Based on the old telephone models, cell phones were designed to transmit voices. The first generation of cell phones were basically radios, transmitting an analogue signal through the air to a cell tower. With the second generation of cell phones, voices were digitized, but still sent the same way, via a circuit-switched network that established a connection between two phones, keeping it open for as long as your conversation lasted.
The most popular standard for the second generation cell phones is GSM, or Global System for Mobile Communications, which was developed in Europe, and it is based on a model called Time Division Multiple Access (TDMA) but it isn't the only one [source: GSM Association]. The CDMA (Code Division Multiple Access) system was developed in the United States by Qualcomm [source: CDMA Development Group]. The two systems differ in how they allow multiple users to transmit information over the same airwaves. Each standard has its adherents. T-Mobile, for instance, uses the GSM standard, while AT&T uses CDMA -- and each standard has its own governing body. You can still call a GSM phone from a CDMA phone, and vice versa, but you can't use one type of phone to connect to the other type of network, so a GSM phone needs GSM coverage to get a signal.
The allure of being able to connect to the Internet wherever you are has led to an increase in data plans offered by cell phone companies, bringing us into the third generation of cell phones (AT&T actually calls its network 3G, but there are others, such as the EDGE network offered by T-Mobile, among others). Third generation networks allow larger chunks of data than just voice, but they still transmit data through the cellular phone system, with the same limitations as second generation phones. As different companies with different network standards head toward uniting the fixed and mobile networks, they've taken a number of different paths, and we'll take a look at some of those next.