Sun, 06 Feb 2000

Cellular phones and the mysteries surrounding them

By Zatni Arbi

You may not have noticed, but increasingly people are asking you for your cell phone number instead of your fixed, or wireline, phone number. Certainly, having your cell phone number will mean that they can reach you anytime, even when you are away from your office or home. Perhaps more importantly, when people have your mobile phone number they can call you directly without first having to struggle to convince your secretary that they are worth your time. It is like having a direct channel to you.

The number of cellular phone users in Indonesia today should really embarrass our beloved "plain old telephone service", or POTS, operator. In the more than a half century since we got our independence, PT Telkom and its predecessors have been able to install less than six million fixed telephone lines throughout the country. However, in the only seven or eight years since we first got our cell phone network, the cell phone business in Indonesia has registered more than one-third of that number.

There were 2,050,000 cell phone users at the end of last year, and do not be surprised if in another two or three years the number of cellular phone subscribers surpasses the number of those who subscribe to PT Telkom's fixed line services.

Data from the International Telecommunications Union (ITU) says that this year there are half a billion people worldwide with cell phones in their pockets or purses. The ITU forecasts that the number of cell phone users in the world will overtake the number of fixed phone lines in the year 2010.

Eventually, though, people will just require one single number to contact you. Products from companies such as Alcatel and Dialogic are already available that will forward your calls to you wherever you are. If you are in the office, the calls will be forwarded to the phone on your desk. If you are at home, they will automatically be forwarded to your home phone number.

But what is it that makes this tiny, increasingly inexpensive gadget in your hand so essential? A cell phone is a low powered radio transmitter and receiver. Radios use frequencies, and frequencies can interfere with one another.

But what if frequencies are reused in different areas that are not directly adjacent to each other? That was the question engineers asked decades ago. They found that radio frequencies would not interfere with one another, as long as the power was kept low. So what they did was divide a certain geographical areas into a number of imaginary "cells" and assign different frequencies to adjacent cells. The same frequencies were used in cells that were far enough away from each other not to interfere with each other. It was the idea of dividing a particular geographic area into a number of cells that led to the service being called "cellular".

In each cell, a number of channels within the allocated frequency range are used to transmit signals and conversations from the Base Transceiver Station, or BTS. As the name indicates, a transceiver is a combination of transmitter and receiver that communicates with every cell phone in the cell. One cell needs one BTS, and all of the stations in a network are connected to a Mobile Telephone Exchange, or MTX.

A cell phone user in his car or on the train is expected to be constantly mobile, and may therefore move from one cell to another. It is the task of the BTS in each cell to keep track of who is in its cell. How does it do that? The moment you turn on your cell phone, it transmits a signal to the BTS, basically saying, "I'm here and this is my number."

The BTS picks up the message from the tiny transmitter in your cell phone and sends it to the network's central computer, which will check the data on your number -- including whether you have paid your bills and are allowed to receive and make calls.

The BTS also measures the strength of the signal from your cell phone. When it gets too weak, it may be time for the BTS of another cell to take over. To find out which other cell you are moving into, the central computer in the network keeps comparing the reports from your current BTS with the reports of an adjacent cell that is also receiving your signal. When Cell X is receiving a weaker and weaker signal from your cell phone while Cell Y is receiving increasingly stronger signals, the network will know that you are leaving Cell X and moving into Cell Y. A slight click indicates the BTS of the original cell has "handed over" control to the BTS of the destination cell.

Each time you use your cell phone to make a call, a series of things happen. Your phone will send out a notice to the BTS that it wishes to connect with a phone number. After the accounting matters are verified, the network computer will determine whether the number you want to call is another cell phone that belongs to the same network, a cell phone that belongs to another network or a number that belongs to POTS.

If you are calling a number on the same network, the network computer will check whether that number is active and, if it is, in which cell it is located. Once the computer finds this information, the cell phone of the called party will receive the necessary signal to activate the ringer. When they press the OK button, the MTX will establish the link between your cell phone and their cell phone and you can start talking.

If the person you are trying to call is on another network, the Mobile Telephone Switching Office of your network will take the necessary actions to connect to that other network, which in turn will check whether the cell phone you are calling is active. If it is, the same procedure to connect the phones will be followed, but this time involving two networks.

Every cellular network is also interconnected with POTS, and this makes it possible for someone to call your cell phone using a fixed line phone. At each step of the process the computers of every network will be busy keeping records of the time of the call, the length and the numbers of both the calling and the receiving parties.

The idea of breaking up an area into cells and reusing frequencies so they do not interfere with each other has been around for a long time. Yet, as you can see, due to the complex process involved in measuring the strength of the signal, in keeping the lines of communication open, in calculating the bill for each cell phone user, etc., the cellular phone network became possible only after sophisticated and powerful computers became available. It is simply impossible to handle all of this manually.

The first cellular network was tested in the U.S. (where else?) in 1978, one year before Ericsson made its first cell phone. The test site was Chicago, where the experimental cellular system developed by Bell Labs served around 2,000 subscribers. The system, which was called Advanced Mobile Phone System, or AMPS, turned out to be very successful. The rest is history.

Today, in addition to the AMPS standard, we have the Global System for Mobile communications (GSM) and Code Division Multiple Access (CDMA). Last year ITU came up with the standard for a third generation system called International Mobile Telephone- 2000 (IMT-2000). Whatever the standards, cellular phone services work only by breaking down a geographical area into cells.