Physical Medium Dependent, also known as PMD, defines the details of transmission and reception of individual bits on a physical medium. These responsibilities encompass bit timing, signal encoding, interacting with the physical medium, and the properties of the cable, optical fiber, or wire itself.
The Ethernet PMD sublayer is part of the Ethernet physical layer (PHY).
Physical Medium Dependent on Fiber Distributed Data Interface
PMD represents the lowest sublayer supported by FDDI. This document specifies the requirements for the optical power sources, photodetectors, transceivers, MIC (medium interface connector), and cabling. This is the only optic (as opposed to electrical) level and corresponds roughly to the lower parts of the physical layer in the OSI Reference Model.
The power source must be able to send a signal of at least 25 microwatts (25 millionths of a watt) into the fiber. The photodetector, or light receptor, must be able to pick up a signal as weak as 2 microwatts.
The MIC for FDDI connections serves as the interface between the electrical and optical components of the architecture. This connector was specially designed by ANSI for FDDI and is also known as the FDDI connector.
The cabling specified at this sublayer calls for two rings running in opposite directions. The primary ring is the main transmission medium. A secondary ring provides redundancy by making it possible to transmit the data in the opposite direction if necessary. When the primary ring is working properly, the secondary ring is generally idle.