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Cell phone signal strength
April 2, 2015
8:18 am
keith cooper

 I’ve been looking for a better way to get a handle on cell phone signal strength. Frankly, a graphical representation of one to five bars really  doesn’t tell me much. Ran across a solution using the field test option on my iPhone. 

Access the field test mode by dialing *3001#12345#* + “Call”

The numeric value for signal strength is now 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, hit the Home button. The mode is available on iPhone’s running iOS 4.1 and all later versions.

If you want your iPhone to always display numerical signal strength instead of signal bars, perform the following process:

1. Once in Field-test mode hold down the power button until you see “Slide to Power Off”, then release it.

2. 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.

3. To exit this permanent field-test mode, simply reboot the phone or re-load Field Test Mode and exit it via the Home button.

The numeric 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. The closer the number is to zero, the better the reception, -80 is a stronger signal than -100. The unit of measurement of RSSI is the decibel (dB), it is a measure of the power of a signal. The decibel scale is logarithmic, 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. Therefore, an RSSI value of -90 is actually ten times stronger than an RSSI of -100. think about the volume of a radio, the numeric RSSI value is really telling you exactly how “loudly” your phone is receiving the signal from your service provider’s cellular network.

Anything numerically less than -80 is good, and is equivalent to five bars. Anything numerically greater than -110 is bad, and would be considered one or two bars.
Once you get the hang of reading the numbers, you’ll find it’s much more accurate, and it becomes easier to predict when you may drop a call or start to get a bad signal or connection, which creates the weird artifacts and sounds on phone calls, often before it starts to cut out or even drop completely. Problems typically starts happening around -110 or so, before dropping the connection or call completely when it hits -120 to -130.

  Signal strength may very frequently _ Carriers pass off signal between towers based on a number of criteria including traffic volume, signal quality as well as legal reasons related to coverage areas. Base ID (Serving Cell Info) will change even when standing in the same place, or be different depending on the time of day. The tower that handles your traffic at any given time may not be the closest one.

you may be wondering why this matters in order to calculate whether a cellular boosting system will help in a particular location it is  useful to perform a site survey. The site survey involves taking several accurate readings of signal strength in and around your coach.

in researching this I Ran across some  Smart phone codes you may find useful:

*#06# – Displays IMEI
*#43# + “Call” – Displays call waiting status
*43# + “Call” – Enables call waiting
#43# + “Call” – Disables call waiting
*#21# – Displays call forwarding status
##002# + “Call” – Disables all call forwarding
*33*pin# – Enables call barring
#33*pin# – Disables call barring
#31#phone-number + “Call” – Blocks caller ID for the current phone call
*3370# + “Call” – Enables “Enhanced Full Rate” and improves voice quality on GSM networks (may impact battery life)
*#5005*7672# + “Call” – Displays your carrier’s message center phone number

April 2, 2015
1:02 pm
Forum Posts: 455
Member Since:
November 3, 2009
sp_UserOfflineSmall Offline

Wow! That is a lot of great information, especially for the real Geeks in the group and valuable for all.


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May 19, 2015
9:53 am
Michael Dorsey

I found something similar on my Galaxy S5. Go to “settings”; “about device”; “status”;. Signal strength is shown in units of “dBm” and “asu”. Readings in my home vary from (-110 dBm 0 asu) to (-87 dBm 13 asu) with the latter being 2 bars and the former no service available. My carrier is AT&T. The network is their suped-up  3G called 4G, not their fastest 4G LTE. I have seen readings nearer my closest cell tower while in my car as high as (-60 dBm 45 asu) a full 5 bars.

May 22, 2015
10:07 am
keith cooper

Just some words of caution about ASU’s ( Arbitrary Strength Units)

  1. In GSM networks, ASU is equal to the RSSI (received signal strength indicator, see TS 27.007 sub clause 8.5). The formula for calculating ASU in a GSM network is dBm = 2 * ASU – 113
  2. In UMTS networks, ASU is equal the RSCP level (received signal code power, see TS 27.007 sub clause 8.69 and TS 27.133 sub clause The formula for calculating ASU in a  UMTS network is dBm = ASU – 116
  3. In LTE networks, ASU is dependent on the RSRQ (Reference Signal Received Quality) level. dBm = ASU – 140

The point is rely on the dBm value not the ASUs.

Another key point is that the radios used in different models of cell phones will vary in both transmit (Tx) power and receive (Rx) sensitivity Which means the power required to generate two bars of signal strength on a Galaxy S5 may be different than the power required to generate two bars of signal strength on an iPhone 5



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