Essentially, the use of Fiber Distributed Data Interface (FDDI) over copper cabling as an alternative to using fiber-optic cabling. Copper Distributed Data Interface (CDDI) can send data over unshielded twisted-pair (UTP) cabling at 100 Mbps, but cable lengths are limited to about 100 meters. The architecture and operation are similar to FDDI, but CDDI is not as commonly implemented as FDDI.
If cost is an issue, CDDI offers an alternative to FDDI. CDDI still provides a 100-Mbps network with redundancy, but at reduced cost because copper cabling is cheaper than fiber-optic cabling. Note that CDDI does not provide the security that FDDI does: copper cabling can be tapped, while fiber-optic cabling cannot.