Varying Downlink Packet size, based on the "datr" attribute of an Uplink Pkt?

[Ajay K]

I am curious to understand the implication of the “datr” attribute that is part of an uplink packet received on the Conduit as shown in the example below. When the “datr” attribute has a value of for example in the case below “SF7BW125“, does it mean the MDot is operating at that data rate when connected to the Conduit, in which case is it safe to assume the Conduit can downlink packets of size larger than the ones at lower data rate? I am guessing this applies for whichever Mode of LORA, Class A or Class C?

{“tmst”:1680476803,”time”:”2021-06-02T16:48:05.981780Z”,”chan”:4,”rfch”:1,”freq”:907.9,”stat”:1,”modu”:”LORA”,“datr”:”SF7BW125″,”codr”:”4/5″,”lsnr”:10,”rssi”:-27,”opts”:””,”size”:2,”fcnt”:84,”cls”:0,”port”:1,”mhdr”:”80308eb201805400″,”data”:”dBc=”,”appeui”:”83-b3-90-3a-72-37-c3-1f”,”deveui”:”00-80-00-00-00-01-6b-b2″,”devaddr”:”01b28e30″,”ack”:false,”adr”:true,”gweui”:”00-80-00-00-a0-00-07-7e”,”seqn”:84}

Thanks,
Ajay

[Jason Reiss]

How much data are you looking to send in the downlink?
In US915 the uplink and downlink datarates and limits are different.

The Rx1 and Rx2 datarates may be different. The Rx2 datarate is fixed and Rx1 can vary based on the uplink datarate and Rx1 Offset.

Class C will use the Rx2 datarate.

Maximum packet sizes for uplink and downlink are detailed in the regional parameters document.
https://lora-alliance.org/resource_hub/rp2-1-0-3-lorawan-regional-parameters/

53 bytes is the maximum payload at the lowest downlink datarate DR8 (SF12BW500) This can only be used if the Rx2 datarate is set to DR8 or the Rx1DROffset is increased above default of 0.

With default settings Rx1 will be at least DR10 (SF10BW500) and can carry up to 242 bytes.

The network may need to send MAC commands for ADR so allowing some bytes below maximum can help with network messages. An ADR command is 5 bytes.

[Ajay K]

Thanks Jason for the detailed explanation. Actually we were looking to send a maximum of 200 bytes in a single downlink packet in class C Mode. Since you mentioned Class C Mode uses RX2 data rate, what would be the Ideal setting in the conduit to achieve this data length and reliability of transmitting a packet of this size , even with MDOT’s are further away in excess of say 1-1.5 miles?

Also we only switch to Class C mode when we are doing large configuration changes to a particular MDOT. So it would be in Class C Mode for not more than a few minutes and it will switch back to Class A. We could use the Lorawan->Operations->Messaging mechanism to transmit as well, however we want this more automated and so we are looking to implement this without using the Operations screen.

Thanks,
Ajay

[Jason Reiss]

Theoretically if a device can join using DR0 (SF10BW125) using 20 dBm then the gateway will be able to reach the same distance using DR10 (SF10BW500) using 27 dBm

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