An In-depth Look at LoRaWAN® Class C Devices
End devices in Class C mode are used when extremely low power consumption is not an issue and latency needs to be minimized. The server-side application determines that it is managing class C devices during the join procedure.
Characteristics of a Class C Device
- Based on Class A foundations
- Devices cannot simultaneously operate in Class B and Class C mode
- Lowest latency among all operating modes
- Uses more power than Class A and Class B devices
Class C: Continuous Reception
End devices operating in Class C mode have receive windows that are almost always open. These windows close only when the device is transmitting. Because of this, Class C end devices use more power to operate than Class A or Class B devices. However, in turn, they offer the lowest latency for communication from the server to an end device.
A device may be switched to Class C mode temporarily. This approach may be used to perform a firmware upgrade of a battery-powered device. A battery-powered Class A device may switch to Class C for a few minutes at a given time in order to receive a firmware update over-the-air (FUOTA) broadcast. Once the broadcast of the update is complete, the device can return to its default Class A, low-power mode of operation.
Class C end devices implement the same two receive windows as Class A devices, but they do not close the RX2 window until they send the next transmission back to the server. Therefore, they can receive a downlink in the RX2 window at almost any time. A short window at the RX2 frequency and data rate is also opened between the end of the transmission and the beginning of the RX1 receive window, as illustrated in Figure 1.
Figure 1: Receive Windows: Packet can be received in RX2 window
Class C end devices may receive multicast downlink frames in a manner similar to end devices in Class B mode. The multicast address and the associated network session key and application session key must come from the application layer.
The FPending bit is used to indicate that there is more multicast data to be sent. Given that a Class C device keeps it receiver active most of the time, the FPending bit does not trigger any specific behavior on the end device.
All commands described in the Class A specification must be supported in Class C end devices.
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