FDDI station classes.
Like Ethernet and Token Ring, FDDI operates at the physical and data link layers of the OSI Model. However, unlike the other two, FDDI's physical layer is split up into two sublayers: the physical level protocol (PHY) and the physical layer medium-dependent interface (PMI).
The PHY is responsible for symbols (data), line states, encoding/decoding, clocking and data framing.
The PMD is responsible for the transmit/receive power levels, transmitter and receiver interface, error rates, and cable and connector specifications.
The MAC sublayer is responsible for data link layer (MAC) addressing, media access, error detection, and token handling.
Station Management (SMT) is defined at the physical layer and at the MAC sublayer. It is responsible for connection management, node configuration, recovery from errors, and encoding SMT frames.
An FDDI ring operates at 100 Mbps.
FDDI stations may be of two types: dual-attachment stations (DAS) and single-attachment stations (SAS). The use of concentrators is not required, but is recommended for most installations.
FDDI station classes.
The cable most often used with FDDI is single-mode (SMF-PMD) or multi-mode fiber (MMF-PMD) optic cable, though twisted-pair copper cable may be used. Each ring is allowed to be 100 km. Twisted-pair is allowed to run 100 m between the network attachment and its concentrator.
Fiber optic cable is made up of three components:
Fiber optic cable: jacket, cladding and core.
Light is transmitted at one end of the cable, and received at the other end. Therefore, the connection of any two attachments is considered a point-to-point connection.
Fiber optic cable is referred to by the diameters of its core and cladding, for example, 50/100 micron, 62.5/125 micron, and 100/40 micron.
Single-mode fiber uses a laser as its light source, and stations may be up to 20 km apart. Multi-mode fiber uses inexpensive LEDs, limiting station separation to 2 km.
Two types of connectors are commonly used:
FDDI fiber optic cable connectors
The FDDI standard defines four port types:
|
V = Valid connection VB = Valid connection, PHY B takes precedence X = Invalid connection P = Prohibited connection |
Sample port type connections.
SAS connections are used in network stations that may be powered off and on periodically. Because the S port is connected to an M port of a concentrator, the M port of the concentrator provides isolation from the rest of the ring.
The DAS connects to both the primary and seconday ring of an FDDI network, using two ports, A and B. The A port connects to another station's B port and the B port connects to another station's A port.
It is important to note that a DAS does not require a connection to a concentrator for attachment to the ring; it is a full-function dual-ring attachment. In case of failure, the DAS can wrap the ring to isolate the failure.
FDDI networks should not be designed using only DAS connections.
An FDDI network can operate on two rings the primary ring and the secondary ring. In normal operation, data will flow counterclockwise on the primary ring only. The secondary is normally idle and is used for backup. It remains inactive until there is a failure, at which point the ring will wrap and the primary and secondary rings will act as one ring. An FDDI dual ring will continue to wrap until the failure has been isolated.
FDDI DAS connections
FDDI breaks
One of the primary goals of an FDDI concentrator is to provide an FDDI service to those devices that may be powered on and off periodically (e.g., desktop computers).
Concentrators may be cascaded to form one of the most popular FDDI topologies: the dual ring of trees. They provide the root of the tree topology.
There are two types of concentrators:
FDDI concentrator topology
FDDI concentrator functions
Stand-alone concentrator
This topology is simplest in design. The concentrator is the root of the topology (like a backbone). All attachments are SAS (including DAS attached as SAS). The concentrator provides fault isolation. Good for small networks.
Dual-ring topology (no concentrators are needed)
This topology consists of DAS attachments directly to a dual ring that can support workstations, bridges or routers. Often used as a backbone for medium to large networks.
Tree-of-concentrators FDDI topology
A concentrator is used as the root of the tree. Attachments may be workstations (SAS or DAS), other concentators (DAC or SAC), bridges and routers. Good for multi-floor or multi-building medium to large networks.
Dual ring of trees FDDI topology
A dual ring backbone is the root of the tree. From here, a second tier of concentrators is placed, with direct attachments to the root concentrators. All further attachments are placed on the second tier of concentrators. The roots are fault-tolerant, and allow for easy expansion at any branch of the tree.
Dual homing FDDI topology
A DAS is allowed to connect to two DACs, but is not considered part of a dual ring. The only active port on the DAS is the B port (by FDDI rules, the B port has precedence over the A port). If port B fails, port A becomes inserted in the other concentrator. This is known as a redundant topology, and it is used in environments where uptime is critical in the event of a station or cable failure.
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