This document summarizes how cellular phone networks work. It explains that cellular phones use different frequencies for transmitting and receiving, allowing two-way communication. This is unlike older walkie talkies which could only transmit or receive on the same frequency at a time. It also describes how cellular networks divide coverage into cells serviced by towers, and phones switch between cells as users move to maintain continuous coverage over large areas. The network tracks users' locations to know which cell to route calls to.

1. It’s not magic, but it’s like magic

3.  CBs
 Walkie talkies
 “Half Duplex”
system
 Both transmitters
use the same
frequency
 Only one person can
talk at a time
 Range dependant on
transmitter wattage

4. • A cell phone is a full-duplex
device. That means that you
use one frequency for talking
and a second, separate
frequency for listening. Both
people on the call can talk at
once.

5.  The power
consumption of
the cell phone is
relatively low
 Low power means
small batteries,
 Small batteries
made handheld
cellular phones
possible

6.  Installed in cars
 Radio
 Required Powerful
Transmitters (big
black box in the car)
 Could transmit 40 to
50 miles
 One Central Antenna
per city
 About 25 Channels

7.  Cell phones operate
within cells, and they
can switch cells as
they move around.
 Cells give cell phones
incredible range.
Someone using a cell
phone can drive
hundreds of miles and
maintain a
conversation the entire
time because of the
cellular approach.

8.  All cell phones have special
codes called SID (system
identification) programmed
into the onboard SIM chip
 When you first power up the
phone, it listens for a SID on
the control channel.
 When the SID is received,
the phone compares it to the
SID on the SIM – if the SIDs
match, the phone knows
that the cell is
communicating on the home
system
The control channel is a
special frequency that the
phone and base station use
to talk to one another about
things like call set-up and
channel changing. If the
phone cannot find any
control channels to listen
to, it knows it is out of
range and displays a "no
service" message.
A SID is a unique,
5-digit identifier
assigned by the
FCC to each carrier

10.  Along with the SID,
the phone also
transmits a
registration request,
and the MTSO in
range keeps track of
your phone's location
in a database
 In this way, the MTSO
knows which cell you
are in when it wants
to ring your phone.

11.  The MTSO gets the
call, and it tries to
find you.
 Searches its database
to determine which
cell you are in
 The MTSO picks a
frequency pair that
your phone will use
in that cell to take
the call.

12.  The MTSO communicates with your phone over
the control channel to tell it which frequencies
to use, and once your phone and the tower
activate those frequencies, the call is
connected and talking by two-way radio.

13.  As you move toward
the edge of your
cell, your cell's base
station notes that
your signal strength
is diminishing.
 The base station in
the cell you are
moving toward
(which is listening
and measuring signal
strength on all
channels) sees your
phone's signal
strength increasing
 Control of the signal
passes to the next
cell
Sprint Coverage Cell
Map

14.  Analog cellular
systems are
considered first-
generation mobile
technology, or 1G.
 Digital cellular
systems are
considered 2G
 3G is the current
technology built for
smartphones
 carries a much more
complex signal carrying
layers of data, voice
and multi-media