• Virtual Circuits
• Dialup Services

Let us characterize each of these classes.  A point-to-point link provides a single, preestablished WAN communications path from the customer premises through a carrier network, such as a telephone company, to a remote network.  A point-to-point link is often called a leased line because its established path is permanent and fixed for each remote network reached through the carrier facilities. The main characteristic of the point-to-point link is that the carrier company reserves these links for the private use of the customer.

In circuit switching, a dedicated physical circuit is established, maintained, and terminated through a carrier network for each communication session.  This method is used in telephone networks.  You can think of circuit switching as a normal telephone call.  The well-known technology, Integrated Services Digital Network (ISDN) is an example of a circuit-switched WAN technology.

Packet switching is also a WAN switching method.  In comparison with circuit switching where the circuit is usually used by a single device or LAN, in packet switching multiple network devices share a single point-to-point link to transport packets from a source to a destination across a carrier network.  The technology that is used in packet switching is called statistical multiplexing.  Examples of packet switching networks are Asynchronous Transfer Mode (ATM), Frame Relay, and X.25.

A virtual circuit is a logical circuit created to ensure reliable communication between two network devices.  This virtual circuit is established over the existing carrier. Some of these virtual circuits are dynamically established on demand and terminated when transmission is complete—they are called switched virtual circuits (SVC).  Another type is a permanently established virtual circuit (PVC). PVCs are used in situations where transfer between devices is constant.

The last type of WAN technology is dialup services.  These services offer cost-effective methods for connectivity across WANs.  There are two popular dialup implementations: dial-on-demand routing (DDR) and dial backup.

Using the DDR technique, a router can dynamically initiate and close a circuit-switched session as transmitting end stations demand.  Certain logic can be implemented on routers.  For example, a router could be configured to consider certain traffic “interesting” (usually this is traffic from a particular protocol) and establish a connection when it senses it.  Other traffic is considered “uninteresting” and does not lead to connection establishment.  The only situation where uninteresting traffic is propagated is when the connection is already established.  If the router receives no interesting traffic before a special idle timer expires, the circuit is terminated.

Dial backup is a service that activates a backup serial line under certain conditions—for example when the primary link fails or when the load on the primary link reaches a certain level.  Dial backup offers some protection against WAN performance degradation and downtime.