All Mobile 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 Mobile phone, you turn it on and someone tries to Mobile 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 Mobile 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 Mobile it is communicating with is part of its homeregistration request, and the MTSO keeps track of your phone's location in a database -- this way, the MTSO knows which Mobile 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 Mobile 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 Mobile is connected. Now, you are talking by two-way radio to a friend.
- As you move toward the edge of your cell, your Mobile’s base station notes that your signal strength is diminishing. Meanwhile, the base station in the Mobile 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 Mobile.
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 Mobile 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 Mobile-phone technology so we can understand how the industry has developed.
