Thursday, October 7, 2010

Cellular telephone system

Cellular telephone system is one of the most important developments in the area of voice networks. Cellular technology supports geographically mobile users, which is not possible in hard-wired network. Cellular Systems accommodate a large number of users over a large geographic area, within a limited frequency spectrum. In cellular networks, the distance between a transmitter and a receiver is unpredictable because it depends on the location of the users. However, cellular telephone systems provide high quality service that is often comparable to that of the land-line telephone systems. High capacity is achieved  by limiting the coverage of each base station transmitter to a small geographic area called a cell, so that the same radio channels may be reused by another base station located some distance away. A sophisticated  switching technique called a handoff enables a call to proceed uninterrupted when the user moves from one cell to another.

 The figure below shows a basic cellular system which consists of mobile stations, base stations and a switching center. The mobile switching center sometime called MTSO (mobile telephone switching office), Since it is responsible for connecting all mobiles to the PSTN in a cellular system. Communication between the base station and the mobile stations is defined by a standard  common air interface (CAI) that specifics four different channel.

 The channels used for voice transmission from the base station to mobiles are called forward voice channels (FVC), and the channels used for voice transmission from the mobiles to the base stations are called revers voice channels (RVC). The two channels responsible for initiating mobile calls are the forward control channel (FCC) and revers control channel (RCC). Control channels often called setup channels because they are only involving in setting up a call and moving it to an unused voice channel.

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