As noted, AMPS was the first generation analogue air-link standard developed in North America and deployed for the first time in the early 1980s. The AMPS air-link was designed primarily for voice service but it did have simple data services including limited short messaging capabilities that were not widely used. In the US and Canada, AMPS-based systems operated in the 800MHz band, and achieved good coverage in most of the populated regions of those countries. Today, with the advent of digital networks, mobile phones that use the AMPS air-link exclusively are now disappearing. In their place have come the `dual-mode’ or Etri-mode’ models that mobile no tracker are backward compatible with AMPS. Although most urban centres will now have digital coverage exclusively, having an AMPS mobile phone may still be necessary when roaming in remote areas that lie beyond the boundaries of digital coverage.
In 1984 the Cellular Telecommunications Industry Association was founded in the United States to provide representation for the growing mobile phone sector and gave its approval for a new digital ‘dual mode’ standard called D-AMPS (ED’ for digital). D-AMPS was the first North American digital cellular standard, which is also known by the official names IS-54 or ANSI-136. An important consideration in creating the first North American digital cellular standard was not only to improve spectral efficiency but also to do so without the need for assigning new spectrum. Spectrum scarcity meant that the digital systems would have to work with the existing allotments for AMPS service. The D-AMPS standard was therefore developed to work with the 30kHz channels established for the analogue system, thus obviating the need for new spectrum allocation and permitting incumbent operators to easily upgrade their 800MHz-based systems from analogue to digital. Like the GSM standard, D-AMPS also uses a time division method for mul-tiplexing and thus earned the name TDMA (Time Division Multiple Access). Whereas each 30KHz of bandwidth in the old AMPS system could support one user only, the D-AMPS version of TDMA could provide service to three callers over the same 30kHz of spectrum because each channel was divided into three unique timeslots. TDMA was developed initially for North America and was adopted by mobile operators in both the US and Canada. It soon faced stiff competition from a second air-link standard called CDMA, which eventually resulted in the rise of two competing standards in those countries.
This fragmentation effectively split North America into two incompatible mobile phone systems, affecting roaming to some degree and, according to some observers, has been partly responsible for the slow adoption of text messaging in North America. Today, former TDMA-based operators, like Cingular in the United States, are changing over to GSM because of its global roaming and widespread appeal GSM also offers improved spectral efficiency over TDMA and a well defined upgrade path to third generation mobile phone service. The transition between these two air-link standards is being made possible with something called GAIT. GAIT is. yet another acronym and stands for ‘GSM ANSI-136 Interoperability Team’, which is a group of experts that has been working on. technical specifications that enable a GSM overlay for TDMA networks. GAIT is also supporting the development of mobile phones capable of roaming on either GSM or TDMA networks.