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at the destination at different times (a phenomenon called modal dispersion ), single-mode fiber is
capable of higher bandwidth and greater cable run distances than multimode 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.
Figure5-1
FDDI Standards
Logical link control
Media access control
Station
management
FDDI
standards
Physical layer protocol
Physical layer medium
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.
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