| In GSM mobile phone system the region to be | | | | concerned BS.o Manage handoff of mobile phones |
| covered by the service is divided into hexagonal cells | | | | from a BS to an appropriate one when a mobile user |
| which are typically 5 km in diameter. The size depends | | | | moves.o Monitor the calls in progress for facilitating |
| on the density of traffic and would be larger in | | | | charging for use. |
| sparsely populated areas. At the centre of each | | | | Channel Allocation |
| hexagonal cell there is a base transceiver station (BS) | | | | Before describing how connection is established |
| which sends and receives calls to /from mobile phones | | | | between a mobile phone and a fixed phone as well as |
| within its range. The cell shape is hexagonal as the | | | | between two mobile phones we will have to |
| distance between centres of neighbouring hexagons is | | | | understand how channels are allocated to mobile |
| constant. This property is useful to ensure easy hand | | | | phones when they want to communicate. The most |
| off, i.e., transfer of control of a mobile phone from one | | | | expensive resource in mobile phone system is the |
| cell to an appropriate adjacent cell when the user of | | | | electromagnetic spectrum allocated for this purpose .A |
| the mobile phone moves. The base station has several | | | | specific band of frequencies is allocated to a cell |
| antennas, a controller and a set of transceivers, one | | | | phone company by the government which has to be |
| transceiver for each channel assigned to that cell. The | | | | used judiciously by the company to support as many |
| Base Stations are all connected to a Mobile | | | | mobile subscribers as possible. In GSM system this |
| Telecommunication Switching Office (MTSO) usually | | | | band is divided into a set of sub bands and one set is |
| by land lines. If the terrain is unsuitable BS may be | | | | allocated to each cell for communication within that cell. |
| connected to MTSO by wireless. MTSO is also | | | | As mobile phones have a limited power (due to small |
| connected to fixed phones through PSTN by a land | | | | size and need to conserve battery life) their |
| line. | | | | transmission has a limited range and thus the sizes of |
| Fixed line to Mobile | | | | the cells are small (a few kms) . Thus a sub band of |
| The functions of MTSO are to: | | | | frequencies allocated to a cell can be reused in other |
| ·o Establish a connection between a fixed phone | | | | cells which are not in their immediate vicinity. This |
| and a mobile phone via BS and vice versa.o Establish | | | | frequency reuse is a very important feature of the |
| a connection between two mobile phones by using | | | | cellular communication system. Now let's see how this |
| their respective BSs.o Allocate appropriate channels to | | | | connection is established. |
| mobile phones to communicate by informing | | | | |