The PMD standard defines the physical characteristics of the media interface connectors and the cabling, and the services necessary for transmitting signals between nodes. The PHY standard defines the rules for encoding and framing data for transmission, clocking requirements, and line states. The MAC standard defines the FDDI timed-token protocol, frame and token construction and transmission on the FDDI ring, ring initialization, and fault isolation. The SMT protocols monitor and control the activity of each node on the ring. Figure 2-4 shows the relationship of the four FDDI standards.
Data flows in opposite directions on the rings.
If a link on the primary ring fails, the secondary ring transmits the data. When the component can function again, the architecture reverts to dual-ring. Failure of Corresponding Links on Both Rings Figure 2-6. An FDDI ring consists of nodes in a ring architecture. The DAS connects to both the primary and secondary rings.