The UDP Packet Forwarder developed by Semtech on a LoRa or LoRaWAN gateway is responsible for bidirectional data transfer between the gateway and the network server. This article describes the JSON objects appended to a message starting from the 12th byte. Further details about the first 12 bytes of each message are explained in this article.

Which JSON objects are there?

For the uplink (transmission from the gateway to the network server), two JSON objects are used: STAT and RXPK. STAT transmits the status of the gateway (location, various counters, and timestamp), while RXPK represents a data packet received by the gateway, including the packets data content and information about the packet itself (size, signal strength, modulation, etc.).
For the downlink (transmission from the network server to the gateway), two JSON objects are used: TXPK and TXPK_ACK. TXPK is used to transmit data packets to the gateway, which are to be sent via radio. If the downlink is accepted by the gateway or can be transmitted, a 12-byte TX_ACK response without a JSON object is sent. In the event of an error, the TXPK_ACK object is appended to the preceding 12 bytes and returned to the network server.

The JSON objects in detail

The following tables describe the individual keys of each JSON object.

STAT (Cyclic status messages from the gateway to the network server):

{
    "stat": {
        "ackr": 0.000000,
        "dwnb": 0,
        "rxfw": 0,
        "rxnb": 0,
        "rxok": 0,
        "time": "2024-11-15 10:45:54 GMT",
        "txnb": 0
    }
}

RXPK (Received data packet or multiple data packets in an array):

{
    "rxpk": [{
        "chan": 0,
        "codr": "4/5",
        "data": "QN3Mu6qATgEBddf3CGO3W+c=",
        "datr": "SF7BW125",
        "freq": 868.100000,
        "lsnr": 9.750000,
        "modu": "LORA",
        "rfch": 1,
        "rssi": -32,
        "size": 26,
        "stat": 1,
        "time": "2024-11-15T10:47:43.674536Z",
        "tmst": 2905060155
    }]
}

TXPK (Downlink: Sending a telegram via a gateway)

{"txpk":{"imme":true,"freq":868.500,"rfch":1,"powe":14,"modu":"LORA","datr":"SF11BW125","codr":"4/5","ipol":false,"size":24,"data":"3UBCTIB9FOa+LyVdGkt63237S2p4CEX/"}}

TXPK_ACK (Antwort vom Gateway zum Netzwerkserver auf ein PULL-RESP):

{"txpk_ack":{"error":"TX_FREQ"}}

Mit dem Key "error" können folgende Fehlermeldungen Übermittelt werden: