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Overview of Line Protocols

1-7

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, and ring initialization and fault

isolation.

The SMT standard deﬁnes the protocols for managing the PMD, the PHY, and the

MAC components of FDDI. The SMT protocols monitor and control the activity

of each node on the ring.

igure 1-5 shows the relationship of the four FDDI standards.

Figure 1-5. Relationship of FDDI Standards

FDDI Dual Counter-Rotating Ring Architecture

FDDI LANs comprise two independent, counter-rotating rings: a

primary

ring

and a

secondary

ring. Data ﬂows in opposite directions on the rings. Both rings

can carry data; however, in high-bandwidth applications, Bay Networks speciﬁes

that the primary ring transmits data and the secondary ring is a backup device.

The counter-rotating ring architecture prevents data loss in the event of a link

failure, a node failure, or the failure of both the primary and secondary links

between any two nodes, as follows:

• If a link on the primary ring fails, the secondary ring transmits the data.

• If a node or corresponding links on both the primary and secondary rings fail,

one ring wraps to the other around the faulty components, forming a single

ring (Figures 1-

6 and 1-7).

When the component can function again, the architecture reverts to dual-ring.

Media Access Control (MAC)

Station

Management

(SMT)

Physical Layer Protocol (PHY)

Physical Layer Medium Dependent (PMD)

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Avaya Configuring Line Services Manual de usuario Pagina 31