 Initially, systems based upon the AMPS mobile phone model were popular, including IS-95 (often known as "CDMA", the air interface technology it uses) and IS-136 (often known as D-AMPS, Digital AMPS, or "TDMA", the air interface technology it uses).Eventually, IS-136 on these frequencies was replaced by most operators with GSM.GSM had already been running for some time on US PCS (1,900 MHz) frequencies.