Chapter 3.3.2 - FDDI

The fiber-distributed data interface (FDDI) is a dual fiber ring (Figure 3.5). Nodes,
known as stations, access the FDDI network using timed tokens (very short messages).
The FDDI token administration uses a Timed-Token Protocol (TTP) and,
based on a prioritized bidding process, it can support both synchronous and asynchronous
service to other stations.

FDDI transports data at 100 Mbps. However, because of a 4B/5B coding, the actual
bit rate on the fiber is 125 Mbps.

The FDDI frame consists of nine fields:

The preamble (64 bits)
The starting delimiter (8 bits)
The frame control (8 bits)
The destination address (16 or 48 bits)
The source address (16 or 48 bits)

Figure 3.4 Ethernet CSMA/CD frame definition.

Figure 3.5 The dual-fiber ring FDDI local area network.

The information (up to 4,500 octets)
The frame check sequence (32 bits)
The end delimiter (8 bits)
The frame status (at least 12 bits or 3 symbols; each symbol consists of 4 bits)

The best feature of FDDI, which has been adopted by most metro networks, is its
protection against link failures. When a link fails, the stations adjacent to the failure
sense it and automatically reconfigure themselves by establishing loops on the
physical layer, from the primary to the secondary ring. The FDDI station standard
consists of four sections, with direct correspondence to the open system interconnect
(OSI) layers, with the exception of the SMT:

The Physical-Layer-Medium-Dependent (PMD)
The Physical Layer Protocol (PHY)
The Medium-specific Access Control (MAC)
The Station Management (SMT)

FDDI networks are theoretically unbounded in size. However, each station and
fiber segment adds to the token delay, affecting the throughput and reconfiguration
time of the network. Thus, each link can be up to 2 km in length. Based on this, a
practical FDDI network can have up to 200 km total fiber length (both primary and
secondary) and it can accommodate more than 500 stations.

FDDI has inspired the two-connected regular ring-mesh network (RMN) architecture,
also known as Manhattan Street Network (MSN) or Manhattan FDDI (MFDDI).
This network utilizes distributed control, packet queuing, adaptive routing,
flow control, deadlock avoidance, and packet resequencing, and exhibits an increased
(superior) throughput performance as well as fault and disaster avoidance.
As an example, a 64 node RMN network outperforms in throughput a bus network
by a factor of 20 to 30, all things being equal. The factor increases in larger networks.
The RMN exhibits a superior routeability or packet deliverability from the
standpoint of the number of available routes between two nodes in the network. For
unidirectional M-FDDI with NXN nodes, the total number of possible routes per
channel between source and destination, without visiting the same node twice, is estimated
to be N!(N – 1). However, in bidirectional RMNs, this number increases
rapidly. For example, a 2 × 2 node network has eight possible routes, a 3 × 3 has
176, and a 4 × 4 has 1592.

FDDI was a fiber ring data network using premature optical technology that
emerged in the mid-80s. Thus, it could not compete in cost with Ethernet. Nevertheless,
FDDI proved that data fiber rings were an excellent solution in metro-size
LAN applications to which Ethernet could not be easily applied. As a result, many
of the topological and protocol features of FDDI were adapted to develop new fiber
data networks, such as the Shared Rings (from RMN or M-FDDI), Two and Four-
fiber Metro rings, the Ethernet over Metro (fiber ring), and the new Resilient Packet
Ring (RPR).

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