Mobile communication has become an inherent part of our lives so much so that it is difficult for us to imagine being unable to connect to the world through our cell phones. A cell phone facilitates duplex communication and automatically switches the link to a new cell tower when the user moves to a new cell area.
An area covered by cellular networks will be divided into different cells. The frequency used in a cell can be reused by some other distant cell without leading to any interference. Reduced power usage, increased capacity, reduced interference from other cells and larger coverage area are some advantages of cellular networks.
What Are The Codes Associated With Mobile Phones?
The codes on the mobile phone include:
- Electronic serial number (ESN): It is a unique 32 bit number programmed in the phone.
- Mobile identification number (MIN): It is the ten digit number derived from the phone’s number.
- System Identification code (SID): It is a unique 5 digit number assigned to each carrier.
ESN is permanent while SID and MIN are programmed in the phone when you activate a service plan.
When we use a mobile phone to make calls, one frequency is used for talking while another frequency is used for listening. This is how a mobile phone works as a duplex device.
Things That Happen When We Make A Call
Here are the things that happen when we make a call.
- As we switch on our mobile phone, it tries for a SID on the control channel. The phone and base station communicate with one another using the special frequency, control channel. If the mobile phone is unable to access the link to the control channel, it will display a “no service” sign.
- If the mobile gets the SID, it makes a comparison between that SID and the SID programmed in the phone. If the SIDs match, the phone recognizes that it is communicating with the cell that is part of the home system.
- Your phone sends a registration request and the SID to the MTSO. The MTSO will keep track of the phone location in a database.
- On getting the signal, the MTSO tries to find the phone. It will check the database to find out which cell the phone is in. The MTSO then selects a frequency pair to take the call.
- The MTSO will initiate a communication with the mobile phone via the control channel to tell what frequencies to use. The call is connected when the mobile phone and tower switch to those frequencies.
Subscriber Identification Module (SIM) is a detachable smart card that contains the user’s subscription information. This ensures that the user information is retained even after they switch off the mobile phone. If the users have to switch between service providers, all they have to do is change the SIM. The SIM card stores the 15 digit service subscriber key.
The first 3 digits of the service subscriber key are the mobile country code, the next 2 digits for the mobile network code and the last 10 digits are the mobile identification number.
When you are using the mobile phone for the first time, it sends a number, International Mobile Subscriber Identity (IMSI) to the network, which then checks up the database to ensure that the IMSI is registered. On recognizing the IMSI, the network creates another number, TMSI (Temporary Mobile Subscriber Identity), which is sent back to the phone after being encrypted.
The GSM System
The GSM system or 2G system is considered as the standard for mobile communications across the world. The five different cell sizes in a GSM network are macro, micro, femto, pico and umbrella cells.
In the case of macro cells, the base station antenna is installed on a mast above the average roof level while the antenna height in micro cells is less than the average roof height. A pico cell has a coverage area that does not exceed a few dozen meters. Femtocells are used in small business and residential environments.
Umbrella cells cover the shadowed regions of smaller cells and fill the coverage gap between the cells. Various factors including the antenna gain, antenna height and propagation conditions influence the propagation radius of the antenna. GSM supports a maximum distance of 35 kilometers.
What Are The Channels Used In GSM?
Two types of channels used in GSM are:
Control channel tells the mobile when a call is coming and tells which frequency to use. To ensure the handover tasks go smooth, the phone keeps constantly monitoring the broadcast channels of about 16 neighboring cells. In case, a phone moves away from a base station, the network checks up the list of neighboring cells and initiates a handover to a neighboring cell that can offer the best signal.
They are used to carry calls and other data from the base station to mobile and vice versa. Voice or text data is carried across the traffic channel in bursts. Each burst has two consecutive strings of bits and each string is 57 bits long.