porfolio ICT

Cellphones are changing the way the world connects. In the early 1990s, only one per cent of the world's population owned a cellphone; today nearly a quarter of people make their phone calls this way. In developing countries, there are on average only five telephones (either land lines or cellphones) per hundred people and cellphones are much more popular; in Cambodia, over 90 percent of all phones are cellphones.

Cellphones are also used in different ways around the world. In the United States, mobiles are still mostly used for voice conversations. In Europe, more people send "texts" (text messages, also known as SMS) from mobile phones than use the internet on personal computers. In Asia, where high-speed "third-generation" (3G) mobile networks and cutting-edge phones are more widely available, more people surf the Web and send emails from mobile phones than in any other way; over a quarter of all Japanese people now use the Internet like this. Since the arrival of high-end cellphones (such as iPhones and Android phones), lots of people now go online by tapping their phones—and "cellphones" have now effectively become fully fledged pocket computers.

The first mobile phones used analog technology. This is pretty much how baked-bean can telephones work too. When you talk on a baked-bean can phone, your voice makes the string vibrate up and down (so fast that you can't see it). The vibrations go up and down like your voice. In other words, they are an analogy of your voice—and that's why we call this analog technology. Some land lines still work in this way today.

What cells do? Suppose several people in your area all want to use their cellphones at the same time. If their phones all send and receive calls in the same way, using the same kind of radio waves, the signals would interfere and scramble together and it would be impossible to tell one call from another. One way to get around this is to use different radio waves for different calls. If each phone call uses a slightly different frequency (the number of up-and-down undulations in a radio wave in one second), the calls are easy to keep separate. They can travel through the air like different radio stations that use different wavebands.

That's fine if there are only a few people calling at once. But suppose you're in the middle of a big city and millions of people are all calling at once. Then you'd need just as many millions of separate frequencies—more than are usually available. The solution is to divide the city up into smaller areas, with each one served by its own masts and base station. These areas are what we call cells and they look like a patchwork of invisible hexagons. Each cell has its base station and masts and all the calls made or received inside that cell are routed through them. Cells enable the system to handle many more calls at once, because each cell uses the same set of frequencies as its neighboring cells. The more cells, the greater the number of calls that can be made at once. This is why urban areas have many more cells than rural areas and why the cells in urban areas are much smaller.

How Cellphone Cells handle Calls?

If a phone in cell A calls a phone in cell B, the call doesn't pass directly between the phones, but from the first phone to mast A and its base station, then to mast B and its base station, and then to the second phone.

Cellphones that are moving between cells (when people are walking along or driving) are regularly sending signals to and from nearby masts so that, at any given time, the cellphone network always knows which mast is closest to which phone.

If a car passenger is making a call and the car drives between cells C, D, and E, the phone call is automatically "handed off" (passed from cell to cell) so the call is not interrupted.

The key to understanding cells is to realize that cellphones and the masts they communicate with are designed to send radio waves only over a limited range; that effectively defines the size of the cells. It's also worth pointing out that this picture is a simplification; it's more accurate to say that the masts sit at the intersections of the cells, but it's a little easier to understand.

How Cellphone Calls Travel?

When you speak into a cellphone, a tiny microphone in the handset converts the up-and-down sounds of your voice into a corresponding up-and-down pattern of electrical signals. A microchip inside the phone turns these signals into strings of numbers. The numbers are packed up into a radio wave and beamed out from the phone's antenna (in some countries, the antenna is called an aerial). The radio wave races through the air at the speed of light until it reaches the nearest cellphone mast.

The mast receives the signals and passes them on to its base station, which effectively coordinates what happens inside each local part of the cellphone network, which is called a cell. From the base station, the calls are routed onward to their destination. Calls made from a cellphone to another cellphone on the same network travel to their destination by being routed to the base station nearest to the destination phone, and finally to that phone itself. Calls made to a cellphone on a different network or a land line follow a more lengthy path. They may have to be routed into the main telephone network before they can reach their ultimate destination.