Overview
The original telephone service consisted of two phones in two separate locations connected by a single line. To call multiple locations required multiple phone lines. Cities across America began to be covered with a network of unsightly telephone wires. After the switchboard was invented, a person could call any other phone using a single telephone line. Half a century later, the same situation occurred, except this time it was with data calls instead of voice calls.
With dedicated private lines, two remote computers connect over a distance using a fixed circuit. That circuit cannot be shared by anyone else. But the phone companies do not like their lines being tied up, so they invented circuit switching.
With circuit switching, the caller (normally a computer) dials the other caller and the two have exclusive use of the phone line until they decide to end the communication. Once the communication is finished, the connection ends and the line is available for another caller. Circuit switching works just like a regular voice phone call between two people; they call, they chat, they hang up.
At the beginning of each circuit switched call, the network determines the route of the call. That path, or circuit, is open for the duration of the call. On the next call, the network may choose an alternate path (see Figure 1). With packet switching, the network establishes a permanent route for the call. On each call, the data travels across the same path in the network. In frame-relay networks, this is called a PVC.
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