How does LoRaWAN Adaptive Data Rate work?

How do LoRaWAN nodes change their data rates? What are the commands for that? Here is the simple and detailed explanation for it.

Let's first talk about the commands for ADR (Adaptive Data Rate). 
In LoRaWAN MAC Layer there are total four different commands for ADR. 
2 of them are MAC commands.

Here are they:

> ADR 
         This is 1 bit long. Node set this bit to ask the gateway to control it's data rate.
         ADR = 1 => Network will control the data rate of the node
         ADR = 0 => Network will not control the data rate of the node no matter what the DR or transmit power is.

> ADRACKReq
         In order to validate that the network is receiving the uplink messages, nodes periodically transmit ADRACKReq message. This is 1 bit long. 

         ADRACKReq = 1 => Network should respond in ADR_ACK_DELAY time to confirm that it is receiving the uplink messages

         ADRACKReq = 0 => Otherwise

> LinkADRReq

        This is 4 bytes long MAC command transmitted by network to request node to change it's transmit parameters. 

- Data Rate and Transmitted power are regional specific and pre stored tables. 

- ChMask stand for Channel Mask. The channel which should be used for next uplink transmissions.

- ChMaskCtrl - This is used to control the interpretation of ChMask

- NbReq - Number of transmissions for all unconfirmed uplink messages. 

> LinkADRAns

       This is 1 byte long MAC command transmitted by nodes in the response of LinkADRReq command.

       If the status bit is 0 then that command is discarded or if it is 1 then it is successfully set.

Now Let's see how does it work.

       Remember Network only increase the Data Rate and Nodes only decrease the Data Rate.  

        First the end-device set the ADR bit to '1' in uplink message. Once the network detect this it starts collecting the max SNR of received signals and keep record of last 20 received signals. If the ADR becomes zero it deletes the table and collect the data again if it becomes 1.  

        After 20 received signal data it computes SNR margin. 

SNRmargin = SNRmax - requiredSNR - margin

where

           SNRmax = max. SNR of 20 received signals

           margin = 10 by default 

and  

            requiredSNR = 

Now from SNRmargin no. of steps are calculated

Nstep = round(SNRmargin/3)

> If Nstep < 0 then transmitted power is incremented in each step by 03 until max. transmitted power is reached (Tx max = 14 dBm)

> If Nstep > 0 first DR is incremented in each step until it reaches DR5 and then transmitted power is decremented until it reaches min. transmitted power (Tx min = 2 dBm)   

And after it network put all these information in LinkADRReq MAC command in next downlink message and request end-node to change it's transmitted parameters.  

         You can see network can only ask for transmitted power increment, it never ask for DR decrements.

Now let's see what happens at node side.

          When the end-device set the ADR  bit to 1 it also periodically validates that the network is receiving uplink messages. Whenever the end-device sends a frame it increments frame counter (not for repeated transmission). Whenever the frame counter increments it also increments ADR_ACK_CNT counter.  If ADR_ACK_CNT reaches to ADR_ACK_LIMIT (= 64 frames for EU) it sets the ADRACKReq bit to 1 to ask the network whether it is receiving the uplink messages or not. The network is supposed to answer (it can be any kind of downlink message) within ADR_ACK_DELAY ( = 32 frames for EU) time. 

         As soon as the node receives the downlink message it reset the ADR_ACK_CNT counter to zero.

         If the end-device doesn't receive the downlink message within the ADR_ACK_DELAY time it decrease it's data rate step by step. 

        Remember the change in Data Rate will reflect from next uplink message. Data Rates of downlink messages (receiver window 01 and 02) are always fixed and controlled by nodes. Data Rates of receiver windows are function of uplink data rates.  

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