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The first generation (1G) of cell phones appeared in the mid-1980s. Known as the Advanced Mobile Phone System (AMPS) in the U.S., these phones used FM in 30-kHz bands and a frequency-division-duplexing scheme in the 800- to 900-MHz range. The system worked well, but subscriber demand overwhelmed the capacity of the spectrum and technology.

To get the kind of growth they wanted, carriers went digital, developing several second-generation (2G) systems. In the U.S., the IS-54/IS-136 time-division multiple-access (TDMA) standard managed three calls per 30-kHz channel using heavy voice compression. Europe developed the Global System for Mobile (GSM) access, which managed eight calls per 270-kHz channel using a TDMA scheme as well.

Meanwhile, Qualcomm developed the IS-95 standard. It employed a sophisticated code-division multiple-access (CDMA) approach also known as spread-spectrum. With direct-sequence spread-spectrum (DSSS), the system could accommodate up to 64 subscribers per 1/25-MHz channel.

Carriers eventually phased out the IS-136 standard in favor of GSM and its greater subscriber capacity. At the same time, Motorola and Nextel developed another TDMA 2G method known as iDEN.

Once 2G phones achieved digital capability, data capability was the next step. Subscribers wanted e-mail and Internet access, prompting improvements. Developed for GSM phones, the General Packet Radio Service (GPRS) provided packet data capability up to about 115 kbits/s; typical throughput ranged from 40 to 70 kbits/s.

The Enhanced Data Rate for GSM Evolution (EDGE) further enhanced GSM phones. It used 8PSK (phase-shift keying) to provide more bits per symbol and data rates up to 384 kbits/s. (The average ranged from 100 to 200 kbits/s.) Sometimes, GPRS is called a 2.5G technology, while EDGE is called a 2.5G or 2.75G technology.

Generally known as cdmaOne, the IS95A/B CDMA standards were upgraded with packet data capability. They eventually morphed into cdma2000, Qualcomm's 2.5G technology. Also known as 1xRTT (Radio Transmission Technology) and EV-DO (Evolution-Data Optimized), it boosted data rates to a maximum of 144 kbits/s with 30 to 70 kbits/s typical.

Today, the two main cell-phone technologies in use are GSM/GPRS/EDGE and cdma2000. AT&T and T-Mobile use GSM, while Verizon and Sprint Nextel employ cdma2000. Sprint Nextel still supports the iDEN system.