Maximum Packet transmit size for Downlink Packets.

[Ajay K]

I am not sure if this question has been asked in the forum before, I couldn’t find one that was relevant.

Inn the mdot today before I can transmit, I can call an API getMaxPacketLength() to determine the maximum packet size I can transmit given the SF/DR. However when It comes to the conduit, how to do I determine what is the max packet size I can transmit in a down link scenario. Also please note I have ADR enabled for my transmissions from the mdot.

Thanks,
Ajay

[Peter Ferland]

There is no function that can tell you this on the downlink side.
The maximum packet size is based on the datarate of the upstream packet, whether or not a DROffset is set and which receive window you use. The details also vary by region and the tables can be found in the official LoRaWAN regional parameters document. In version 1.0 of that document the table is in section 2.1.7 on page 10 for EU 868 and section 2.2.7 on page 15 for US915.

If you are not using a DROffset:
EU868: RX1 is transmitted using DR0, RX2 is configurable, defaults to DR0 (DR0 = 11 bytes max payload)
US915: RX1 is transmitted using:
|Upstream|Downstream|
|DR0|DR10|
|DR1|DR9|
|DR2|DR8|
|DR3|DR8|

Max payload sizes:
|DateRate| Payload size (bytes)|
|DR8 | 33|
|DR9 | 109|
|DR10| 222|

Note that the gateway cannot listen to incoming messages while transmitting downstream so even if larger payloads are possible they may not be advisable.

[Ajay K]

Thanks Peter for taking the time to respond.

I am using Conduit AEP and the node-red LORA output node to transmit. I don’t think I have the ability to configure “DROffset and which receive window to use”, if my understanding of your response is correct. Also the conduit is being used in the US Market. Most of the times I see the SF/DR for my upstream packets depending on where it is situated in the field has been either SF10 or SF8BW500 and plus i have ADR enabled on the mdot.

But based on your comments although the LORA input does tell me what SF/DR the upstream packet was delivered, I am guessing that can not be used reliably to determine the packet size for future transmissions or I think it would be hard. So my best bet is to set the least of the packet size of 11 bytes?

Thanks,
Ajay

[Peter Ferland]

DROffset is set in the Lora Network server configuration page on the Conduit’s config webpage. The offset will not change unless you change it. So in the US you have at least a downlink payload size of 33 bytes. In Europe that indeed is only 11 bytes. So if you want to send the same packets in all situations in all locations 11 bytes is the minimum size, just like for uplink.

[Ajay K]

Thanks Peter that is good know. I am guessing if we can use the 33 bytes, I would definitely want to use it. Thanks once again for your inputs.

Thanks,
Ajay

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