How slave knows when master received the notification response message?

[vasilje]

I have 2 devices connected, working in notification mode. Using the nRF Sniffer for Bluetooth LE I was able to check all the messages that are exchanged. When in close proximity and good signal strength the messages looks like as follows:

However, when increasing distance between the two the following, as the signal gets weaker, the following can be observed:

On the picture you can see that master is having several retransmissions when sending write request. Also, slave is sending several Write Responces on different connection events, but how does the slave know that it has to send several Write Responces, since sometimes it sends only one, sometimes several? I dont see how this is indicated in Empty PDU messages or in any other way. How can we clarify this?

[Ivan Herrera]

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. 

[vasilje]

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?

[vasilje]

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:

1. It must be on the same channel
2. 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.