====== FAQ - Multicast ======


===== What are Unicast and Multicast? =====
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.

===== How can I know that multicast messages are correctly received by my end-devices? =====
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.

===== What are the requirements to create a Multicast group? =====
The mandatory requirements are : \\
  *  a cluster name
  * a multicast address: DevAddr, a 32 bit network address (must be unique and not belong to an existing end-device)\\
  * the application session key (AppSKey) and network session key (NWkSKey) used for frames encryption, 32 digits hexadecimal string.
  * radio settings: a fixed frequency in MHz (float32) and a DataRate (a string formatted as "SFxxBWyyy" with xx from 7 to 12 and yyy = 125/250/500 for LoRa modulation or yyy=5000 for FSK modulation)
  * a frame counter: FCntDown
  * the LoRaWAN MAC version: for Europe 868 MHz, select 1.0.2 MAC version and Regional parameters B revision to be compatible with WMC 2.x 

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.

===== Can several Multicast groups have the same multicast adress? =====
Yes. The same multicast adrress can be used by several Multicast groups.

===== Can an end-device belong to several Multicast groups? =====
Yes. An end-device can belong to several Multicast groups.

===== Which gateways are chosen for a C-Multicast DataDown? =====

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.


===== Which gateways are choosen for a Unicast DataDown? =====
The LNS selects gateways according to their SNR (Signal/Noise ratio). The gateway with the higher SNR (lowest noise) is choosen in priority.

===== Is it possible that a end-device receive a C-Multicast DataDown from multiple gateways at the same time? =====
This case is not managed by the LNS yet.