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mobile phone tutorial


The mobile phone is an electronic device used for mobile communication that uses a network of specialized base stations known as cell sites.The mobile phones have a low-power transceiver that transmits voice and data to the nearest cell sites, usually not more than 8 to 13 km (approximately 5 to 8 miles) away. When the mobile phone or data device is turned on, it registers with the mobile telephone exchange, or switch, with its unique identifiers, and will then be alerted by the mobile switch when there is an incoming telephone call. The handset constantly listens for the strongest signal being received from the surrounding base stations. As the user moves around the network, the mobile device will “handoff” to various cell sites during calls, or while waiting (idle) between calls it will reselect cell sites.

The working of mobile phones is just like a two way radio that uses radio frequescy waves which are a type of “Electromagnetic field” to communicate information.
The radio signal from the Mobile Phone is transmitted through the air to the antenna of the nearest base station, which then passes the signal on to the network and through to its destination.

There are three common technologies used by 2G cell-phone networks for transmitting information

1) Frequency division multiple access (FDMA)
2) Time division multiple access (TDMA)
3) Code division multiple access (CDMA)

The first word tells you what the access method is. The second word, division, lets you know that it splits calls based on that access method.

1) FDMA puts each call on a separate frequency.
2) TDMA assigns each call a certain portion of time on a designated frequency.
3) CDMA gives a unique code to each call and spreads it over the available frequencies.

The last part of each name is multiple access. This simply means that more than one user can utilize each cell.

1) FDMA : FDMA separates the spectrum into distinct voice channels by splitting it into uniform chunks of bandwidth. To better understand FDMA, think of radio stations: Each station sends its signal at a different frequency within the available band. FDMA is
used mainly for analog transmission. While it is certainly capable of carrying digital information, FDMA is not considered to be an efficient method for digital transmission.

2) TDMA : TDMA is the access method used by the Electronics Industry Alliance and the Telecommunications Industry Association for Interim Standard 54 (IS-54) and Interim Standard 136 (IS-136). Using TDMA, a narrow band that is 30 kHz wide and 6.7 milliseconds long is split time-wise into three time slots.

Narrow band means “channels” in the traditional sense. Each conversation gets the radio for one-third of the time. This is possible because voice data that has been converted to digital information is compressed so that it takes up significantly less transmission space. Therefore, TDMA has three times the capacity of an analog system using the same number of channels. TDMA systems operate in either the 800-MHz (IS-54) or 1900-MHz (IS-136) frequency bands.

GSM :: TDMA is also used as the access technology for Global System for Mobile communications (GSM). However, GSM implements TDMA in a somewhat different and incompatible way from IS-136. Think of GSM and IS-136 as two different operating systems that work on the same processor, like Windows and Linux both working on an Intel Pentium III. GSM systems use encryption to make phone calls more secure. GSM operates in the 900-MHz and 1800-MHz bands in Europe and Asia and in the 850-MHz and 1900-MHz (sometimes referred to as 1.9-GHz) band in the United States. It is used in digital cellular and PCS-based systems. GSM is also the basis for Integrated Digital Enhanced Network (IDEN), a popular system introduced by Motorola and used by Nextel.

3) CDMA :: CDMA takes an entirely different approach from TDMA. CDMA, after digitizing data, spreads it out over the entire available bandwidth. Multiple calls are overlaid on each other on the channel, with each assigned a unique sequence code. CDMA is a form of spread spectrum, which simply means that data is sent in small pieces over a number of the discrete frequencies available for use at any time in the specified range.





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