Maximum Packet size for downlink packets in Class A mode.

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  • November 23, 2020 at 11:57 am #31375
    Ajay K
    Participant

    I have asked this question before and I have never been able to figure out what is the largest packet size one can transmit from the conduit to the end device (mDot) in our case. In the mDot you have an API which tells you the packet size you can transmit based on the current data rate, however with the Conduit how do you determine that for a class A scenario?

    Let’s say we use the maximum packet size of 242 bytes and downlink the packet to the downlink queue on the conduit, does the conduit automatically figure out how much to transfer per downlink transmission to the end device based on the data rate it is maintaining with the end device or should we curtail the packet size to certain number? Btw we have ACK & ADR enabled for the end devices.

    Also fyi we are on firmware version 5.2.1

    Thanks,
    Ajay

    November 23, 2020 at 12:07 pm #31376
    Jason Reiss
    Keymaster

    With ADR enabled it may vary. On the Conduit when a packet is queued for Class A it does not know what the datarate of the next uplink will be.

    In US915 53 bytes is the allowed packet length for downlink in Rx1 when DR0 is used in the uplink.

    Rx2 can be set to DR10 for most use cases and 242 bytes of application payload are allowed. The GW can transmit at a higher power than end-devices so it can reach end-devices easier with higher datarates. Performance may vary per environment of deployment.

    The size will depend on the downlink datarates configured for Rx1 and Rx2. Rx1 is dependent on the uplink DR used and the Rx1Offset setting.

    https://lora-alliance.org/resource-hub/rp2-102-lorawan-regional-parameters

    The N column on page 34 shows the maximum size for each datarate.

    November 23, 2020 at 1:41 pm #31377
    Ajay K
    Participant

    Thanks so much for taking the time to respond. Just was wondering what happens from a conduits perspective if we queue a payload size in the downlink queue greater than the maximum permissible payload size for a given data rate?

    Thanks,
    Ajay

    November 23, 2020 at 1:48 pm #31378
    Jason Reiss
    Keymaster

    The packet will wait in the queue until there is a chance to send in a packet at the required datarate.

    There is no protocol to allow automatic fragmentation. The application layer could implement a protocol. Still the lowest size is easiest to handle. Or adjusting the Rx2 datarate so max is always available. Still there occasional MAC commands for ADR to consider.

    November 23, 2020 at 2:58 pm #31379
    Ajay K
    Participant

    Awesome thanks for the clarification.

    Thanks,
    Ajay

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