Important note:
Downlinks should only be used to a limited extent depending on the infrastructure. In the free version of TTN, the Fair use policy says, that no more than 10 downlinks should be made. You also have to observe the regulations within the Limit of the duty cycle limitation. That is why Downlinks are only to be carried out with caution and when absolutely necessary. The number of possible characters that can be sent is limited by the respective spreading factor (With a SF of 12 only 51 characters).
The applicable and possible downlinks depend on the device and the firmware version.
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