The first in a family of cdma2000 1x digital wireless standards designed to extend and replace the IS-95 CDMA standard. 1xRTT is sometimes referred to as a "2.5G" or "3G" standard.
Compared to the IS-95 CDMA technology it replaces, 1xRTT offers increased network voice capacity. This benefit requires support on both the base station (tower) and handset (phone). If everyone using a given tower has a 1xRTT phone, twice as many people can use that tower at the same time, compared to the old IS-95 standard.
1xRTT also offers much faster data speeds. The initial release - release 0 (zero) - supports data speeds peaking at 144 kbps. Typical real-world speeds are closer to 60-80 kbps.
A second release of 1xRTT is planned - release A. Release A doubles data rates at 288 kbps peak.