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In addition to the wireline telephones described above, there exist wire­less instruments. Naturally, the wireless telephones have no wire transmis­sion lines. Nevertheless they have connections of their own: they are all con­nected to the public switched telephone network (PSTN). There are wireless telephones of different kinds. Among them there are cordless telephones and cellular systems. It is supposed that these cellular radio systems will be ex­panded to include global satellite-based telephony.

 

CORDLESS TELEPHONES

Cordless telephones are devices that are employed mostly within a home ₀r office. Cordless telephones have a very limited mobility — up to a hun­dred metres. They essentially serve as a wireless extension to the wiring that exists in a home or office because they are plugged directly into the existing telephone jack. Cordless telephones operate over a pair of frequencies in the 46- and 48-megahertz bands or over a single frequency in the 902-928-mega-hertz band.

 

CELLULAR TELEPHONES

Cellular telephones are transportable by vehicle or personally portable devices that may be used in motor vehicles or by people.

Cellular telephones communicate by radiowave in the 800-900-mega-hertz band. They allow a great degree of mobility within a service region that

may occupy hundreds of square kilometres in area.

It should be noted that all communication with a mobile or portable device within a given cell is made to the base station that serves the cell. Because of the low transmitting power of battery-operated portable devices, specific sending and receiving frequencies of a cell may be reused in other cells within larger geographic areas. Thus, the spectral efficiency of a cellular system is increased by a factor equal to the number of times a frequency may be reused within its service area.

It is interesting to note that the first mobile and portable cellular systems were huge and heavy. But because of the progress in component technology, their weight and size were considerably decreased. For example, the weight of light portables in 1990 was equal to 310 grams. By 1994 their weight was reduced and became equal to 120 grams. This process is continuing. The weight of one of the models of cellular telephones produced in Russia — Motorola 70V — is only 81 grams!

Fig.1. Motorola 70V

Ответьте на вопросы

 

· Do wireless telephones have any connections?

· Where can cordless devices be employed?

· Into what element are they plugged?

· How large may be serving regions of cellular telephones?

· What can you say about the weight of portable cellular systems?