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DollarDollar (Semtech)  
From the definition of source code, the MAX_RX_WINDOW is 3000ms, it means that if detected a preamble during Rx1 window or Rx2 window, the max receiving duration is 3000ms? If the data packet exceed 3000ms, then the data can't be received? That's right? 
But, after tx completed, we set the Rx1 windower timer and Rx2 window timer simultaneously. So, if end-device detected the preamble during the Rx1 window and start to receive data, but the data packet duration over 1s. Then, after 1s from the beginning of RECEIVE_DELAY1, the receiving status will be abort and enter the Rx2 window timer event. So, even if end-device has been detected the preamble during Rx1 window, but as the data packet duration over 1s, and the RECEIVE_DELAY2 is 1s later after RECEIVE_DELAY1. Thus, end-device can't receive data packet correctly during the first receive window. 
So, if the end-device wants to receive data completely during Rx1 window, the time on air should not be exceed 1s? That's right?
But if time on air not exceed 3s, it's will be ok in the Rx2 window, that's right?
Best Answer chosen by Dollar (Semtech) 
GregoryGregory (Semtech Corporation) 
Your question is in two parts:

"it means that if detected a preamble during Rx1 window or Rx2 window, the max receiving duration is 3000ms"

Yes, this is correct. In the LoRaWAN stack, the packet time will never exceed 3 seconds in TX or Rx. 
On the node side, this is ensured by the function "ValidatePayloadLength (...)" which verify the amount of data to be transmitted compared to the data rate. 
If the frame is longer than the predefined length, the LORAMAC_STATUS_LENGTH_ERROR status will be returned.
On the GW side, the same kind of mechanism is used so that the time on air of any given packet over the network will never attain the 3 seconds.

"So, if the end-device wants to receive data completely during Rx1 window, the time on air should not be exceed 1s"

No, the limitation is at 3 seconds. 
After a Tx, the timer event "OnRxWindow1TimerEvent( )" will occur precisely after 1 second and the radio will go into Rx mode.
After 2 seconds, the timer event "OnRxWindow2TimerEvent( )" will occur and will lead to the configuration of the radio for the second reception window in the function "RxWindowSetup()".
However, before changing the radio, the code checks the radio status with the call to Radio.GetStatus( ) 
At this stage, or the radio is in "RF_IDLE" state and the code will configure the radio and go into Rx mode. 
Or the radio is in another state (meaning the radio is already busy) and the code will simply drop the command until the next irq from the radio.
DollarDollar (Semtech)  
For Class C device, we set RxWindowTimer1 of RxWindow1Delay after Tx done. If RxWindow1Delay was expired, then jump to OnRxWindow1TimerEvent() to set Class C device as
singl receive mode for time of symbTimeout to detect preamble in Rx1 window. We assume that the Class C detected the preamble in Rx1 window, but the data packet time
on air over 3s. After 3s, then jump to the SX1276OnTimeoutIrq() in SX1276.c, and call RadioEvents->TxTimeout( ), as it's the function of OnRadioRxTimeout() in
LoRaMac.c. Our original idea is that to call OnRxWindow2TimerEvent() to set Class C device as Rx2 status for continuous receive mode. But, in OnRadioRxTimeout(), we
didn't set the SX127x RF status as RF_IDLE when receive timeout. So, when we call OnRxWindow2TimerEvent(), we can't set the SX127x as continuous receive mode. Thus, we
should watiting for receiveing complete no matter how long the data packet duration is. Then go into SX1276OnDio0Irq() in SX1276.c to set the SX127x RF status as
RF_IDLE, and reset the Class C device as Rx2 continuous receive mode by calling RadioEvents->RxError() or RadioEvents->RxDone(). 
Best Answer chosen by Dollar (Semtech) 
GregoryGregory (Semtech Corporation) 
In the LoRaWAN stack, the packet time will never exceed 3 seconds in TX or Rx. 
On the node side, this is ensured by the function "ValidatePayloadLength (...)" which verify the amount of data to be transmitted compared to the data rate. 
If the frame is longer than the predefined length, the LORAMAC_STATUS_LENGTH_ERROR status will be returned.
On the GW side, the same kind of mechanism is used so that the time on air of any given packet over the network will never attain the 3 seconds.
DollarDollar (Semtech)  
For Join the network according to OTAA process, if the end-device send the Join_Request but tx timeout. Then will call OnMacStateCheckTimerEvent() to set the DeviceState as DEVICE_STATE_SLEEP to stop transmit cycle due to tx timeout. Then the end-device will go to DEVICE_STATE_SLEEP in main function. 
My question is: How to start to the second attempt to join the network when above scenario occurred?
Best Answer chosen by Dollar (Semtech) 
GregoryGregory (Semtech Corporation) 
The Tx Timeout is really a legacy verification from the early days of the LoRaWAN stack.
Actually, you should never have this irq unless you have a strong HW issue. The status of the radio is continuously monitored so that this event cannot happen.
Andre SchwarzmeierAndre Schwarzmeier 

Hello!

I would like to know, if here is any integrity check for downlink messages foreseen in the LoRaWAN?
Refering to the LoRaWAN Specification 1.0.2. in section 4 decribes, that the CRC for integrity check is only available for uplink messages. This would mean, that a corrupted payload, e.g., caused by collisions or other effectes of the channel, might never be detected if there is no further integrety check on application level, right?Hello!

I would like to know, if here is any integrity check for downlink messages foreseen in the LoRaWAN?

Referring to the LoRaWAN Specification 1.0.2 in section 4 describes, that the CRC for integrity check is only available for uplink messages. This would mean, that a corrupted payload, e.g., caused by collisions or other effects of the channel, might never be detected if there is no further integrity check on application level, right?
I hope, I did not misinterpret the standard and someone can help me to shed a lot of light on :-)

Thanks!
 

Best Answer chosen by Andre Schwarzmeier
GregoryGregory (Semtech Corporation) 
Hello Andre,
You are correct, in the specifications, the CRC is only applied to the uplink messages. This was made so that the GW can discard packets straight away and not forward "known bad packet" to the server. This way, we are sure that the GW only forward valid packets to the server, the server will then check the MIC to validate the content of the payload.
On the node side, the integrity of the messge is only checked with the MIC but this is sufficient to discard errors on downlink messages and adding the CRC would be redundant.

 
Xuan Dung LEXuan Dung LE 
Hello everyone,
I read the LoRaWAN specification version 1.0.2 that can be found in LoRa Alliance website.
I found out that the application payload for uplink message can reach up to 242 bytes
Unfortunately, I could not find out the maximum size of application payload for downlink message.

I saw that the reciever window size is smaller than the transmitted package. So does this means that the downlink payload size is smaller than uplink one? 
Could you give me more information about the size of downlink application message, please.?
Another question: I found out that it can be possible to communicate between sx127x. Could anyone tell me how many end-devices with sx127x chipsets can communicate together? that means, if I create a base station by sx127x, how many end-devices can communicate with the base station?
Thank you
Best Answer chosen by Xuan Dung LE
GregoryGregory (Semtech Corporation) 
Hello,
One by one:
>>I could not find out the maximum size of application payload for downlink message...So does this means that the downlink payload size is smaller than uplink one?
Maximum payload size is depending of the datarate and the size limitations are identical on the Tx and Rx side. The receive windows are actually only used to detect the preamble and one a LoRa preamble is detected, the window will remain open until the reception is fully done.
>>Could anyone tell me how many end-devices with sx127x chipsets can communicate together?
Two devices can communicate in a point to point way or you can create a small GW with one sx127x but you will not be able to create a LoRaWAN base station with the sx127x. The sx127x is a one frequency, one datarate device while the GW is able to cope with several channels and all datarate at the same time.
Vyacheslav KhudyakovVyacheslav Khudyakov 
Hello, I have a question about using real-time data transmission in LoRaWAN. LORIOT company have shown data feed visualization in real-time (in their Youtube channel). Many experts prove that for LoRaWAN it is impossible. If it is possible, what is the maximum number of nodes can work in real-time per 1 gateway. I hope to get the answer. 
Best Answer chosen by Vyacheslav Khudyakov
GregoryGregory (Semtech Corporation) 
Hello Vyacheslav,
I would be glad to know who are these experts who "proved" LoRaWAN is impossible :-) There is no magic at play here, only technology and maths!

Lets go back to basics:
Each GW has at least one sx1301, the sx1301 is a huge digital baseband chip which emulates 49 LoRa demodulator and 1 FSK demodulator, all running in parallel.
The sx1301 has 8 channels and it is continuously scanning for the presence of preambles of any datarate on any of these 8 channels. Due to the orthogonalities of LoRa signals between them, the sx1301 can demodulate 8 LoRa packets at different datarate (DR0, DR1, ...DR7), at the same time, and on the same channel (providing you have a sufficient link margin between each packet). If you multiplex these parameters over time and the 8 channels, the number of nodes per GW can become staggering, even more as the network densify and the time on air of each packet decrease (due to ADR).

Everybody wants a definite answer as to how many nodes a GW can support but the answer is 4 dimensional:
- rssi/SNR of the received packets (simultaneous reception on the same channel)
- time on air of the packets (equivalent to datarate, the longer the packet, the longer one demodulator of the GW is used)
- frequency of the packets (two packets with the same datarate and the same RSSI/SNR will collide unless they are on 2 different frequencies).
- number of times per day a node will send a packet (taking resources another node could take)

The answer is thus incredibly complex to calculate because these 4 parameters all have an impact one on another. If you add the legislations which are different for each region (Duty Cycle, LBT,... ), the calcul becomes even more difficult to compute. So in the end you will be disappointed, there is no definite answer and even simulations can go from figures to figures depending on the input parameters. You can of course set your input parameters for your specific case and you will have a certain number but this number won't necessarily be representative of a real life deployment.

PS: to add some fun into the equation, LoRaWAN is all about country wide network deployment. At this level, the geographical distribution of the GWs over the territories also have an impact on the number of nodes the network can support :-)
 
Krzysztof ChojnowskiKrzysztof Chojnowski 
According to the datasheet the PpmOffset is calculated as PpmOffset = 0.95 * measured Offset [PPM]. Which frequency should be taken into account to get the measured Offset [PPM]: the carrier, the bandwidth or the external crystal?
Best Answer chosen by Krzysztof Chojnowski
SebastienSebastien (Semtech Corporation) 
Dear Kryzsztof,

The offset in ppm can be computed from either the carrier frequency, or the reference (XTAL oscillator) frequency. This would give you the same results.

Best,
Krzysztof ChojnowskiKrzysztof Chojnowski 
I observed that payload CRC error occures quite often when the two modems are very close to each other, especially with SF = 2048. What can be the reason of this behaviour? Please advise where to start debugging of this issue.
Best Answer chosen by Krzysztof Chojnowski
SebastienSebastien (Semtech Corporation) 
Dear Krzysztof,

Could you confirm that the CRC errors disappear when the modems are separated by a longer distance? This could be related to saturation of the receiver, although we have an internal AGC which takes care of saturation. It may fail for extremely large coupled powers, exceeding the max ratings of the chip. Also, do you see a difference when the "LowDataRateOptimize" is set?

Best,
Harry GoodmanHarry Goodman 
In our experiments, when CAD is performed with RxTimeout set to 40 symbols or more, PER may exceed 1%.
If we repeat reception without executing CAD, PER is less than 0.3% regardless of the setting value of RxTimeout.
  
 Does PER become worse when CAD is implemented?
 And also, we set the RxTimeout setting value to less than 40. Is this set value appropriate?

Please advice.
 
Best Answer chosen by Harry Goodman
GregoryGregory (Semtech Corporation) 
Hello Hiroshi,
Yes, CAD has around 3 dB less sensitivity than the standard RX and it is also prone to false detection.
jet sujet su 
Hi,

I have  some questions about CAD model of SX1278 .I am testing the reliability and usability of CAD in RF shielded room.when CADDone completed, then entering the CAD model, Continue to repeat .
In the shield room,I can confirm that there is no other external Lora signal, but the RF still able to produced CADDetected.
According to the datasheet, "Once the calculation is finished the modem generates the CadDone interrupt. If the correlation was successful,CadDetected is generated simultaneously.",  I used 
the counter to record CadDone and CadDetected's values, At the end of each test, the ratio of CadDetected to CadDone was found to vary from 1/100 to 1/10000.
Is there a problem with my configuration or test method?
The following is part of my CAD test code.


       irqflagMask = RFLR_IRQFLAGS_RXTIMEOUT |
                            RFLR_IRQFLAGS_RXDONE |
                            RFLR_IRQFLAGS_PAYLOADCRCERROR |
                            RFLR_IRQFLAGS_VALIDHEADER |
                            RFLR_IRQFLAGS_TXDONE |
                           // RFLR_IRQFLAGS_CADDONE |
                            RFLR_IRQFLAGS_FHSSCHANGEDCHANNEL ;//|
                          //  RFLR_IRQFLAGS_CADDETECTED;
        SX1278Write( REG_LR_IRQFLAGSMASK, irqflagMask);

    while(1)
    {
        SX1278LoRaSetOpMode(RFLR_OPMODE_CAD);  /*op mode*/

        SX1278Write(REG_LR_IRQFLAGS, 0xFF);    /*clear irqFlag*/

        while(1)
        {
            SX1278Read(REG_LR_IRQFLAGS,&irqFlg);
            if((irqFlg & RFLR_IRQFLAGS_CADDONE) ==               RFLR_IRQFLAGS_CADDONE)
            {
            cadDoneCnt ++ ;
            SX1278Read(REG_LR_IRQFLAGS,&irqFlg);
            break;
          }
          else
          {
            readCnt ++ ;
          }
        }
        
        if((irqFlg & RFLR_IRQFLAGS_CADDETECTED) == RFLR_IRQFLAGS_CADDETECTED)
        {
            cadDetectedCnt ++;
        }
    }


Thanks!
Best Answer chosen by jet su
GregoryGregory (Semtech Corporation) 
Hello,
This is an expected behavior as the CAD is over sensitive and detecting LoRa signal in the noise while there isn't any. Against this, we usually recommend to perform a dual CAD by doing a second CAD is the first was successful. This way, the number of false CAD detection should be near 0.