fieldtester.io

Understanding the readings

RSSI, SNR, packet loss, number of gateways and distance, and how to read them together.

RSSI: how strong the signal arrives

RSSI (received signal strength indicator) is the received power in dBm. The values are negative; the closer to 0, the stronger.

limitusablegoodvery good-135-120-105-90-75-60dBmweakerstronger
RSSI Rating
above −90 dBm very good reception
−90 to −105 dBm good reception
−105 to −115 dBm usable, margin getting thin
below −115 dBm at the limit, packets get lost

On its own the RSSI says little, because it also measures interference. That is why the SNR always belongs with it.

SNR: how clearly the signal stands above the noise

SNR (signal-to-noise ratio) is the gap between signal and noise in dB. LoRa can receive signals below the noise, so negative values are normal.

below the noiseabove the noise-20-15-10-50510dBSF7SF8SF9SF10SF11SF12Lowest SNR a gateway can still decodeA packet is received as long as its SNR stays above the line of its spreading factor.More than 5 dB above the line leaves a stable reserve.
  • positive SNR: clear signal, margin available,
  • SNR around 0 to −10 dB: reception with shrinking margin,
  • SNR close to the minimum of the spreading factor: at the limit.

Rule of thumb: if the SNR is more than 5 dB above the minimum, the link is stable.

Look at uplink and downlink separately

  • Uplink values are measured by the gateway: how well does the network hear your device?
  • Downlink values are measured by the device: how well does your device hear the network?

For sensors the uplink is what matters. A gateway usually has a better antenna and sits higher, so the downlink is often better than the uplink. A field tester only shows both sides with a backend such as fieldtester.io.

Packet loss

Packet loss follows from gaps in the frame counter: if a number is missing, that packet reached no gateway. Single losses are normal. From about 10 % it is worth looking more closely at the location, antenna or spreading factor.

Number of gateways

How many gateways heard a packet. More gateways mean more resilience: if one fails, the packet arrives through another. For important sensors two or more gateways are recommended.

Distance

The distance to a gateway can only be calculated when two things are available: a GPS fix on the field tester and a publicly entered location of the gateway.