How much frame count deviation is allowed?

jeroen2 · March 16, 2017, 8:46am

When using ABP the LoRa node stores the FC locally. Now one of the things I would like to find out is how much deviation is allowed between the FC which the LoRa nodes sends out and the FC stored on the backend server.

In normal situations there is no deviation however due to local radio outage or other circumstances there might be a deviation. How will the TTN network react and how should it according to the LoRa standards?

BoRRoZ · March 16, 2017, 12:09pm

according to LoRaWAN specs
https://www.lora-alliance.org/portals/0/specs/LoRaWAN%20Specification%201R0.pdf#page=19

jeroen2 · March 16, 2017, 12:41pm

Thanks. Re-read that sub section and extracted the following:

4.3.1.5 Frame counter (FCnt)
…
At the receiver side, the corresponding counter is kept in sync with the value received provided the value received has incremented compared to the current counter value and is less than the value specified by MAX_FCNT_GAP¹ after considering counter rollovers.

¹ Actual value for MAX_FCNT_GAP, RECEIVE_DELAY1 and RECEIVE_DELAY2 can be found at 7.1.7 for EU863 - 870 or 7.2.7 for US902 - 928

7.1.8 EU863-870 Default Settings
…
MAX_FCNT_GAP 16384

7.2.8 US902-928 Default Settings
…
MAX_FCNT_GAP 16384

So does this also apply to TTN. Can we have a MAX_FCNT_GAP of 16384? Can somebody from TTN confirm?

Cheers!

jmarcelino · March 16, 2017, 12:53pm

Did you know you can just look that up in the TTN source code?

github.com

TheThingsArchive/ttn/blob/62557cc2a2e0434ab21c6abb88d8e551d3811c33/core/broker/uplink.go#L26


      
          	"github.com/TheThingsNetwork/ttn/api/fields"
          	"github.com/TheThingsNetwork/ttn/api/networkserver"
          	pb_lorawan "github.com/TheThingsNetwork/ttn/api/protocol/lorawan"
          	"github.com/TheThingsNetwork/ttn/api/trace"
          	"github.com/TheThingsNetwork/ttn/core/types"
          	"github.com/TheThingsNetwork/ttn/utils/errors"
          	"github.com/TheThingsNetwork/ttn/utils/fcnt"
          	"github.com/brocaar/lorawan"
          )
          
          const maxFCntGap = 16384
          
          func (b *broker) HandleUplink(uplink *pb.UplinkMessage) (err error) {
          	ctx := b.Ctx.WithFields(fields.Get(uplink))
          	start := time.Now()
          	deduplicatedUplink := new(pb.DeduplicatedUplinkMessage)
          	deduplicatedUplink.ServerTime = start.UnixNano()
          	defer func() {
          		if err != nil {
          			if deduplicatedUplink != nil {
          				deduplicatedUplink.Trace = deduplicatedUplink.Trace.WithEvent(trace.DropEvent, "reason", err)

You can also change it if running a private server.

jeroen2 · March 16, 2017, 1:26pm

Did not cross my mind to check the source code … but I guess that is the easiest