Error Characteristics of Fiber Distributed Data Interface (FDDI)

Fiber distributed data interface (FDDI) is a 100 Mbps fiber optic local area network (LAN) standard being developed by the American National Standard Institute (ANSI). We analyze the impact of various design decisions on the error detection capability of the protocol. In particular, we quantify frame error rate, token loss rate, and undetected error rate. Several characteristics of the 32 bit frame check sequence (FCS) polynomial, which is also used in IEEE 802 LAN protocols, are discussed. The standard uses a “nonreturn to zero invert on ones” (NRZI) signal encoding and a 4 bit to 5 bit (4b /5b) symbol encoding in the physical layer. Due to the combination of NRZI and 4b /5b encoding, many noise events are detected by code (or symbol) violations. A large percentage of errors is also detected by framing violations. Some of the remaining errors are detected by FCS violations. The errors that escape these three violations remain undetected. The probability of undetected errors due to creation of false starting delimiters, false ending delimiters, or merging of two frames is analyzed. It is shown that every noise event results in two code bit errors, which in turn may result in up to four data bit errors. The FCS can detect up to two noise events. Creation of a false starting delimiter or ending delimiter on a symbol boundary also requires two noise events. This assumes enhanced frame validity criteria. We justify the enhancements by quantifying their effect. This analysis here is limited to noise events not resulting in a change of symbol boundaries. Extensions to the case of changed symbol boundaries is continuing and will be presented at a later time.