Calculation of cellular signal indicator in web interface

The Particle web interface of a Boron shows in the vitals section a cellular signal indicator:
image
The network section of the raw json vitals data looks like this:

"network":{
  "signal":{
    "rssi":-83,
    "strength":39.58,
    "quality":63.26,
    "qualityv":-9,
    "at":4,
    "strengthv":-83
  }
}

What algorithm is used to calculate the number of bars from this data?