I (partially) figured out why a node with the LMIC library, e.g. the RFM95W doesn’t join instantly when using OTAA. The standard code will first try to join the network at SF7 at certain frequency. However for some reason this will fail. After a failed attempt the node will try another frequency with the same datarate (SF7), this will fail again. Now the node will decrease the datarate to SF8 and again do two attempts which both will fail. Consequently the node decreases the datarate to SF9. This time the node will succeed at joining at the first attempt with this datarate.
After reading this comment of @kersing it seems explainable why SF7 and SF8 are not working (for me).
SF7 and SF8 should have a join reply in RX1 using the parameters of the request. SF9 + response is in RX2 at SF12
Based on this, reporting of others who also can’t join instantly with LMIC OTAA, and on tests with the Microchip RN2483, I conclude that the LMIC library doesn’t parse the received data in RX1 (correctly), please correct me if I am wrong, @matthijs @Thomas? As I will be on holidays for a while I can’t dig deeper into the code so I hope this information will help.
.
p.s. If you want a dirty fix for the instant join you can adjust the LMIC library to start with at SF9 by editing the following at line 686 in the LMIC library:
setDrJoin(DRCHG_SET, DR_SF7);
Change to:
setDrJoin(DRCHG_SET, DR_SF9);