transmit on the member's Selected MCPTT Group.
Following an MCPTT Request for permission to transmit, an Affiliated MCPTT Group Member that is not allowed to transmit on the Selected MCPTT Group shall be given an indication that permission to transmit was rejected or queued.
Following an MCPTT Private Call (with Floor control) request for permission to transmit, the MCPTT User that is allowed to transmit shall be given an indication that the user is allowed to transmit to the targeted MCPTT User.
Following an MCPTT Private Call (with Floor control) request for permission to transmit, an MCPTT User that is not allowed to transmit shall be given an indication that the permission to transmit was rejected.
An MCPTT UE shall be pre-provisioned by an MCPTT Administrator and/or authorized user with the necessary information in order that Floor control override may operate during off-network MCPTT.
The MCPTT Service shall provide a mechanism for MCPTT Administrators to create a priority hierarchy for determining what Participants, Participant types, and urgent transmission types, when operating off the network, be granted a request to override an active off-network MCPTT transmission.
The priority hierarchy used for granting a request to override an active MCPTT transmission shall contain at least four (4) levels.
The MCPTT Service shall provide a mechanism for Participants, to override an active MCPTT transmission of a transmitting Participant when the priority level of the overriding Participant or call type are ranked higher than the priority level of the transmitting Participant or call type.
If an authorized Participant overrides an MCPTT transmission, the MCPTT Service shall provide a means of notifying the overridden Participant(s) that the transmission has been overridden.
The MCPTT Service shall provide a mechanism to enable an MCPTT Administrator to configure which MCPTT Group transmission a Participant(s) receives, overriding and/or overridden for cases where an authorized Participant overrides an off-network MCPTT transmission. This mechanism, at the receiving Participant, shall also determine which transmission should be presented to the MCPTT User when an unauthorized transmission override has occurred due to a failure of transmit Floor control (e.g., due to the best effort nature of ProSe direct communication).
If the MCPTT Group has been configured to only allow the overriding transmitting Participant to transmit, the MCPTT Service shall revoke the transmit permission of the overridden transmitting Participant.
If the MCPTT Group has been configured to allow both overriding and overridden transmitting Participants to transmit, the MCPTT Service shall provide a mechanism for authorized receiving Participants to be able to listen to both the overriding transmission and any overridden Participant transmissions, dependent on configuration.
For MCPTT Users, one of the most important performance criteria is the MCPTT Access time (KPI 1). The MCPTT Access time is defined as the time between when an MCPTT User requests to speak (normally by pressing the MCPTT control on the UE) and when this user gets a signal to start speaking. This time does not include confirmations from receiving users.
The Mouth-to-ear latency (KPI 3) is the time between an utterance by the transmitting user, and the playback of the utterance at the receiving user's speaker. Figure 22.214.171.124-1 illustrates the MCPTT Access time and Mouth-to-ear latency.
An MCPTT User is able to join or leave an already ongoing MCPTT Group Call. Late call entry is the activity when an Affiliated MCPTT Group Member joins an MCPTT Group Call in which other Affiliated MCPTT Group Members are already active. The Late call entry time (KPI 4) is the time to enter an ongoing MCPTT Group Call measured from the time that the user decides to monitor such an MCPTT Group Call, to the time when the UE's speaker starts to play the audio. The performance requirements for Late call entry time only applies to when there is ongoing voice transmitted at the time the MCPTT User joins the call.
In a Late call entry there might be an initial lost audio of the voice burst sent to the new Receiving MCPTT Group Member.