Nothing. Or, almost nothing.
For decades, we’ve been using the bar indicators on our phones to try to have some idea whether or not we have cell reception. The problem is that they’re almost meaningless. Not only is there no fixed translation between bars and real power measurements like dBm, the bars themselves vary from manufacturer to manufacturer and sometimes even from software version to software version.
As a general rule, you would think that one bar would equal about 15dB of signal, because again, just in general, there’s about a 75dB difference between the strongest signal you can get without climbing the pole. Yet, this isn’t true. I went out and read a lot of articles where people compared bar measurements to actual signal measurements, and in some cases the leftmost bar represented 10dB of signal while the rightmost bar represented 40dB. In others they’re mostly linear but still you find that the more bars you have, the more each bar represents.
Bars can’t tell you the whole story because they’re way too simple. Your phone measures receive power as well as transmit power on each of the radios it uses, and it could use two radios for voice and up to six for data. Add to that, the radios on the cell towers themselves are about 20 times more powerful so you could have a strong incoming signal while the outgoing signal might not be strong enough. There are so many variables, it’s amazing. There’s all the filtering that takes place, and less expensive phones might not have good filters so that they can’t make good use of a weak signal.
Of course the bar measurements aren’t completely useless. I personally use them as an excuse – “It can’t be me, I have 5 bars on my phone.” See, that gets me off the hook when someone else says they can’t hear me. But if I really want to know how my phone is doing, the first thing I do is go into test mode. Even though I can’t be sure which radio is being tested, turning off all cellular data is supposed to give you the strongest voice signal reading. There’s no substitute for actual dBm numbers.
But really, the real measure of whether or not you can make a phone call… is whether or not you can make a phone call. And the good news is that there are more cell towers in use now than ever before, so the chances are pretty good. If your home or office is built so that you get bad reception inside, a cell booster is going to help you get better reception, and luckily they’re available at SolidSignal.com.