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iOS 4.1 restores limited Field Test mode iOS devices
In older versions of the iPhone OS software, it was possible to enter Field Test mode to get a lot of useful data about the cellular connection. This capability was removed in iOS 4.0, and caused some angst while trying to examine the iPhone 4 antenna reception issues.

The ability has been restored, although to a greatly limited degree, with iOS 4.1. Once again, if you dial the number *3001#12345#* in the keypad you will enter FTM. What you will see now is a negative number (presumably representing the signal loss in dB) in the upper left that replaces the cellular signal strength bars. A more negative number means lower relative signal strength, so the -121 I have at the moment is worse than the -92 I had in a different room in the house. Zero would be no signal loss.

To exit Field Test mode, just press the Home button. It may take a moment for the number to go away and be replaced by the bars again.

I think we all hope that when iOS 4.2 is released (Apple says in November) it will restore full Field Test mode with all the diagnostic data available, and maybe bring it to the 3G enabled iPad as well.
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staying in numerical signal strength mode
Authored by: hayne on Sep 10, '10 08:47:38AM

If you force-quit the Field Test app (hold down the Sleep/Wake button for several seconds until you see the “slide to power off” prompt and then release the Sleep/Wake button and hold the Home button for several seconds), the numerical display of signal strength will persist.

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staying in numerical signal strength mode
Authored by: brucio on Sep 10, '10 04:00:02PM

On my phone it only persisted for about a minute :-/

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staying in numerical signal strength mode
Authored by: fool4drummin on Nov 22, '10 06:45:37PM

So, that worked for me. And I haven't been able to get it to go away. Any suggestions? I decided I would wait for 4.2 to come out to see if that would automatically make it flip back to only the bars, but it didn't. And though it was fun seeing the numerical values for awhile, I'd like my bars back.

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iOS 4.1 restores limited Field Test mode
Authored by: felibb on Sep 10, '10 10:41:14AM

Tapping on the number in the upper left corner while in Field Test mode will switch between bars and dB.

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What the numbers mean
Authored by: kevinp on Sep 10, '10 11:59:16AM
Actually your description, "the signal loss in dB," is not quite correct. A "loss" would have to be relative to something. The confusion comes about because the numbers are negative and because smaller (less negative) values indicate a stronger signal.

The display indicates signal strength in dBm, which compares the received signal to 1 milliwatt (an enormously strong value for a received radio signal). Zero doesn't mean "no loss", it mean a 1mW signal. Since the scale is logarithmic, all vales less than 1mW are shown as negative numbers. For example, -92 dBm is about one billionth of a milliwatt and -121 is about one thousand times smaller than that!

This enormous range of useful values explains the use of a logarithmic scale and, incidentally, the controversy about how best to condense this information into a meaningful five-bar scale.

Kevin Patfield

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What the numbers mean
Authored by: crarko on Sep 10, '10 12:11:06PM

Thanks for the clarification!

Craig A

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iOS 4.1 restores limited Field Test mode
Authored by: westont on Sep 10, '10 12:41:13PM

Most likely this is showing the signal level in dBm which is the decibel representation of the signal power relative to a 1mW reference level. i.e.1mW = 0dBm. Negative values indicate levels less than 1mW, positive values indicate levels greater than 1mW. A 10dBm difference means a factor of 10 difference in signal level. For example +30dBm = 1000mW = 1W. Likewise -30dBm = 0.001mW = 1μW.

The receive sensitivity of mobile phones will vary according to the technology in use (GSM, CDMA, W-CDMA etc.) but as reference point, the GSM standards specify a mobile receive sensitivity of -102dBm to -104dBm depending on the band of operation (900, 1800, 1900 etc.) and mobile power class. This figure accounts for Rayleigh fading and an implementation margin and assumes a receiver Noise Figure of 6dB but, of course, some mobiles perform better than this.

It is important to remember that this Rx sensitivity figure represents performance in a thermal noise limited environment. The presence of network generated noise on the operating channel (especially at cell edges) results in a interference limited environment which the network operator must allow for in the design and planning of the network.

Hope that helps !

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