Inside the iPhone field test mode

 Want to take a look at what's going on inside your iPhone? Dial *3001#12345#* and hit Call and you'll get access to the special field test mode of the iPhone. This mode is designed to allow technicians and Apple reps to get information on the inner workings of the iPhone and the cell phone network it talks to.

[Field test code via MacOSXHints]

NOTE: Although it seems that most of the information is read-only (so you can't change anything), field modes like this have the potential to damage your phone and possibly interfere with the phone network. We are providing this information as-is; we cannot be held responsible if anything you do in this field mode damages your phone or the phone network.

The iPhone field mode shows a lot of information. In fact, it is more comprehensive than many other phone field modes, allowing you to see the details of the individual cell towers and a lot of detail about the cell phone network. To access it, dial *3001#12345#*. If you are already in a call, just hit "add call", enter the number above and hit call; the phone will go into test mode, but keep your call connected.

For a phone that tries to hide much of the technical details of how it works, that's a litte surprising. [main screen] The first screen that you see is the main menu for the field mode. This presents you with the following choices: Network Information, Cell Information, GPRS Information, PDP information, Call Information and Versions. Let's look at each of these in turn.

The network information screen shows a large number of technical details on the cell phone network you are connected to. This is, of course AT&T (unless you've managed to hack the phone, in which case we'd love to hear how). Most of these things are only of interest to cell phone engineers and AT&T technicians, but the particular things of interest are RX LevFull, which shows the received signal strength if you are in a call. TX Powr would seem to be the transmitting power, but it isn't clear what that number means.

This is an interesting one; cell information shows the details of the cell phone towers that the iPhone can currently see. The items listed by each cell site are RX (the received signal stength), the FQ (frequency band), ST (station id), CI (the cell tower ID), N (the network ID) and # (a status code).
You can click on any of the listed cell towers to get more details (as shown above), such as the network ID (which will be mostly 410; that's the code for AT&T), the cell ID (each tower has a unique ID) and the strength of the signal it is receiving from that tower. AT&T technicians can use this data to analyze the network and determine if there are any weak spots; you can use this data to look at what cell towers are nearby and how strong the signal is. Remember that the iPhone is locked to AT&T, though, and that this won't show the cell towers for non-GSM networks like Verizon and Sprint.

The GPRS information screen shows you various technical details of the state of the GPRS connection to the network. Again, most of this information is not overly inttereting or useful unless you happen to work for AT&T [PDP information] PDP is short for Packet Data Protocol, and refers to the way that the iPhone (and other phones) send Internet data over the cell phone network. This screen shows you the type of connection (which should always be IP), the name of the APM (Access Point Name) that your iPhone talks to on the AT&T network, and the IP address of the iPhone. You cannot, unfortunately, use this information to access the iPhone directly; AT&T filters all of the traffic.

 The Call information screen shows the details of the call that you are currently in, including the codec that it is using to compress your voice. On this example, we see that the iPhone is using the v3 AMR full-rate codec; there are several different codecs that can be used that use different amounts of bandwidth and offer various levels of voice quality.

This shows the firmware version of the iPhone (note the misspelling of firmware; Apple aren't perfect after all) and the LCD panel ID. This last one is interesting; it seems to be unique to each iPhone, and presumably identifies the individual touch panel. Why apple would do this isn't clear, but they may want to track this information for quality control purposes.