Brighthand FAQ: What’s the Difference Between CDMA and GSM?

by Reads (16,800)

Often when a new member of the Brighthand forum asks for advice on which device to buy, one of the questions that trips them up has to do with cell phone technologies: which is better, and what these various acronyms and technologies even mean.

CDMA and GSM are the names of competing cellular phone standards. They’re about equal in the U.S. in terms of users, but overseas there’s only GSM other than a few places like Japan, India, and Korea. Out of the four national carriers in the U.S., two — Sprint and Verizon — use CDMA, the other two–AT&T and T-Mobile — use GSM.

The two biggest differences between these standards are international compatibility, and how the networks handle activating phones. Outside the U.S. and Canada, most GSM phones will still work, while almost all CDMA phones simply can’t be used overseas. The only ones that can are those that also have a GSM radio. These are sold for people regularly traveling overseas on business, and are priced accordingly.

CDMA phones are activated remotely, by the carrier, using the phone’s serial number, known as the ESN. Since each carrier has a database of all the ESNs that are approved for its network, this lets most CDMA carriers refuse to activate phones not originally intended for their network.

GSM phones are activated differently. Each account is associated with what’s called a SIM card, or Subscriber Identity Module. This card, about the size of a fingertip and the thickness of a piece of paperboard, carries an encrypted version of all the information needed to identify your wireless account to the network. You slip it into the appropriate slot on a GSM phone (usually under the battery) and that phone is ready to use.

Naturally, this means that you can use pretty much any compatible phone on a GSM network, with or without the carrier’s permission. This is limited only by the phone supporting the right radio frequencies (850 and 1900 MHz for North America, 900 and 1800 MHz for overseas), and on the phone not being "locked" to a provider other than the one you’re trying to use it with.

Not only can you use a wider variety of phones with your service, it also allows you to take any GSM hardware and make it "your phone" instantly, including the ability to carry your contacts right on the SIM card. You could literally take your phone, smash it with a hammer, extract the SIM card, and with a backup phone in your glove compartment be back up and running in 30 seconds. Or you could carry a pre-paid SIM with you and pop it into your phone in case there was ever a time when you wanted to have phone service but not be reachable by your usual number. GSM’s strength is in its flexibility.

That’s not to say CDMA providers don’t have advantages. In the U.S., at least, Sprint and Verizon are well ahead of AT&T and T-Mobile on deploying high-speed Internet: both of the CDMA carriers have reasonably extensive wireless broadband footprints, while AT&T’s is relatively small, and T-Mobile is still preparing to launch such a system. Thus, some users in the U.S. have to chose whether flexibility now is worth having to wait for the broadband coverage to catch up.

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