Fiber Distributed Data Interface (FDDI)

• 100 Mbps token passed ring network that uses Fiber optic cable
• used for Metropolitan Area Networks (MAN) to connect within the same city so this isn't really a WAN technology
• 100 km (62 miles) max. length => not really a WAN technology
• FDDI uses fiber optic cable to serve as
• "backend network" that handle file transfer
• serve as a backbone for other low capacity LANs
• LANs that require large bandwidth
• Video
• CAD
• CAM
• Token Passing
• not the same as token passing in 802.5
• here a computer can transmit as many frames as it can produce within a predetermined time before letting the token go. When it is finished transmitting, it lets the token go.
• Because the computer releases the token when it's finished, there may be several frames on the ring at once.
• FDDI is not like a regular Token Ring network because more than one computer at a time can transmit a token so that multiple tokens can circulate on the ring at any one time.
• This is why FDDI is faster than regular Token Ring 802.5 => 802.5 only allows one token at a time to transmit.
• Topology
• dual-ring
• primary ring is for traffic; a redundant second ring for backup
• when the primary ring breaks down, the secondary ring reconfigures itself and flows in the opposite direction
• REDUNDANCY is one of the key features of this technology.
• 500 computers max.
• more than one computer can transmit at a time - they share the bandwidth; for example, when 10 computers transmit, each does so at 10 Mbps
• there must be a repeater every 2 Kms or less
• computers connected to both rings are CLASS A stations and help to reconfigure the network if the first ring fails. CLASS B stations are only connected to the one, primary ring.
• FDDI can have point-to-point links to a hub => it can be set up using a star ring topology
• Beaconing
• all computers on an FDDI network are responsible for monitoring faults in the network
• a computer that detects a fault sends a signal called a BEACON onto the network. If it sees it's upstream neighbour is sending a beacon it stops. This goes on until the only computer sending a beacon is the one directly downstream from the faulty computer. This process stops when a beaconing computer receives its own beacon => this means the beacon made it around the ring
• Media
• FDDI uses fiber-optic. This means
• immune to electromagnetic interference
• secure because fiber optic doesn't emit a signal that can be monitored and cannot be tapped
• able to transmit long distances before needing a repeater
• FDDI on copper wire is called CDDI => can be done, but it has FAR less distance