PSDN stands for public switched data network, this is another alternate to WAN. A PSDN is a way to lease time to acces other information by other computers. It is maintained and developed by a vendor. They might lease this "line" for personal use or to an organization, which is more likely. Any type of data they access is of no concern to the lessees as all they provide is the "line" or bridge for you to get to where you need to go to get that data or information.
PSDNs save cost because a company does not have to lease the entire network for that given time, all they have to do is pay for the access to the network.
The location where the PSDN networked is accessed is called POP (point of preference). An example of this is the phone number you dial to reach someone else. Once a site has connected to the proper PSDN POP it also has obtains access to other sites connected to the PSDN. Basically the company or organization is paying for the traffic it is sending via the lines to PSDN. There are three protocols affiliated with PSDN, Frame Relay, ATM,, and Ethernet. ATM supports the fastest speed and also can support voice and data communications. They oporate at different speeds and the client can choose which one fits them the best.
