Class B downlink TX aborted no mandatory "txpk.tmst"

stanvn · January 14, 2021, 9:22am

Hi All,

I’m trying to set up LoRaWAN class B multicast using the LoRaMac-node repo. Joining the network and receiving beacons from the gateway all work fine. However, when I want to send a downlink using the Chirpstack Application server the gateway gives the following error (or actually a warning):

JSON down: {"txpk":{"imme":false,"rfch":0,"powe":14,"ant":0,"brd":0,"tmms":1294649730750,"freq":869.525,"modu":"LORA","datr":"SF12BW125","codr":"4/5","ipol":true,"size":13,"data":"YEvrFwCABgACLNaSjw=="}}
WARNING: [down] no mandatory "txpk.tmst" or "txpk.time" objects in JSON, TX aborted

The result is that the downlink is never send. Sending the same data using Class A works perfectly.

The gateway I’m using is the Kerlink Wirnet Station with the Chirpstack Gateway and the Semtech packet forwarder installed. For the packet forwarder I use this configuration file.

Any ideas what this could be? If more information is needed please let me know.

Thanks in advance,
Stan

brocaar · February 10, 2021, 12:29pm

Are you sure you are using a recent version of the Semtech UDP Packet Forwarder?

ChirpStack uses the tmms (which is the time since GPS epoch) for scheduling Class-B downlinks. This field is documented here:

github.com

Lora-net/packet_forwarder/blob/master/PROTOCOL.TXT#L355


	"txpk": {...}
}
```
That object contain a RF packet to be emitted and associated metadata with the following fields: 
 Name |  Type  | Function
:----:|:------:|--------------------------------------------------------------
 imme | bool   | Send packet immediately (will ignore tmst & time)
 tmst | number | Send packet on a certain timestamp value (will ignore time)
 tmms | number | Send packet at a certain GPS time (GPS synchronization required)
 freq | number | TX central frequency in MHz (unsigned float, Hz precision)
 rfch | number | Concentrator "RF chain" used for TX (unsigned integer)
 powe | number | TX output power in dBm (unsigned integer, dBm precision)
 modu | string | Modulation identifier "LORA" or "FSK"
 datr | string | LoRa datarate identifier (eg. SF12BW500)
 datr | number | FSK datarate (unsigned, in bits per second)
 codr | string | LoRa ECC coding rate identifier
 fdev | number | FSK frequency deviation (unsigned integer, in Hz) 
 ipol | bool   | Lora modulation polarization inversion
 prea | number | RF preamble size (unsigned integer)

However, it seems that your packet-forwarder version does not consider this field forthe downlink timing.

stanvn · February 10, 2021, 4:12pm

Thanks for you’re answer. Your right, Semtech has changed to protocol a bit since version 4.0.0, while I installed version 3.1.0.

github.com

Lora-net/packet_forwarder/blob/master/readme.md#L95


### v4.0.1 - 2017-03-16 ###
* Class-B: Added xtal error correction to beacon frequency
* Class-B: Added support for all regions to beacon frame format (various
datarates imply different frame sizes), as defined by LoRaWAN v1.1.
### v4.0.0 - 2017-01-10 ###
* Added Class-B support, as defined in LoRaWAN v1.1
* Downlink only support "tmst" or "tmms" timestamp. "time" is not supported
anymore ("time" field is kept in Uplink as an informative field).
* Reworked thread_gps to handle GPS UBX messages for native GPS time.
* Updated Gateway <-> NetworkServer protocol to describe the new "tmms" field.
* Updated global_conf.PCB286*.json to remove indexes of the TX gain LUT above
20dBm. Use PCB336 (aka GW v1.5) to comply with ETSI TX mask between 20dBm and
27dBm.
### v3.1.0 - 2016-09-07 ###
* Updated "Listen-Before-Talk" JSON configuration to match with LBT rework.

The Chripstack gateway documentation for the Kerlink IoT Station tells to install version 3.1.0, maybe a good idea to update this. Or maybe state that the Kerlink IoT Station has reached it EOL and might not function as expected.

I ended up building my own gateway out of a Raspberry Pi because compiling for the Kerlink can be a bit tricky, and because the IoT Station has reached its EOL.