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Unit 11: Wireless WAN

transmissions nearly anywhere in the world. Now that network is being used more and more Notes
for data instead of voice. This is pushing the cellular telephone carriers to switch from analog
to digital transmissions. Among other advantages, digital signals can be encrypted for better
security and can have better error control.
The cellular system uses radio waves to transmit signals. A call on a cellular system uses two
frequencies for each call, one frequency for each direction the signal travels between the base
station and the mobile user. When a call is made using a cellular phone, the phone searches for
the nearest base station. The call is routed through the antenna for that base station to the public
switched telephone network where it is transmitted over that network like a regular phone call.
As the phone moves from one cell to another during a call, the network automatically roams and
switches the call to the next cell.

Did u know? Cellular technology was developed to overcome the problem of more and
more people using mobile phones. In the beginning, engineers would solve the problem
of network growth by adding frequencies or by subdividing the frequencies they had into
narrower and narrower channels. But it became obvious that there would never be enough
individual frequencies to carry all of the calls.

Cellular networks offer a different solution called frequency reuse. Each frequency is only used
in a limited area called a cell. Two cells can use the same frequency if they are separated by a
certain amount that depends on the power of the transceiver and the topography. This increases
the number of “available” frequencies without actually increasing the number of frequencies.
The smaller the cells, the more the frequencies can be reused. In fact, when the size of the cell is
reduced by 50%, capacity will increase four times. Frequency reuse has always been used for radio
and television. Two television stations can use the same frequency if they are at least 150 miles
apart. It is not uncommon to experience radio stations reusing frequencies while driving between
cities with the car radio tuned to a particular station. At a certain point the signal changed from
one station to another with a short time in between when the two signals overlapped.
Figure 11.4: Diagram of Cells

Source: http://k-12.pisd.edu/currinst/network/11_806A_4-2_SG.pdf
Cell sizes (figure 11.4) range from one kilometer to 50 kilometers in radius. Larger cells are used
for less densely populated places and especially for highways where mobile telephone users are

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