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How do cellphones work?
IN TODAY’S high-tech world we tend to take for granted many gadgets around us. Most people who find these “magical” devices essential to their everyday life are only too happy to use them, without fully appreciating the technology behind them.
Cellphones perhaps top the list of such gadgets and appliances. As ubiquitous as they are, to most of us, their workings are nothing less than an enigma. However, there is more to this small piece of equipment than just a phone line. Depending on your cellphone model, you can:
— Store contact information;
— Create tasks or to-do lists;
— Keep track of appointments and set reminders;
— Use the built-in calculator for simple calculations;
— Send or receive e-mails;
— Get information (entertainment, news, stock quotations) from the internet;
— Play simple games, and;
— Integrate other devices such as PDAs, MP3 players and GPS receivers.
But have you ever wondered how a cellphone works? What makes it different from a regular telephone? Let’s start with the basics. In essence, a cellphone is a radio — an extremely sophisticated radio, but a radio nonetheless.
The telephone was invented by Alexander Graham Bell in 1876, and wireless communication can trace its roots to the invention of radio by Nikolai Tesla in the 1880s (formally patented in 1894 by a young Italian named Guglielmo Marconi). It was only natural that these two great technologies would eventually be combined.
The genius of the cellular system is in the division of a city into small cells. This allows extensive frequency reuse across a city, so that millions of people can use cellphones simultaneously. In a typical analogue cellphone system, the cellphone carrier receives about 800 frequencies to use across the city.
The carrier chops up the city into cells. Each cell is typically sized at about 10 square miles (26 square kilometres). Cells are normally thought of as hexagons on a big hexagonal grid. Each cell has a base station that consists of a tower and a small building containing the radio equipment.
A single cell in an analogue system uses one-seventh of the available duplex voice channels. That is, each cell (of the seven on a hexagonal grid) is using one-seventh of the available channels so it has a unique set of frequencies and there are no collisions. A cellphone carrier typically gets 832 radio frequencies to use in a city.
Each cellphone uses two frequencies per call — a duplex channel — so there are typically 395 voice channels per carrier. The other 42 frequencies are used for control channels. Therefore, each cell has about 56 voice channels available.
Cellphones have low-power transmitters in them. Many cellphones have two signal strengths: 0.6 watts and 3 watts. The base station is also transmitting at low power.
Low-power transmitters have two advantages: The transmissions of a base station and the phones within its cell do not make it very far outside that cell. Therefore, both the cells can reuse the same 56 frequencies. The same frequencies can be reused extensively across the city. Secondly, the power consumption of the cellphone, which is normally battery-operated, is relatively low. Low-power means small batteries, and this is what has made handheld cellular phones possible.
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 cellphones, costs remain low per user.
Each carrier in every city also runs one central office called the Mobile Telephone Switching Office (MTSO). This office handles all the phone connections to the normal, land-based phone system and also controls the base stations in the region.
Now, what happens as you (and your cellphone) move from cell to cell? All cellphones have special codes associated with them. These codes are used to identify the phone, their owners and the service providers.
When you turn it on and someone tries to call you, it listens for a Signal Identification (SID) 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 in 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 MTSO keeps track of your phone’s location in a database. This way, MTSO knows which cell you are in when it wants to reach your phone.
When MTSO gets a call, it tries to find you. It looks in its database to see which cell you are in. MTSO picks a frequency pair that your cellphone will use in that cell to take the call.
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 to those frequencies, the call is connected. You are talking via a two-way radio, to a friend.
As you move towards 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 towards (which is listening and measuring signal strength on all frequencies) sees your phone’s signal strength increasing. The two base stations coordinate with each other through MTSO, and at some point, your cellphone gets a signal on a control channel, telling it to change frequencies. This switches over your phone to the new cell and now for roaming calls. If the SID on the control channel does not match the SID programmed in your phone, then it knows it is roaming. The cell MTSO that you are roaming in contacts the MTSO of your home system, which then checks its database to confirm that the SID of cellphone you are using, is valid. Your home system verifies your phone to the local MTSO, which then tracks your phone as you move through its cells. And the amazing thing is that all of this happens within seconds.
It’s fair to say that cellphones have changed the way we go about our business. People talk to each other through technology that was unthinkable just thirty years ago. From teenagers to businessmen to vegetable sellers, scores of people use them to stay connected. The world should thank the makers of this handy gadget. And just think. Only a few decades ago the system described above would have required a roomful of equipment, instead of a contraption that fits in the palm of your hand.
The writer
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