FDDI specifies the physical and media-access portions of the OSI reference model. FDDI is not actually a single specification, but it is a collection of four separate specifications each with a specific function. Combined, these specifications have the capability to provide high-speed connectivity between upper-layer protocols such as TCP/IP and IPX, and media such as fiber-optic cabling.
FDDI’s four specifications are the Media Access Control (MAC), Physical Layer Protocol (PHY), Physical-Medium Dependent (PMD), and Station Management (SMT).
- Physical Layer Medium Dependent Protocol
The physical layer medium dependent protocol (PMD) defines the lowest FDDI protocol; it occupies the lower sublayer of the physical layer. PMD specifies the requirements for the cable (for example, fiber optic), the transmitter and receiver, the media interface connectors (MIC), and the optional optical bypass switch. PMD functionality is contained within a chip on the FDDI board.
- Physical Layer Protocol
The physical layer protocol (PHY) defines the upper sublayer of the physical layer. It establishes the connection between the PMD and MAC. In addition, the PHY provides encoding and decoding of data and control symbols. The PHY synchronizes incoming and outgoing code-bit clocks. This functionality is contained inside a chip on the FDDI board.
- Media Access Control Protocol
The media access control protocol (MAC) schedules and performs data transfer on the FDDI cable. The MAC is the FDDI component that contains the FDDI connection’s identity, commonly referred to as a MAC address.When a MAC begins to receive a block of information (a frame) from the FDDI cable, it checks the destination address field of the frame to see if the address is one of its own addresses. If the address matches one of its own addresses, the MAC simultaneously repeats the frame onto the physical medium and copies the frame into its local memory.While repeating the frame, the MAC modifies the frame’s status to indicate that the frame has been seen and received. The modified frame continues along the ring until it reaches the original transmitting station, which interprets the modified frame as an acknowledgment. This functionality is handled by a chip on the FDDI board.
- Station Management Protocol
The station management protocol (SMT) monitors and controls all FDDI activity on its station. SMT manages processes in the various FDDI layers (PMD, PHY, and MAC) at the station level and ensures the correct operation of the station on the ring. SMT’s responsibilities include overseeing station insertion and removal from the ring, initializing the station to conform with the current ring status, and identifying, isolating, and recovering from faults on the ring.
FDDI is similar to IEEE 802.3 Ethernet and IEEE 802.5 Token Ringin its relationship with the OSI model. Its primary purpose is to provide connectivity between upper OSI layers of common protocols and the media used to connect network devices.
Figure1: FDDI Components of FDDIXPress and an FDDI Board
Figure2: FDDI Components map to the OSI hierarchical model