The Improved Mobile Telephone Service
) 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 original US mobile telephone system includes three frequency bands, VHF Low (35–44 MHz, 9 channels), VHF High (152–158 MHz, 11 channels), and UHF (454–460 MHz, 12 channels). See IMTS Frequencies
for the complete channel list. A given provider might have offered service on one, two, or all three bands, although IMTS was never offered on low band (only MTS). 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. 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
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, and the 7 digit mobile number (3 digits of area 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 area 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.
IMTS systems typically had 25 watts of transmitter power at the mobile station and 100-250 Watts at the terminal — unlike the older cellular car telephones that had maximum power output of 3 watts and modern cellular handsets with power outputs of 0.6 watts. Mobile installations normally consisted of a "head unit" or the telephone handset which sat in a cradle with a direct dialing keyboard. These looked and functioned much like a landline, or hardwired, telephone. Unlike cellular handsets, these units passed through a dial tone when the receiver was lifted from the cradle and in this way seemed more like a landline telephone than a cellular handset. There was a separate large radio transceiver chassis, typically measuring at least a foot square and 6 inches high, that was mounted either in the trunk or under the seats of an automobile. These transceivers were connected to the handset cradle with a multi-conductor cable usually around .5 inch thick. The mobile antennas almost always required a hole to be drilled in the body of the car to mount the antenna in; until the 1970's there were no "on-glass" antennas - these were developed later for the cellular car-mounted telephones. The antennas looked much like those used for CB radios and were about 19 in. long (1/4 wavelength at 155 MHz). These mobile telephone systems required a large amount of power (10 to 15 ampères at 12 volts) and this was supplied by thick power cabling connected directly to the automobile's battery. It therefore was quite possible and not uncommon for a IMTS telephone to drain an automobile's battery if used for moderate periods of time without the automobile engine running or if left on overnight. Optionally these units were also connected to the car's horn and were switched to honk the horn as a ringer when receiving an incoming call if the user was away from the car at the time of the call.
The IMTS units were full duplex, meaning that a user could both talk and hear the other party at the same time. This was an improvement over the earlier MTS systems, most of which were half duplex, allowing only one party to transmit at a time; the user had to "push to talk" to speak and then "unkey" the transmitter to hear the other party on the line. In 1960 General Electric introduced the "Progress Line" DTO- series MTS mobiles which were full duplex, although subscribers were still required to press the "push to talk" bar on the handset to speak.
There were also IMTS handheld transceivers (Yaesu's 1982 vintage Traveler) that operated on 2–4 watts, and these were all half duplex. These were essentially a modified "walkie-in the IMTS systems, which fooled the terminal into believing an IMTS mobile was using the system. A compromise existed with the briefcase phone, which had somewhat higher power in the range of 20 watts, and which was full duplex. Typical IMTS briefcase phones were made by Canyon, GCS and Livermore Data Systems.
IMTS base station sites generally covered an area 40-60 miles in diameter. This extended range was due to both their large transmitter power and in many cases higher antenna placement at 200–500 ft. IMTS base stations in larger cites had as many as 7 or 8 channels while rural stations had as few as one or two channels. Each telephone connection required the exclusive use of a channel. Because of this limitation these systems had a much lower capacity than do the cellular systems and all channels busy conditions were common. In larger cities this dictated a very limited number of simultaneous calls.
Each subscriber was given a packet of dialing and use instructions. Roaming (receiving calls out of the “Home area” was achieved by selecting the specific channels used by the tower, and service provider that you were traveling in and then dialing a three-digit code, thereby logging in your land number at that location. This process had to be repeated at each tower, as noted, usually had a range of 50 miles. Some areas only had half-duplex (one-way) communications and required the push-to-talk switch in the handset, between the mouthpiece and the earpiece. Two lights on the “head” indicated busy (red) if no channels were idle and in-use (green) if you were connected to the tower, or depressing the push-to-talk switch.
IMTS technology severely limited the total number of subscribers. In the 70s and the early 80s, before the introduction of cellular phones, there were "waiting lists" of up to 3 years for those wishing to have mobile telephone service. These potential subscribers were literally waiting for other subscribers to disconnect their subscription in order to obtain a mobile telephone number and mobile phone service.
These limitations resulted in low quantity sales and production of IMTS phones and the mobile units were therefore very expensive ($2000 to $4000). Prior to the divestiture of AT&T in 1982, Bell System IMTS subscribers usually leased the equipment at a monthly rate of up to $ 120. Availability of the channels was scarce hence airtime was also quite expensive at $0.70–1.20 per minute. Following the divestiture, customer owned equipment was required by Bell companies and monthly rates then typically ran $ 25 plus air time. Also since there were so few channels it was common for the phones to "queue up" to use a channel and IMTS manufactures competed for the speed with which the units would seize an available channel.
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.