ANSI Standard X3T9.5
Application
Presentation
Session
Network
Data Link
Physical
Data Link
LLC
MAC
Media Access Control
•Medium Addressing
•Data Checking
•Data Framing
Physical
PHY
Physical Layer Protocol
•Symbol Coding/Decoding
•Symbol Framing
•Clock Rate
PMD 
Physical Layer Medium Dependent
•Power Levels
•Transmitter
•Receiver
•Optical Interface
•Connector Types
SMT
Station Management
•Fault Isolation and Recovery
•Station Configuration
•Scheduling Procedures
Figure A-3. FDDI Structure and the OSI Network Model
The PMD standard establishes the physical characteristics of the network connection, including the fiber optic transmitter power levels, receiver sensitivity, the fiber optic cable type, the type of connectors, and acceptable losses between nodes. The PMD converts the optically encoded information that it receives to electrically encoded information and presents it to the PHY sublayer. The layers reverse this process when transmitting information.
The PHY entity implements the physical layer protocol. The PHY receives data frames from the MAC as a series of 4-bit symbols and encodes each 4-bit MAC symbol as a 5-bit symbol for transmission. This encoding occurs to ensure each symbol has at least two bit transitions for bit-cell synchronization at the remote receiver. Decoding reverses this process for the received frames.
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