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Communication.md

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Communication

This document describes how the lorawan-server handles LoRaWAN communication with active network Nodes.

Uplink

LoRaWAN gateways act like one distributed antenna. When a Node sends a LoRaWAN frame (message) it gets received by one (or multiple) gateways. The gateways are very simple and just forward all received frames to the server.

Upon receiving an uplink frame the server:

  • Filters duplicate frames, which were received by multiple gateways.
  • Verifies the message integrity check and decrypts application data.
  • Compares the received Frame Counter (FCnt Up) with the previously received FCnt Up to identify retransmissions:
    • If the FCnt Up increased the respective Application Handler gets invoked.
    • If the FCnt Up is the same the device just retransmitted the previous frame. The frame gets logged in the Received Frames list, but handler is not invoked.
    • If the FCnt Up decreased an error Event is generated, unless the FCnt Up did reset and the Node FCnt Check is set to Reset on zero. Such frames are processed as if the FCnt Up increased.

Every LoRa device can transmit for a short time only, usually 0.1% or 1% depending on frequency. LoRaWAN is thus not suitable for continuous transmissions.

Duty Total TX in 1 hour Each TX Gap between TX
0.1% < 3.6s < 0.72s > 0.72s
1% < 36s < 3.6s > 1.8s

Downlink

LoRaWAN devices can be Class A, B or C.

Class A

Class A devices listen for downlink frames only for few seconds after sending an uplink frame. This is the default and most common behaviour.

Upon receiving a downlink frame for a given Node:

  • When the Node just sent an uplink frame the server sends the downlink frame immediately to the closest Gateway, which forwards the frame to the device.
  • When the Node did not send anything the frame gets queued in the server and waits for the Node to send a next uplink. The queued Downlinks can be viewed via the Node Administration.

The server uses the standard downlink frequencies and data rates. To modify the RX2 parameters you need to add a regions section into your lorawan-server/releases/<VERSION>/sys.config and modify it accordingly. The default values are:

{regions, [
    {<<"EU863-870">>, [
        % default RX2 frequency (MHz) and data rate (DRx)
        {rx2_rf, {869.525, <<"SF12BW125">>}}
    ]},
    {<<"US902-928">>, [
        {rx2_rf, {923.3, <<"SF12BW500">>}}
    ]},
    % Multitech Private Hybrid Mode
    % http://www.multitech.net/developer/software/lora/introduction-to-lora
    {<<"US902-928-PR">>, [
        {rx2_rf, {undefined, <<"SF12BW500">>}}
    ]},
    {<<"CN779-787">>, [
        {rx2_rf, {786, <<"SF12BW125">>}}
    ]},
    {<<"EU433">>, [
        {rx2_rf, {434.665, <<"SF12BW125">>}}
    ]},
    {<<"AU915-928">>, [
        {rx2_rf, {923.3, <<"SF12BW500">>}}
    ]},
    {<<"CN470-510">>, [
        {rx2_rf, {505.3, <<"SF12BW125">>}}
    ]},
    {<<"KR920-923">>, [
        {rx2_rf, {921.9, <<"SF12BW125">>}}
    ]}
]},

Class B

Currently not supported.

Class C

Class C devices listen for downlink frames constantly.

The server has no knowledge if a particular device is Class A or C. It assumes the applications (somehow) know this.

When initiating a downlink applications can set requested downlink time. This can be either a specific timestamp, or an immediately flag. See the JSON Payload documentation for more details.

Upon receiving a downlink frame with the time parameter set the server sends the frame to the gateway and relies on its scheduling mechanism to transmit the frame.

Multicast

It is possible to send one downlink frame to multiple Class C devices. First, a multicast channel needs to be defined both in the network Infrastructure and also in the device (see struct sMulticastParams in LoRaMac-node). The multicast DevAddr is a special dedicated address that must not collide with any Node address.

To send a multicast downlink the application simply initiates a downlink with:

  • DevAddr that was previously defined as a multicast channel.
  • time parameter set to a given timestamp, or to the immediately flag.