Strange extra packet in network server log

[Tamas Bondar]

Hi All,

This is an excerpt from my network server log (mLinux, v1.0.32):

17:24:25:894|INFO| ED:55-66-77-88-00-00-00-03|CHECK-PKT|FCNT: 00000366 LAST-FCNT: 00000365 Duplicate: no
17:24:25:894|INFO| ED:55-66-77-88-00-00-00-03|CHECK-MIC|ADDR: 00:00:00:03 passed
17:24:25:895|INFO| ED:55-66-77-88-00-00-00-03|PER|13.076923%
17:24:25:895|INFO| ED:55-66-77-88-00-00-00-03|FCTRL|ADR:1 ADRACK:0 ACK:0 CLASS:A OPTS:0 
17:24:25:912|INFO| ED:55-66-77-88-00-00-00-03|SCHED-TX|Use RX1 TOA:1165 ms
17:24:25:915|INFO| ED:55-66-77-88-00-00-00-03|CHECK-PKT|FCNT: beec010c LAST-FCNT: 00000366 Duplicate: yes
17:24:25:916|INFO| ED:55-66-77-88-00-00-00-03|CHECK-MIC|ADDR: 00:00:00:03 passed
17:24:25:916|INFO| ED:55-66-77-88-00-00-00-03|PER|13.026820%
17:24:25:917|INFO| ED:55-66-77-88-00-00-00-03|FCTRL|ADR:1 ADRACK:0 ACK:0 CLASS:A OPTS:0 
17:24:26:649|INFO| ED:55-66-77-88-00-00-00-03|FRAME-TX|Nothing to transmit

First a packet is received with FCNT 366 and processed without problem. Then it seems, that (at timestamp 17:24:25:915) another packet is received from the same node with FCNT beec010. I see these double transmissions occasionally from some of my nodes but not all of them.

Anyone seen something similar before? Just wondering if this might be a network server bug or the node is actually sending two packets? Could it send two packets so close to each other at all? The node sends one packet every 5 minutes, probably with SF12, confirmation turned off.

Thanks,
-Tamás

[Jason Reiss]

Transmitting close to a gateway can cause the a packet to be received by the radio on adjacent channels.
The second packet will have much lower RSSI and SNR but can still be demodulated.

Using SF12 makes this more likely with increased redundancy transmitted.

If you increase the distance or remove the antenna this should stop.

[Tamas Bondar]

Thanks for the hint Jason, we will try moving the nodes farther away from the gateway. What I still don’t understand though, how can the same packet on a different channel decoded differently (i.e. frame counter beec010c instead of 00000366 in the example above)?

[Jason Reiss]

That is a logging error. The frame counter is not filled as it has been marked duplicate so a garbage value is logged. I will mark this to be fixed.

[Tamas Bondar]

Thanks Jason, it’s clear now. I think the logs will be more intuitive when this is fixed.

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