While the iPhone is offered by carriers around the globe, all of them rely on the Global System for Mobile Communications (GSM) protocol.

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In 1993, NTT DoCoMo, Japan's principal mobile-phone carrier, opted for a technical standard called Personal Digital Cellular (PDC), which was incompatible with the Global System for Mobile Communications (GSM), the standard adopted by most of the rest of the world.

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Americans are at a disadvantage when it comes to cell phones overseas, because most of the world uses a GSM (Global System for Mobile Communications) standard, Schukai said.

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