Improved Mobile Telephone Service

The Improved Mobile Telephone Service (IMTS) was a 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 1964 as a replacement to Mobile Telephone Service or MTS and improved on most MTS systems by offering direct-dial rather than connections through a live operator.

The original Bell System US and Canadian mobile telephone system includes three frequency bands, VHF Low (35-44 MHz, 9 channels), VHF High (152-158 MHz, 11 channels in the U.S., 13 channels in Canada), and UHF (454-460 MHz, 12 channels). Alternative names were "Low Band", "High band" and "UHF". In addition to the Bell system (wireline incumbent) channels, another 7 channels at VHF, and 12 channels at UHF were granted to non-wireline companies designated as "RCCs" (Radio Common Carriers). These RCC channels were adjacent to the Bell System frequencies.

RCCs were also allowed to offer paging services to "beepers" or "pagers" on a secondary basis on the same channels, but soon, with the growth of paging, RCC mobile phone services were given lower priority. Some RCCs utilized IMTS technology, but most adopted the "Secode-2805" system which allowed for simultaneous paging, so after a few years, the predominant provider of mobile telephone service was the Bell System companies.

A given provider might have offered service on one, two, or all three bands, although IMTS was never offered on low band (only MTS, but Whidbey Telephone in Washington State had a custom-designed direct-dial system.) 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 disadvantage of this is 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.

The basic operation of IMTS was very advanced for its time, considering that integrated circuits were not commonly available. The most common IMTS phone, the Motorola TLD-1100 series, used two circuit boards about 8 inches square, to perform the channel scanning and digit decoding process, and all logic was performed with discrete transistors. In a given city, one IMTS base station channel was "marked idle" by the transmission of a steady 2000 Hz "idle" tone. Mobiles would scan the available frequencies and lock on to the channel transmitting the idle tone. When a call was placed to a mobile, the idle tone would change to 1800 Hz "channel seize" tone (the idle tone would appear on another frequency, if available), and the 7 digit mobile number (three digits of the NPA and the last four digits of subscriber number, the NXX was not sent) would be sent out as rotary dial pulses, switching between 2000 and 1800 Hz to represent digits. Any mobile recognizing that the call was for someone else would resume scanning for mark idle tone, while the called mobile would then transmit 2150 Hz "guard" tone back to the base station. This would also initiate ringing at the mobile, and when the mobile subscriber picked up the phone, 1633 Hz "connect" tone would be sent back to the base station to indicate answer supervision and the voice path would be cut through. When the mobile hung up, a burst of alternating 1336 "disconnect" and 1800 Hz "seize" tones would be sent to allow the base station to service another call.

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Wikipedia