Fiber Distributed Data Interface (FDDI)
Computer Networks
FDDI Oultine
FDDI
100 Mbps Dual Ring Multiple-Token Self-Healing Ring
Target Token Rotation Time (TTRT) 4B/5B Encoder
Computer Networks
FDDI
FDDI
FDDI uses a ring topology of multimode or single mode optical fiber transmission links operating at 100 Mbps to span up to 200 kms and permits up to 500 stations. Employs dual counter-rotating rings.
In FDDI, token is absorbed by station and released as soon as it completes the frame transmission {multi-token operation}.
16 and 48-bit addresses are allowed.
Computer Networks FDDI 3
FDDI: Dual Token Ring
A
E B
C
Computer Networks
D
FDDI
Leon-Garcia & Widjaja: Communication Networks
FDDI Repair
Self-healing Dual-Ring
Computer Networks FDDI 5
FDDI Ring Operation
Multi-token implies more than one frame on the ring at the same time.
Computer Networks
FDDI
FDDI
To accommodate a mixture of stream and bursty traffic, FDDI is designed to handle two types of traffic:
Synchronous frames that typically have tighter delay requirements (e.g., voice and video). Asynchronous frames have greater delay tolerances (e.g., data traffic).
** First occasion when design considered QoS.
FDDI uses TTRT (Target Token Rotation Time) to ensure that token rotation time is less than some value.
Computer Networks FDDI 7
FDDI Data Encoding
Cannot use Differential Manchester because 100 Mbps FDDI would require 200 Mbaud! Instead each ring interface has its own local clock.
Outgoing data is transmitted using this clock. Incoming data is received using a clock that is frequency and phase locked to the transitions in the incoming bit stream.
Computer Networks FDDI 8
FDDI Data Encoding
Data is encoded using a 4B/5B encoder.
For each four bits of data transmitted, a corresponding five-bit codeword is generated by the encoder. There is a maximum of two consecutive zero bits in each symbol.
The symbols are then shifted out through a NRZI encoder which produces a signal transition whenever a 1 bit is being transmitted and no transition when a 0 bit is transmitted. Local clock is 125MHz. This yields 100 Mbps (80% due to 4B/5B).
Computer Networks FDDI 9
FDDI
Computer Networks
FDDI
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SD ED
FDDI
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4B/5B Codes
DCC 9th Ed. Stallings Computer Networks FDDI 12
FDDI Frame Structure
Token Frame Format Data Frame Format 8 PRE PRE SD FC ED
1
SD
FC
2 or 6 4 2 or 6 Destination Source Information FCS Address Address
1 ED
1 FS
Preamble Frame Control CLFFZZZZ C = Synch/Asynch L = Address length (16 or 48 bits) FF = LLC/MAC control/reserved frame type
Leon-Garcia & Widjaja: Communication Networks
Computer Networks
FDDI
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More FDDI Details
FDDI Transmission on optical fiber requires ASK (e.g., coding is done via the absence or presence of a carrier signal {Intensity Modulation}.) Specific 5-bit codeword patterns chosen to guarantee no more than three zeroes in a row to provide for adequate synchronization. 1300 nm wavelength specified. Dual rings (primary and secondary) transmit in opposite directions. Normally, second ring is idle and used for redundancy for automatic repair (self-healing).
Computer Networks FDDI 14
IEEE 802.5 versus FDDI
802.5 Token Ring Shielded twisted pair 4, 16 Mbps No reliability specified Differential Manchester Centralized clock Priority and Reservation bits Three distinct token operations are possible.
FDDI Optical Fiber 100 Mbps Reliability specified (dual ring) 4B/5B encoding Distributed clocking Timed Token Rotation Time (TTRT) Multi-token operation
Computer Networks
FDDI
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FDDI Summary
FDDI
100 Mbps Dual Ring Multiple-Token Self-Healing Ring
Target Token Rotation Time
Two classes of traffic
4B/5B Encoder
Computer Networks
FDDI
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