A network structure in which the nodes are laid out in a physical ring, or closed loop, configuration, as illustrated in Figure R-2. Information travels around the ring in only one direction, with each attached station or node serving as a repeater. Rings generally employ coaxial cable or optical fiber as transmission media. In the local area network domain, rings are characterized as being deterministic in nature, employing token passing as the method of medium access control (MAC) to ensure that all nodes can access the network within a predetermined time interval. Priority access is recognized. A master control station controls access to the transmission medium by generating tokens, without which a station cannot access the network. Generally, any station can assume backup control responsibility in the event of a master failure. The IEEE 802.5 standard is a specification for LANs based on an electrical ring topology. Fiber Distributed Data Interface (FDDI) is a specification from the American National Standards Institute (ANSI) for a LAN based on a fiber optic dual counter-rotating ring. In the metropolitan area network (MAN) and wide area network (WAN) domains, Resilient Packet Ring (RPR), Synchronous Digital Hierarchy (SDH), and Synchronous Optical Network (SONET) each specifies several fiber optic dual ring configurations. See also 802.5, ANSI, deterministic, FDDI, IEEE, MAC, node, ringlet, RPR, SDH, SONET, token passing, token-passing ring, Token Ring, and topology.