Radio Link Systems

    This is a connection via a chain of transmitters and receivers.

    There are radio links for both analogue and digital transfer.

    Analogue radio systems can be used to transfer pulse modulated signals while digital
        systems are purely design for digital transmission.

    Each radio link connection requires two radio channels, one in each direction. The
        transmission frequency and the receiving frequency are separated by a few MHz. This is
        a very small difference, bearing in mind the frequency band use.
 
 

     Satellite Systems

    Satellite transmission is similar in principle to the ordinary radio link. Instead of having
         all the stations earthbound, we send some up into the space.

    Communications Satellite rotate at almost exactly the same rate as the earth rotate.
         Compared to the radio link, the satellite has a considerable large range. They are used
         for both in the national network and in the international network.

    There are only a few problems in the transmission characteristics of the satellite link.
         Due to the long distance that the signals have to travel, resulted in a delay (echo) which
         have to be counteracted by the echo suppressors. It has to be recognized that this is a
         communication between two floating bodies in space thus there is always a relative
         movement between the earth and the satellite which can cause errors in digital transmissions.
         However, this may be compensated for by intermediate storage of the information in buffer
         memories.

    The capacity of the telephone channels increase as time goes on as compared to the
         time when the satellite was first launch into space. The Intelsat which was first launch
         in 1965 have 75 duplex telephone channel but today the basic version of the new
         Intelsat VI satellite can handle 80 000 telephone channel.
 
 
 
 
 

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