Field Test Mode: db Signal Strength vs Bars

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Everyone knows that more bars is a good thing - but very few people realize that different phones and operating systems calculate how many signal bars to display very differently.

This means that comparing "bars", unless you are on the same phone and same carrier, is actually a very poor way to compare coverage and signal quality between different devices.

The bars your phone or hotspot is displaying sometimes do not even directly correspond to the actual signal strength - in addition to raw signal strength, the phone may be measuring network congestion and other variables to calculate how many bars to display.

This can make it harder to measure the impact a cellular booster is having - boosting the signal strength might not register as "more bars" if the phone is focusing on network congestion.

In general - the iPhone puts more weight on network congestion when calculating how many bars (or - "dots" as of iOS 7) to display, and Android focuses more on raw signal strength.


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Understanding Raw Signal Strength

To get a sense of the real signal strength being received over the air (and thus the impact a booster is having) - it is a good idea to learn how to look up the raw received signal strength on your mobile devices, which is measured in decibels (dB), instead of relying on counting bars.

The decibel is a logarithmic scale - every change by 10 represents a 10x change in received signal power. Wireless signal strength will be recorded as a negative number, and the numerically lower the number, the weaker the actual signal.

  • -50dBm would be considered an awesome signal.
  • -60dBm is 10x weaker, but still great.
  • -70dBm is 100x weaker.
  • -80dBm is 1,000x weaker.
  • -90dBm is 10,000 weaker.
  • -100dBm is 100,000x weaker - and is when you are likely to start seeing a serious impact.
  • -110dBm is a million times weaker than -50dBm, and is usually barely usable.
  • And by the time you would see -120dBm the device has probably already given up and switched to "No Service".

signal_strength_chart

For a great video demonstration of how bars reflect signal strength on both iPhone and Android, look here: http://vimeo.com/59084995

iPhone Tip: Field Test Mode

A hidden feature found in every iPhone running iOS 4.1 or newer is the ability to enable a special "Field Test Mode" that displays a real signal strength reading rather than arbitrary "bars" or "dots".

This mode is enabled by going to the dial pad, and dialing: *3001#12345#*

Now - instead of bars, you see an actual signal strength reading in dBm in the upper left corner of the screen. You can also click on the "Serving Cell Info" to find the "Freq Band Indicator" to look up what LTE band you are connected to.

Most of the rest of the information you will find in Field Test Mode is useless to anyone but a cellular engineer - but you can't hurt your iPhone, so feel free to look around.

To enable Field Test Mode's dBm readout to be your default display all the time, replacing the signal dots with a number, you can do this:

  • Dial *3001#12345#* and hit “Call” if you haven’t done so already to launch Field Test
  • Now hold down the Power button until the “Slide to Power Off” message appears, then release the Power button and hold the Home button until Field Test quits
  • From now on - you can tap the signal bars or signal numbers to switch between the two

To remove this capability, you can either reboot the iPhone, or go back into Field Test and close out of it as usual.

iOS 9 Note: iOS 9 changed the behavior of the system status bar, and the raw signal strength is now no longer displayed at the top of the Field Test Mode screen. But when you quit Field Test Mode, the signal strength in dBm will be displayed briefly in the top left before the regular "dots" display returns.

Easier on Android

For Android devices, this seems to work on most phones:

Go to Settings -> About Phone -> Status.

The 'Signal Notification' app for Android is also an option without going through the Settings menu - and there are plenty of other signal status apps that will give you access to raw signal strength and even more wireless diagnostic information.


Carrier Aggregation Notes

Devices that support LTE-Advanced carrier aggregation can combine multiple LTE bands together for blistering fast speeds.

Carrier aggregation is a technology supported by LTE-Advanced modems to combine multiple frequency bands together to enable vastly faster speeds.

When Carrier Aggregation is enabled, one band is the primary band for both upload and download connections, and one or two other bands are added into the connection to turbo-charge the download speed.

But for some reason device makers and the networks make it very hard to tell when carrier aggregation is active.

We've yet to see a hotspot that reveals this at all.

Usually the only easy way to even guess that Carrier Aggregation is engaged is to run a speed test side-by-side with a device that has CA support tested against something that does not.

The only cellular device we've used that will explicitly tell you that CA is active and what bands are in use is the MoFi 4500 SIM4, if you dig deep enough into the diagnostic menus.

The iPhone Field Test Mode does reveal a bit of info though:

Go into field test mode by dialing: *3001#12345#*

Selecting "Serving Cell Info" gives you the primary LTE Band next to "Freq Band Indicator" if you're lucky. You may need to quit and restart Field Test Mode several times before you see a sensible value here.

Back at the top level, selecting "Connected mode LTE Intra-frequency Meas" will let you know if CA is enabled. Look for "E-ARFCN" at the bottom of this page.

If it's 0, you're not CA. If it says 1, then you are in 2x CA mode, and if it says 2 you are in 3x CA mode. Only the iPhone 7 will support 3x CA.

The iPhone Field Test Mode though is often unreliable.

There are some Android apps that attempt to calculate the LTE bands in use too, but the ones I've seen are often just guessing unless they are running on a rooted phone.

Frustrating indeed for geeks like us.

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