Myriam Joire (tnkgrl) was born wearing combat boots and holding a keyboard; moments later she picked up a soldering iron. She's been stomping, typing and hacking ever since. In her regular series, she'll share her tips, tricks, and mods on everything from phones to netbooks -- and be sure to catch her weekly on the Engadget Mobile podcast.

Things used to be simple: you either had a CDMA device with no SIM, or a GSM device with a SIM. If your GSM device was unlocked you could swap SIMs between carriers, and if was locked, you could swap SIMs between accounts on the same carrier. Basically, an unlocked phone meant mobile freedom. At first, the only complication was the different frequencies used for GSM in different parts of the world, which was remedied with quadband radios. Then 3G was introduced, with different frequencies used by different carriers in the same geographic area, like North America. Today, this proliferation of bands makes it difficult to build universal radios, and makes it less appealing to switch carriers. In fact, the only phone that currently supports all 3G frequencies used in North America is the upcoming pentaband Nokia N8.

The second obstacle was the introduction of special data plans, either requiring additional service layers (such as BlackBerry or Sidekick plans) or restricting the types of devices permitted as the iPhone and iPad plans do. What used to be as simple as changing APNs is now a Kafkaesque nightmare, complete with nasty "you are using the wrong data plan and we are going to send you a bill" text messages from carriers. Strike three was the micro SIM (also known as the 3FF SIM), popularized by Apple's iPad and the iPhone 4. This smaller form-factor SIM is electrically compatible with the standard SIM we all know and love -- and as such, it's easy to convert a standard SIM into a micro SIM and vice-versa (but still, it's one more hurdle standing in the way of happiness).

Let's dive in and take a look at how it's done.

Cutting your standard SIM



Beyond band limitations and data plan restrictions, there are many reasons to convert a standard SIM to a micro SIM. The iPad is sold unlocked, and the iPhone 4 is (or will soon be) available unlocked in several countries, including the UK, Australia, and Canada. Both devices support 3G on most carriers worldwide, including AT&T in the US, Rogers / Fido / Telus / Bell in Canada, and 2G on almost all carriers around the world.

You may want to use these devices with an existing standard SIM -- and even if your iPhone 4 is locked to a carrier, you may want to use it with an existing standard SIM from the same carrier. The converse is also true: you may want to use the micro SIM from your iPad or iPhone 4 in a device with a standard SIM slot. Fortunately, cutting a standard SIM into a micro SIM is pretty simple, as illustrated in the video below.



All that's required is a standard SIM, a cutting board, a knife (kitchen or utility), scissors, a file, a template (ideally an actual micro SIM), some care, and some patience. The basic idea is to place the template over the SIM to be trimmed, carefully lining up the contacts, then mark the SIM at the edges of the template with the knife and cut along the scored edges using the scissors. Fine-tuning is achieved by filing the edges and corners of the freshly minted SIM and checking fit in an iPad or iPhone 4 SIM tray.

Always keep in mind that it's best to cut less than you have to and file off any excess than to over-cut and cry. A word of caution: we won't be responsible if you damage your SIM or yourself while attempting to follow these steps; your carrier can replace your SIM if you make a mistake, but can't replace your fingers. Alternatively there's a SIM punching tool, but it runs $30 or more to accomplish a task that you might have to do once or twice a year, if that.

Using your micro SIM in a standard SIM device


Now, say you've got your micro SIM -- the one you made, or the one you were provided by your carrier -- and you want to use it in a device that isn't an iPhone 4 or iPad. Using a micro SIM in a device with standard SIM slot only requires a simple plastic adapter. There's also a DIY solution (PDF) that uses discarded SIM packaging or a credit card, but it's more involved than the cutting method above, and the results are less polished.


APNs


Now that you know how to swap between standard and micro SIM devices effortlessly, it's time to learn about changing APNs. Many unlocked devices today auto-detect APN settings when a SIM is inserted, but if not, most support manual and / or OTA (over the air) APN settings:
When using a SIM provisioned with the iPad data plan in another device, you'll be unable to send or receive calls and text messages, but data will work after changing APNs. Ditto when using a SIM provisioned with both voice and a regular data plan in an iPad (note that using an iPad or iPhone with a T-Mobile USA SIM only provides 2G access in the US because of band limitations). You may want to be mindful of data plan restrictions when using an iPhone with an AT&T SIM provisioned for another device; it's hit or miss, but you might get dinged with additional plan or overage charges at some point.

Here's a comprehensive list of APNs for carriers worldwide. If you're in the US, check out the table below:

APN name Username Password
AT&T iPad broadband -- --
AT&T iPhone, BlackBerry, and other phones wap.cingular WAP@CINGULARGPRS.COM CINGULAR1
AT&T data modems, netbooks isp.cingular ISP@CINGULARGPRS.COM
or
ISPDA@CINGULARGPRS.COM
CINGULAR1
T-Mobile epc.tmobile.com -- --
T-Mobile (old, but still works) wap.voicestream.com
or
internet2.voicestream.com
or
internet3.voicestream.com
-- --
T-Mobile Sidekick hiptop.voicestream.com -- --

And here's a neat trick: not only can you use a SIM provisioned for the iPad data plan in another device by converting SIMs and changing APNs, but in the US you can order a replacement micro SIM from AT&T, sign up for an iPad data plan, and manage it without purchasing an iPad. It sure makes getting that unlocked iPhone 4 a significantly more attractive proposition for those crazy data-centric users amongst us -- you know who you are.