Starting from boss’ rebuke to mom’s scolding.....from friends’ missed calls to girlfriend’s texts.....from exam reminders to its results.....all comes from this little electronic box...If today we make a list of 5 things that we can’t live without...I suspect that the word CELLPHONE can be excluded...but have you ever thought how this thing works...how by just dialling three numbers you can save lives (and if you are powerful enough like Ob(s)ama, then maybe you can blow up two countries too)...or how the world has been reduced to the keypads...Let’s see...

The first thing comes to mind is who and how on earth someone thought of making a device which can update your facebook status by a couple of flicks of our fingers while roaming in an African safari. Well , history takes us to the late Nineteenth Century...first in 1876 when Alexander Graham Bell invented telephone, then in 1894 when Marconi established the foundation of wireless communication with his radio...so it was just matter of time for someone to combine both these technologies (and some more time to add Google into the potpourri).But nevertheless, finally a Motorola researcher named Martin Cooper officially presented the portable telephone; winning the race against the Bell labs ...and on April 3  1973 he made his first call on his so called portable headset to his rival  Dr. Joel S. Engel of Bell Labs (poor doctor...)

Mobile phone communication is very similar to radio communication. We can consider the mobile phone as a highly sophisticated radio or walkie-talkie but unlike a walkie-talkie which uses a single frequency (that’s why only one person can talk at a time in walkie-talkie not simultaneously) mobile phone uses two different frequencies- one to transmit what we are speaking and other to receive what we are listening to(so theoretically, you CAN talk while your girlfriend is chattering about her new dress).That’s why a CB radio or a walkie-talkie is called a half-duplex device while a mobile is called a full-duplex device. The combination of two frequencies is called a CHANNEL.  

The initial problem for mobile communication was the bandwidth- since the bandwidth (i.e. the frequency range in which the mobile will transmit or receive) was limited (since we can’t simultaneously send information in the same frequency otherwise they’ll mix up creating a new absurd signal), number of channels available was small hence it could only provide mobile communication facility to a small number  of people, but soon engineers found a way to overcome it by reusing the same frequency at different places. What they actually did was this- instead of using up the whole bandwidth in a particular place they created a  number of regions in that place and divided the frequency spectrum(the frequencies available in the bandwidth) among them...places which were distant enough for the frequency to interfere were allotted the same frequency. As we can see here in the diagram below

The same frequency can be used in regions 1,8,9,10,11 since they are far apart so service can be provided here to large number of people by dividing the bandwidth into seven parts each using one seventh of the available channels (denoted by seven colours) these hexagonal regions are called CELLS and due to these cells, mobile phones are also called cell phones.

First generation mobile communication 1G used the analog cell-phone standard called AMPS (Advanced Mobile Phone System) was approved by the FCC and first used in Chicago. AMPS uses a range of frequencies between 824 megahertz (MHz) and 894 MHz The frequencies used in analog voice channels are typically 30 kHz wide 30 kHz was chosen as the standard size because it gives you voice quality comparable to a wired telephone. Normally government provides 832 distinct (i.e. non-interfering) frequencies to the mobile service provider usually 790 for voice and 42 for data (these 42 channels are known as CONTROL CHANNELS). A pair of frequencies (one for transmit and one for receive) is used to create one channel. Thus no. of available channel is 365. These 365 channels are evenly distributed among the seven cells as described and each cell gets around 56 channel thus in AMPS maximum 56 mobile users can talk simultaneously. In second generation 2G instead of using analog signal digital signal was used....which utilised TDMA method which enables each channel to have 168 channels (3 times of analog) thus enabling 168 user to talk simultaneously.

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

•  Frequency division multiple access (FDMA)

•  Time division multiple access (TDMA)

•  Code division multiple access (CDMA)                                

•  FDMA puts each call on a separate frequency.

•  TDMA assigns each call a certain portion of time on a designated frequency.

•  CDMA gives a unique code to each call and spreads it over the available frequencies

In TDMA, 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.

The cellular approach requires a large number of base stations in a city of any size. A typical large city can have hundreds of towers. But because so many people are using cell phones, costs remain low per user. Each carrier in each city also runs one central office called the Mobile Telephone Switching Office (MTSO). This office handles all of the phone connections to the normal land-based phone system, and controls all of the base stations in the region.

A cell phone has following codes

Electronic Serial Number (ESN) - a unique 32-bit number programmed into the phone when it is manufactured

Mobile Identification Number (MIN) - a 10-digit number derived from your phone's number

System Identification Code (SID) - a unique 5-digit number that is assigned to each carrier by the FCC

While the ESN is considered a permanent part of the phone, both the MIN and SID codes are programmed into the phone when you purchase a service plan and have the phone activated.

Let's say you have a cell phone, you turn it on and someone tries to call you. Here is what happens to the call:

• When you first power up the phone, it listens for an SID (see sidebar) on the control channel. The control channel is a special frequency that the phone and base station use to talk to one another about things like call set-up and channel changing. If the phone cannot find any control channels to listen to, it knows it is out of range and displays a "no service" message.

• When it receives the SID, the phone compares it to the SID programmed into the phone. If the SIDs match, the phone knows that the cell it is communicating with is part of its home system.

• Along with the SID, the phone also transmits a registration request, and the MTSO keeps track of your phone's location in a database -- this way, the MTSO knows which cell you are in when it wants to ring your phone.

• The MTSO gets the call, and it tries to find you. It looks in its database to see which cell you are in.

• The MTSO picks a frequency pair that your phone will use in that cell to take the call.

• The MTSO communicates with your phone over the control channel to tell it which frequencies to use, and once your phone and the tower switch on those frequencies, the call is connected. Now, you are talking by two-way radio to a friend.

• As you move toward the edge of your cell, your cell's base station notes that your signal strength is diminishing. Meanwhile, the base station in the cell you are moving toward (which is listening and measuring signal strength on all frequencies, not just its own one-seventh) sees your phone's signal strength increasing. The two base stations coordinate with each other through the MTSO, and at some point, your phone gets a signal on a control channel telling it to change frequencies. This hand off switches your phone to the new cell.

All these happen in just few seconds...and this just 2G service method (hence of course your facebook status can’t be updated here). So next time when you call someone....do think of the long, amazing, lightening fast yet unnoticeable journey of your voice.

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written by Minakhi Prasad Misra, June 19, 2010

Welcome, buddy. Quite an article. Looking forward to your next.
How about taking us on a quick trip through the intricacies of 3G, the new 'in' thing in the cellular market?

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