Virtual Circuits
Frame Relay provides significant advantages over simply using point-to-point leased lines. The primary advantage has to do with virtual circuits (VCs) which define a logical path between two Frame Relay DTEs. The VC acts like a point-to-point circuit, providing the ability to send data between two endpoints over a WAN. However, there is no physical circuit directly between the two endpoints. VCs share the access link and the Frame Relay network. Each VC has a committed information rate (CIR), which is a guarantee by the provider that a particular VC gets at least that much bandwidth. Service providers can build their Frame Relay networks more cost-effectively than for leased lines. Therefore, Frame Relay is more cost-effective than leased lines for connecting many WAN sites.
Two types of VCs are allowed-permanent (PVC) and switched (SVC). PVCs are predefined by the provider, while SVCs are created dynamically. A Frame Relay network which includes PVCs between each pair of sites is called a full mesh Frame Relay network. In such a network, any two sites are connected by a PVC. When not all pairs have a direct PVC, it is called a partial mesh network. In such networks packets must be forwarded through other sites when packets are to be transmitted between two sites that are not directly connected by a PVC. This is the major disadvantage of partial mesh networks, however, partial mesh networks are cheaper because the provider charges per VC.
Frame Relay uses an address to differentiate one PVC from another. This address is called a data-link connection identifier (DLCI). The name is descriptive: The address is for an OSI Layer 2 (data link) protocol, and it identifies a VC, which is sometimes called a virtual connection.