Mobile Telephone System

Mobile Telephone System Points : mobile telephone system This system was introduced in the 1980s and designed to support mobile activities, it allows one to use the system in a car, boats, airplanes and other locations. The most successful mobile telephone system is the cellular telephone system developed by. Bell’s laboratories. It is the cellular technology, which made mobile system possible. In this system each geographical area is divided into small regions called cells. Each cell is equipped with low powered radio transmitter and control equipment located in a building called a cell site. The communication in each cell is performed by using a given bandwidth and a centre frequency.

Two cells can be given the same centre frequency if they are widely separated, so that the signal in one cell can not be detected in the other cell. This technique is used to conserve the frequency spectrum and is called frequency reuse.

As shown in the layout the cell site is connected to the Mobile Telephone Switching Office (MTSO). The MTSO is connected to the regular telephone network through the telephone exchange. The MTSO monitors the identification of a call to ensure that it is proper and to see that it refers to a mobile or fixed subscriber. As a mobile telephone travels from one cell into another, the MTSO automatically, with the help of computer controlled equipment, transfers the call from one transmitting and receiving device to the next, i.e., from one cell to another cell. During this transfer no interruption in the call or conversation in progress is made.

When a mobile user wants to make a call. The mobile unit first enquires about the availability of a channel and its centre frequency. After determining the centre frequency, the mobile unit then sends a data message. This data message contains the user’s identification and the number to be called. The speed of this message is 10 Kb/s. The message is accompanied with an error correcting code and is repeated 5 times to minimise errors in the signal due to fading which occurs when tall buildings are present in the area.

The cell site then forward the data message to an MTSO. The MTSO identified the call and check the called number that whether it is a mobile or fixed. When the call is to a mobile unit. The MTSO sends the data message containing the called number to each cell site. Each mobile unit scans, the set up channels in the cell in which it is located. After sensing the data message, the unit which is being called, responds to the corresponding cell site and the cell site responds to the MTSO. The MTSO then provides the two subscribers with voice line connection and the connection is completed and the conversation can begin. When the conversation is terminated and either user “hang up” the MTSO releases both voice lines. The mobile-to-mobile call situation.

When the mobile user wants to make a call to a fixed subscriber. The process of entering of .a cell to MTSO is the same as discussed before. However, the MTSO after ensuring the identification of the called number as being “fixed”. The call is then routed to the local switching office where it is forwarded to its destination. When a fixed subscriber wants to make a call to a mobile user. The call is forwarded to the corresponding toll office. This routing is determined by the area code of the called number. The toll office routes the call to the local switching office and then to the MTSO. The MTSO processed as discussed above and connection is finally completed.