ght expect, a class B station is referred to as a single-attachment station. Figure 5.4 illustrates the use of several single-attachment stations and one dual-attachment station on an FDDI ring. Note that this configura- tion is designed as an economy measure since it would have been too expensive during the 1980s, when FDDI was developed with SMT capa- bility for each device connected to the LAN. Thus, restricting SMT to dual-attachment stations results in the wrapping of communications during a cable break or another impairment supporting redundancy for (a) FDDI Ring (b) Defective Station Figure 5.3 causing fiber break Compensating for a cable break. Defective secondary break Station (inactive) break primary (active) Legend: workstation Team-Fly® Fiber in the LAN multiple stations. Because of the high degree of redundancy afforded through the use of FDDI, it remains a popular infrastructure at Inter- net peering points. Port Types and Rules To prevent the incorrect configuration of a network, ANSI FDDI stan- dards specify certain connection rules. Those rules cover the functions of four defined port types: A, B, M, and S. Those ports govern connectivity between workstations and a concentrator as well as the connection of a concentrator to the primary and secondary rings. Figure 5.5 is a schematic diagram of the configuration of these ports in an FDDI network. Primary Ring Figure 5.4 A dual-attachment sta- tion acts as a connec- Secondary Ring tion for the indirect attachment of single- attachment stations. Dual-Attached Station Legend: Single-Attached Station Secondary Figure 5.5 Secondary FDDI port types. Primary Port A Concentrator Port B Primary Port M Port M Port M Port S Port S Port S Workstation Workstation Workstation Chapter Five In examining Figure 5.5, it is important to note that each port con- nector has a specific mechanical keying interface defined by the PMD standard. For example, port A connects to the incoming primary ring and outgoing secondary ring. In comparison, port B connects to the outgo