Today data communication may occur also over the air using GSM mobile systems, or using radio bridges, which can be either terrestrial or satellite.

Geographical surface of a nation is divided into many areas of the shape of cells, usually hexagonal, whence the name of cellular telephones.

In the middle of each cell there is a BTS which connected to cellular telephones present in its action area, by electromagnetic waves.

Each cellular telephone is connected via radio to the nearest BTS, the one which is the middle of its cell.

Many BTS placed in adjoining cells are usually connected in the shape of a star by optical fibre cables to a digital switching exchange called MSC.

This station receives the cellular callings sent by the BTS and switches them to the PSTN network, to which the station is directly connected.

Switching and service stations, MSC, connect the radio base station BTS to the PSTN Network.

The MSC also allows  connections between two cellular telephones hence between two BTS near the calling telephones so that MSCs are interconnected as shown in the following picture.

In this way, every single cellular telephone can be connected to any telephone of the PSTN, but also to any other cellular telephone.

WIDENINGS:

• GSM CODING AND WEB
• FREQUENCIES
• GSM SYSTEM
• DECT SYSTEM
• DCS SYSTEM

Radio bridges are bi-directional connections between two fixed exchanges carried out by microwaves using parabolic antennas.

Connections may occur directly between two points of the earth, and so they are called TERRESTRIAL RADIO BRIDGES.

They can connect two distant points on the earth by an artificial telecommunication satellite, then they are called SATELLITE RADIO BRIDGES.

Radio bridges are classified, according to their modulation, in ANALOG and DIGITAL.

The powers varies from 1 Watt to tens of Watts and distances vary from 1 Km to tens of Km.

WIDENINGS:

• PARABOLIC ANTENNAS
• TRANSMISSION CAPACITY
• DIGITAL RADIO BRIDGES
  

Artificial satellites sent into orbit round the earth, from the early sixties till today, are, for the most part, of military kind, while others have scientific purposes.

There are also some hundreds of telecommunication satellites in equatorial geostationary orbit, which work as repeaters between two terrestrial stations very distant from each other, connected by a radio bridge.

According to Keplero laws, the satellite's orbit around a celestial body, applied to the Earth, may be either a circle with the Earth in the middle, or an ellipse with the Earth in one of the two focuses.

WIDENINGS:

• TRANSPONDER
• GEOSTATIONARY ORBITS
• SATELLITE STRUCTURE