I found these abbreviations in an official documentation and I don't know what they are stand for.
Can you explain it to me please? I know that it is basic question.
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Unfortunately this is not a complete answer, I will update it if I get more information. As for now, based on the Sigfox AT command descriptions.
AT$SB=bit[.bit]Send a Bit Status (0 or 1).
AT$SF=frame[.bit]Used to send payload data or frame of size 1 to 12 bytes. For example:
SF means Sigfox Frame
An uplink message has up to 12-bytes payload and takes an average 2s over the air to reach the base stations which monitors the spectrum looking for UNB signals to demodulate. For a 12-byte data payload, a Sigfox frame will use 26 bytes in total. source
ACK, a device can request acknowledgement from the network by setting the ACK flag in the message.
The Downstream messages are, in fact, frame acknowledgment. It means that to receive a message on the device, you have to send one indicating the Sigfox network you are expecting a ack.
This ack is a 8bytes message the network can send directly with predefined value like current time or any used defined fixed value. It can also be a dynamic message sent by your own backend.
The device is sending a message with ack flag activated and then turn into receive mode during 25 seconds, looking for the network acknowledgment. This one received it will go back sleeping. Between the end of the send and the begin or the receive you have about 15 seconds sleep. source