Telephony Radiotelephone Essay Research Paper RADIOTELEPHONEIn addition

Telephony Radiotelephone Essay, Research Paper

RADIOTELEPHONE

In addition to the wireline telephones described in The telephone instrument , there exist a number of wireless instruments that are connected to the public switched telephone network (PSTN). At the present time, these wireless telephones generally fall into one of three categories: cordless telephones, cellular radio systems, or personal communication systems. Eventually these systems will be expanded to include global satellite-based telephony. Cordless telephones.

Cordless telephones are devices that take the place of a telephone instrument within a home or office and permit very limited mobility (up to a hundred metres). Because they are plugged directly into an existing telephone jack, they essentially serve as a wireless extension to the existing home or office wiring. Cordless transceivers communicate with the plugged-in base unit over a pair of frequencies in the 46- and 48-megahertz bands or over a single frequency in the 902-928-megahertz band.

Cellular radio.

Cellular telephones are transportable by vehicle or personally portable devices that may be used in motor vehicles or by pedestrians. Communicating by radiowave in the 800-900-megahertz band, they permit a significant degree of mobility within a defined serving region that may be hundreds of square kilometres in area. In this section, the concept of cellular radio and the development of cellular systems are discussed. cellular telecommunication.

All cellular radio systems exhibit several fundamental characteristics, as summarized in the following:

1. The geographic area served by a cellular radio system is broken up into smaller geographic areas, or cells. Uniform hexagons most frequently are employed to represent these cells on maps and diagrams; in practice, though, radiowaves do not confine themselves to hexagonal areas, so that the actual cells have irregular shapes.

2. All communication with a mobile or portable instrument within a given cell is made to the base station that serves the cell.

3. Because of the low transmitting power of battery-operated portable instruments, specific sending and receiving frequencies assigned to a cell may be reused in other cells within the larger geographic area. Thus, the spectral efficiency of a cellular system (that is, the uses to which it can put its portion of the radio spectrum) is increased by a factor equal to the number of times a frequency may be reused within its service area.

4. As a mobile instrument proceeds from one cell to another during the course of a call, a central controller automatically reroutes the call from the old cell to the new cell without a noticeable interruption in the signal reception. This process is known as handoff. The central controller, or mobile telephone switching office (MTSO), thus acts as an intelligent central office switch that keeps track of the movement of the mobile subscriber.

5. As demand for the radio channels within a given cell increases beyond the capacity of that cell (as measured by the number of calls that may be supported simultaneously), the overloaded cell is split into smaller cells, each with its own base station and central controller. The radio-frequency allocations of the original cellular system are then rearranged to account for the greater number of smaller cells.

Frequency reuse between discontiguous cells and the splitting of cells as demand increases are the concepts that distinguish cellular systems from other radiotelephone systems. They allow cellular providers to serve large metropolitan areas that may contain hundreds of thousands of customers. The first mobile and portable subscriber units for cellular systems were large and heavy. With significant advances in component technology, though, the weight and size of portable transceivers have been significantly reduced. For example, lightweight portables in 1990 may have weighed 310 grams (10 ounces); by 1994 they weighed as little as 120 grams.

Development of cellular systems.

In the United States, interconnection of mobile radio transmitters and receivers (transceivers) with the PSTN began in 1946, with the introduction of mobile telephone service (MTS) by AT&T. The MTS system employed frequencies in either the 35-megahertz band or the 150-megahertz band. A mobile user who wished to place a call from a radiotelephone had to search manually for an unused channel before placing the call. The user then spoke with a mobile operator, who actually dialed the call over the PSTN. The radio connection was simplex i.e., only one party could speak at a time, the call direction being controlled by a push-to-talk switch in the mobile handset.

In 1964 AT&T introduced a second generation of mobile telephony, known as improved mobile telephone service (IMTS). This provided full-duplex operation, automatic dialing, and automatic channel searching. Initially 11 channels were provided in the 152-158-megahertz band, but in 1969 an additional 12 channels were added in the 454-459-megahertz band. Since only 11 (or 12) channels were available for all users of the system within a given geographic area (such as the metropolitan area around a large city) and since each frequency was used only once in that geographic area, the IMTS system faced a high demand for a very limite