what happens when a mobile phone powers up?

Question

How does a mobile come to know that which operator is meant for it. What exactly happens and how a mobile phone attach to the network.
How it is different if the mobile is in roaming area?

Answer 1

A cell phone network is made up of the cell phones themselves, base stations, and the mobile switching center (MSC) hardware which does call routing, etc. The base stations (cell towers) are always broadcasting out control signals that contain a System ID (SID) and how it expects cell phones to identify themselves.

When you power up a cell phone, it hears this and "registers" with the nearest base station and your cell phone sends its Mobile ID Number. This Mobile ID Number is more than just your cell phone number, it also contains its own system ID (SID) number, and it also contains the ESN, Electronic Serial Number that identifies that specific phone. If the base station's SID and cell phone's SID match, then the phone is on its "home" network. If not, then you are "roaming" and connect to another provider's network, usually for additional charges. Your phone usually can "hear" multiple cell towers but connects to the one that has the best signal (signal to noise ratio).

After your phone is registered, your cell phone number plus ESN are forwarded to the MSC, which sends this to a "home location register" or HLR. If it turns out you're not on your home provider's network, it goes to a VLR, Visiting Location Register. This way the cellular network always knows where you are, that is, which cell you're in (what cell tower) so calls can be router to/from your cell phone.