Short for Copper Distributed Data Interface, CDDI is officially called the Twisted-Pair Physical Medium Dependent (TP-PMD) and is a data transmission standard. CDDI uses either STP or UTP copper wire and based on FDDI (Fiber Distributed Data Interface) which uses fiber optic lines instead of copper wires.
CDDI can be supports distances up to 200 meters and has dual-ring capacity of 200 Mbps, unlike FDDI, which supports distances much greater than 200 meters, but only has a capacity of 100 Mbps. It conforms to the OSI model and is defined by the American National Standards Committee X3-T9.5. CDDI was adopted into the X3-T9.5 standard in 1994.