ATM Operation - 10 : Virtual Circuit Switching
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Now let's take a look at how circuit switching works. An important thing about VPI and VCI indicators is they are only valid between switches. This means they change all the time as you travel across the network. This isn't so strange if you compare it to a cross-country trip in a car. As you go across the country, you travel on different highways. Similarly, as an ATM packet goes across the network, it uses different circuit identifiers.
So from the computer to the first switch I use connection 31/34. The first number is the VPI, and the second is the VCI. So, 31/34 means virtual path 31 and virtual channel 34. Then from switch to switch I use 77/291 and then finally, I use 126/453 to reach the final destination. The VPI/VCI values have no value beyond just identifying the route. VPI/VCI values are also randomly selected-the switch just selects which values are available when needed and uses them.
Each switch is configured during the connection set-up phase about what to do with the traffic. It is instructed that if a packet comes in with a specific VPI/VCI, it should go out with another specific value.
Once again, this is like a driving trip. Take I-95 from Philadelphia to New York, then take I-80 from New York to Chicago, then on until you reach the end of your trip.