GSM and CDMA are two different technologies that are competing in the US. AT&T and T-Mobile are the major players using GSM, while Sprint and Verizon are the ones using CDMA.

There is a huge difference in how SIM cards work between CDMA and GSM. With GSM, the subscriber data is stored on the SIM card. This means you can pop it into any phone with a SIM card slot and it'll just work without needing to even contact your carrier. With CDMA, subscriber data is stored with the carrier. This means to switch a phone over to a different CDMA carrier you'll need for both carriers to work together in swapping over your data, or to use a different phone with the carrier you need to go through them and have them pre-approve it. The reason CDMA carriers use SIM cards at all is because they were forced to adopt it when 4G became a big thing.. It is not even close to what SIM cards do with typical GSM carriers. What's the problem? Well, the majority of the world uses GSM as a global standard and a lot of phones are not built CDMA compatible because the USA is one of the weird cases where CDMA is popular. It's a lot easier for an OEM to build a GSM-compatible phone and just let the user swap a SIM card in and have it work. This is why you normally don't see OEM's selling two unlocked versions of their phones, CDMA and GSM versions. Because CDMA carriers need to allow and support unlocked phones, while GSM carriers can just hand out a SIM card. Recently there have been unlocked phones released for CDMA thanks to popular demand, but to get an unlocked phone working on a CDMA network you still have to get it activated with the carrier instead of just swapping a SIM card. Besides, most of the world uses GSM anyway.

Image source: PC Mag

Project Fi isn't exactly a household name. It's a new wireless network created by Google that only currently supports Nexus devices. But not only is it one of the best things to come out of Google in a long time, if it gains enough popularity, it could surely change the world of wireless networks forever.

So, what makes Project Fi different? Fi uses two different networks, Sprint and T-Mobile, and intelligently switches between them to find the best option. Not only does it switch between data networks, it can also connect to specific WiFi hotspots that Google deems safe, such as Starbucks WiFi. Even though these networks are safe, Google still automatically connects you through a VPN so there is no question whether your data is secure. This intelligent data-hopping is something I have never seen before, and it could really help save precious GB at the end of the month.

Speaking of GB, Fi offers an interesting take on what data plans in the 21st century should be. Instead of a fixed monthly price for an allocated amount of data, Fi lets you pick what you think is best for you, and then refunds you the difference. Fi starts at $20/mo, and then it's +$10/mo for each GB. Let's say you buy 3GB, so you're paying $50/mo. But let's also say that you only use 1GB. That extra $20 worth of data is credited to your account, so you only pay $30 for the next month! It's a brilliant system that provides a level of flexibility I have never seen.

Not to mention all the other amazing features that Fi offers, such as the same $10/GB model internationally, the ability to connect with Google Hangouts to talk/text on multiple devices, WiFi calling, and a simple to use management app. Project Fi also heavily discounts the Nexus 5X to a price of only $199. That's incredible! Personally, I'll be switching to Fi the second my current plan runs out. I can't wait!