6.3.2 Controlling MCVideo function procedures at MCVideo call initialization
24.5813GPPMission Critical Video (MCVideo) media plane controlProtocol specificationRelease 18TS
6.3.2.1 General
The clause 6.3.2.2 describes the initial procedures when a new SIP session is establishing a group session or a private session with transmission control.
6.3.2.2 Initial procedures
When an MCVideo call is established a new instance of the transmission control server state machine for ‘general transmission control operation’ is created.
For each MCVideo client added to the MCVideo call, a new instance of the transmission control server state machine for ‘basic transmission control operation towards the transmission participant’ is added.
When an MCVideo call is established a new instance of the transmission control server state machine for ‘general reception control operation’ is created.
For each MCVideo client added to the MCVideo call, a new instance of the transmission control server state machine for ‘basic reception control operation towards the transmission participant’ is added.
If the optional "mc_queueing" feature is supported and has been negotiated as specified in clause 14, the transmission control server could queue the implicit transmission control request for the media-transmission control entity.
The original initial SIP INVITE request or SIP REFER request to establish an MCVideo chat group call or to rejoin an ongoing MCVideo call is not handled as an implicit transmission control request message by the transmission control server unless explicitly stated in the SIP INVITE request or in the SIP REFER request.
The permission to send media to the inviting MCVideo client due to implicit transmission control request is applicable to both confirmed indication and unconfirmed indication.
When the first unconfirmed indication is received from the invited participating MCVideo function (see 3GPP TS 24.281 [2]) the transmission control server optionally can give an early indication to send RTP media packets, to the inviting MCVideo client.
If an early indication to send RTP media packets is given to the inviting MCVideo client, the transmission participant is granted the permission to send media and the MCVideo server buffers RTP media packets received from the MCVideo client at least until the first invited MCVideo client accepts the invitation or until the RTP media packet buffer exceeds it maximum limit to store RTP media packets.
If the MCVideo server does not support or does not allow media buffering then when an early indication to send RTP media packets is not given to the inviting MCVideo client, the transmission participant is granted the permission to send media when the first invited MCVideo client accepts the media.
Before the transmission control server sends the first transmission control message in the MCVideo call, the transmission control server has to assign itself a SSRC identifier to be included in media transmission control messages and quality feedback messages if the MCVideo server is supporting that option. A suitable algorithm to generate the SSRC identifier is described in IETF RFC 3550 [3].
The transmission participant and the transmission control server can negotiate the maximum priority level that the transmission participant is permitted to request. The transmission control server can pre-empt the current sender based on the negotiated maximum priority level that the transmission participant is permitted to request and the priority level included in the Transmission Media Request message.
NOTE: The maximum priority level that a transmission participant can use is negotiated as specified in clause 14.3.3 and is based on group configuration data retrieved by the controlling MCVideo function from the group management server as described in 3GPP TS 24.481 [12] and service configuration data retrieved by the controlling MCVideo function from the configuration management server as described in 3GPP TS 24.484 [13].
The transmission participant and the transmission control server can negotiate queueing of Transmission requests using the "mc_queueing" fmtp attribute as described in clause 14. If queueing is supported and negotiated, the transmission control server queues the transmission control request if a Transmission Media Request message is received when another transmission participant has the transmission and the priority of the current speaker is the same or higher.
6.3.2.3 Switching from a non-controlling MCVideo function mode to a controlling MCVideo function mode
When the MCVideo server switches from the non-controlling MCVideo function mode to controlling MCVideo function mode a new instance of the transmission control server state machine for ‘general transmission control operation’ is created.
For each MCVideo client in the MCVideo call a new instance of the transmission control server state machine for ‘basic transmission control operation towards the transmission participant’ is added.
Any transmission request in the passive transmission request queue is moved to the active transmission request queue.
NOTE: The passive transmission request queue is a transmission request queue used by the non-controlling MCVideo function as specified in clause 6.5.4 to monitor transmission requests sent by transmission participants controlled by the non-controlling MCVideo function.