How slave knows when master received the notification response message?

RE: How slave knows when master received the notification response message?

vasilje — Mon, 30 Nov 2020 16:28:49 GMT

I found an answer in this article:

Understanding SN and NESN in a BLE Link Layer packet

Basically: The Empty PDU's on the pictures are ACKs. How they work is that once the package is sent there should be two conditions satisfied in the response:

It must be on the same channel
Original message (from Master) contained NESN (next expected sequence number) and SN (sequence number of the current message). So Slave answer's SN needs to match the NESN of message from Master. The NESN in this Slave answer should say what is the SN of the following message from the Master.

If these two conditions are not satisfied, then the Master (or Slave) will know that the message has not been delivered, and it will retransmit.

RE: How slave knows when master received the notification response message?

vasilje — Mon, 30 Nov 2020 14:52:31 GMT

Thanks for the answer. Can you be more specific, keeping in mind the picture that is attached? This is the Sniffer output, I believe it shows Link Layer messages. How ACK's are represented there?

RE: How slave knows when master received the notification response message?

Ivan Herrera — Mon, 30 Nov 2020 14:37:04 GMT

All BLE notifications, writes (even without response afaik), are still ACK'ed on the link layer. So if there is no ACK from a notification or write, it will retry.