If you ve been following our cable telephony series, you know that we ve covered a wide variety of topics relating to traditional switched-circuit telephony and IP telephony. One of the most obvious differences between switched-circuit and IP telephony is the way a call is sent to its destination. Subscribers A through H are shown. The horizontal lines represent connections from the subscriber to the switch. The vertical lines represent connections within the switch itself.
Circles represent connections between subscribers who are talking to each other.
In this scenario, subscriber A is talking with subscriber B, tying up the resources represented by the second vertical line from the left. Similarly, C is talking to H, and E is talking to G. Subscribers D and F are not talking to anyone. When A dialed B s number, the connection represented by those two circles is made in the switch. The trunk resources between the two switches are tied up for the duration of the call, and cannot be shared with other users. Routing is the basis for the Internet, so when we talk about IP telephony, for the most part we are talking about the call being routed rather than switched.