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Fiber Distributed Data Interface
(FDDI) came about because system managers became
concerned with network
reliability issues as mission-critical applications were
implemented on high-speed
networks.
FDDI is frequently used as a
backbone technology and to connect high-speed computers in a LAN.
FDDI has four specifications:
MediaAccessControl—
defineshowthemediumis
accessedframe formattoken handlingaddressing algorithm for
calculating a cyclic redundancy
check and error-recovery mechanisms
FDDI has four specifications:
Physical Layer Protocol—defines
data encoding/decoding procedures
clocking requirements framing
FDDI has four specifications:
Physical Layer Medium—defines the
characteristics of the transmission medium fiber
optic link power levels bit error
rates optical components connectors
FDDI has four specifications:
Station Management—defines the
FDDI station configuration ring configuration ring
control features stationinsertion
and removal initialization fault isolation and recovery
Recovery collection of statistics
Unlike CSMA/CD networks, such as
Ethernet, token-passing networks are deterministic-
-you can calculate the maximum
time that will pass before any end station will be able to
transmit. FDDI's dual ring makes
FDDI very reliable.
FDDI supports real-time
allocationof network bandwidth, making it ideal for a variety of
different application types. FDDI
provides this support by defining two types of traffic –
synchronous and asynchronous.
• Synchronous traffic can consume
a portion of the 100 Mbps total bandwidth of an
FDDI network, while asynchronous
traffic can consume the rest.
• Synchronous bandwidth is
allocated to those stations requiring continuous
transmission capability. This is
useful for transmitting voice and video
information.
• The remaining bandwidth is used
for asynchronous transmissions.
• The FDDI SMT specification
defines a distributed bidding scheme to allocate
FDDI bandwidth
• Asynchronous bandwidth is
allocated using an eight-level priority scheme. Each
station is assigned an
asynchronous priority level.
• FDDI also permits extended
dialogues, in which stations may temporarily use all
asynchronous bandwidth.
• The FDDI priority mechanism can
lock out stations that cannot use synchronous
bandwidth and that have too low
an asynchronous priority.
• FDDI uses an encoding scheme
called 4B/5B. Every 4 bits of data are sent as a 5
bit code. The signal sources in
FDDI transceivers are LEDs or lasers.
• FDDI specifies a 100 Mbps,
token-passing, dual-ring LAN that uses a fiber-optic
transmission medium.
• It defines the physical
layer and media access portion of the data link layer,
which is similar to IEEE 802.3
and IEEE 802.5 in its relationship to the OSI
Model.
• Although it operates at faster
speeds, FDDI is similar to Token Ring.
• The two networks share a few
features, such as topology (ring) and media access
technique (token-passing). A characteristic
of FDDI is its use of optical fiber as a
transmission medium.
• Optical fiber is exploding in
popularity as a networking medium, being installed
at a rate of 4000 miles per day in the United
States.
• Single-mode fiber is capable of
higher bandwidth and greater cable run distances
than multi-mode fiber.
• Because of these
characteristics, single-mode fiber is often used for interbuilding
connectivity while multi-mode
fiber is often used for intra-building
connectivity.
• Multi-mode fiber uses LEDs as
the light-generating devices while single-mode
fiber generally uses lasers.
• FDDI specifies the use of dual
rings for physical connections. Traffic on each
ring 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; the secondary ring is generally
used as a back up.
• Class B, or single-attachment
stations (SAS), attach to one ring; Class A, or dual
attachment
stations (DAS), attach
to both rings.
• SASs are attached to the
primary ring through a concentrator, which provides
connections for multiple SASs.
The concentrator ensures that a 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.
• Each FDDI DAS has two ports,
designated A and B. These ports connect the
station to dual FDDI ring;
therefore each port provides a connection for both
primary and secondary rings.
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