Permanent Virtual Circuit (PVC)
A virtual circuit references a nonphysical aspect of our Frame Relay communications. The users of the Frame Relay WAN connection see the connectivity just like it is an actual physical link or connection they have with the remote device. In reality, this link is not physical and there is no actual bandwidth allocated to a clear cut physical connection between the devices. This is often why we choose to represent the Frame Relay environment of the Service Provider as a cloud. Any path or particular technology can be going on within this cloud. A Permanent Virtual Circuit (PVC) has an engineer set up the endpoints of the circuit when a customer signs up. These are then typically left in place for the duration of the contract. With a Switched Virtual Circuit (SVC), the circuit is set up when there is a desire to communication. Notice that this is much like a phone call. CCNA focuses on the PVC because it is much more commonly used with Frame Relay.
Data Link Connection Identifier (DLCI)
This term is critical for you to understand fully. The DLCI is a locally significant number to identify the path you need to send traffic to in the network. Locally significant means that there it is only relevant for your local router. In other words, there could be the same DLCI used by another router somewhere in the cloud. An analogy for this concept that I love is like Gate Numbers at an airport. We are told that in order to fly to Madrid, Spain, we need to go to gate A10. This is only locally significant to us. We go to that gate, travel down the jetway, get on an airplane, and some time later, we are smiling in beautiful Spain. We do not care what gate number is in use when we arrive, or what gate number will be used in the future for our return trip. The only thing we needed to care about when we departed was gate A10. DLCIs are represented using 10 bits, so there are 2^10, or 1024 different DLCI addresses possible. Keep in mind that some of these are reserved, however. For example, DLCI 0 is used for Local Management Interface (LMI) signaling. A common approach for Frame Relay providers is to start DLCI numbering at 100, and to count by 5 or 10.
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