In GATT Packet transfer, is there occurs any packet loss and retrial?

Question

In GATT Packet transfer, is there occurs any packet loss and retrial? 
 I'm trying to connect about -90 Rssi dBm peripheral device with my nRF51-dk. 
 In this far range, does it may occur with packet loss in this far range, and if there is packet loss, does it automatically retry to send it's packet? 
 The reason why I'm curious about this is, there is no single log that It failed to send its packet or it had retried sending packet because the packet has been loss. 
 (I tried to find out if there are any packet loss in this circumstances, but I can't find any single log. Tried with PC NRF Connect, Master Control Pannel, Android NRF Toolbox ) 
 Can't I find any logs about it? or Is there any ways that I can find whether it has failed transferring packet? (and also after failure it has succeed with Packet send Retrial). 
 Lastly, If it retries, can I manually change the retrial count with Softdevice s130? 
 I'm using nRF51822, S130, nRF51-dk...

RK · Accepted Answer

BLE link layer is guaranteed to deliver reliably and in order and it will retry as often as it needs to to uphold that guarantee. Eventually if the link fails to get a packet across it will timeout and fail. Those are the only two possibilities. 
 You can't tell if it retried a packet from anything on the user side, you can run a sniffer and look to see if the underlying BLE packets are re-transmitted, or you can deduce it from the amount of data you're getting down the link. 
 You can't change the retry count. This is all part of the BLE link layer spec.

In GATT Packet transfer, is there occurs any packet loss and retrial?

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