We have explicitly shown the multiplexing symbols to emphasize the division of
the link into channels even though multiplexing can be implicitly included in the switch
fabric.
The end systems, such as computers or telephones, are directly connected to a
switch. We have shown only two end systems for simplicity. When end system A needs
to communicate with end system M, system A needs to request a connection to M that
must be accepted by all switches as well as by M itself. This is called the setup phase;
a circuit (channel) is reserved on each link, and the combination of circuits or channels
defines the dedicated path. After the dedicated path made of connected circuits (channels)
is established, the data-transfer phase can take place. After all data have been transferred,
the circuits are torn down.