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Even though we use our cell phone everyday, most of us do not know how a cell phone works. We are not aware as to what makes this cell phone different from a regular phone. We hear about the terms, such as, SIM, GSM, PCS, GSM, CDMA and TDMA, but most of us do not know what they stand for or what they are. In here, we shall discuss the basic technology which makes a cell phone work in this amazing way. If you are thinking of buying a new cell phone, there are few points that you must be aware of, which are discussed in this article, which will be discussed in following articles.

One of the basic interesting point about a cell phone that it is a sort of very sophisticated radio In the history of telephony inventions, there were two great landmarks, one, when the telephone was invented by Alexander Graham Bell in 1876, and two, when wireless invention can be traced to Nikolai Tesla in the 1880s, which was eventually formally presented by a young Italian named Guglielmo Marconi in 1894. Today in mobile telephony, the two inventions have integrated together to give you the features that you find in your cell phones.

Before the advent of mobile telephony, people had two-way radio system installed in their cars, which provided the much needed mobile-communications ability. These were called Radio Telephones. With this mobile telephony system, you would find a single central antenna in each of the cities, which would have meant that the radio telephone system installed, had to have a powerful transmitter in order to transmit over 50 miles radius. The systems had about 25 channels altogether and this restricted the number of people using the radio telephone simultaneously. As discussed earlier, this system worked on half-duplex. In later days we had the hand held radios, which are portable and are still being used. The frequencies used in this kind of radio telephones were mainly in the VHF and UHF bands.

The concept of cellular technology, divides a city into small cells. Each cell is typically sized at about 10 square miles. Imagine a city described by small hexagon overlapping each other at its ends and is spread all through the city. It is more like a grid formation that is spread across the city area. This concept allows optimum frequency re-use across the city, and there-by the system is available to millions using a cell phone. In a typical analog system, used in the United States, the 'carrier' company, or the 'operator' as known in Europe, receives a set of frequencies to use for their services that they are providing.

In the analog system, each cell uses one-seventh of the available duplex voice channels available, and therefore each of the cells would have a unique set of frequencies which will not be shared at any one time. Typically a network operator gets 832 radio frequencies to operate in a city and out of this, for a mobile phone call to mature each cell would need two frequencies out of these 832. These two frequencies set up the duplex voice channel.

In using two frequencies per duplex channel, which is required to set up a call, the network operator in effect uses 395 frequencies with the rest of 42 frequencies being used for control of channels. These control frequencies play an important part in mobile telephony, which would be clear as we proceed with this article. If the grid is divided into 7 hexagonal cells, each cell is allocated with about 56 duplex frequency sets for voice channels.

Global System for Mobile Communications (GSM) was first introduced in 1991. It is the first open structured, second generation (2G), digital mobile telephony technology, which replaced the old analog first generation (1G) system. GSM was developed in Europe and is used in more than 214 countries and has more than 80% subscribers, out of the total mobile users in the world. It is estimated that there are over 2 billion people across the globe, using GSM system. The technology uses Time Division Multiplexing (TDMA) transmission system, which provides far more channel capability than the old Analog system, with the availability of 168 channels in each cell.

Coming back to the hexagonal cell formation in a city, each of these cells have a tower, which is a switching station, connecting one mobile to the other, either in the same cell or different one, and establishing a call. If you are driving and having a cellular conversation, as you move from one cell to the other, the previous tower hands over the call to the tower area you have ventured into, providing you with a seamless connectivity in your conversation. Cell phones are low power transmitters, having two signal strengths, 02 watts and 3 watts. In comparison, a citizen's band (CB) radio had 4 watts transmitting power. The base station strength is transmitting at low power too.

There are two advantages having low power transmitters. One, the transmission of the cell phone does not make it further than the particular cell within which it is situated. While this true, the other cells can re-use this very frequency and as such all the frequencies allocated to the individual cells can be re-used in this way. Secondly, a cell phone uses a battery and hence it is designed to consume low power, as otherwise, the battery would run out in no time. The batteries itself are of low power made especially for the cell phones. Therefore, cell phones are designed for low power transmission, in conjunction with the relative area of the cell within which it is transmitting. That is why the network tower has to be within the reach of any of the cell phones that may be within that cell. Hence, in a large city, there are hundreds of these towers, belonging to different service providers. Each such operator would have a central switching office, which is called Mobile Telephone Switching Office (MTSO). This office would generally handle setting up of all phone connections to the land-based phone system, and controls all of the base stations in the region.

The Subscriber Identity Module (SIM) card is a smart card that turns your cell phone intelligent, as much as to let you make and receive phone calls and use the other features provided by your service provider. A SIM card is of the size a little smaller than a postage stamp and goes into a slot behind your cell phone. As you open the back cover, you shall be able to locate the slot where the SIM card is placed. It is a printed circuit board, having a microprocessor circuitry and memory. It contains all your unique information, such as, you as the phone's owner, the SIM card number, your subscription information, the International Mobile Subscriber Identity (IMSI) of your mobile phone and other security details. As soon as you switch on the phone, the information is transmitted to the nearest tower and this is checked for its authenticity and security before you are logged into the network. When you are through, you are ready to make or receive calls and do what-ever other usage of your cell phone.

As you switch on your cell phone, it tries to identify the SID in one of the 42 control channels we had mentioned before. The base station and your cell phone uses this special frequency to talk to each other regarding matters of call set up, channel changing, etc. Each of the service providers is assigned with a System Identification Code (SID), consisting of 5 digits, by FCC. As your phone receives the SID, it compares that with the SID programmed into its SIM card. On matching of the SIDS, your phone concludes that it is communicating with a part of its home system. If your cell phone does not receive the SID, it knows that it is out of range of the transmission and displays a 'no service' message.

Your cell phone also transmits for a registration request, and the Mobile Telephone Switching Office (MTSO) keeps a track on your cell phone's location in its database. When a call comes to your cell phone, the MTSO knows exactly which cell you are in, so that it can send the call alert to you. In the process to mature this incoming call, the MTSO receives the call and tries to locate you. It looks at its database to find you where you are. A frequency pair is picked by MTSO, which is allocated to your phone. The MTISO communicates with your phone to convey the frequencies to be used for the incoming call. This is conveyed to your cell phone over one of the control frequencies and this process starts establishing the call. At this point, your cell phone and the tower, switch on to that frequency when the call gets established.

As mentioned above, as you move to the edge of the cell that you are in, while being connected to your incoming call, the base station of the call senses that your signal strength is going down. You will be soon moving into a next cell. At this time, the next cell senses that your frequency is gaining its strength as you move towards that cell. At this point, the base stations of these two cells co-ordinate through the MYSO, and at some point, as your travel into the next cell, your cell phone is intimated through another control channel to switch to another two sets of frequencies. Again, the relevant tower and your cell phone co-ordinate to switch on the new set of frequencies. This being transparent to you, you move into the next cell having a seamless connectivity to your call. If the cell you that move into, is an area managed by another service provide, your call is not dropped. Simply, the call is handed over to the tower of the other service provider.

If in case the SID, as discussed before, does not match the SID programmed in your cell phone, the phone comes to know that it is on roaming. The MTSO of the cell that you are roaming then contacts your home MTSO and exchanges information to check your validity. Your home MTSO has a look in its database to find out if you have been granted the facility of roaming. This is again done through SID information. The local MTSO, that is the MTSO of the cell that you are roaming, gets verification from your home MTSO and if found correct, the connection is established. The bad side of this is, you will be charged heavily when you venture into an area which puts you into a roaming status. This is how you are connected with your outgoing and incoming calls when you visit abroad, with your cell phone on roaming. If you wish to roam internationally, you would need a phone which works both at home and the country of your visit.

Your home service provider has an arrangement with the service provider of the country that you are visiting, for rendering roaming facility to its subscribers. This is essentially done by the two service providers entering into a service contract with financial implications. This would mean that a mutual arrangement has been set up so that roaming service is provided to the respective subscribers visiting each others country.

The next generation of mobile telephony, the third generation (3G), is already operating in few countries in the world. The 3G technology is an International Telecommunication Union (ITU) specification of the third generation mobile communication technology, offering greater bandwidth, up to 384 Kbps when a device is stationary or as you take your cell phone for a walk, and up to 128 kbps while you are in a car. 3G technology has been designed to work with GSM, TDMA and CDMA.

About the Author:

John Dulaney is an international traveler and an expert in overseas audio video equipment and cell phone communications when traveling abroad. His website Planetomni, carries thousands of multisystem TVs, VCRs, region free DVD players at deeply discounted prices. Planetomni also offers a large selection of prepaid cell phones, cell phone rental and satellite phones for rent or purchase.

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