Permanent virtual circuits (PVCs)
are dedicated virtual paths through the Frame Relay network
that are up and running 100 percent of the time (well, at least in theory!). Unlike an SVC, a PVC
does not require the call establishment and call teardown phases. However, when the circuit initially
comes up, some parameter negotiations do pass over the wire; these communications
should occur only when the dedicated circuit goes down.
The two phases for PVCs are as follows:
Data exchange
Data is transmitted between two devices, and each device can transmit data as
needed because it doesn’t need to wait for a call to be established to do so. The data exchange
can happen at any time because the virtual connection is permanent and always available.
Idle
The connection is still active, but data is not being transmitted. The idle time can be indefinite:
the circuits will not time out. The idle time keeps the VC up and keeps the line from timing
out when no data is present. This is done by the transmission of idle frames, the sole purpose
of which is to keep line synchronization in the absence of data.
PVCs have gained in popularity as the price for dedicated lines has decreased. They are the
types of links that we will configure later in this chapter.