How stuff works

How Mobile Phones work?
Most of you have asked yourselves how is it possible that mobile phone providers – Celtel, Safaricom and Telco Wireless are able to pinpoint exactly where you are. Even though we don’t care to admit it, it scares the hell out of many of us that there is a virtual Big Brother watching over everyone as long as you have a cell phone in your bag, pocket, car or house.
So how is it possible? Cellular systems are made up of numerous transmitting and receiving sites, whose individual coverage areas partially overlap. The concept of frequency reuse, the same frequency is used by several sites, allows a high traffic density in a wide area. Due to the limited transmission range of terminals, cellular systems are based on a large number of base stations on the infrastructure side, scattered over the area to cover, with each covering a fairly small geographical zone called cells.
The GSM network that most of us use can be broadly divided into three parts: Mobile Station (MS) carried by the subscriber. This is the phone that you have in your pocket. Next, we have Base Station Sub-System (BSS) that folks here call Booster which controls the radio link with your handset. Finally, we have Mobile Switching Centre (MSC) which performs the switching of call between mobile phone users, and between mobile and fixed network users.
So how do Safaricom/Celtel know where you are?
Each mobile station (phone) is given a unique identity. As soon as the mobile phone is turned on, it registers with the network and is authenticated; as such the network could always find the mobile phone. A larger amount of data is being exchanged between your phone and the Mobile Switching Centre at Safaricom/Celtel.
At the service provider (Safaricom/Celltel) there is a Visitor Location Register (VLR). This VLR stores information about mobile subscribers that enter its coverage area, which is associated with the geographical area where the mobile is currently roaming. When there is an incoming call for the mobile, the computers is interrogated about the present address of the VLR.
There is also Home Location Register-HLR. This is a database that contains all data concerning the subscription of the mobile subscriber, i.e. access capabilities, subscribed services and supplementary services like M-Pesa,. It contains information about the VLR that is handling your mobile phone. When you move from area covered by the BSS (Booster) the HLR is updated accordingly. It also provides the Safarcion/Celtel Computers with information about the Mobile Switching Centre area where your phone is actually located to allow incoming calls to be routed immediately to the called party.
At the back of each phone there is 15 digit number that is unique to your phone. This can also be accessed by pressing *#06#. This is called International Mobile Equipment Identifier (IMEI). This is what identifies your phone on the network. In addition tot his and most importantly there is Subscriber Identity Module (SIM). This is a smart card which has a computer, and a memory chip that is permanently installed in the mobile equipment. The subscriber information which includes a unique number called the International Mobile subscriber Identity (IMSI) is stored in the SIM card. SIM card identifies the subscriber to the network.
Every new phone (after year 2000) comes equipped with a Global Position System (GPS) chip that gives geographic coordinates of the phone. Essentially, every time your phone is on, the service provider knows exactly where you are with accuracy of less than ten meters! To those who are fond of threatening others over the phone, desist because the BIG brother is watching.
Most people have heard of how police arrest people based on the information from a mobile phone. This is how it is possible. Every time you switch on a mobile phone, the IMEI number and your IMSI are paired and the information is logged at safaricom/celtel computers. If you steal or buy a stolen phone and possibly there was a major crime related with that phone, the police will capture you based on the IMEI and IMSI pairing. Do not bother to change a SIM card because no matter what you do, the log exists. Take my advice because you might need it, DO NOT BUY stolen mobile phones, they could land you in prison.