Outdoors, in a vehicle, and the signal strength figure
Each carrier filing marks every covered hexagon with one of two environments and a minimum signal strength. Those three values are what the address lookup shows, so they are worth understanding before switching carriers over them.
Outdoor stationary
The model assumes a phone held in the open, at about head height, not moving. That is the easiest case a phone ever gets: no walls, no window tint, no body of a car around it. A hexagon marked outdoors only is one where the carrier expects the speed tier to hold for that phone, and does not promise it inside a vehicle.
In-vehicle mobile
The model subtracts a vehicle penetration loss, a fixed allowance for the metal and glass of a car, and assumes the phone is moving. A hexagon marked in-vehicle is covered in both environments. Because the allowance is a few decibels, the in-vehicle area is a shrunken copy of the outdoor one: the same towers, with the edge pulled in. On a county page the gap between the two columns is a fair guide to how much of a carrier's claimed area is marginal.
There is no indoor figure
The FCC does not ask for one, because buildings vary too much to model. Rules of thumb from field engineering: a wood-frame house with ordinary windows costs about 10 dB, a brick or concrete building 15 to 20 dB, metal roofing or low-emissivity window glass can cost more than that. So an outdoor signal predicted at -85 dBm can be -100 dBm at the kitchen table. Low bands (600 to 900 MHz) get through walls better than the mid-band spectrum most 5G runs on, which is one reason a 4G-only hexagon sometimes gives better indoor service than a 5G one.
Reading the signal strength
The filing gives the lowest signal the model predicts anywhere in the hexagon, in 10 dB steps: RSRP for 4G and 5G, RSSI for 3G. dBm is a logarithmic scale and negative, so -70 dBm is a strong signal and -110 dBm a faint one; every 10 dB is a tenfold change in power. The words the lookup attaches to each value follow the usual field bands:
| Predicted RSRP | Word | What it usually means |
|---|---|---|
| -80 dBm or better | strong | Full bars. Calls, video and hotspot use should all work, outdoors or in. |
| -80 to -90 dBm | good | Three or four bars outside. Indoors it usually still works, in an upstairs room or near a window. |
| -90 to -100 dBm | fair | Two or three bars outside. Walls can take it below usable; a wood-frame house is better than concrete or metal-clad. |
| -100 to -110 dBm | weak | One or two bars outside. Calls may hold; data is slow. Expect little or nothing indoors without a booster. |
| below -110 dBm | very weak | The edge of the model. A phone may register and drop. Often a different carrier covers the spot better. |
Two cautions. The figure is the minimum inside the hexagon, so most of the cell is better than it says. And it is the carrier's prediction for its own network: it says nothing about congestion, which is capacity rather than signal, and nothing about which band a given phone will land on.
Checking a number against a phone
Both major phone platforms show the live RSRP in their field-test or network-settings screens, and some carriers' apps show it too. Stand outside where the address is, let the phone settle for a minute, and compare the reading with the filed minimum. If the phone reads much worse than the filing in the open, the filing is the kind the FCC's challenge process exists for. How to file a challenge.