switching almost obsolete. It was used sometimes as backbone for other services, such as X.25 or IP traffic. Where Frame Relay was used in the USA also as carrier for TCP/IP traffic, in Europe backbones for IP networks often used ATM or PoS, later replaced by Carrier Ethernet Relationship to X.25 X.25 was an important early WAN protocol, and is often considered to be the grandfather of Frame Relay as many of the underlying protocols and functions of X.25 are still in use today (with upgrades) by Frame Relay. X.25 provides quality of service and error-free delivery, whereas Frame Relay was designed to relay data as quickly as possible over low error networks. Frame Relay eliminates a number of the higher-level procedures and fields used in X.25. Frame Relay was designed for use on links with error-rates far lower than available when X.25 was designed. X.25 prepares and sends packets, while Frame Relay prepares and sends frames. X.25 packets contain several fields used for error checking and flow control, most of which are not used by Frame Relay. The frames in Frame Relay contain an expanded link layer address field that enables Frame Relay nodes to direct frames to their destinations with minimal processing. The elimination of functions and fields over X.25 allows Frame Relay to move data more quickly, but leaves more room for errors and larger delays should data need to be retransmitted. X.25 packet switched networks typically allocated a fixed bandwidth through the network for each X.25 access, regardless of the current load. This resource allocation approach, while apt for applications that require guaranteed quality of service, is inefficient for applications that are highly dynamic in their load characteristics or which would benefit from a more dynamic resource allocation. Frame Relay networks can dynamically allocate bandwidth at both the physical and logical channel level. 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. Local management interface Initial proposals for Frame Relay were presented to the Consultative Committee on International Telephone and Telegraph (CCITT)