Unicast messages are sent to a single end-device and multicast messages are sent to multiple end-devices. All devices of a multicast group must share the same multicast address and associated encryption keys. All end-devices must listen to the multicast channel in order to receive multicast messages (end-devices must be C-class). The best use-cases for Multicast are Lighting domain and Firmware update.
There is no ACK for Multicast messages. So from the WMC side, we cannot know if the multicast message has been received by all end-devices.
The mandatory requirements are :
The multicast address and associated network session key and application session key must come from the application layer.
Class-C multicast downlinks SHALL NOT carry MAC commands.
Yes. The same multicast adrress can be used by several Multicast groups.
Yes. An end-device can belong to several Multicast groups.
The LNS selects gateways according to the dutycycle and the coverage of each gateway.
In any case, a gateway cannot be selected if its duty cycle is over the maximum limit or if downlink is not allowed by its owner.
If an end-device has never transmitted data, it won't be considered when doing the selection of gateways used for Multicast DataDown.
The LNS selects gateways according to their SNR (Signal/Noise ratio). The gateway with the higher SNR (lowest noise) is choosen in priority.
This case is not managed by the LNS yet.