Phones: Just How Do They Work?
Entirely Reproduced From an article that first
appeared in NewScientist:15/02/03.
There are many other good related mobile phone
articles in NewScientist
Phones: Just How Do They Work?
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.
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.
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?").
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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).
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.
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.
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.
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
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").
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.
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?
there a health risk?
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.
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.
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.
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.
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.
can only get faster
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.
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.
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.
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