[Reader-list] How cell phone works

[Supreet Sethi]

     * 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 turned 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 what 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
       will note 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) will be able to see your phone's signal strength
       increasing. The two base stations coordinate themselves 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!