One of the first known communications networks was built by the Arabs and consisted of a series of successive towers with a distance between them of some 5 to 12 km. A message could be coded and transmitted from the first tower to the second using optical signals, and then be passed on along the line until it reached its final destination.

Already in this primitive system we can see all the elements of a genuine communications network:

• Information: This must be meaningful for the end users of the communications network. In order to cross the network, the information needs to be transmitted by means of a signals code.
  
  
• Signals: In the case of the towers, the signals were usually produced using coloured flags or mechanical devices whose shape could be changed. In telecommunications a signal is a function that represents a physical magnitude (voltage, electric current or electromagnetic waves) that changes with respect to an independent variable (time). Signals are transmitted through a transmission channel or medium.
  
  
• Transmission media: In our towers system, the atmosphere, the mechanisms for presenting the signals and even the people who operated the signalling devices themselves were all part of the transmission media, together with the various difficulties that could arise such as noise (fog, rain), attenuation (when the distance was too great and the optical signal could no longer be clearly seen), capacity (the speed at which the people could communicate the signals from one tower to the next).
  
  
• Nodes: These were the towers that received the signals, regenerated them (eliminating errors) and transmitted them further along the line to the next tower. In telecommunications systems there are basically two types of node: repeaters, which simply receive signals and pass them on; and switches, which route each of the signals received in their inputs towards one of their outputs, depending on its destination.