3Com LANPLEX 2500, Operation Manual

Page: 80 / 139

9-2 C
HAPTER
9: FDDI O
VERVIEW AND
I
MPLEMENTATION
The industry guideline for FDDI technology is divided into four major
standards:
■
Physical Medium Dependent (PMD) — PMD specifies the characteristics of
the fiber optic medium, the connectors that attach stations to the fiber optic
medium, the transmission wavelength, the power requirements for
transmitters, and the methods for optically bypassing inactive stations.
■
Physical (PHY) — PHY specifies data encoding and decoding, clock speed
and clocking scheme, data framing, and the control symbols used in the
network.
■
Media Access Control (MAC ) — MAC specifies access to the medium,
token passing, addressing, data checking, frame generation and reception,
error detection and recovery, and the bandwidth allocation among the
stations.
■
Station Management (SMT ) — SMT specifies the FDDI station and ring
configurations, initialization and maintenance of station-to-station
connections, and the control required for the proper operation of stations in
an FDDI ring.
These four standards are always described in relation to the Open Systems
Interconnect (OSI) Reference Model. This model was established by the
International Standards Organization (ISO) to standardize digital data
communications. Each FDDI station is made up of logical entities that
conform to the four standards. These entities represent the active services or
management elements within OSI.
Figure 9-1 illustrates the relationship of FDDI entities to the OSI Reference
Model. Network attachments communicate with each other using
predetermined protocols. The model divides these communication
protocols into seven layers, defined so that each layer only requires services
from the layer below it.