CELLULAR NETWORK SYSTEM - V.J.T.I.
A cellular network allows cellular subscribers to wander
anywhere in the country and remain connected to 'Public Switched Telephone Number'
(PSTN) via their mobile phones. A cellular network has a hierarchical structure and is
formed by connecting the following components:
Mobile Phones, Base Station, Mobile Switching centre. The base station serves a cell,
which could be a few kilometres in diameter. The cells when grouped together form a
cluster. All BSs within a cluster are connected to a Mobile Switching Centre (MSC) using
land - lines. The MSC stores information about the subscribers located within the cluster
and is responsible for directing calls to them. Cell: Shape and Size. Hexagonal shaped
cells are artificial and cannot be generated in real world. The shape is chosen to
simplify planning and design of cellular system as hexagons fit together without any
overlap. Another advantage is that it approaches a circular shape that is ideal power
coverage area. The size of cell largely depends on area in which cell is located.
Generally rural areas have few subscribers as compared to urban areas. If each cell has
fixed number of channels the cell size in urban area would have to be smaller to
accommodate more channels. However reducing the size too much would result in using
similar channel frequency to be located closer to each other causing co-channel
interference.
Allowing Mobility of Subscriber.
In order to allow mobility to a subscriber:
 | The cellular network has to have the ability to track the subscriber down when a call is made. |
| Allow subscriber to make a call while not in the home market. |
Tracking down is possible if:
| The system maintains information about the location of subscriber's mobile phone. |
| The mobile phone knows the appropriate channels to await signals from the system. |
There are two channels that are searched by mobile phone:
| Strong dedicated control channel- channel used for transmission of digital control information from base station to mobile phone and vice - versa. |
| Strong paging channel- a channel used by MSC for seeking the mobile phone when a call is made to it. |
During a call, the base station would monitor the signal level from mobile phone. When the mobile phone is moved into a new cell, the signal level will fall to a critical value causing the base station to inform the MSC about this event. The MSC would instruct all the surrounding base station to measure the mobile phone's signal level and transfer control to base station receiving he strongest signal level. This is known as hand-over and occurs within 400ms, so subscribers are hardly aware of the break.
