Illustration 2.1

To establish a link between two telephone network subscribers, the system establishes a connection through several exchanges, using for each connection one of the limited number of lines available.

For the duration of the call, these lines serve exclusively for the link between the two subscribers. The lines in use are therefore occupied for the whole duration of a call, independently of the amount of information actually being exchanged.

Long distance lines can also be satellite or terrestrial microwave links.

Illustration 2.2

The mobile telephone service uses the conventional telephone network.

Only the line to the mobile subscriber is replaced by a radio link, from the handheld subscriber unit to a radio base station, and a radio or fixed line conection from there to the local exchange as the entry point into the conventional network.

A mobile phone might have additional features, supported by additional installations connected to the base stations. For actual communications it nevertheless remains dependent on the conventional telephone network(s).

Illustration 2.3

The hand-held satellite phones on the left side look and work just like terrestrial cellular phones. If they are used in a location with GSM coverage, they can also connect to such systems. Their antennas need not be pointed to the satellite, but all satellite phones need to « see » the satellite. Consequently they do not work indoors.

The two terminals on the right need to be positioned in a place where the antenna can be pointed into the direction of the satellite. The lower picture shows, how the antenna, connected with the terminal through a cable,  can be placed in such a location, while the actual terminal can be located indoors or in another convenient location.

Illustration 2.4

Different from the telephone network, the Internet does not establish one exclusive link between subscribers, but divides information into small « packets », which are « routed » through any number of intermediaries, automatically choosing the fastest and least congested route.

Illustration 2.5

By using any one of many possible routes, the service provider sends the « packets » over the best possible alternative route. Disrupted or overloaded links are automatically bypassed.