GSM vs CDMA

This question came up on the expatriot list.  My answer turned out to be so long that I thought I'd post it here just in case someone links to my blog to get info about phone services in New Zealand...

The debate is largely a struggle between technological superiority and cost.  CDMA works better over longer distances than GSM--don't believe the commercials.  AT&T's network will drop more calls and have much slower data speeds than the CDMA carriers like Verizon and Sprint.  Dropped calls has been AT&T's focus the last couple of years and they have responded with more towers (more coverage) which has greatly improved service, but the underlying flaws in the technology remain.

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But you as a customer don't care about any of that.  You want a phone that works where ever you are, is easy to use and is easy understand.  Only the first one is GSM or CDMA specific.  The last 2 are specific to the device you select and all carriers will have a range of devices from simple phones to complex PDA's with tethered data and Windows Mobile OS.  I will only focus on a phone that works where ever you are.

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The very short answer is that GSM has about 10 times the world coverage of CDMA which really only has excellent coverate in North America.  Outside North America CDMA coverage will always be a question of will it or won't it.  With GSM you'll never have that problem.  If there is any cell phone capability, it will 9 times in 10 be GSM.  While that sounds great, you should still consider how often that will be important to you. 

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If you plan to spend 95% of your time in one large metro location.  Then really you will be much better served by a CDMA device for 95% of the time.  If you're never going to continent hop, then CDMA will always be a better choice.  But if you are going to seriously travel, especially through Europe, then GSM is the way to go.  Very exotic destinations like Africa, China, Australia, remote islands, will generally only support GSM or only have very spotty CDMA.

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There is one gotcha... data services.  CDMA is a great data delivery service.  GSM is not.  If you really depend on data services, you have to be a CDMA customer and take your chances with coverage.

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GSM does have one feature that falls in to the "easy to use" list, the SIM card.  GSM phones have a little chip you buy with the handset that contains all your personal information (phone lists, account details, etc).  When you buy a new GSM phone, you just pop in your SIM card and presto-chango you got a new phone with all your contact list info.  It's that easy.  Swap between 10 phones a day... no problem, just swap in your SIM card.  You never have to talk to customer service or visit a store.

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So the question is, How much do you roam?  If you are a globe trotter, then GSM.  If you're a home body, then CDMA.  The second question is, How dependent on data are you? If you really need data access (like for a laptop) then CDMA is better.  Then the last ugly fact, that CDMA data roaming is not supported in New Zealand.  None of the US CDMA carrier devices will be able to access data in NZ and none of the Telecom NZ devices will be able to access data in the US (or anywhere else).  GSM data will be more wide-spread, but very slow and sometimes very costly when roaming ($10 a megabyte in some cases).  However, if data roaming is needed at any cost, to and from NZ, then GSM is the only choice (Vodafone NZ).

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There is one last consideration... WorldMode devices.  These phones can attach to both CDMA and GSM.  But it is not as slick as it sounds.  The device is basically two phones duct taped together. There are issues using the phone between networks.  You will be able to make phone calls in both systems, but features beyond that will not be guaranteed (voicemail, data applications, contact lists, etc.)  These devices are harder to use for the less technically interested.  If you hate dinking around with your phone, this won't be a good choice for you.  If you're a travelling uber techno-geek, then it will get the job done.

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Technical mumbo jumbo for the morbidly interested...  CDMA has much better radio gain properties.  It will always be able to maintain a stronger and clearer signal with equivalent coverage and power to GSM.  GSM carriers compensate by using more cell towers to increase coverage, but this philosophy only works in certain markets.  In some places, it is not possible (or cost effective) to install enough towers to match a CDMA deployment. 

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The biggest impact of the CDMA radio gain is in data performance.  The latest high speed CDMA based data offering is EVDO Revision A.  These devices can easily hit 2 megabits/second download speeds throughout the US.  Other markets will vary.  Telecom New Zealand also supports EVDO Rev A (but won't support data roaming).  EVDO Rev A is the best data service in the world at the moment.  It probably will be for the next year or two.  WiMAX is better, but it is a long way from wide spread coverage and even farther from world acceptance.  GSM data offerings (even at the highest speeds) are a joke compared to EVDO--espeically when compared to general acceptance.  Most CDMA carriers around the world support EVDO so there is a high chance that if you have service at all you will also have high speed data.  Whereas with GSM, your phone calls will work, but universially your data performance is slow--roaming or not.

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One last tidbit of interest to my fellow Kiwi's... Telecom NZ is standing up a GSM network.  So very soon the choice will be moot as you can use either technology on Telecom's network *and* benefit from world wide voice and data roaming.  While the data offering won't be as good as EVDO Rev A, it will be the best GSM can offer (probably 700k a second).