Improved Mobile Telephone Service
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- "IMTS" redirects here. For Intelligent Multimode Transit System, see Guided bus.
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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 (35–44 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 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 was known to offer IMTS service, and Northwestel was known to offer MTS service in northern Canada.[citation needed] In areas where IMTS has been discontinued, the frequencies used were often allocated to paging systems.
IMTS systems typically had 50–100 watts of transmitter power at both the base station and the mobile subscriber 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 that 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 produced 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 about 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, 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 2–3 ft. long. 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. 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.
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, many 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.
There were also IMTS handheld transceivers that operated on 2–4 watts and these were all half duplex. These were essentially modified "walkie-talkies" with a DTMF (dual tone multi-frequency) keypad attached on the front panel. These units were not very common or practical because they lacked the power to reliably connect to the base station over the distances common in the IMTS systems. A compromise existed with the briefcase phone which had somewhat higher power in the range of 10 watts.
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 11, 12, or 13 radio 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. This also imposed a limit to the total number of subscribers to these systems. 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). Availability of the channels was scarce hence airtime was also quite expensive at $0.70–1.20 per minute while the basic monthly subscription charge was upwards of $100. 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.
