IS-54

IS-136 is a second-generation (2G) mobile phone system also called D-AMPS. It is now mostly being superseded by GSM systems, but even in its heyday was primarily used only in the Americas, particularly the U.S.A.. IS-136 was designed as a digital extension to the Advanced mobile phone system (AMPS). IS-54 was an earlier version of D-AMPS which used the same timeslot structure and encryption, but lacked some extra features.

D-AMPS was designed to use existing AMPS channels and allow a smooth transition since both systems could co-exist in the same area. IS-54 increased capacity over the preceding analogue design by dividing each 30kHz channel pair into three time slots (using TDMA technology) and digitally compressing the voice data, yielding three times the call capacity at some expense of voice quality. Moving to a digital system also made calls more secure because analogue scanners could not access digital signals, and new scanners in the U.S. were prohibited by the FCC from accessing cellphone frequencies. Calls were also encrypted, although the algorithm used, CMEA was later found to be weak (see links below).

IS-136 superseded IS-54, a previous extension to AMPS, adding features such as text messaging and data capabilities borrowed from competing GSM and IS-95/CdmaOne standards. IS-136 is used primarily by Cingular Wireless and AT&T Wireless, although both companies are now migrating to GSM technology.