Subject: [lp-wan] LoRaWAN profile - Retransmission timer

From: Olivier Gimenez <[email-masked]>
Date: Tue, 19 May 2020 20:53:54 +0000

Hi,

As discussed during the interim this afternoon I will change the uplink (Ack-on-Error) text to:
Retransmission timer: Set by the implementation depending on the application requirements.

For downlink fragmentation (Ack-Always) we did not agree on a solution. Context:

*         LoRaWAN class A devices opens a reception window only after a transmission, for few seconds.

*         LoRaWAN class B devices opens a reception window after a transmission + regular windows every [2;128] seconds (depending on configuration)

*         LoRaWAN class C devices are always in reception mode if not transmiting.

For class B & C we can have a retransmission timer "set depending on the application requirements"; but regarding the class A we cannot as the SCHC gateway will never know when the device will open a reception window.
Three options:

1.       Queue a downlink when the retransmission tiler timer expires. Next time it expires, clear the previous downlink, queue an new one, repeat up to MAX_ACK_REQ or until the inactivity timer expires, then clean the queue

a.       Pro: Respect the usual layer separation architecture

b.       Con: Not all LoRaWAN network servers allows to select the element to be deleted in the queue

2.       Not use retransmission timer: If the device sends an uplink the SCHC gateway will be able to send a SCHC ACK REQ that the device should receive during its reception window. The state machine can go out of this "waiting for uplink" state thanks to the inactivity timer

a.       Pro: It will work with all network servers, SCHC gateway does not have to manage the queue.

b.       Con: The SCHC gateway have to know if the device is a Class A or Class B/C device

Personally I would go to option 2, what is your opinion ?

Thank you
Olivier

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