The Telephone Network

• In the telephone network, when a number is dialed a series of switches is configured to create a path for the voice signal to be transmitted.
• Once, the path is switched there exists a point-to-pont link or circuit between the two phones.
• When a dedicated circuit is established for each connection, we say the network uses circuit switching.
• This method of switching is most appropriate when there is a very steady stream of information to be transmitted, e.g., in voice or video communications.
• Such traffic is sometimes called synchronous.
• This path is (nearly) indistinguishable from a direct connection between the two phones, i.e., a cable connecting them.
• The dialed number and the location of the calling phone determines the sequence of switches in the path.
• With mechanical switches, there used to be a deterministic way for selecting a series of switches from the number that was dialed.
• Today, electronic switches are used that can take many other factors into considerations, such as loads or failures in parts of the network.

  

Figure 2.38: A series of switches allows for arbitrary connections through a network.