6 - Class C: Continuously Listening for Downlinks
End devices in Class C mode listen for downlinks except when they are broadcasting Class A uplinks, or the Class A receive windows are open.
Class C mode is enabled by identifying the end device as a Class C capable device when you register it on the network server. The network server will then immediately send any downlinks it receives over a single gateway.
Send Messages to Multiple End Devices Simultaneously
Class C downlinks may be unicast, sent to a single end device; or multicast, sent to a group of end devices simultaneously. Multicasting is useful when you need to send a message to a group of devices at the same time, for example turning off all the streetlights in an area or turning off different valves in the event of an early earthquake warning. Multicasting is also useful when performing firmware upgrades over the air, as you will need to update many devices and it would be time-consuming to update them all individually.
End devices that will receive multicast messages must first be placed into a multicast group, where each device in the group shares an identical multicast network address, multicast session keys, and a multicast frame counter.
The LoRaWAN® Link Layer Specification v1.0.4 does not define a standard approach for onboarding an end device into a multicast group but specifies that this process should be established using the application layer, external from the core LoRaWAN functionality. In practice, this means that it is up to you to define the format and contents of messages transmitted as application downlinks (not MAC commands), and program your end device to receive these messages and act upon them. The LoRaWAN Remote Multicast Setup Specification TS005-2.0.0 proposes a standard secure approach to constructing these messages, as well as a set of predefined actions the end device should take when receiving each message. If you are going to implement multicasting, you should implement and follow this standard, which may also allow you to take advantage of code libraries and SDKs that others may produce.