Spreading factor of join accept message with data rate offset

ChirpStack Network Server
1

Quick question about the expected spreading factor for a join accept message. The LoRa Server is configured for a data rate offset of 3, but when a join request with SF10 (DR0) is received, the server replies with a join accept with SF10 (DR10). From the regional parameters v1.1a, if the device emits an upstream packet using SF10 (DR0) and the data rate offset is set to 3, the downstream packet should use SF12 (DR8).

The data rate offset works as expected for normal tx and rx pairs, but seems to be overridden for join request/join accept messages. Maybe there is something in the specification I am missing?

I’ve copied here the relevant sections of the 1.0.2 spec and 1.1a regional parameters as well as examples of the issue as shown in the raw frame logs. EDIT: new users aren’t allowed to upload more than one image, so I’ll have to wait until a moderator unlocks this

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Screen Shot 2018-01-03 at 3.35.35 PM.png1414×354 99.3 KB

2

Does your end-device receive join accept message?

4

The DR offset does not apply to the join-accept message as this would not be possible. The device only knows the DR offset after an OTAA activation as the DR offset information is sent to the device as part of the join-accept message.

5

Thank you for the clarification, perhaps my misunderstanding came from the data sheet and command reference for the Microchip LoRa module. I thought that when listening to a specific frequency, a node radio could demodulate a plurality of spreading factors. It seems strange then that the module does not provide any commands to set the DR offset for use in an ABP scenario where the offset is not exchanged in the OTAA accept message.