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FDDI
(Fiber Distributed Data Interface)
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FDDI Media
FDDI specifies a 100 Mbps, token-passing,
dual-ring LAN that uses a fiber-optic
transmission medium.
FDDI is a local area network protocol.
The copper version of FDDI is known as
CDDI.
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FDDI Media Access Control:
Token passing
FDDI's dual ring makes FDDI very reliable.
FDDI supports real-time allocation of network
bandwidth.
FDDI provides this support by defining two
types of trafficsynchronous and
asynchronous.
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Synchronous frame refer to informationthat is real time. ( audio and video
information)
It is also called S-frames
Asynchronous frame refers to informatinthat is not real time.
It is also called as A- frames.
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3 Time Registers FDDI defines three time registers to
control circulation of the token and todistribute link access opportunities amongthe nodes equally.
They are
Synchronous Allocation (SA) register.
Target Token Rotation Time (TTRT) register.
Absolute maximum Time (AMT) register.
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SA register indicates the length of time allowedeach station for sending synchronous data.
This value differ from node to node.
TTRT registers indicates the average timerequired for a token to circulate around the ring
exactly once. ie; elapsed time between a token arrival at a
given statin and its next arrival at the samestation.
AMT register holds a value equal to twice theTTRT.
A token may not take longer than this time to
make one rotation of the ring.
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2 Timers
Token Rotation Timer(TRT) runcontinuously and measures the actual timetaken by the token to complete a cycle.
Token Holding Timer (THT)beginsrunning as soon as the token is received.
Its function is to show how much timeremains for sending asynchronous framesonce the synchronous frames have beensent
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FDDI Frame Format
The FDDI frame is similar to that of a Token Ring frame.
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Dual ring
FDDI specifies the use ofdual 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.
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One of the two FDDI rings is called theprimary
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.
The secondary ring works if the primary ring fails.
.
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FDDI Fault Tolerance
A ring recovers from a station failure by wrapping.
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Nodes connect to one or both rings usinga media interface connector(MIC)
FDDI defines 3 types of nodes.
1. Dual Attachment station (DAS)
2. Single Attachment station(SAS)
3.
Dual Attachment concentrator (DAC)
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DAS has two MIC( called MIC A and MIC B) andconnect to both rings.
Faults are bypassed by a stations making a wrapconnection from the primary ring to the secondaryto switch signals from one input to another output.
SAS are used to connect workstations, serversand minicomputers.
SAS has only one MIC(called MIC s) and there
fore can connect only to one ring.
DAC connects SAS to dual ring as well as controlfunctions.
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FDDI Layers:
FDDI has four specifications:1. Media Access Control - defines how the medium
is accessed
2. Physical Layer Protocol
defines dataencoding/decoding procedures
3. Physical Layer Mediumdefines the
characteristics of the transmission medium
4. Station Management
defines the FDDI station
configuration
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Networks: Token Ring and FDDI16
Differences between 802.5 and
FDDIToken Ring Shielded twisted pair
4, 16 Mbps
No reliability specified Differential Manchester
Centralized clock
Priority andReservation bits
New token afterreceive
FDDI
Optical Fiber
100 Mbps
Reliability specified (dualring)
4B/5B encoding
Distributed clocking Timed Token Rotation
Time
New tokenafter transmit