SACK Mechanism

The frame format of SACK is here.

If the SACK function is enabled (please check the ‘SACK Mode’ field in the AT@ACK command), the terminal will perform a special mechanism:

  1. Terminal will wait for the backend server to respond SACK message after sending a report. The maximum timeout for each waiting is 40 seconds.

  2. Once the terminal receives the correct SACK response message, it means that this report has been successfully received by the backend server. The terminal will end the sending process of this report.

  3. If the terminal does not receive the correct SACK message within the waiting time from the backend server, the terminal will try to resend the report. After resending 3 times (i.e., sent a total of 4 times), it still does not receive a correct SACK, it will reconnect to backend server.

The flowchart is shown below:

../_images/air-appendix-1.png