Cell Phone Codes
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.
|
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. |
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. 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.
Roaming
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!
|
