Virtual circuits

As a WAN protocol, frame relay is most commonly implemented at Layer 2 (data link layer) of the Open Systems Interconnection (OSI) seven layer model. Two types of circuits exist: permanent virtual circuits (PVCs) which are used to form logical end-to-end links mapped over a physical network, and switched virtual circuits (SVCs). The latter are analogous to the circuit-switching concepts of the public switched telephone network (PSTN), the global phone network. While SVCs exist and are part of the frame relay specification, they are rarely implemented in practice.[citation needed] SVCs are often considered hard to configure and maintain and are generally avoided without appropriate justification.[citation needed]