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Physical Interface

Physical Interface

• No active ring monitor (unlike token ring IEEE 802.5)

• Each ring interface has its own clock

• Clocks synchronised to incoming data

• Outgoing data is transmitted using local clock

• FDDI is not synchronous but plesiochronous

Encoding/Decoding

4B/5B

• All data is encoded prior to transmission

• Encoding uses 4 of 5 group code known as 4B/5B
  • Every 4 bit group (16 different combinations) is mapped onto a 5 bit code (symbol)

  • 5B symbols for 4B data groups are chosen such that a maximum of 2 successive zeros occur

  • 5B symbols which are not used for data encoding are used as control symbols - symbols such as 0001, 00010 ... are not used

Control Symbols

• 5B symbols are named using a single alphanumeric - data symbols are named 0..9 and A..F

• There are 8+1 control symbols
  • I,H,Q - state of the optical link (Idle -11111, Halt -00100, Quiet -00000)

  • J, K, T frame delimiters

  • R, S logical indicators (`0', `1')

  • L reserved for FDDI II

NRZI

• 5B symbols are passed through a NRZI (Non Return to Zero Inverter) which produces a signal transision for a 1 and none for a 0

• The 4B/5B encoding with NZRI modulation guarantees that there is a signal transison every 3 bits

Frame Formats

Token

• PA - preamble - 16 or more Idle (I) symbols - line changes at maximum frequency

• SD - Start Delimiter - J and K symbol - receiver fixes correct symbol boundaries

• FC - Frame Control - 2 symbols see data frame

• ED - End Delimiter - 2 T symbols

Data Frame

• Data frame may contain MAC, SMT or LLC information depending on a bit set in FC

• PA and SD same as for token

• FC - describes frame type and features (synchronous; asynchronous; address field size - 16 or 48 bits)

• DA and SA - Destination and Source Address - 4 or 8 symbols - see FC

• Information - empty or an even number of symbols to a maximum of 9000 (4500 bytes) including all the fields

• FCS - Frame Check Sequence - (8 symbols)

• ED - End Delimiter - 1 symbol

• FS - Frame Status - check frame validity and reception

Frame Status

• The following indicators are defined in the FS field
  • E - error detected

  • A - address recognised

  • C - frame copeid

  • Other symbols may be added possibly followed by a T symbol

  • For each indicator an S symbol (logic 1) represents a Set and R (logic 0) a Reset symbol, thus EAC == RSS indicates that some station has matched the address in DA and some station has copied the frame into its buffer

Buffering

• 2 symbols J and K in the SD field establish the correct symbol boundaries

• A 10 bit buffer is needed for reception - this is known as the latency buffer

Protocol

• To transmit
  • Station captures the token

  • Transmits information frame(s)

  • Immediately after transmits a new token followed by idle symbols - early token release

• Transmitting station must retrieve its own frames from the ring when it recognises the SA field - the small buffer size (10 bits) mean that a frame fragment (PA, SD, FC, DA) has already been transmitted

• Frame fragments remain in the ring until a station transmits

Station Modes

• FDDI attached stations can be in different modes:
  • Reception: reception and copying information after analysing the DA field

  • Withdrawal: repetition up to SA field followed by I

  • Transmission: in possession of token

Timed Token

• Access to the ring is based on the following times:
  • TRT - Token Rotation Time

  • TTRT - Target Token Rotation Time

  • TS - Transmit Synchronous

  • TA - Transmit Asynchronous

• FDDI token access is similar to that of a token bus (IEEE 802.4) rather than a token ring (IEEE 802.5)

• Stations access the ring for up to a negotiated and/or preset time - timed token

TRT

• A continuously altering value calculated as the time between 2 passages of the token

• TRT is an indicator of the total load on the ring:
  • Low TRT - light load

  • High TRT - heavy load

TTRT

• At ring initialisation stations negotiate TTRT using `MAC' claim frames based on their response time constraits

• Lowest value is set ring-wide ie. most demanding is taken as the common reference

• If TRT exceeds TTRT over several consecutive circuits of the ring (or if a new station is added) the ring is re-initialised

TS and TA

• Data such as voice must be transmitted within time constraints - such data may be sent as synchronous data

• Synchronous data is treated as high priority as late delivery would render it useless

• Stations are allocated a synchronous time TS

• Asynchronous or `normal' data generated at random time intervals is sent at lower priority in a time slot TA (see below)

Timing Example

• Consider a ring of 3 stations where TTRT = 10ms and TS = 2ms

• 60% of the time is allocated for synchronous traffic and 40% remains for asynchronous

• Transmission times are calculated w.r.t. TRT and TTRT
  • Token early (TRT < TTRT) TS is available plusTA = TTRT -TRT

  • Token late (TRT >= TTRT) synchronous transmission only; TA = 0

Characteristic values

• TRT has a mean value less than or equal to TTRT

• If the maximum delay for synchronous data is T then a station requests that TTRT = T/2

• The maximum of the sum of the TSs = TTRT therefore the maximum value of TRT = 2xTTRT

Priority Levels

• Asynchronous data may be prioritised

• Priorities are defined in terms of threshold times:
  • Up to 8 priority levels

  • Lower priority data is transmitted if TRT is below threshold ie. ring is lightly loaded

Maximum Utilisation

• FDDI can typically deliver aggragate usage of up to 90-95%

• High usage implies longer queuing times

• Some stations do as little as 20 Mbps while others can achieve up to 90 Mbps (vendor/system dependent)