Can somebody explain to me the signalling procedure of the group call setup for unicast and multicast path?

Unicast Path

 1) UE 1 sends an INVITE message to initiate a group call.

2) The eMDC obtains a list of all interested UEs of the group, and detects that the UEs support only the On-Demand mode and have no pre-established SIP sessions. The eMDC then determines to establish a unicast path for these UEs.

3) The eMDC sends a 200 OK message to UE 1, the speaker, to establish an uplink session, and notify the speaker that floor is granted.

4)  The eMDC sends INVITE messages (carrying the group ID and speaker MCPTT ID, with Answer-Mode set to auto) to UE 2 and UE 3, the listeners, to establish a unicast path in the downlink. If a UE returns a 4XX message, it refuses to receive downlink group data. If a UE returns a 200 OK message, it will receive downlink group data on the unicast path.

5) If UE 2 or UE 3 determines to receive downlink group data, the eMDC sends a TBCP Floor Taken message on the user plane and on the unicast path to notify them of speaker information

Multicast Path

Prerequisites:

 - SIP sessions have been pre-established for all UEs.

- All UEs have received the Announcement message from the eMDC and start to listen to multicast signaling messages in the local SA.

Group setup procedure

1) When the PTT key is pressed, UE 1 sends a REFER message to the eMDC to initiate a group setup request.

2) The eMDC returns a 200 OK message and starts to send a MapToBearer message on the multicast signaling channel in the SA bound to the group.

3) After receiving the floor indication from the eMDC, UE 1 starts to send uplink voice data.

4) After receiving the MapToBearer message, UE 2 and UE 3 determine whether to listen to the group (based on the priorities of interested groups).

5) If UE 2 and UE 3 determine to listen to the group, they receive voice data on the user plane and on the multicast path based on the indication carried in the MapToBearer message.