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Cellular Telephone Basics

by Tom Farley with Mark van der Hoek

Introduction. Cellular radio provides mobile telephone service by employing a network of cell sites distributed over a wide area. A cell site contains a radio transceiver and a base station controller, which manages, sends, and receives traffic from the mobiles in its geographical area to a cellular telephone switch. It also employs a tower and its antennas, and provides a link to the distant cellular switch called a mobile telecommunications switching office. This MTSO places calls from land based telephones to wireless customers, switches calls between cells as mobiles travel across cell boundaries, and authenticates wireless customers before they make calls.

Cellular uses a principle called frequency reuse to greatly increase customers served. Low powered mobiles and radio equipment at each cell site permit the same radio frequencies to be reused in different cells, multiplying calling capacity without creating interference. This spectrum efficient method contrasts sharply with earlier mobile systems that used a high powered, centrally located transmitter, to communicate with high powered car mounted mobiles on a small number of frequencies, channels which were then monopolized and not re-used over a wide area.

Complex signaling routines handle call placements, call requests, handovers, or call transfers from one cell to another, and roaming, moving from one carrier's area to another. Different cellular radio systems use frequency division multiplexing (analog), time division multiplexing (TDMA), and spread spectrum (CDMA) techniques. Despite different operating methods, AMPS, PCS, GSM, E-TACS, and NMT are all cellular radio. That’s because they all rely on a distributed network of cell sites employing frequency reuse. Is your head spinning yet? Let’s ease into this cellular discussion by discussing some history first.

History. United States cellular planning began in the mid I940s - after World War II, but trial service did not begin until 1978, and full deployment in America not until 1984. This delay must seem odd compared to today’s furious pace of wireless development, but there were many reasons for it. Early technology, Bell System ambivalence, and government regulation limited radiotelephone progress.

As the vacuum tube and the transistor made possible the early telephone network, the wireless revolution began only after low cost microprocessors, miniature circuit boards, and digital switching became available. And while AT&T personnel built the finest landline telephone system in the world, Bell System management never truly committed to mobile telephony. The U.S. Federal Communications Commission also contributed to the delay, stalling for decades on granting more frequency space. This limited the number of mobile customers, and thus prevented any new service from developing fully since serving those few subscribers would not make economic sense. For different reasons cellular was delayed overseas as well. Scandinavia, Britain, and Japan had state run telephone companies which operated without competition. But these telcos could not do everything they wanted, whenever they wanted. They, too, suffered under their own state and regional regulatory and bureaucratic interference.

What, then, most limited cellular development? I think it's very simple. No one knew how popular cellular radio would become nor how cheap the service would eventually be. If anyone suspected such a great demand then funding would certainly have flowed. No one knew; cellular instead was thought of as an evolution of early radio telephones, a better way to provide a few people with a telephone for their cars. It was not thought that cellular would revolutionize communications. But indeed it did.

Although theorized for years before, Bell Laboratories' D.H. Ring articulated the cellular concept in 1947 in an unpublished company paper. W.R.Young, writing in The Bell System Technical Journal, said Ring’s paper stated all of cellular’s elements: a network of small geographical areas called cells, a low powered transmitter in each, traffic controlled by a central switch, frequencies reused by different cells and so on. Young states that from 1947 Bell teams “had faith that the means for administering and connecting to many small cells would evolve by the time they were needed.” While cellular waited to evolve, a more simple system was used for mobile telephony, a technology that, as it finally matured, originated some practices that cellular radio later employed.

On June 17, 1946 in Saint Louis, Missouri, AT&T and Southwestern Bell introduced the first American commercial mobile radiotelephone service, it was called simply Mobile Telephone Service or MTS. Car drivers used newly issued vehicle radio-telephone licenses granted to Southwestern Bell by the FCC. These radios operated on six channels in the 150 MHz band with a 60 kHz channel spacing, twice the size of today's analog cellular. 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.

Installed high above Southwestern Bell's headquarters at 1010 Pine Street, a centrally located antenna transmitting 250 watts paged mobiles when a call was for them. Automobiles responded not by transmitting to the headquarters building but to a scattering of receiving sites placed around the city, usually atop neighborhood central switching offices. That’s because automobiles used lower powered transmitters and could not always get a signal back to the middle of town. These central offices relayed the voice traffic back to the manually operated switchboard at the HQ where calls were switched. So, although the receiver sites were passive, merely collecting calls and passing them on, they did presage the cellular network of distributed, interactive cell sites.

One party talked at a time with 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. This is not simplex operation as many people say it was. Simplex, used in business radio, shares a single frequency for both people talking. In MTS and IMTS transmitting and receiving frequencies were different, and offset from each other to prevent interference. Base to mobile might be on 152 MHz and mobile to base might be on 158. This is what we call half duplex, whereby different frequencies for transmit and receive are employed, but only one party talks at a time.

Operators placed all calls so a complex signaling wasn’t required. The Bell System was not interested in automatic dial up and call handling until decades later, instead, independent wireless companies or Radio Common Carriers, pioneered these techniques.

On March 1, 1948 the first fully automatic radiotelephone service began operating in Richmond, Indiana, eliminating the operator to place most calls. In 1964 the Bell System began introducing Improved Mobile Telephone Service or IMTS. In January, 1969 the Bell System employed frequency reuse in a commercial service for the first time.

The chief difference between IMTS and cellular: In older mobile telephone services a single frequency serves an entire area; in cellular that frequency is used again and again. More exactly, a channel is used again and again, a radio channel being a pair of frequencies, one to transmit on and one to receive. (6470 p.s.)

Комментарий:

transceiver – радиопередатчик и радиоприемник в общем корпусе; traffic – поток данных в передающей среде; tower – башня; boundary – граница, межа; to authenticate – проверять подлинность; interference – помехи; to mount – устанавливать, монтировать; routine – программа; to handle – управлять; request – просьба, требование; handover – передача; roaming – странствование; carrier – носитель; division – разделение, распределение; multiplexing – уплотнение; reuse – повторное использование; to spin – кружиться; to ease into – осторожно устанавливать; trial – испытательный; deployment – развёртывание; delay – отсрочка; furious – неистовый; pace – темп; ambivalence – двойственность, неопределённость; to contribute – способствовать; to stall – терять скорость; to grant – предоставлять; to prevent – предотвращать, предупреждать; to delay – задерживать, откладывать; overseas – иностранный; run – управлять; eventually – в итоге, со временем; to suspect – предполагать, подозревать; to articulate – формулировать; to evolve – развиваться; to mature – тщательно обдумывать, прорабатывать, развивать; to originate – порождать; vehicle – транспортное средство; license – лицензия; to install – устанавливать; монтировать; собирать; to respond – реагировать; to scatter – разбрасывать, раскидывать; atop – наверху; to relay – передавать, транслировать; presage – прогнозирование; handset – телефонная трубка; to release – освобождать; simplex - симплекс (редкий режим работы линии связи, когда передача данных от устройства к устройству идёт только в одном направлении); to share – делить, распределять; offset – ответвляться, отходить от чего-л.; to prevent – предотвращать, предупреждать; whereby - посредством чего, при помощи чего; в соответствии с чем.