Fiber Distributed Data Interface (FDDI) is a set of ANSI protocols for sending digital data over fiber optic cable. FDDI networks are token-passing (similar to IEEE 802.5 Token Ring protocol) and dual-ring networks, and support data rates of up to 100 Mbps. FDDI networks are typically used as backbones technology because of its support for high bandwidth and great distance. A related copper specification similar to FDDI protocols, called Copper Distributed Data Interface (CDDI), has also been defined to provide 100-Mbps service over twisted-pair copper.
An extension to FDDI, called FDDI-2, supports the transmission of voice and video information as well as data. Another variation of FDDI, called FDDI Full Duplex Technology (FFDT) uses the same network infrastructure but can potentially support data rates up to 200 Mbps.
FDDI uses dual-ring architecture with traffic on each ring flowing in opposite directions (called counter-rotating). The dual rings consist of a primary and a secondary ring. During normal operation, the primary ring is used for data transmission, and the secondary ring remains idle. As will be discussed in detail later in this chapter, the primary purpose of the dual rings is to provide superior reliability and robustness.
FDDI specifies the physical and media-access portions of the OSI reference model. FDDI is not actually a single specification, but it is a collection of four separate specifications, each with a specific function. Combined, these specifications have the capability to provide high-speed connectivity between upper-layer protocols such as TCP/IP and IPX, and media such as fiber-optic cabling.
FDDI"s four specifications are the Media Access Control (MAC), Physical Layer Protocol (PHY), Physical-Medium Dependent (PMD), and Station Management (SMT) specifications. The MAC specification defines how the medium is accessed, including frame format, token handling, addressing, algorithms for calculating cyclic redundancy check (CRC) value, and error-recovery mechanisms. The PHY specification defines data encoding/decoding procedures, clocking requirements, and framing, among other functions. The PMD specification defines the characteristics of the transmission medium, including fiber-optic links, power levels, bit-error rates, optical components, and connectors. The SMT specification defines FDDI station configuration, ring configuration, and ring control features, including station insertion and removal, initialization, fault isolation and recovery, scheduling, and statistics collection.
Protocol Structure
| 2 | 6 | 6 | 0-30 | Variable | 4 bytes |
| Frame control | Destination address | Source address | Route information | Information | FCS |
Frame control - The frame control structure is as follows:
| C | L | F | F | Z | Z | Z | Z |
- C Class bit: 0 Asynchronous frame; 1 Synchronous frame
- L Address length bit: 0 16 bits (never); 1 48 bits (always).
- FF Format bits.
- ZZZZ Control bits.
Destination address - The address structure is as follows:
| I/G | U/L | Address bits |
Source address - The address structure is as follows:
| I/G | RII | Address bits |
- I/G Individual/group address: 0 Group address; 1 Individual address.
- RII Routing information indicator: 0 RI absent; 1 RI present.
Route Information - The structure of the route information is as follows:
| 3 | 5 | 1 | 6 | 1 | 16 | 16 | 16 | |
| RT | LTH | D | LF | r | RD1 | RD@ | ... | RDn |
- RC Routing control (16 bits).
- RDn Route descriptor (16 bits).
- RT Routing type (3 bits).
- LTH Length (5 bits).
- D Direction bit (1 bit).
- LF Largest frame (6 bits).
- r reserved (1 bit).
Information - The Information field may be LLC, MAC or SMT protocol.
FCS - Frame check sequence.
Related protocols: IEEE 802.5, 802.2
Sponsor Source: FDDI is defined by ANSI (http://www.ansi.org) X3T9.5.
Reference: http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/fddi.htm: Fiber Distributed Data Interface