What
are Cell-Phone Channels?
A single cell in an analog cell-phone system
uses one-seventh of the available
duplex voice
channels. That is, each cell (of
the seven on a hexagonal grid) is using
one-seventh of the available channels so it
has a unique set of frequencies and there
are no collisions:
·
A cell-phone carrier typically gets
832 radio frequencies
to use in a city.
·
Each cell phone uses two frequencies per
call -- a
duplex channel
-- so there are typically
395 voice channels
per carrier. (The other 42 frequencies are
used for
control channels
-- more on this later.)
Therefore, each cell has about
56 voice channels
available. In other words, in any cell, 56
people can be talking on their cell phone at
one time. Analog cellular systems are
considered first-generation mobile
technology, or
1G.
With
digital
transmission methods
(2G), the number of available channels
increases. For example, a
TDMA-based
digital system (more on TDMA later)
can carry three times as many calls as an
analog system, so each cell has about 168
channels available.
Cell phones have
low-power transmitters
in them. Many cell phones have two signal
strengths: 0.6 watts and 3 watts (for
comparison, most CB radios transmit at 4
watts). The
base station
is also transmitting at low power. Low-power
transmitters have two advantages:
·
The
transmissions
of a base station and the phones within its
cell do not make it very far outside that
cell. Therefore, in the figure above, both
of the purple cells can
reuse the same 56 frequencies.
The same frequencies can be reused
extensively across the city.
·
The
power consumption
of the cell phone, which is normally
battery-operated, is relatively low. Low
power means small
batteries,
and this is what has made handheld cellular
phones possible.
The cellular approach requires a large
number of base stations in a city of any
size. A typical large city can have hundreds
of
towers.
But because so many people are using cell
phones, costs remain low per user. Each
carrier in each city also runs one central
office called the
Mobile Telephone Switching Office
(MTSO). This office handles all of the phone
connections to the normal land-based phone
system, and controls all of the base
stations in the region.
Cell-phone Codess
Cell Phone Codes
Electronic Serial Number
(ESN) - a unique 32-bit number
programmed into the phone when it is
manufactured
Mobile Identification Number
(MIN) - a 10-digit number derived
from your phone's number
System Identification Code
(SID) - a unique 5-digit number that
is assigned to each carrier by the
FCC
While the ESN is considered a
permanent part of the phone, both
the MIN and SID codes are programmed
into the phone when you purchase a
service plan and have the phone
activated |
All cell phones have special
codes
associated with them. These codes are used
to identify the phone, the phone's owner and
the service provider.
Let's say you have a
cell phone,
you turn it on and someone tries to call
you. Here is what happens to the call:
·
When you first power up the phone, it
listens for an
SID
(see sidebar) on the
control channel.
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.
·
When it receives the SID, the phone
compares it
to the SID programmed into the phone. If the
SIDs match, the phone knows that the cell it
is communicating with is part of its
home
system.
·
Along with the SID, the phone also transmits
a
registration request,
and the MTSO keeps track of your phone's
location in a database -- this way, the MTSO
knows which cell you are in when it wants to
ring your phone.
·
The
MTSO
gets the call, and it tries to
find you.
It looks in its database to see which cell
you are in.
·
The MTSO
picks a frequency pair
that your phone will use in that cell to
take the call.
·
The MTSO communicates with your phone over
the
control channel
to tell it which frequencies to use, and
once your phone and the tower switch on
those frequencies, the call is
connected.
Now, you are talking by two-way radio to a
friend.
·
As you move toward the edge of your cell,
your cell's
base station
notes that your
signal strength
is diminishing. Meanwhile, the base station
in the cell you are moving toward (which is
listening and measuring signal strength on
all frequencies, not just its own
one-seventh) sees your phone's signal
strength increasing. The two base stations
coordinate with each other through the MTSO,
and at some point, your phone gets a signal
on a control channel telling it to change
frequencies. This
hand off
switches your phone to the new cell.
As you travel, the signal is passed from
cell to cell.
Let's say you're on the phone and you move
from one cell to another -- but the cell you
move into is covered by another service
provider, not yours. Instead of dropping the
call, it'll actually be handed off to the
other service provider.
If the SID on the control channel does not
match the SID programmed into your phone,
then the phone knows it is
roaming.
The MTSO of the cell that you are roaming in
contacts the MTSO of your home system, which
then checks its database to
confirm
that the SID of the phone you are using is
valid. Your home system
verifies
your phone to the local MTSO, which then
tracks your phone as you move through its
cells. And the amazing thing is that all of
this happens within seconds.
The less amazing thing is that you may be
charged insane rates for your roaming call.
On most phones, the word "roam" will come up
on your phone's screen when you leave your
provider's coverage area and enter
another's. If not, you'd better study your
coverage maps carefully -- more than one
person has been unpleasantly surprised by
the cost of roaming. Check your service
contract carefully to find out how much
you're paying when you roam.
Note that if you want to roam
internationally, you'll need a phone that
will work both at home and abroad. Different
countries use different cellular access
technologies. More on those technologies
later. First, let's get some background on
analog
cell-phone technology so we can
understand how the industry has developed.
Learn
how to protect yourself from
harmful radiation emitted by
your cell phone. Click on
the picture below..
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