Technical data

www.westermo.com Theoretical and general applications 55
A comparison
In order to give a clearer image of OSI we can make a comparison with an everyday
telephone call.
… The physical layer is the telephone network and definitions of the signals that are
transferred.
… The data link layer’s logical link control (LLC) corresponds to the telephone’s speak-
er and microphone. The link layer’s media access control (MAC) corresponds to
the components in the telephone that convert the microphone’s signals to signals
that the telephone can transmit on the network and the reverse for the speaker.
… The network layer corresponds to the telephone’s key pulsing.
… The transport layer can be compared with when you call another subscriber you
dial the telephone number and are then connected up through the transport layer,
which ensures you make contact with the recipient.
… The session layer corresponds to the actual call.
… The conversation has its counterpart in the presentation layer.
… The application layer is the entire call.
B A C K