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Network Configuration

FDDI - Network Schematic

FDDI - 2 Counter Rotating Rings

• Two rings of fibre installed - Primary and Secondary ring

• Token and data flow in opposite directions in each ring

• Secondary ring may be used as additional transmission path or as a backup should the primary ring fail (see below)

• Stations in the FDDI rings are connected to both rings

Fault tolerance

• Fibre break - two ports adjacent to break are disconnected and these stations enter wrap state - rings collapse to single ring

• Station failure - two nearest stations enter wrap state - rings collapse to single ring

• Multiple breaks - rings fragment into wrapped single rings

Connection Modes

Workstations

• Dual ported devices directly attached to both rings are called DAS (Dual Attachment Station). Physical attachment is disruptive and this is rarely used to connect workstations on an FDDI backbone

• SAS - Single Attachment Station - connected to primary ring

Concentrator

• DAC Dual Attachment Concentrator - SASs in a star
  • Pros
    • Fault tolerant eg. powering down workstation

    • Better planning of network

    • Lower cost of connection

    • Non disruptive connection

    • Troubleshooting is easier

  • Cons
    • Single point of failure for multiple workstations

• Concentrators may also be cascaded to give a tree like connection structure. SAC - Single Attachment Concentrator - connected to DAC type concentrator.

• Dual homing - a DAS may alternatively be connected to concentrator ports where the A port is a hot standby polling the state of the B port and taking over if it fails

Port types

• 4 distinct type of port A, B, S, M

• DASs have 2 ports A and B which are attached in an alternate fashion, ie. the port A of one DAS is connected to B of the next DAS

• SASs have one S port

• Concentrators provide multiple M ports to which the S ports of SASs are attached (or A and B ports of DASs)

Port connections

• Illegal connections

• Prefered Connections

• Connections which may cause problems (Vendor dependent)

Cables

• Multimode fibre 62.5/125 micron (fibre/casing)
  • Also supported 50/125, 85/125 ...

• Single mode 8-12 micron

• 62.5/125 is favoured as LED/photodiode technology used to drive/detect light in cable needs wider fibre

• 50/125 used by many PTTs in Europe is satisfactory

• 8-12 micron cable used with Laser

Cabling

• 4 fibres (2 transmit and 2 receive)

• Installation of spares recommended for replacement of faulty fibres

FDDI over Copper - CuDDI

• Use copper UTP (Unshielded Twisted Pair) or STP (Shielded Twisted Pair) cables for desktop FDDI

• 100 metres maximum cable length

• Standard: ANSI TP-PMD (Twisted Pair - Physical Medium Dependent)

• Low cost cable, installation and termination

• Copper transeivers are cheaper, smaller and require less power