Field Test Mode: What it is and How To Enable it on Your Phone

When most people talk about cell phone signal strength, they talk about “bars” in reference to the signal strength bar indicator on the phone. While bars are an easy way to talk about signal strength, it turns out that it’s not a very accurate way. While many phones show the signal strength on a 5-bar scale, some phones only have 4 bars while others have 8. Even among phones with the same number of bars, there’s no standardization, so the strength of a 4 bar signal on one phone can be very different than a 4 bar signal on another. Finally, bars aren’t very granular so for example, saying 3 bars of signal is not being very specific.

When experts discuss cell phone signal strength, they measure the signal in decibels. Decibels are a logarithmic unit of measuring signal strength and are very precise making them ideal for communicating just how strong of a signal you’re currently receiving.

By default, most phones do not display the signal strength in decibels. Most phones, however, have with we call a Field Test Mode which you can use to view useful information about your phone, including the signal strength in decibels.

iPhone Field Test Mode

Accessing Field Test Mode on the iPhone is relatively simple, just open the Phone app, switch to the keypad and dial the following code: *3001#12345#* and then press call. If you dialed it correctly, your iPhone will enter Field Test Mode and you’ll see the numerical value for signal strength in the upper left hand corner of the screen where the signal strength was previously displayed in bars. To exit and return your iPhone to normal status, all you need to do is hit the Home button. The mode is available on any iPhone running iOS 4.1 and all later versions.

If you want your iPhone to always display numerical signal strength instead of signal bars, you can perform the following process. Once in Field-test mode (accessed by entering and dialing the code above), hold down the power button until you see “Slide to Power Off”, then release it. Then hold the Home button until you’re returned to your main app screen. You’ll now see your numerical signal strength while you use your phone, and you’ll be able to tap the signal numbers to switch to signal bars, and vice versa. To exit this permanent field-test mode, simply reboot the phone or re-load Field Test Mode and exit it via the Home button.

Android Field Test Mode

Accessing Field test mode on Android phones is also straightforward. You simply need to find your way to “Settings” > “About Phone”, and your numerical signal strength will be available under either Network or Status, depending on the model of the phone you own.

Field Test Mode on Other Phones

Most other phones also support a Field Test Mode. Some more popular phones are listed in the following document: Field Test Modes.

What Is It Used For?

The most common use of Field Test Mode is to conduct a special kind of field test known as a site survey. If you find yourself living or working in an area with weak signal strength on your device, and you want to boost your signal, performing a site survey will be useful before you choose a signal boosting system. The site survey involves taking several accurate readings of signal strength in and around your building, in order to calculate whether a cellular boosting system will help, and if so, what sort of equipment will be necessary.

Field Test mode is an immensely useful tool in such a situation - switching from unreliable signal bars to a precise numerical value is essential if you want to quantitatively assess your smartphone’s signal strength in different places.

Although a site survey field test is a multi-step process, it isn’t overly complex, and it’s possible to follow the steps and conduct your own test in and around you home or office, using the field test mode on your phone.

The Numbers Explained

This numerical value is known as RSSI, which stands for Received Signal Strength Indication. It will generally be double or triple digits, and it will be negative. Some phones don’t show a negative sign so if your phone shows just a number such as 90, the signal strength is really -90. The closer the number is to zero, the better the reception, so -80 is a stronger signal than -90. The unit of measurement of RSSI is the decibel (dB), which is a measure of the power of a signal. Similar to the volume of a stereo, the numerical RSSI value is really telling you exactly how “loudly” your phone I receiving the signal from your provider’s cellular network.

The decibel scale is not linear – an increase of 3 dB corresponds a signal that is twice as strong while a 10 dB increase corresponds to a 10 times increase in signal strength. So an RSSI value of -60 is actually ten times stronger than an RSSI of -70, for example.

Having an accurate reading of your Received Signal Strength is crucial when you’re assessing an area with the purpose of installing a possible signal boosting system. Being able to see exactly how strong your signal is, outside and inside your building, is essential in finding out how a cellular signal boosting system can improve your reception.