jueves, 29 de julio de 2010


The first commercial American radio-telephone service

On June 17, 1946 in Saint Louis, Missouri, AT&T and Southwestern Bell introduced the first American commercial mobile radio-telephone service to private customers. Mobiles used newly issued vehicle radio-telephone licenses granted to Southwestern Bell by the FCC. They operated on six channels in the 150 MHz band with a 60 kHz channel spacing. [Peterson] Bad cross channel interference, something like cross talk in a landline phone, soon forced Bell to use only three channels. In a rare exception to Bell System practice, subscribers could buy their own radio sets and not AT&T's equipment.

A simplified picture of Radio Telephone Service -- A Non-Zoned System
The diagram above shows a central transmitter serving mobiles over a wide area. One antenna serves a wide area, like a taxi dispatch service. While small cities used this arrangement, radio telephone service was more complicated, using more receiving antennas as depicted below. That's because car mounted transmitters weren't as powerful as the central antenna, thus their signals couldn't always get back to the originating site. That meant, in other words, you needed receiving antennas throughout a large area to funnel radio traffic back to the switch handling the call.. This process of keeping a call going from one zone to another is called a handoff.

The 1946 Bell System Mobile Telephone Service in St. Louis -- A Zoned System
M: mobile R:receiver. PSTN: Public switched telephone network.
As depicted above, in larger cities the Bell System Mobile Telephone Service used a central transmitter to page mobiles and deliver voice traffic on the downlink. Mobiles, based on a signal to noise ratio, selected the nearest receiver to transmit their signal to. In other words, they got messages on one frequency from the central transmitter but they sent their messages to the nearest receiver on a separate frequency.

Placed atop distant central offices, these receivers and antennas could also "be installed in buildings or mounted in weather proof cabinets or poles." They collected the traffic and passed it on to the largest telephone office, where the main mobile equipment and operators resided. [Peterson2]

Installed high above Southwestern Bell's headquarters at 1010 Pine Street, a centrally located antenna transmitting 250 watts paged mobiles and provided radio-telephone traffic on the downlink or forward path, that is, the frequency from the transmitter to the mobile. Operation was straightforward, as the following describes:

How Mobile Telephone Calls Are Handled

Telephone customer (1) dials 'Long Distance' and asks to be connected with the mobile services operator, to whom he gives the telephone number of the vehicle he wants to call. The operator sends out a signal from the radio control terminal (2) which causes a lamp to light and a bell to ring in the mobile unit (3). Occupant answers his telephone, his voice traveling by radio to the nearest receiver (4) and thence by telephone wire.

To place a call from a vehicle, the occupant merely lifts his telephone and presses a 'talk' button. This sends out a radio signal which is picked up by the nearest receiver and transmitted to the operator.[BLR1]
The above text accompanies a Bell Laboratories Record illustration (346K), from the 1946 article that first described the system. It gives you a good idea of how the system worked. Click on the link to view this big, but slow to load graphic.)

Simple block diagrams can be hard to follow. Click here to see another MTS illustration; it is from Bell Labs and my cellular telephone basics article.)

One party talked at a time with Mobile Telephone Service or MTS. You pushed a handset button to talk, then released the button to listen. (This eliminated echo problems which took years to solve before natural, full duplex communications were possible.) Mobile telephone service was not simplex operation as many writers describe, but half duplex operation.
Simplex uses only one frequency to both transmit and receive. In MTS the base station frequency and mobile frequency were offset by five kHz. Privacy is one reason to do this; eavesdroppers could hear only one side of a conversation. Like a citizen's band radio, a caller searched manually for an unused frequency before placing a call. But since there were so few channels this wasn't much of a problem. This does point out greatest problem for conventional radio-telephony: too few channels.
Shortly after this cartoon appeared the July 1948 BLR reported that a taxi cab driver with a mobile phone reported a stuck car on a railroad crossing, thus saving the broken down car and its motorist from disaster. Possibly the first radio-telephone rescue of its kind. This incident happened at a "grade crossing of the Nickel Plate Railroad at Dunkirk, New York." Dr. Scott Savett has found a photograph on the web of a representative Dunkirk rail crossing. The Dr. says, "According to a source on the Web, there were about five grade crossings in Dunkirk, so there's no guarantee that the one shown above is actually the one where the call was made." Still, this photo gives you an idea of the country. Click here to view. I wonder if the county history museum knows of the crossing's place in mobile telephone history.
Art imitating life below. This cartoon is from the April, 1948 issue of The Bell Laboratories Record. It reads, "Hello, Mr. Bunting. I've changed my mind -- I'll take that accident policy!"

Things to come. "All equipped with telephones so that the minute you catch anything you can call all your friends and start bragging." From the September, 1950 Bell Laboratories Record.

C.I 18878408

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