Interphase Tech iSPAN - Manual (Page 207)

Appendix D: Protocols Overview
iSPAN Serial WAN Adapter Users Guide 187
PVCs
PVCs are similar to leased lines because the connection is
always present. The logical connection is established
permanently by the Packet Switched Network administration.
Therefore, data may always be sent, without any call setup.
Frame Relay Protocol
Frame Relay is defined as a protocol-oriented packet-switched
technology offered by telephone companies.
Frame Relay depends on the concepts of Permanent Virtual
Circuits (PVCs) and Committed Information Rate (CIR).
A PVC is virtual because no wires or cables directly hook up
the local and remote peers (for example, your branch library
and the central library). The connection occurs in the cloud or
switch at the telephone company. A PVC is permanent because
as long as the computer at the telephone company is turned on,
the circuit remains established.
The CIR is the contracted communication rate (bandwidth)
you have negotiated with the telephone company, guaranteeing
your ability to transmit at a specified minimum speed all the
time. It is generally possible to transmit at much higher speeds,
by virtue of the nature of frame relay, but you will always
attain the CIR you negotiated.