RFM69 ATC Not working as expected? [explanation]

[DRBRODIE]

I have two Moteinos - one running the DS18B20 example from your github page without modification (except to change the node # to 2) using your coil antenna and the other running your Gateway example without any modification using an SMA foldable whip antenna.  Both sketches have the ATC enabled.
First, I do not see the RSSI dropping down to the -90 setpoint over time.  Second, on the third transmission I see "ACK...nothing" as shown below:

10:20:23.845 -> #[664][2] 1] F:81.05   [RX_RSSI:-68] - ACK sent.
10:20:29.017 -> #[665][2] 1] F:81.05   [RX_RSSI:-68] - ACK sent.
10:20:34.101 -> #[666][2] 1] F:81.05   [RX_RSSI:-67] - ACK sent. Pinging node 2 - ACK...nothing

Can you suggest what I need to do to correct this?

[Felix]

Well it depends what you mean by "not as expected".

RSSI can be misleading because a strong signal is a higher number. So a -65dBm RSSI is many times stronger than -90. The -100db is where the "noise floor" starts. We want to be above that so the radio has a chance to still distinguish the useful signal from the noise. If your module is well within range, it might yield a very strong signal even at the lowest transmit power setting. At that point ATC can't make it transmit a weaker signal (ie "closer to the target" of -90 or whatever threshold).

So here's what you can expect.
If the RSSI reaches -90 (or whatever threshold is set) at a transmit power above 0, then ATC will keep it at that level to meet or exceed the RSSI threshold.
If the RSSI is already stronger than the threshold at power level 0, ATC can't ramp power down any more. It means you have a very strong signal at the lowest transmit power. Good thing, and ideal case.

The DS18B20 is a sleepy example, it transmits then sleeps. It does not actively listen for packets from the Gateway example. For that, use the Node example instead, which listens and does an sendACK() when a packet is randomly received from the gateway.

[DRBRODIE]

The fact that the RSSI stayed static and did not go down to the -90 target was the first thing that I did not expect. You can see that it stayed around -67 to -68. The second thing I didn’t expect was the “ACK - nothing” response. I think you have explained the second one - thanks!

[Felix]

Please revisit my comment, I explained everything.

The fact that the RSSI stayed static and did not go down to the -90 target was the first thing that I did not expect. You can see that it stayed around -67 to -68.

That can be normal, if you consider what I explained, perhaps a misunderstanding (or incorrect expectation) how transmit power level and RSSI work.

[DRBRODIE]

I guess I misunderstood.  I was just going by: the "RFM69_ATC: Automatic Transmission Control" article posted on November 11, 2015 which stated that "the node will start to progressively dial down power (assuming it starts transmitting at full power) with each packet sent, until the RSSI meets the target". Anyhow, I'm very happy with the Moteinos and their range and plan to use them throughout my home automation upgrade from a wi-fi based system. Thanks for responding so quickly!