System information

FDDI Technology Basics
Book Title
5-66
fiber. Because of these characteristics, single-mode fiber is often used for interbuilding connectivity,
and multimode fiber is often used for intrabuilding connectivity. Multimode fiber uses light-emitting
diodes (LEDs) as the light-generating devices, whereas single-mode fiber generally uses lasers.
FDDI Specifications
FDDI is defined by four separate specifications (see Figure 5-1):
• Media Access Control (MAC)—Defines how the medium is accessed, including frame format,
token handling, addressing, an algorithm for calculating a cyclic redundancy check value, and
error recovery mechanisms.
• Physical Layer Protocol (PHY)—Defines data encoding/decoding procedures, clocking
requirements, framing, and other functions.
• Physical Layer Medium (PMD)—Defines the characteristics of the transmission medium,
including the fiber-optic link, power levels, bit error rates, optical components, and connectors.
• Station Management (SMT)—Defines the FDDI station configuration, ring configuration, and
ring control features, including station insertion and removal, initialization, fault isolation and
recovery, scheduling, and collection of statistics.
Figure 5-1 FDDI Standards
Physical Connections
FDDI specifies the use of dual rings. Traffic on these rings travels in opposite directions. Physically,
the rings consist of two or more point-to-point connections between adjacent stations. One of the
two FDDI rings is called the primary ring; the other is called the secondary ring. The primary ring
is used for data transmission, and the secondary ring is generally used as a backup.
Class B or single-attachment stations (SASs) attach to one ring; Class A or dual-attachment stations
(DASs) attach to both rings. SASs are attached to the primary ring through a concentrator, which
provides connections for multiple SASs. The concentrator ensures that failure or power down of any
given SAS does not interrupt the ring. This is particularly useful when PCs, or similar devices that
frequently power on and off, connect to the ring.
A typical FDDI configuration with both DASs and SASs is shown in Figure 5-2.
Logical link control
Media access control
Physical layer protocol
Station
management
FDDI
standards
Physical layer medium