Service manual

6. Theory Of Operation
6.1GSM System Overview
Note
The following description is intended only as a
preliminary general introduction to the Global
System for Mobile communications (GSM)
cellular network. This description is greatly
simplified and does not illustrate the full
operating capabilities, techniques, or
technology incorporated in the system.
6.2General Cellular Concept
The cellular systems are used to provide radiotelephone service in the frequency range
890-960MHz. A cellular system provides higher call handling capacity and system
availability than would be possible with conventional radiotelephone systems (those
which require total system area coverage on every operaing channel) by dividing the
system coverage area into several adjoining sub-areas or cells.
Each cell contains a base station (cell site) which provides transmitting and receiving
facilities, for an allocated set of duplex frequency pairs (channels). Since each cell is a
relatively small area, both the cell site and the radiotelephone that it supports can operate
at lower power level than would be used in conventional system.
Using this technique, radiation on a given channel is virtually contained in the cell
operating on that channel and, to some extent, those cells directly adjacent to that cell.
Since the coverage area of a cell on a given channel is limited to a small area (relative to
the total system coverage area), a channel may be reused in another cell outside the
coverage area of the first. By this means, several subscribers may operate within the same
geographic area, without interference with each other, on a single channel.
6.3GSM Description
Unlike previous cellular systems, GSM uses digital radio techniques. The GSM system
has the following advantages over previous analogue systems:
• International Roaming – Due to international harmonization and standardization,
it will be possible to make and receive calls in any country which supports a
GSM system.