Improved Mobile Telephone Service (IMTS)

In the beginning, there was dispatch mobile service. The base operator broadcasted a message to you. Everyone could hear it. You responded. Then they had mobile telephone service. You picked up the phone in your car, the operator responded. You asked for the number you wanted and she/he dialed it and connected you. You had the channel to yourself but others could still tune in. Then came Improved Mobile Telephone Service (IMTS). Now you could dial from your car without using an operator with some assurance of privacy. IMTS was the pre-cellular mobile telephone service enhancement introduced in 1965, which permitted full duplex mobile radio communications, as well as other enhancements. The original Mobile Telephone Service was introduced in 1946.

IMTS can be considered “0 G” (zero G) as it is a pre-cellular VHF/UHF radio system that links directly to the Pubic Switch Telephone Network (PSTN). The original US mobile telephone system included 3 frequency bands, VHF Low (35-44 MHz, 9 channels), VHF High (152-158 MHz, 11 channels), and UHF (454-460 MHz, 12 channels). The low band was prone to network congestion and interference. Cellular networks remedied this problem by decreasing the area covered by one tower and increasing the number of cells in that area.

IMTS technology severely limited the number of subscribers able to use the service. In the 70s and 80s, there were “waiting lists” of up to 3 years for those wishing to have mobile telephone service. This limited the sales of IMTS devices driving the prices up to $2000 - $3000. The limit of customer numbers on MTS and IMTS was the driver for investment in cellular networks. In remote regions, this is not the case. In remote regions, obsolescence is the driver, but the lack of a suitable and affordable alternative has resulted in regulatory obstacles: customers do not want the MTS/IMTS service to be withdrawn. Increasing affordability of satellite service, and government investment in cellular expansion is slowly allowing MTS and IMTS to be removed.