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Mobile Phones: Just How Do They Work?

Article Entirely Reproduced From an article that first appeared in NewScientist:15/02/03. There are many other good related mobile phone articles in NewScientist

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Mobile Phones: Just How Do They Work?

IF YOU own a mobile phone, how do you think you 'd cope without it? A recent study by the Italian consumer association looked at the effect of depriving 300 volunteers of their phones for two weeks. Nearly 1 in 6 reported loss of appetite or depression. And a quarter confessed that being phoneless was a blow to their confidence that led to sexual problems with their partners.

It seems cellphones have become an indispensable part of our everyday lives. In Britain, around 70 per cent of the population own a mobile, and in Finland 98 per cent of 18 to 24 year-olds have one. Last year the number of users around the world surged past the billion mark - outstripping landlines for the first time. Their impact is hard to overstate, leading to the emergence of new social behaviours and etiquettes. And the ability to contact anyone from just about anywhere has helped many a stranded traveller and saved more than one soul drifting helplessly out to sea or lost on a mountain range.

But the revolution isn 't all positive. Mobiles are inviting to criminals : Britain 's Home Office estimates that a mobile phone is stolen on average every three minutes. About one third of street robberies in London involve mobile phone theft. And then there 's the health issue. Questions remain about the long-term effect of regularly pressing a mobile phone to your ear, especially for children (see "Is there a health risk?").

The popularity of mobiles is arguably a direct result of an industry decision in 1987 to push ahead with new digital technology in Europe. Until then, mobile phones - which could only fit into the most roomy, reinforced pocket - used analogue technology. Now known as first-generation phones, they worked much like radios that can be tuned into radio stations broadcasting on a particular frequency, except that they could transmit as well as receive. Speech was converted into an analogue electrical signal (which, unlike digital, carries data as a range of values rather than just 1s and 0s). This signal was then used to "modulate" a radio wave called a carrier wave - the wave that actually transmits the signal. Modulation involves raising or lowering the frequency of the carrier wave in proportion to the analogue signal. The signal can then be reconstructed by the receiver by repeatedly checking how much the frequency of the carrier wave has been changed.

Inner Workings Of A Mobile Phone

But first-generation phones had major problems. In the early 1980s, many countries developed their own systems and they were mostly incompatible with each other. Analogue was also inefficient - like radio stations broadcasting on a set frequency, only one conversation could be carried on a given frequency. This severely restricted the number of people who could use a network, which had the knock-on effect that the cost to each user was relatively high. Analogue phones were also prone to interference and were easy to eavesdrop on, leading not only to embarrassing revelations from the private calls of public figures but also to phone "cloning". An analogue phone sends information to the network telling it who you are (so it knows who to charge for the call), but by eavesdropping on the call, your identity could be stolen and programmed into another phone. So you 'd be charged for any calls from it.

It became evident that if mobiles were ever to become ubiquitous, analogue wasn 't up to the job. Going digital was seen as the best way to overcome the problems, handle the anticipated surge in users and be flexible enough to allow text messages and other data to be sent.

In 1982, the European Conference of Postal and Telecommunications Administrations set up the GSM (Groupe Sp Écial Mobile) to develop a Europe-wide standard for second-generation mobile communications. After five years of wrangling and testing, the group voted to pursue digital technology and, in changing GSM to stand for "Global System for Mobile Communications", proposed the standard for worldwide adoption.

Although there are other kinds of digital network in place around the world, GSM networks are now by far the most common. Catering for more than 70 per cent of all digital mobile phone users, GSM is the only system used throughout Europe, Australia, the Arab world and sub-Saharan Africa. It 's the dominant network in Asia and also covers North America and several South American countries.

In Europe, GSM networks and phones send and receive data over radio waves at around 900 or 1800 megahertz. In the US, the frequency used is around 1900 MHz. A lot of mobile phones are designed to work in other countries and are either "dual band", meaning they work on 900 MHz and 1800 MHz networks, or "tri-band", meaning they can work on 1900 MHz networks as well.

Each GSM network is allocated two frequency ranges or bands of up to 25 MHz each. One band is used by phones to contact the network and the other band is used by the network to contact phones. The capacity of each band is limited, so if each person registered with a network in France, for example, had to use a specific frequency to make a call, the two batches of 25 MHz allocated to French networks would quickly be used up. So network operators devised ways of squeezing more out of the scarce bandwidth available.

The first trick was borrowed from the old analogue systems and involves dividing the entire region that the network covers into a patchwork of cells (see Figure). People in different cells can use the same frequencies without their calls interfering. Each cell has a base station that transmits and receives signals over just a small fraction of the frequencies to which the network operator has access. To avoid interference, neighbouring cells must use different frequencies, so the available radio spectrum is effectively divided up between a cluster of cells. In this way, frequencies can be re-used in other cell clusters, allowing far more users onto the airwaves without any risk of their signals interfering.

The power of a base station determines the size of its cell. In areas with few people, high-power base stations are used to produce hyper cells that can provide coverage up to about a 20-kilometre radius. In densely populated areas such as cities, low-power base stations produce micro cells that usually cover a 50 to 300-metre radius. While cells are often thought of as circular, they can also be long and narrow. These selective or directional cells are produced by base stations that send out narrow beams at the entrances to tunnels or along roads in rural areas.

To squeeze even more capacity out of the available airwaves, each band is divided up further into carrier waves, each 200 kilohertz wide (see Figure). Dividing up the spectrum like this is called Frequency Division Multiple Access (FDMA). Each carrier wave is then split up again, but instead of being divided by frequency, it is divided into eight equal time slots called bursts, where each burst lasts less than half a millisecond - a system called Time Division Multiple Access or TDMA. Each burst represents a new channel, so up to eight calls can be conducted at the same time on one carrier wave frequency. Your mobile phone just needs to know what frequency to tune into and what burst number in the repeating frame represents the channel it can use.

Networks Splitting available bandwidth

There are two kinds of channel used in GSM : control channels and traffic channels. Control channels are responsible for housekeeping tasks such as telling the mobile when a call is coming in and which frequency to use. Whenever your phone is powered up, the network records which cell you are in. When a call arrives, it sends a message to your phone in the cell you were last recorded as being in, and usually its immediate neighbours. If you have wandered out of that group of cells, your network will have registered this. If need be, the location of your phone can be determined even more accurately, to a few tens of metres. The network does this by comparing how long it takes a signal from your phone to reach three or more of the base stations nearest to you.

A call often has to be "handed over" to a neighbouring cell as the user moves around, especially in cities where lots of small, low-power cells are common. To ensure this handover works, the phone constantly monitors the broadcast control channel of up to 16 neighbouring cells. The phone works out which signals are strongest and sends a list of the top six back to the base station to which it is currently connected. In normal operation, phones continually adjust the power of the radio waves they send out to be the minimum needed for the base station to receive a clear signal. If a phone moves so far away from its base station that boosting the power no longer improves the signal, the network consults the list and triggers a handover to whichever neighbouring cell should get the best signal. The system isn 't infallible though, as you 'll know if you 've ever made a call from a moving train.

Traffic channels - the second type of channel - are used to carry calls or other data from the mobile phone to the base station and vice versa. On a traffic channel, voice or text data is carried in bursts. Each comprises two consecutive strings of bits (a series of signals representing 1s and 0s), each 57 bits long. But in between these strings of data, the burst carries another string of bits called a training sequence that allows digital phones to overcome one of the problems that plague analogue phones. Radio waves bounce off things like buildings and hills. This can cause interference in analogue phones because it means the waves from the base station follow different paths of different lengths on their way to the phone, so some arrive later than others. Digital phones get round this problem by comparing the training sequence they receive with a copy of the sequence stored in their memory. The phone can then work out how interference has corrupted the signal and correct it. Interference in the voice data is removed using the same corrections.

When the GSM system was being designed, security was a big issue. The upshot is that whenever you use your phone, a complex series of checks is done to ensure three things : that you are who you say you are; that your conversation or other data is encrypted to deter eavesdroppers; and that should it be stolen or lost, your mobile is useless to anyone else. What makes a mobile phone unique to you is the postage stamp-sized SIM card or subscriber identity module that slots into it. Keeping this safe is paramount because, to the network, you are your SIM card. It holds secret numbers that tell the network who you are and that carry out vital calculations confirming your identity and encrypting your calls.

When you use the phone for the very first time, it sends a number held on your SIM card called the International Mobile Subscriber Identity (IMSI) to the network, which looks it up in a database to ensure the card is registered. If the IMSI is recognised, the network creates another number called a Temporary Mobile Subscriber Identity (TMSI), which is encrypted and sent back to the phone. In all subsequent calls, the phone identifies itself by broadcasting the TMSI. This puts in train a series of elaborate authentication and security processes (see Figure).

What Happens When You Make A Call

 

 

Once the TMSI has been broadcast, the network finds the corresponding IMSI for your phone, which tells it what services you have signed up for, like news updates and so on. A part of the network called the Authentication Centre then broadcasts a random number to your phone. This number and a secret authentication number held on the SIM card are fed into an algorithm - a sequence of mathematical functions - to produce a new number. The phone sends this result back to the network. Meanwhile, the network runs the same random number and the user 's authentication code through the same algorithm to give its own result. If the two results match, the phone is given the all-clear. By using this elaborate "challenge-response" approach, the user 's identity can be checked without the phone ever having to send its secret authentication code. If this code were ever broadcast, or even known to the user, it could be used to set up fraudulent calls on the network.

To generate an encryption key for encoding and decoding the data sent and received during the subsequent connection, the SIM card feeds the random number from the network and authentication number into a second algorithm.

Another security check ensures that the user isn 't calling from a stolen handset. Periodically, the network beams a signal to the phone asking it to send in the International Mobile Equipment Identity (IMEI) number held in its memory. The network checks this in an equipment identity register. If the phone is listed as stolen, the network cuts the connection. In Britain, all network providers use a common register, so a stolen phone can be banned from all of them at once. The IMEI is the number you 're supposed to note down when you buy your phone.

While GSM networks were primarily designed to handle voice communications, they increasingly carry other forms of data. Text messaging, which allows blocks of text up to 160 characters long to be sent, has been a huge success with 50 million being sent in Britain alone every day. Texting has led to the evolution of a stripped-down lexicon for communication, and innovations like text voting and news bulletins - as well as a good number of scams.

Despite their tiny screens, it 's also possible to access Web pages from some mobiles. The first mode of access to be developed was WAP ( Wireless Application Protocol). But only pages that have been converted to a WAP format can be downloaded. This severely limits the pages available and at present only text can be displayed. Because of the slow data rates - it takes a minute to download a page - "surfing" with WAP can be time-consuming and expensive (see "Things can only get faster").

In Japan, the hugely successful I- mode phones made by a company called DoCoMo get around the delays WAP users commonly experience by shifting data differently. Standard GSM phones transmit and receive data by circuit switching, which means that a dedicated connection between the phone and the base station must be established. I-mode, on the other hand, uses a system borrowed from the Internet called packet-switching. Data transferred is divided into blocks called packets, each labelled with the address of its final destination. This makes use of all the available bandwidth, rather than reserving channels for specific users. The result is that downloads are quicker and the user pays for the amount of data they receive, rather than the time it takes to download it.

All GSM networks will soon be able to carry packet-switched calls. But improvements in technology will not stop there. Phone makers need to find new reasons for you to upgrade. Right now their hopes are based on camera phones. After video, who knows what 's next?

Cellphone Coverage Cartoon
Is there a health risk?

A vast number of experiments have been performed to see if the electromagnetic (EM) radiation emitted by mobile phones and base stations can damage our health. While there is no compelling evidence of a risk, there are some uncertainties.

Electromagnetic radiation is certainly capable of damaging biological tissue, but precisely how depends upon its frequency. High-frequency EM radiation, such as ultraviolet, gamma or X-rays, can break chemical bonds in living tissue. Lower frequency EM radiation is too weak to cause this kind of damage but is still capable of damaging tissue.

Microwave ovens illustrate what high-power, low-frequency EM radiation can do to raw meat, operating at up to around 900 watts and using EM waves of 2.45 gigahertz. GSM mobiles, on the other hand, use lower frequencies and are limited to a maximum average power output of 0.25 watts at 900 megahertz and 0.125 watts at 1800 megahertz. But most of the time they transmit at just one tenth of this.

The heating effect of radio frequencies is due to tissues absorbing the oscillating field of the wave. EM fields exert a force on charged ions and dipoles such as water in the tissues, producing heat from electrical resistance as they try to move or reorient themselves. Computer models have shown that radiation from a typical mobile phone can cause a maximum temperature rise of around 0.1 °C in the brain.

Base stations, with antennas on masts between 10 and 30 metres high, produce more powerful beams of EM radiation. But the power of the beams falls rapidly with distance. The main beam from a base station hits the ground around 50 metres away, and at this distance the maximum power from a typical 60-watt antenna is around 100 milliwatts per square metre. The heating effect from this is about 5000 times less than that produced by a mobile phone antenna.

Things can only get faster

With the advent of picture messaging, the clamour for improved data transfer rates has become even louder. Basic GSM phones send and receive data at a paltry 9.6 kilobits per second (kbps). This has forced the development of new systems.

One of the first was called High Speed Circuit Switched Data, which lets users receive roughly five times as much data by giving them access to more than one channel. Unfortunately, because multiple channels are devoted to a single user, HSCSD rapidly eats up available bandwidth for a cell.

Among the latest ways to achieve higher data rates is a system called General Packet Radio Service (GPRS). This also allows each phone to use several channels, but they 're shared among many users. Data is simply chopped up into packets, tagged with the address it 's being sent to, and broadcast when a channel is free. The data is then pieced together at the other end. In theory it can provide rates of up to 171 kbps.

The long-delayed third generation or 3G mobile phones, which may finally be available later this year, promise even faster data rates. These will use either the Universal Mobile Telecommunications System (UMTS) that evolved from today 's GSM system, or another called CDMA2000 based on the IS-95 standard common in North and South America. Both systems will be packet-switched and send data using "code division multiple access", which enables "bursts" to carry several signals simultaneously. Maximum rates are expected to be up to 2 megabits per second for UMTS and 70 kbps for CDMA2000 - in theory making video phoning possible.

 
 
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