The radios in the iPhone sometimes get described in terms of how many different frequencies they support. A quad-band radio can support four different bands of frequencies, while a dual-band radio supports only two. Generally speaking, the more frequencies that a phone's radio can support, the more cellular networks it will be able to support. This makes the phone more useful for roaming both globally and locally, since different companies use differnet frequencies and having access to multiple frequencies gives you access to multiple different signal sources.
Cellular phones like the iPhone transmit data and voice over radio waves. Each carrier has a set block of frequencies that it can use, which it then splits into smaller channels. The cellular spectrum is a lot like a bookcase. Each shelf is equivalent to a block of frequencies, while the space occupied by each book on the shelf is equivalent to a channel. Using this analogy, a quad-band phone can access all of the books or empty spaces on four different shelves while a dual-band phone can only work with two shelves.
GSM or CDMA
Apple doesn't only differentiate iPhones based on which frequencies they can use. IPhones also differ based on which cellular encoding system they use. The Global System for Mobile Communication used by AT&T and T-Mobile in the United States is the world's most prevalent cellular system. The other system, Code Division Multiple Access, is used by Verizon, Sprint, U.S. Cellular, Cricket and MetroPCS. The two systems are completely incompatible, even though they can both be used on many of the same radio bands.
IPhone Cellular Radios
The iPhone 5 is a quad-band phone in both GSM and CDMA flavors. The CDMA version of the phone is both tri-band and quad-band, depending on whether it's use on an EV-DO or HSDPA network. GSM phones are quad-band. The CDMA iPhone 4, on the other hand, supports only dual-band connections.
The cellular radio in the iPhone isn't its only radio. One of the iPhone 5's innovations over previous generations of iPhone is the introduction of dual-band Wi-Fi support. Prior to the iPhone 5, the phone could only connect to wireless networks over the relatively crowded 2.4-GHz frequency band. The iPhone 5 added support for the less congested 5-GHz band, giving the phone more opportunities to find a good Wi-Fi connection.
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