jueves, 29 de julio de 2010


Cellular radio provides mobile telephone service by employing a network of cell sites distributed over a wide area. Cell sites incorporate a radio transceiver to manage, send and receive traffic from the mobiles in its area, a tower and its antennas, and a link to a distant switch called an MTSO. This mobile telecommunications switching office 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 sharply contrasts 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 frequenices, 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 re-use. Is your head spinning yet? Take it easy. Let's ease into this cellular discussion by discussing some history first.

    United States cellular planning began in the mid 1940s-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. Limited technology, Bell System ambivalence, and government regulation limited radio-telephone progress.
    As the vacuum tube and the transistor made possible the early telephone network, the wireless revolution began only after low cost microprocessors, minature 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 since serving those few customers would not make economic sense. But in Europe, Scandinavia, Britain, and Japan, where state run telephone companies operated without competition, and where regulatory interference was minor, cellular came at the same time or later, not sooner than in America. It remains a question, then, on what the biggest factor limiting cellular development truly was.

    Although theorized for years before, Bell Laboratories' D.H. Ring articulated the cellular concept in 1947 in an unpublished 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 radio-telephone 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, of course, 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 collectng calls and passing them on, they did presage the cellular network of distributed, interactive cell sites.
    Saint Louis Experiment
    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. Transmitting and receiving frequencies were different, offset from each other to prevent interference. Operators placed all calls so a complex signaling routine 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.  The Richmond Radiotelephone Company bested the Bell System by 16 years. AT&T didn't provide automated dialing for most mobiles until 1964, lagging behind automatic switching for wireless as they had done with landline telephony. (As an aside, the Bell System did not retire their last cord switchboard until 1978.) Most systems, though, RCCs included, still operated manually until the 1960s.
    In 1964 the Bell System introduced Improved Mobile Telephone Service or IMTS, a replacement to the badly aging Mobile Telephone System. It worked in full-duplex so people didn't have to press a button to talk. Talk went back and forth just like a regular telephone. It finally permitted direct dialing, automatic channel selection and reduced bandwidth to 25-30 kHz. . Operating details foreshadowed analog cellular routines, the complexity of which we will see soon enough. Here's how AT&T described automatic dialing:
Control equipment at the central office continually chooses an idle channel (if there is one) among the locally equipped complement of channels and marks it with an "idle" tone. All idle mobiles scan these channels and lock onto the one marked with the idle tone. All incoming and outgoing calls are then routed over this channel. Signaling in both directions uses low-speed audio tone pulses for user identification and for dialing
    In January,1969 the Bell System employed frequency reuse in a commercial service for the first time. On a train. From payphones. As we've mentioned before, frequency re-use is the defining principle or concept of cellular. "[D]elighted passengers" on Metroliner trains running between New York City and Washington, D.C. "found they could conveniently make telephone calls while racing along at better than 100 miles an hour."[Paul] Six channels in the 450 MHz band were used again and again in nine zones along the 225 mile route. A computerized control center in Philadelphia managed the system. The main elements of cellular were finally coming into being, and would result in a fully functional system in 1978.

Let's not dismiss early radio systems too quickly, especially since we need to contrast them with cellular radio, to see what makes cellular different. IMTS or the Improved Mobile Telephone System (and its variants) is still around, serving isolated and rural areas not well covered by cellular. Cellular service may be in 90% of urban areas, but it only reaches 30% to 40% of the geographical area of America. [See IMTS] Most IMTS equipment operates in the UHF band. Again, it uses a centrally located transmitter and receiver serving a wide area with a relatively few frequencies and users. Only larger areas will have additional receiving sites like in Saint Louis. Most areas allow you to dial out directly from your car, however, there are still places where the operator comes up on frequency to place the call for you. [See IMTS2] A single customer can drive 25 miles or more from the transmitter, however, only one person at a time can use that channel.

This limited availability of frequencies and their inefficient use were two main reasons for cellular's development. The key to the system is, to be stupidly and offensively repetitive, the concept of frequency reuse. It is 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.

C.I 18878408


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