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A little bit about mobile phones

A little bit about mobile phones

We all use them every day, but most of us have no idea how they work. We asked Dr Tomasz Kawalec from the JU Institute of Physics to explain the science behind one of the most popular modern appliances.





Everything starts with our voice. The sounds we produce, which are nothing more than simply vibrations moving through the air, are transformed into electric signal by a microphone. The next stage depends on whether the signal’s transmission is analogue or digital. In the case of analogue transmission, the signal is sent to a modulator – a device which adjusts radio waves to electronic oscillation. The first mobile phones used frequency modulation (FM); however, this technology has become obsolete.

The following generations of mobile phones, including the current one, as well as numerous other signal transmission systems use digital transmission. This allows our appliances to send not only voice, but also other data, and thanks to encryption, our conversations are significantly harder to intercept. When we speak, electric oscillation is converted to digital data, compressed, encrypted and finally sent to the modulator. The most basic type of digital modulation is called Amplitude Shift Keying (ASK), where radio wave is turned on and off based on the signal’s frequency. This is used in some electric gates. Frequency Shift Keying (FSK) is a more complicated modulation, in which zeroes and ones (off – on) are sent by increasing or decreasing radio wave frequency. Another type of modulation – Phase Shift Keying (PSK) – occurs by varying the signal inputs at a precise time. Mobile phones and Wi-Fi transmitters use complex combinations of digital modulations of amplitude and phase or various types of digital modulations of frequency. Of course, no matter what modulation type is used, the receiving device needs to contain a demodulator, which reverts the incoming signal back to voice or data.

How do mobile networks work?

Modern mobile phones communicate only with Base Transceiver Stations (BTS) and never between themselves, even if they’re extremely close to each other. It ensures a good connection quality, since the stations are always placed on tall buildings, chimneys and poles. Sending and receiving data is quick and efficient, since it is done on different frequencies.

The mobile phone radio network consists of numerous stations that transmit radio waves to and from our phones. These Base Transceiver Stations are operated by controllers which pass the digital signal on to the ‘backbone network’. It contains several key elements:

  • Home Location Register (HLR) – the main database of permanent subscriber information for a mobile network,
  • Visitor Location Register (VLR) – a database that contains information about the subscribers roaming within a mobile switching centre’s (MSC) location area,
  • Equipment Identity Register (EIR) – a database of the IMEI numbers of blacklisted appliances,
  • Mobile Switching Centre (MSC) – the centrepiece of a network switching subsystem,
  • Network Switching Subsystem (NSS) – the component of a GSM system that carries out call switching and mobility management functions for mobile phones roaming on the network of base stations.

Trivia

  • The name “cellular network” comes from the fact that it consists of land areas called cells, each served by at least one fixed-location transceiver. What’s interesting is the maximum size of those cells – about 35 kilometres – is indirectly determined by the speed of light.
  • The battery in the first mobile phone only lasted for 20 minutes of use. But it wasn’t really that terrible, since the phone weighed more than one kilogram and it was really hard to hold near the ear.
  • The communication systems used in the first half of the 20th century, precursors of cellular networks, were only installed in cars due to their excessive weight and energy consumption.
  • We say that cells are ‘breathing’: it means that the area serviced by one BTS can dynamically enlarge or shrink depending on the current number of users.
  • Cellular network makes use of the challenge–response authentication method. The network sends a randomly generated number to our phone. The SIM card uses that number and a secret key to solve certain mathematical equations. The result is then sent back and if it’s verified, the phone can log in.
  • The first cellular network was called Nordic Mobile Telephone and was based in Saudi Arabia.

Behind the scenes: the differences between 2G and 3G phones

2G cellular network (in Poland – GSM):

  • The encryption used in 2G networks is relatively easy to crack. It was designed like this because of the need to force weak encryption or removing it entirely, making sure the network wasn’t used for illegal communication, e.g. terrorist activities in certain countries.
  • By today’s standards, the Internet access offered by GSM is very limited. The service itself was added during the network’s development.
  • The digital data is sent in short impulses. In this way, several phones can operate on one frequency, with each one sending data in its own time slot.

3G cellular network (in Poland – UMTS):

  • Strong encryption and, to a certain extent, control over transmission integrity. It means that the data we send and receive may not be accessed and modified by a third party.
  • 3G phones authenticate the network they access. This may seem strange, but it’s really important, since it eliminates the ‘man in the middle’ threats, in which the attacker uses their own BTS to infiltrate our phone.
  • As opposed to 2G network, fast Internet access was planned from the start.
  • All phones can be logged into one BTS and operate on one frequency thanks to spread-spectrum techniques. This technology offers increased protection from interference.

Original text: www.nauka.uj.edu.pl

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