Fibre optics, communication and navigation satellites, cellular radio

Modern networks are composed of different channels, of which the three mentioned in the above heading seem to have the greatest potential. Being the least ‘glamorous’, fibre optics have already brought about a silent revolutionary change in the global communication system. The invention of fibre optics in fact neutralized the emerging ‘copper crisis’ during the beginning of the 1970s. Copper was then considered a serious global shortage problem and the diminishing reserves were important parameters in Forrester’s World simulations (see p. 43).

Optical fibres were first commercially available in 1970 and are tiny strands of pure glass no wider than a human hair. This strand can simultaneously carry thousands of digitized telephone calls or their equivalents. Many strands can be joined together into a cable which is only one-sixth of the diameter of a conventional copper cable. Information travels along the strands in the form of extremely fast light pulses zigzagging inside the cladding of the strand. These pulses travel at a higher frequency than electric current and are generated by laser diodes, pulsing at a rate of many million per second. Technically, the process is called PCM or pulse code modulation, which implies that a quantified analogue signal is transmitted in digitally coded form by means of light pulses (see the sampling theorem on page 250). At the receiving end, the light is converted back to an analogue signal.

Compared with ordinary copper cables, optical fibres have a number of advantages, including the following:

  • Higher capacity and speed.
  • Cheap to produce (raw material is sand).
  • Light and easy to handle (takes little space in crowded underground ducts).
  • Need few repeaters to amplify the light pulses.
  • Free of electrical interference.
  • Practically impossible to tap.

From a relatively slow start the progress of installing fibre cables is now accelerating. Several transatlantic submarine optic cables requiring repeater stations only every 150 km have already been laid. These cables compete with satellite links and it is expected that their cost-efficiency will soon surpass that of a corresponding satellite system covering the same distance.

Communication satellites are another technology that has made possible the telecommunications revolution. Such satellites have a geosynchronous orbit 36 000 km above the equator which makes them appear to be stationary when viewed from earth.

Something which the world seems both to need and like is terrestrial cellular radio telephony. The widespread use of mobile phones has already revolutionized the global communication pattern. Cellular radio works by dividing city areas and counties into transmission zones called cells. In every cell there is a low-powered transmitter with a range adapted exactly to the cell. This means that transmitters in cells not adjacent to each other can share the same frequency; a kind of frequency reuse.

More than ten years ago, a global telephony system called IRIDIUM consisting of hundreds of satellites orbiting the Earth, was planned. Their handsets were not intended to be used in cities but in remote places where cellphones should not work. Today it is up and running although its consortium declared itself bankrupt in autumn 1999. One of the reasons was that we now have well built out systems like GSM (Global System for Mobile telephony) that enable cellphones used in one country to work in another. The need for a system like IRIDIUM with its high charges is not so urgent as it was a decade ago. From a military point of view, however, the system seems very attractive and US military has already bought substantial amounts of handsets. This system should be a resistant substitute of conventional cellular telephony supplying the military with (besides telephony): fax, data communication, staff locating, and position fixing.

Another satellite system, originally developed by the US military for navigation of cruise missiles, is the GPS (Global Positioning System). Although there exists similar systems, for example the Russian GLONASS, GPS has set a common standard and is used by the world society. The system which covers the whole world consists of 24 low flying satellites (from which 3 is in reserve) in six orbits with a circulation period of 12 hours. Depending on the sophistication of the receiving equipment, a position can be defined with an accuracy between fifty metres to a few centimetres. Today, the receivers are mass-produced and used, for example, by berry pickers to register the coordinates of the place where wild strawberries grow in the forest.

Source: Skyttner Lars (2006), General Systems Theory: Problems, Perspectives, Practice, Wspc, 2nd Edition.

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