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FDDI II

FDDI II

• Extension of FDDI to support isochronous traffic
  • Isochronous service required when there are strict timing constraints

  • Application areas such as multimedia where data, digitized sound, graphics and video are integrated

• Two modes are supported: Basic - FDDI; Hybrid - FDDI plus isochronous

• HRC - Hybrid Ring Controller - required to interface between PHY and MAC layers
  • HMUX - Hybrid Multiplexer splits data to P-MAC (packet MAC) or I-MAC (Isochronous) MAC sublayers

Periodic Cycle

• Information is carried in periodic frames called cycles

• One cycle is generated every 125 microseconds

• At 100 Mbps a 125 us cycle can carry 12000 bits or 3125 symbols

Cycle Structure

• A cycle is divided into:
  • 5 preamble symbols

  • 24 symbol (12 byte) cycle header

  • 24 symbols for packet type data channel DPG - Dedicated Packet Group

  • 16 WBCs - Wide-Band Channel - each 96 bytes wide, 3072 symbols

  • Total 5+24+24+(16*96*2) = 3125

Transmission Rates

• DPG - 12 bytes per cycle - 768 kbits/s (kbps)

• Each WBC transmits 96 bytes per cycle - 6144 kbps
  • WBCs may be allocated to packet or isochronous traffic

  • Cycle header indicates traffic type of each WBC

  • Each WBC may be sub-divided into low capacity channels in multiples (or sub-multiples) of 64kbps ie. 8,16,32,64, n*64 kbps

• Total 0.768+(16*6.144) = 99.072 Mbps

Cycle Header

• SD - Start Delimiter - 2 symbols

• C1, C2 - Synchronisation control, Sequence Control - 1 symbol each

• CS - Cycle Sequence - 2 symbols

• P0 -> P15 - WBC programming template

• IMC - Isochronous Maintenance Channel - 2 symbols

Cycle Header

• Principally a key to the WBC modes

• P0 ..P15
  • S (logic `1') isochronous

  • R (logic `0') packet

• Other fields provide synchronisation, management and maintenance of the ring

• L symbol (FDDI II only) is used to ensure the uniqueness of the cycle delimiter, SD, within the cycle - packet type data is delimited by I and L instead of J and K

FDDI Epilogue

• MANs are defined to span the gap between LANs and WANs

• FDDI may be used as MAN but DQDB is preferred

• FDDI is used for private networks, DQDB for public
  • FDDI ring not suited to sharing by multiple clients (data passes through all clients - confidentiality issues)

  • Cost sharing problems

  • Therefore FDDI is used as a campus(premises)-wide MAN - 1 client