Improved Mobile Telephone Service - Technical Information

Technical Information

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). See IMTS Frequencies for the complete channel list. In addition to the Bell system (wireline) 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" on a secondary basis on the same channels, but soon, with the growth of paging, RCC mobile phone services were given back seat. 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 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 2008, at least one U.S. company near the Canadian border was known to offer IMTS service. Also Schuykill Mobile Phone offers IMTS in the Pocono Mountains of Eastern Pennsylvania. Northwestel was known to offer MTS service in northern Canada, but has been withdrawing service from some areas. In areas where IMTS has been discontinued, the frequencies used were often allocated to paging systems.

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 (3 digits of central office code and 4 digits of subscriber number) 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.

Mobiles would originate calls by sending a burst of connect tone, to which the base station responded with a burst of seize tone. The mobile would then respond with its identification, consisting of its central office code and last four digits of the phone number sent at 20 pulses per second, just as in inward dialing but with the addition of rudimentary parity checking. Digits are formed with a pulsetrain of alternating tones, either connect and silence (for odd digits) or connect and guard (for even digits). When the base station received the calling party's identification, it would send dialtone to the mobile. The user would then use the rotary dial, which would send the dialed digits as an alternating 10 pps pulse train (originally, directly formed by the rotary dial) of connect and guard tones.