Pre-Cellular (MTS & IMTS)

This section is about early mobile telephone systems pre-cellular. (Yes boys and girls, there was a time before cellular!) Here we’ll talk about the original Mobile Telephone System (MTS) and the improved version called the Improved Mobile Telephone System (IMTS). IMTS was in fairly widespread use until analog cellular (AMPS) came into widespread use in the 1990s.

Mobile Telephone System (MTS)

The Mobile Telephone Service (MTS) was a pre-cellular VHF radio system that linked to the Public Switched Telephone Network (PSTN). MTS was the radiotelephone equivalent of land dial phone service.

The Mobile Telephone Service was one of the earliest mobile telephone standards. It was operator assisted in both directions, meaning that if one were called from a land line the call would be routed to a mobile operator, who would route it to one’s phone. Similarly, to make an outbound call one had to go through the mobile operator, who would ask for the mobile number and the number to be called, and would then place the call.

This service originated with the Bell System, and was first used in St. Louis on June 17, 1946. The original equipment weighed 80 pounds (36 kg), and there were initially only 3 channels for all the users in the metropolitan area, later more licenses were added bringing the total to 32 channels across 3 bands (See IMTS frequencies). This service was used at least into the 1980s in large portions of North America. On October 2, 1946, Motorola communications equipment carried the first calls on Illinois Bell Telephone Company’s new car radiotelephone service in Chicago. Due to the small number of radio frequencies available, the service quickly reached capacity.

MTS was replaced by Improved Mobile Telephone Service (IMTS), introduced in 1964.

Channels

MTS uses 25 VHF radio channels in the United States and Canada. The channels are identified by pairs of letters taken from positions on a North American telephone dial that, when changed to digits, form (for 12-channel mobile sets) 55, 57, 95 and 97.

In the 1960s plan, the VHF high-band allocations provided for 11 channels in the United States: JL, YL, JP, YP, YJ, YK, JS, YS, YR, JK and JR. In Canada, two additional channels were available: JJ and JW.

12-Channel	24-Channel
Mobile 	ID 	Mobile 	Base Station MHz
			Transmit 	Receive
	JJ 		152.480 	157.740
	XJ 	1 	152.495 	157.755
1 	JL 	2 	152.510 	157.770
	XK 	3 	152.525 	157.785
2 	YL 	4 	152.540 	157.800
	XL 	5 	152.555 	157.815
3 	JP 	6 	152.570 	157.830
	XP 	7 	152.585 	157.845
4 	YP 	8 	152.600 	157.860
	XR 	9 	152.615 	157.875
5 	YJ 	10 	152.630 	157.890
	XS 	11 	152.645 	157.905
6 	YK 	12 	152.660 	157.920
	XT 	13 	152.675 	157.935
7 	JS 	14 	152.690 	157.950
	XU 	15 	152.705 	157.965
8 	YS 	16 	152.720 	157.980
	XV 	17 	152.735 	157.995
9 	YR 	18 	152.750 	158.010
	XW 	19 	152.765 	158.025
10 	JK 	20 	152.780 	158.040
	XX 	21 	152.795 	158.055
11 	JR 	22 	152.810 	158.070
	XY 	23 	152.825 	158.085
12 	JW 	24 	152.840 	158.100

Frequency problems and elimination of the service

These channels are prone to network congestion and interference since a radio closer to the terminal will sometimes take over the channel due to having a more powerful signal. The service uses technology that has been manufacturer discontinued for more than three decades.

The driver for replacement in most of North America, particularly large cities, was congestion, the inability of the network to carry more than two dozen channels in a geographic area. Cellular service resolved this congestion problem very effectively, especially since cellular frequencies, typically UHF, do not reach as far as VHF frequencies and can therefore be reused. The ability of a cellular system to use signal strength to choose channels and split cells into smaller units also helps expand channel capacity.

The driver for replacement in remote areas, however, is not network congestion, but obsolescence. Because the equipment is no longer manufactured, companies still using the service must struggle to keep their equipment operating, either by cannibalising from retired equipment or improvising solutions. Due to insufficient traffic, cellular is not a cost-effective replacement. Currently, the only viable solution is satellite telephony, as the small number of “base stations” which orbit the planet serve large geographic regions as they pass over. Cost, however, has been an issue, and the replacement has become acceptable to VHF mobile customers gradually, as the cost of satellite telephony has been dropping and will continue to drop.

Many MTS frequencies are now used for local paging services. They are only found in some parts of rural North America, having been replaced in most areas by cellular service in the 1980s or later.

Operation

All calls were placed by a suitably equipped telephone operator.

Outgoing calls were placed when the operator connected to a base station (originally using a cord board, but by the 1990s could be done by dialing a code sequence from a TOPS position), then announced the call over the channel (giving the channel’s name first), e.g.,

“Dawson Channel calling 2M-2368, 2M-2368, 2M-2368.”

The page would usually be repeated twice more after a pause. The called party had to have their unit on and the volume set at a level that allowed them to notice a call and then listen to the called number. If the called party heard an incoming call, they would then use the microphone to announce they were receiving the call, and the operator would allow the two parties to speak, monitoring for the end of the call and marking a manual ticket for billing.

The format of such “voice-called” mobile customer numbers varied by jurisdiction. B.C. Tel, for example, used seven-character numbers starting with N or H, a digit (often 1), then five more digits for the individual customer. AGT in Alberta used seven-character numbers.

Soon enough, customer equipment, “selective call”, was developed that had a built-in circuit that could be programmed to recognize a five-digit code as its own, in a manner similar to IMTS systems and crude compared to cellular phones. The phone company would assign the customer a number and the customer would have it programmed into the set by the phone company or by a dealer.

The operator would connect to the base station by cord board and key the five-digit number; in the 1990s, phone operators at TOPS positions would key the five digits after dialing the code to initiate the call and then identify the base station; a typical TOPS operator code would consist of a two digit sequence for voice-call or selective call, a three digit sequence for the base station, then the customer number.

The base station would signal the five digit sequence; any and all radios tuned to that base station would detect the sequence and compare it with their own; if it matched, the unit would signal its user with a bell or buzzer, and the user could then answer and announce the identity of their unit, similar to how a voice-call user would respond.

Calls from mobile units to a base station were started by signaling the operator by pressing the microphone button. The operator would plug into the channel (automated TOPS systems routed the incoming call through the normal queue), announce availability and the channel name, and the customer would identify their mobile number. (Security systems were sometimes adopted later to prevent fraud, by the customer giving a confidential number. In an automated connection to the operator, other radios on the base station could hear the transmission from the station, not what the radio user was transmitting.) The customer would then specify the details of their call, either to another mobile or to a landline number, which the operator would then place. The call was manually ticketed prior to the use of TOPS, and automatically ticketed if the mobile user was automatically connected to the TOPS operator.

Very few companies automated MTS to use TOPS as most were able to discontinue MTS services due to the reasons above: they could not meet the service demand except by switching to cellular. Northwestel was one company still offering MTS that tied the base stations into TOPS.

A variant of MTS used short-wave frequencies, and was known as High Frequency or High Frequency-Single Sideband, so named for using frequencies between 3 and 30 MHz. These services required far fewer base stations and were used to reach distant locations over vast territories. The drawback was that the frequencies were extremely noisy from various interference, and were subject to propagation problems due to time of day, mostly due to the sun’s effect on the ionosphere. Northwestel, which discontinued the service shortly after 2000, could not tie this system into TOPS, and had to aurally monitor the channels using speakers to listen for incoming calls.

For billing purposes, many MTS base stations were identified with a very close-by rate center of an automatic exchange. However, if there was no nearby rate center, they became an “Other Place Point”. Phone companies typically identified these, and single points such as an individual telephone in a rural area, using a six-digit combination of 88T-XXX, where T is a digit from 6 to 9, and X is any digit 0-9. For example, Fox Lake, Yukon Territory, was 889-949. This combination would serve the same purpose as the NANP area code and central office code, with its own latitudinal-longitudinal coordinates, to allow a distance to be calculated for rating of a call. This coding system, which was in its zenith of usage during the 1980s and 1990s, was rendered unusable when the North American Numbering Plan administrator withdrew the 88X codes for future use as toll free (e.g. 1-800-) services.

By the end of the 1990s, very few companies still had need of the Other Place Point codes, and other rating arrangements were made. For example, Northwestel would use the nearest exchange for calls to its mobile points from other phone companies, and would code the locations in its billing software for calls within the company’s operating territory. The rate impact was negligible for calls over longer distances.

The move to IMTS and later AMPS Cellular

Due to the lack of usable freqencies, obsolete equipment, and the move to practical cellular, both MTS and IMTS were eliminated completely by and large before 2000. AMPS, the replacement for both MTS and IMTS was later eliminated around 2009.

Improved Mobile Telephone System (IMTS)

The Improved Mobile Telephone Service (IMTS) was a pre-cellular VHF/UHF radio system which linked to the public telephone network. IMTS was the radiotelephone equivalent of land dial phone service. Introduced in 1964, it replaced Mobile Telephone Service (MTS) and improved on most MTS systems by offering direct-dial rather than connections through a live operator, and full-duplex operation so both parties could talk at the same time.

Control head for a General Electric IMTS phone

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). 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.

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.
Terminal

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. There was a separate large radio transceiver chassis, typically measuring at least a foot square and 6 inches high, 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 1970s there were no “on-glass” antennas – these were developed later for the cellular car-mounted telephones. These whip 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 amperes 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 an 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 could honk the horn as a ringer to summon a user who 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, 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 modified “walkie-talkies” with a DTMF (dual tone multi-frequency) keypad attached on the front panel, which fooled the terminal into believing an IMTS mobile was using the system. 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 to 20 watts (depending on how much battery was in the briefcase), and which was full duplex. Typical IMTS briefcase phones were made by Canyon, GCS, SCM Melabs and Livermore Data Systems.
Base station

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 anywhere from 100-500 ft. IMTS base stations in larger cities had as many as 7 or 8 channels while rural stations had as few as one or two channels. Each telephone conversation (connection) required the exclusive use of a channel for the duration. Because of this limitation these systems had a much lower capacity than 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 the user was traveling in and dialing a three-digit code, thereby logging the user’s land number at that location. This process had to be repeated at each tower which, as noted, usually had a range of 40-60 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 connected to the tower, or depressing the push-to-talk switch. There was no encryption and all conversations were public.

Frequencies

The frequencies listed below (in MHz) are those formerly used in the US & Canadian Mobile Telephone Service and the Improved Mobile Telephone Service. The low band “Z” prefixed channels were always operated in the MTS, or manual mode. The “Z” channels were sold at auction by the FCC in approximately 2003 to other services and remain largely unused. The VHF and UHF frequencies have been opened to other services unrelated to mobile telephony and largely reassigned.

The two VHF high-band channels designated JJ and JW were used only in Canada, and were not available for use in the United States.

Channel	Base	  Mobile
 	Frequency Frequency

VHF Low Band
ZO 	35.26 	43.26
ZF 	35.30 	43.30
ZM 	35.38 	43.38
ZH 	35.34 	43.34
ZA 	35.42 	43.32
ZY 	35.46 	43.46
ZR 	35.50 	43.50
ZB 	35.54 	43.54
ZW 	35.62 	43.62
ZL 	35.66 	43.66

VHF High Band
JJ 	152.48 	157.74
JL 	152.51 	157.77
YL 	152.54 	157.80
JP 	152.57 	157.83
YP 	152.60 	157.86
YJ 	152.63 	157.89
YK 	152.66 	157.92
JS 	152.69 	157.95
YS 	152.72 	157.98
YR 	152.75 	158.01
JK 	152.78 	158.04
JR 	152.81 	158.07
JW 	152.84 	158.10

UHF Band
QC 	454.375 459.375
QJ 	454.40 	459.40
QD 	454.425 459.425
QA 	454.45 	459.45
QE 	454.475 459.475
QP 	454.50 	459.50
QK 	454.525 459.525
QB 	454.55 	459.55
QO 	454.575 459.575
QR 	454.60 	459.60
QY 	454.625 459.625
QF 	454.65 	459.65

Limitations

IMTS technology severely limited the total number of subscribers. In the 1970s and the early 1980s, before the introduction of cellular phones, there were “waiting lists” of up to three years for those wishing to have mobile telephone service. These potential subscribers were 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 ($2,000 to $4,000). Prior to the divestiture of AT&T in 1984, 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 to $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 manufacturers 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 did not want the MTS/IMTS service to be withdrawn. Increasing affordability of satellite service, and government investment in cellular expansion allowed MTS and IMTS to be removed.

The end of MTS/IMTS

Advanced Mobile Telephone System (AMPS) finally made headway in the 1990s to the point where MTS and IMTS systems were ultimately shut down. Many of the towers were reused for cellular antennas. Though the physical distance of AMPS was way lower than either MTS or IMTS, the amount of calls per tower increased significantly. But in the end, AMPS eventually was shut down since it too was analog (though it did have digital control channels). Using digital methods such as GSM and CDMA, the amount of calls per channel could increase significantly. So AMPS itself didn’t last much longer than IMTS. IMTS went away by the late 1990s, and AMPS was officially shut down in 2009.

Technical Video on IMTS

Someone made a “Blue Box” that could have conceivably been used on an IMTS system to make “free” phone calls. This person goes into some good details on how the system worked as well as how the device he was using worked. Pretty clever!