This article is about the functionality of the LoRaWAN UDP Packet Forwarder. It is software developed by Semtech for the LoRaWAN gateway, enabling efficient data packet exchange with the network server using the UDP protocol. With a special example code in C++, which functions as a UDP packet listener on port 1700 to receive data from a gateway, received data packets can be displayed. This example allows for a closer analysis of the precise workings of the Semtech Packet-Forwarder, providing a deep insight into the world of LoRaWAN.

The Gateway

For this example, a MikroTik LR8 LoRaWAN Gateway is used. The MikroTik LR8 comes with the UDP Packet Forwarder pre-installed, which serves as the interface between the gateway and the LoRaWAN network server. The LoRaWAN gateway operates bidirectionally, meaning it both receives data from the end device and sends it to the LoRaWAN network server via the Packet Forwarder, and vice versa. The network server can communicate with the gateway to send data packets to its end devices through the gateway. In this example, a unique, easily recognizable ID (Gateway EUI) is assigned to the LoRaWAN gateway: AA AA AA AA AA AA AA FF. This ID is immediately identifiable in raw data analysis, making it easy to trace.

Raw Data Analysis

If a simple UDP listener is started on port 1700 on a computer or server, and the IP address is entered in the gateway, the listener typically receives a data packet multiple times per minute. When these data are displayed in plaintext, some "cryptic" characters initially appear, followed by readable information in JSON format. The readable JSON data contains information about the gateway’s status or represents a received LoRaWAN packet. However, first, attention is given to the "cryptic" characters, as seen in the following image:

The unreadable characters here always have a specific length of 12 bytes. When converted to hexadecimal, the previously assigned Gateway EUI and additional hexadecimal characters become visible:

Bytes 0 to Byte 11 are always transmitted before each JSON message, and sometimes packets are sent consisting only of these 12 bytes. The Gateway EUI consists of 8 bytes (Bytes 4 - 11), and the bytes are composed as follows:

This example uses protocol version 2. Each message from a gateway contains a random token consisting of two bytes. When the network server responds to a message from the gateway, these exact two bytes are sent back, allowing the gateway to match the message. This is mostly used for acknowledgments of receipt.

The 5 message types (identifiers) are as follows:

JSON from Byte 12

The first 12 bytes in hexadecimal format always precede each JSON message, whether it is sent from the gateway to the network server or vice versa. Further details on the structure of JSON messages can be found in the next article: XXXX