Digital AMPS

IS-54 and IS-136 are second-generation (2G) mobile phone systems, known as Digital AMPS (D-AMPS), and a further development of the north-American 1G mobile system AMPS. It was once prevalent throughout the Americas, particularly in the United States and Canada since the first commercial network was deployed in 1993. D-AMPS is considered end-of-life, and existing networks have mostly been replaced by GSM/GPRS or CDMA2000 technologies.

This system is most often referred to as TDMA. That name is based on the abbreviation for time division multiple access, a common multiple access technique which is used in most 2G standards, including GSM, as well as in IS-54 and IS-136. However, D-AMPS has been competing against GSM and systems based on code division multiple access (CDMA) for adoption by the network carriers, although it is now being phased out in favor of GSM/GPRS and CDMA2000 technology.

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 (hence time division) and digitally compressing the voice data, yielding three times the call capacity in a single cell. A digital system also made calls more secure in the beginning, as analogue scanners could not access digital signals. Calls were encrypted, using (CMEA), which 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.

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