Telephone on 450 MHz (NMT450), and the UK in 1985 with the Total Access
Control System (TACS). Japan has its own unique analog cellular system
developed by NTT, and Germany and France both have unique systems that are
incompatible with other countries. A list of cellular phone networks can be found
in Appendix A, “Cellular Networks Worldwide” on page 151.
1.2.2 Cellular Systems
Figure 2. Cellular Structure
The diagram above shows how a cellular telephone network is organized. (The
hexagon shapes are a convenient way of illustrating the network, but in real life,
the cells can be very irregular shapes.) The letters in each cell represent the
transmitting and receiving frequencies assigned to each cell. The center cell of
each group of seven cells has a range of frequencies indicated by the letter “A”.
Other cells have different frequencies assigned to them indicated by “B”, “C”,
and so on. You can see that no two adjacent cells have the same set of
frequencies; there are always at least two intervening cells before the
frequencies are re-used. The diagram shows a very simple arrangement with
only seven different sets of frequencies. In real cellular networks, the patterns
can be much more complex with many sets of frequencies.
At first, only the major cities were covered with cellular transmitters. It was not
always economical to cover less populated areas as the initial costs were high
(prices ranged upwards to tens of millions of dollars).
In the early 1980s the capacity of urban cells reached their limits and were
divided up into smaller and smaller cells with lower power transmitters. Since a
mobile cellular telephone (for example, in a car) can receive signals from
different cell transmitters (known as base stations) near cell boundaries, a
mobile switching center (MSC) was created to coordinate the frequencies used
by the different mobile users and cell transmitters. As the mobile user moves
out of a cell, the MSC must decide which is the proper cell to take over the call.
Chapter 1. Introduction 7