A co-worker let me take a look at his Cell Ranger Port yesterday. This device somehow amplifies GSM and CDMA radio signals for nearby cellphones. While reading a review of it in TestFreaks Blog...

Cell Ranger Port

...I noticed they provided the keypad sequence to place an iPhone 3G in what is called Field Test Mode. This is a hidden diagnostic tool that can, if you know how to read and interpret its output, can tell you a lot about the radio signal quality your phone sees. Here's how you get to the iPhone's Field Test Mode:

- Open the iPhone's dialer keypad
- Enter *3001#12345#*
- Press the Call button

There's a lot of info you can see in this mode. But, the number of interest is displayed right away in the upper left hand corner. TestFreaks Blog categorized the dBm (power ratio in decibels) this way:

-80 to -70 dBm: Reasonably strong signal
-107 to -90 dBm: Weak signal
-113 dBM: No servicable signal

You can see three readings I recorded above. The first (2:04pm) was taken in my office where I saw a reasonably good -79 dBm signal. However, at my home, where I often see No Service in the iPhone signal status bar, I recorded the next two readings. The first (6:17pm) is from inside of my home with 101 dBm (weak signal). The second (6:19pm) was noted just outside of my home with a pathetic -113 dBm signal. Paradoxically, however, you can see that the iPhone says I had a 3G connection outside while I only had an EDGE wireless data connection inside.

I'm planning to try data transfer speed tests soon.