I was watching a video today and noticed that on T-Mobile (USA) using a Samsung Galaxy S4, when you have -109dbm LTE or better, it shows as full signal (4 bars), when it drops to -110dbm it shows 3 bars.

See the video at about 6:15 :
https://www.youtube.com/watch?v=_9pDB1kGmLY

I have the same phone, and in the UK you get 1 bar of 4G when you are at -104dbm or worse, and 2 bars when you are -103dbm or better. That T-Mobile phone shows 3 bars at -110dbm, but my Galaxy S4 shows 0 bars at -116dbm and promptly falls back to 3G, sometimes I will see it hang on until around -120dbm but it almost always drops before that.

So with T-Mobile, what's going on? Do they want customers to believe they have a better signal than they really do, or does their network actually let the signal get worse than ours without falling back to 3G?

I've found -120dbm on LTE to be plenty usable, maybe UK networks are falling back to 3G much quicker than any US LTE network would.

Its possible - or they're trying to show the same number of bars as Verizon, who really want their customers on 4G LTE and not dropping back to the pain of CDMA 3G (fine for voice, but awful for data).

T-Mobile and AT&T have our style 3G, but Verizon and Sprint have CDMA, which generally gets data speeds of 700kbps. (yes, 0.7mbps) in my sisters experiences. You can see why Verizon has rolled out 4G LTE very fast!

Is Verizon's rollout complete yet? I know they claim the biggest network in the USA.

I think they're claiming parity with their 3G but not necessarily their 2G (equivalent) yet; but its any day now. In most areas they're working on increasing capacity as the original rollout was quite thin. VoLTE was enabled about the time of the iPhone 6 launch, and I suspect they will want to start turning off CDMA in a year or so's time.

I'd see them turning off their EVDO before 1x to refarm the spectrum but their XLTE network has had some good results where it's been deployed.

It's got the issue Three might be facing soon of lots people grandfathered at unlimited data still but you get pretty good speeds I believe.

The network's quite costly though off the top of my head and they love their bloatware.

Edit: Wait my bad. Just checked and 1x and EVDO run off the same band so won't work. See what you mean now about turning off the whole of CDMA. They're planning to reallocate their 1900mhz though I see.

My nexus 6 recently updated to 5.1 and it had a similar effect to what you see in the video.

-98 dBm or higher with a SNR of around 13-14 or higher is displayed as full bars

-98 to -108 dBm with a SNR of around 13-14 is 3 bars

-108 to -118 dBm (not sure on the SNR) is 2 bars.

Any less then -118 dBm is 1 bar.

Your correct at around -120 dBm the handset drops back to 3G.

If you can force your handset to 4G only you will find its still usable below -120 dBm. I've had usable data at -128 dBm but speeds suffer massively. Not sure of the exact figure but I think a handset will still connect to a 4G cell as low as -135 dBm.

I think this closer matches the signal bars you recieve on 3G/2G, for example, full bars is -89 dBm or higher. -20 dBm to compare to 4G and your at -99 dBm which is just under the full signal threshold for 4G (assuming a good SNR).

I remember when I had a HTC One M7 which was totally inaccurate for bars on 4G. It was impossible to get it to display full bars even standing next to a mast. Most of the time it sat on 1 or 2 bars.

I know for a fact Verizon has refarmed 20MHz in New York from there 3G service to there 4G network.

Just done some research and these American networks are strange! AT&T using 6 different bands for LTE. Perhaps it's just me used to one band on Three but that seems an awful lot.

I do think AT&T made the right call by announcing very early when they were turning their 2g network off (2017), gives plenty of time to switch even for M2M and it allows that spectrum to be freed.

It's because of how spectrum is licensed in america sometimes they will have 1900MHz in one area but will only have 1700/2100 in another. So they need a lot of LTE bands so they can cover the US.

They had no choice as I understand it. In a lot of big cities they only had 1900mhz spectrum and were using it for both 2G and 3G. Turning off 2G means they can support more 3G calls and connections. This is where LTE is helping.

When reading US stuff don't forget they call high speed 3G (UMTS) by the term 4G on the AT&T and T-mobile networks. :-/