The PMD sublayer(Physical Medium Dependent) forms the lowest sublayer of the physical layer in local networks. It describes the physical properties, defines the transmission parameters such as the transmission speed, and ensures that the signaling matches the transmission medium.
The PMD sublayer is specified for FDDI and also for Fast Ethernet. For FDDI, there are PMDs for multimode fiber (MMF-PMD), for single-mode fiber (SMF-PMD), for twisted pair cable (TP-PMD) and for low-cost fiber (LCF-PMD) for the different physical media.
In the SMF-PMD, the single mode fiber is specified. This approach was chosen in order to be able to use FDDI as an alternative in MANs, with correspondingly longer distances without additional nodes.TP-PMD specifies TP cables as the transmission medium. This variant is also referred to as Copper Distributed Data Interface (CDDI), Greenbook or Twisted Pair Distributed Data Interface (TPDDI). This method uses the Sub-D connector and the RJ45 connector as connection technology. TP-PMD uses Multi Level Transmission (MLT-3) as the coding method and, in contrast to the standard, NRZ coding.