Interphase Tech iSPAN User Manual

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 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.