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The Main GSM And CDMA Frequencies
If you're a Verizon Customer, an AT&T Customer, a T-Mobile customer, and so on and so forth, you're already using GSM or CDMA technology and you may not even realize it. GSM and CDMA are the two main cellular technologies used by mobile operators around the world. Admittedly CDMA is reserved mostly for the U.S. and Verizon, Sprint, and Alltel Customers, while GSM is the preferred vendor by most carriers worldwide.
When purchasing a mobile phone from outside of your carrier the first thing you'll want to determine is what type of network your cellular company operates on, and which frequencies they use on their network. For instance AT&T Wireless operates on the 850mhz GSM network, with only a few 1900mhz towers nationwide, for this reason you'll want to make sure you purchase a device that offers the 850Mhz frequency. On the other hand Verizon Wireless uses the CDMA 850/1900Mhz frequencies. While Verizon Wireless phones are available outside of normal channels, most CDMA devices whether from Verizon, Sprint, Alltel, Etc. are carrier locked, meaning you'll only find those devices via the carrier they are branded for.
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Frequencies by Carrier
So now we know what GSM and CDMA means, so here's a quick list of the frequencies you'll see on most U.S. based networks.
GSM: 850Mhz (AT&T) and 1900Mhz (T-Mobile) - Of course there are other regional networks you'll have to find out which one of those two frequencies they run on.
GSM: 900Mhz, 1800Mhz - These frequencies are used in several international regions and are included on Tri-Band and Quad-Band GSM devices.
This brings us to an important point. Devices usually carry more than one frequency. The most popular options are Tri-Band GSM devices (900/1800/1900Mhz frequencies) and Quad-Band GSM devices (850/900/1800/1900Mhz frequencies).
In terms of CDMA the main frequencies of importance are the 850/1900Mhz frequencies which you'll find in use at Verizon, Sprint, Alltel, etc.
Simply put, you'll want to find out which frequency your cellular carrier implements and make your choices from that point.
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What are 3G, EDGE and GPRS Frequencies?
So now you know that your carrier, let's say you're using AT&T Wireless operates on the 850Mhz frequency. If you plan on using that device with a data plan of some sort you're probably also curious to know what data speeds are available to you, and how to determine if your carrier supports those offerings. In order to accomplish this goal you'll need to have an understanding of the data connections available and the frequencies they work on.
GSM technology offers several types of connections. Starting from the fastest and degrading from there we have: 3G, Edge and finally GPRS. What technology you can use depends on your carrier and ultimately your device. When choosing a cell phone you'll first want to see if your device offers 3G data, if it does you'll want to find out which frequency of 3G it offers, for instance AT&T runs on the HSDPA 1900Mhz frequency so if your device, let's say the HTC Touch Pro 2 is 2100Mhz based you won't be able to take advantage of 3G in the U.S. yet if you went to Europe you could easily connect to their 3G network on their 2100Mhz frequency.
If your device doesn't use 3G you can always connect to the Edge network or the slowest of the 3 technologies known as GPRS, you don't have to worry about frequencies for the last two offerings, if your phone supports Edge/GPRS or just GPRS or Edge you'll be able to connect.
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CDMA and EVDO
For CDMA users the high speed offering is known as EVDO, the speed of your EVDO will depend on the REV number, which is basically the "version" of your high speed. You may see Rev.0 or Rev.A. At the time of this article Rev. A is the fastest of the EVDO connections.
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Other Frequencies Used Inernationally
In terms of HSDPA/UMTS connections AKA 3G the frequencies available include 850/900/1700/1800/1900/2100Mhz. Most of these are not used in the U.S. in fact the 1900Mhz for AT&T and the 1700Mhz AWS frequencies are the main frequencies for U.S. use, the rest are international offerings.
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While the information listed above should give you a pretty good understanding of the questions to ask when you decide to buy a new device from outside of your carrier, it's in no way meant to be a complete list of frequencies supported by all carriers, nor should it be considered a static examination of technologies, as more and more new offerings are introduced on an almost daily basis.
Remember when choosing an unlocked device from outside of your carrier its important that you find out what type of calling frequency your network uses and which type of data connections are available, you can then easily determine if those offerings are available on the phones of your choice.
As always, if you have any questions feel free to post them below in the comment area.