| Mobile phone|
IS-54 and IS-136 are second-generation (2G) mobile phone systems, known as Digital AMPS (D-AMPS). It is used throughout the Americas, particularly in the United States and Canada. D-AMPS is considered end-of-life, and existing networks are in the process of being replaced by GSM/GPRS and cdma2000 technologies. Although most often referred to as TDMA, this is an incorrect reference to the protocols. IS-54 and IS-136 do, however, use Time Division Multiple Access as an air interface.
D-AMPS uses existing AMPS channels and allows for smooth transition between digital and analog systems in the same area. Capacity was increased over the preceding analog design by dividing each 30 kHz channel pair into three time slots and digitally compressing the voice data, yielding three times the call capacity in a single cell. A digital system also made calls more secure because analog scanners could not access digital signals. Calls were encrypted, although the algorithm used (CMEA) was later found to be weak. 
IS-136 added a number of features to the original IS-54 specification, including text messaging, circuit switched data (CSD), and an improved compression protocol. SMS and CSD were both available as part of the GSM protocol, and IS-136 implemented them in a nearly identical fashion.
Large IS-136 networks include Cingular Wireless and U.S. Cellular in the United States, and Rogers Wireless in Canada. Cingular (who purchased AT&T Wireless in October 2004) and Rogers Wireless are upgrading their existing IS-136 networks to GSM/GPRS, while US Celluar is migrating most of their network to cdma2000. Rogers Wireless removed all 1900 MHz IS-136 in 2003, and is slowly doing the same with their 800 MHz spectrum as the equipment fails. It is not unusual for a Rogers D-AMPS customer to have coverage in a specific location one day, and then none in that same area the next.