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The Orientation: The differences between GSM and CDMA

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It’s come to my attention that not everyone understands the subtle yet significant differences between GSM and CDMA. We’ve received random e-mails from folks asking how they can get XY phone from AT&T to work on Sprint’s network and it makes me scratch my head. My first instinct is to curse and hit the Spam button, but I slowly realize this individual just doesn’t know the difference. It’s not their fault. In our line of work we don’t always remember that everyone is as savvy or immersed in tech on a daily basis. For this we apologize. To make up for our indiscretions here is your Orientation for the week.

A few people I spoke with knew that the main difference had something to do with a SIM card, which is the Subscriber Identity Module card. Said SIM cards are used in the US by T-Mobile and AT&T. I’m hoping most of you know this already. Here in lies one of the positives for subscribers with GSM carriers. You’re not tied down to a single handset that’s attached to one network like you are with CDMA carriers/phones ie Sprint and Verizon. It’s quite evident with the mayhem surrounding the iPhone. That’s the bare bones difference.

To be exact, GSM stands for Global System for Mobile Communications, which is based on an older standard called TDMA, which we won’t get into. CDMA stands for Code Division Multiple Access. Neither of those terms is very easy to digest at all, now is it? Without getting into the nitty gritty of it, simply because you needn’t worry about it, I’ve broken it down as best I can for you (and me) to understand. If you must know the technical specs then I have one morsel for you to chew on: they share bandwidth differently. GSM allocates a bit of bandwidth for each user on a local network and they can do as they please, whereas with CDMA each device gets a chunk of bandwidth only when they need it.

Some say that CDMA is a superior technology for the following reasons: It allows for more users across a given frequency because it’s only being used when needed, and because battery life on CDMA phones is far superior to that of any GSM device because it’s not transmitting anything when sitting by idle.

Those are the two main factors and I could talk about the cancer causing aspect of mobile phones, but we’re beyond that and everything causes cancer these days.

However, GSM’s constant contact means that acquiring a signal isn’t as tricky. When a CDMA phone connects to the network, it must negotiate with the local tower what phone it is, what service, what number, etc. This is that pause between when you hit “send” and it starts ringing. The more interference there is, the longer this takes. With GSM, that’s taken care of when you roam into a tower’s area, the phone is “registered” with the tower already and starts transmitting much faster. Also, the already-connected state means that GMS can work in areas (such as elevators, underground) where CDMA generally can not.

The only downside to CDMA at this point in time is it’s lack of mobility across continents. GSM networks are used in more countries (roughly 3/4 of the world market) than CDMA so roaming is hardly ever an issue thanks to that SIM card.

In terms of data transmission, GSM has the upper hand with HSDPA as it is capable of transferring up to 7.2 Mbps while EV-DO Rev. A can only muster 3.1 Mbps. A GSM signal can also be shoddy at times depending on where you are. For instance, in Manhattan AT&T doesn’t get the best coverage while Verizon and Sprint are rock solid. In my experience T-Mobile works perfect in the city, but not very well out on Long Island. It also depends on where you live in the country because in the Bay Area, AT&T is a champ while T-Mobile eats it. In general, CDMA carriers are more reliable nationwide because of the larger tower footprints.

The overall point that you should take away from this is that you need to assess your current needs if you’re in the market for a new phone. Will you be traveling or staying home? Of course, CDMA providers like Verizon are releasing dual-band phones like the BlackBerry 8830 World Edition that allow for a SIM card when you travel overseas, so it may one day be a moot point. Be sure to check the network coverage in your area as it differs from region to region. Voice/data plans and hardware themselves are going to be the biggest factors in your decision. Do you want better coverage (Verizon), better phones (AT&T), cheap plans (T-Mo) or faster data (Sprint)?

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