https://devzone.nordicsemi.com/f/nordic-q-a/9292/nrf51-ack-retransmissionHi there Can you clarify a point for me? Quoting from Gazell link layer user guide: &quot;If only the ACK packet sent from the Host to the Device is lost, but both the initial packet and the subsequent retransmission attempts are being successfullyen-USTelligent Community 13Fri, 18 Sep 2015 09:08:18 GMThttps://devzone.nordicsemi.com/thread/34281?ContentTypeID=1Fri, 18 Sep 2015 09:08:18 GMT137ad170-7792-4731-bb38-c0d22fbe4515:5b15c6b0-fa25-4c43-b147-916fdfa704a6Hung Bui<p>Hi Elliott,</p> <p>I assume you were talking about the figure with the description &quot;Example on failing package transaction.&quot; <a href="http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.sdk51.v9.0.0/gzll_02_user_guide.html?cp=4_1_0_5_0">here</a>.</p> <p>In the figure we wanted to demonstrate when a transaction is failed. The second ACK packet was not &quot;not sent&quot;, it was sent but got lost, not received by Device, because of interference for example.</p> <p>And when the last packet from Device to Host also went missing, and the third ACK was not sent, we will have nrf_gzll_device_tx_failed() call back return.</p> <p>The figure doesn&#39;t really demonstrating what discussed in the paragraph right above it.</p><div style="clear:both;"></div>