(15) FDDI finaal

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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