Improved Mobile Telephone Service
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The Improved Mobile Telephone Service (IMTS) is a "0G" pre-cellular VHF/UHF radio system that links to the PSTN. IMTS was the radiotelephone equivalent of land dial phone service. It was introduced in 1969 as a replacement to Mobile Telephone Service or MTS and improved on most MTS systems by offering direct-dial rather than connections through an operator.
The IMTS system includes three frequency bands, VHF Low (35-44Mhz, 9 channels), VHF High (152-158Mhz, 11 channels), and UHF (454-460Mhz, 12 channels). See IMTS Frequencies for the complete channel list. A given IMTS provider might have offered service on one, two, or all three bands. These were prone to network congestion and interference since a radio closer to the terminal would sometimes take over the channel because of its stronger signal. Cellular networks remedied this problem by decreasing the area covered by one tower (a "cell") and increasing the number of cells. The obvious disadvantage of this is that more towers are required to cover a given area. Thus, IMTS and MTS systems still exist in some remote areas, as it may be the only feasible way to cover a large sparsely-populated area. As of 2005, at least one US company near the Canadian border is known to offer IMTS service, and NorthwesTel is known to offer MTS service in northern Canada. In areas where IMTS has been discontinued, the frequencies used were often allocated to paging systems.
