The Fiber Distributed Data
Interface (FDDI) specifies a 100-Mbps token-passing, dual-ring LAN using
fiber-optic cable. FDDI is frequently used as high-speed backbone
technology because of its support for high bandwidth and greater
distances than copper. It should be noted that relatively recently, a
related copper specification, called Copper
Distributed Data Interface (CDDI) has emerged to provide 100-Mbps
service over copper. CDDI is the implementation of FDDI protocols over
twisted-pair copper wire. This chapter focuses mainly on FDDI
specifications and operations, but it also provides a high-level
overview of CDDI.
FDDI uses a dual-ring architecture with
traffic on each ring flowing in opposite directions (called counter-rotating).
The dual-rings consist of a primary and a secondary ring. During normal
operation, the primary ring is used for data transmission, and the
secondary ring remains idle. The primary purpose of the dual rings, as
will be discussed in detail later in this chapter, is to provide
superior reliability and robustness. Figure 8-1 shows the
counter-rotating primary and secondary FDDI rings.
Figure 8-1: FDDI uses
counter-rotating primary and secondary rings.
FDDI was developed by the
American National Standards Institute (ANSI) X3T9.5 standards committee
in the mid-1980s. At the time, high-speed engineering workstations were
beginning to tax the bandwidth of existing local area networks (LANs)
based on Ethernet and Token Ring). A new LAN media was needed that could
easily support these workstations and their new distributed
applications. At the same time, network reliability had become an
increasingly important issue as system managers migrated
mission-critical applications from large computers to networks. FDDI was
developed to fill these needs. After completing the FDDI specification,
ANSI submitted FDDI to the International Organization for
Standardization (ISO), which created an international version of FDDI
that is completely compatible with the ANSI standard version.
FDDI uses optical fiber as the primary
transmission medium, but it also can run over copper cabling. As
mentioned earlier, FDDI over copper is referred to as Copper-Distributed
Data Interface (CDDI). Optical fiber has several advantages over
copper media. In particular, security, reliability, and performance all
are enhanced with optical fiber media because fiber does not emit
electrical signals. A physical medium that does emit electrical signals
(copper) can be tapped and therefore would permit unauthorized access to
the data that is transiting the medium. In addition, fiber is immune to
electrical interference from radio frequency interference (RFI) and
electromagnetic interference (EMI). Fiber historically has supported
much higher bandwidth (throughput potential) than copper, although
recent technological advances have made copper capable of transmitting
at 100 Mbps. Finally, FDDI allows two kilometers between stations using
multi-mode fiber, and even longer distances using a single mode.
FDDI defines two types of optical fiber:
single-mode and multi-mode. A mode
is a ray of light that enters the fiber at a particular angle. Multi-mode
fiber uses LED as the light-generating devices, while single-mode fiber
generally uses lasers.
Multi-mode fiber allows multiple modes of
light to propagate through the fiber. Because these modes of light enter
the fiber at different angles, they will arrive at the end of the fiber
at different times. This characteristic is known as modal
dispersion. Modal dispersion limits the bandwidth and distances
that can be accomplished using multi-mode fibers. For this reason,
multi-mode fiber is generally used for connectivity within a building or
within a relatively geographically contained environment.
Single-mode fiber allows only one mode of
light to propagate through the fiber. Because only a single mode of
light is used, modal dispersion is not present with single-mode fiber.
Therefore, single-mode is capable of delivering considerably higher
performance connectivity and over much larger distances, which is why it
generally is used for connectivity between buildings and within
environments that are more geographically dispersed.
Figure 8-2 depicts single-mode fiber using
a laser light source and multi-mode fiber using a light-emitting
diode (LED) light source.
Figure 8-2: Light
sources differ for single-mode and multi-mode fibers.
FDDI specifies the physical and media-access
portions of the OSI reference model. FDDI is not actually a single
specification, but it is a collection of four separate specifications
each with a specific function. Combined, these specifications have the
capability to provide high-speed connectivity between upper-layer
protocols such as TCP/IP and IPX, and media such as fiber-optic cabling.
FDDI's four specifications are the Media
Access Control (MAC), Physical Layer Protocol (PHY), Physical-Medium
Dependent (PMD), and Station Management (SMT). The MAC specification
defines how the medium is accessed, including frame format, token
handling, addressing, algorithms for calculating cyclic redundancy check
(CRC) value, and error-recovery mechanisms. The PHY specification
defines data encoding/decoding procedures, clocking requirements, and
framing, among other functions. The PMD specification defines the
characteristics of the transmission medium, including fiber-optic links,
power levels, bit-error rates, optical components, and connectors. The
SMT specification defines FDDI station configuration, ring
configuration, and ring control features, including station insertion
and removal, initialization, fault isolation and recovery, scheduling,
and statistics collection.
FDDI is similar to IEEE
802.3 Ethernet and IEEE 802.5 Token Ring in its
relationship with the OSI model. Its primary purpose is to provide
connectivity between upper OSI layers of common protocols and the media
used to connect network devices. Figure 8-3 illustrates the four
FDDI specifications and their relationship to each other and to the
IEEE-defined Logical-Link Control (LLC) sublayer. The LLC sublayer is a
component of Layer 2, the MAC layer, of the OSI reference model.
Figure 8-3: FDDI
specifications map to the OSI hierarchical model.
One of the unique characteristics of FDDI
is that multiple ways actually exist by which to connect FDDI devices.
FDDI defines three types of devices: single-attachment station (SAS), dual-attachment
station (DAS), and a concentrator.
An SAS attaches to only one ring (the
primary) through a concentrator. One of the primary advantages of
connecting devices with SAS attachments is that the devices will not
have any effect on the FDDI ring if they are disconnected or powered
off. Concentrators will be discussed in more detail in the following
discussion.
Each FDDI DAS has two ports, designated A
and B. These ports connect the DAS to the dual FDDI ring. Therefore,
each port provides a connection for both the primary and the secondary
ring. As you will see in the next section, devices using DAS connections
will affect the ring if they are disconnected or powered off. Figure
8-4 shows FDDI DAS A and B ports with attachments to the primary and
secondary rings.
Figure 8-4: FDDI DAS ports
attach to the primary and secondary rings.
An FDDI concentrator (also called a dual-attachment
concentrator [DAC]) is the building block of an
FDDI network. It attaches directly to both the
primary and secondary rings and ensures that the failure or power-down
of any SAS does not bring down the ring. This is particularly useful
when PCs, or similar devices that are frequently powered on and off,
connect to the ring. Figure 8-5 shows the ring attachments of an
FDDI SAS, DAS, and concentrator.
Figure 8-5: A concentrator
attaches to both the primary and secondary rings.
FDDI provides a number of fault-tolerant
features. In particular, FDDI's dual-ring environment, the
implementation of the optical bypass switch, and dual-homing support
make FDDI a resilient media technology.
FDDI's primary fault-tolerant feature is
the dual ring. If a station on the dual ring fails or is
powered down, or if the cable is damaged, the dual ring is automatically
wrapped (doubled back onto itself) into a single ring. When the
ring is wrapped, the dual-ring topology becomes a single-ring topology.
Data continues to be transmitted on the FDDI ring without performance
impact during the wrap condition. Figure 8-6 and Figure 8-7
illustrate the effect of a ring wrapping
in FDDI.
Figure 8-6: A
ring recovers from a station failure by wrapping.
Figure 8-7: A
ring also wraps to withstand a cable failure.
When a single station fails, as shown
in Figure 8-6, devices on either side of the failed (or powered
down) station wrap, forming a single ring. Network operation continues
for the remaining stations on the ring. When a cable failure occurs, as
shown in Figure 8-7, devices on either side of the cable fault wrap.
Network operation continues for all stations.
It should be noted that FDDI truly
provides fault-tolerance against a single failure only. When two or more
failures occur, the FDDI ring segments into two or more independent
rings that are unable to communicate with each other.
An optical bypass switch
provides continuous dual-ring operation if a device on the dual ring
fails. This is used both to prevent ring segmentation and to eliminate
failed stations from the ring. The optical bypass switch performs this
function through the use of optical mirrors that pass light from the
ring directly to the DAS device during normal operation. In the event of
a failure of the DAS device, such as a power-off, the optical bypass
switch will pass the light through itself by using internal mirrors and
thereby maintain the ring's integrity. The benefit of this capability is
that the ring will not enter a wrapped condition in the event of a
device failure. Figure 8-8 shows the functionality of an optical
bypass switch in an FDDI network.
Figure 8-8: The optical
bypass switch uses internal mirrors to maintain a network.
Critical devices, such as routers or
mainframe hosts, can use a fault-tolerant technique called dual
homing to provide additional redundancy and to
help guarantee operation. In dual-homing situations, the critical device
is attached to two concentrators. Figure 8-9 shows a dual-homed
configuration for devices such as file servers and routers.
Figure 8-9: A dual-homed
configuration guarantees operation.
One pair of concentrator links is
declared the active link; the other pair is declared passive. The
passive link stays in back-up mode until the primary link (or the
concentrator to which it is attached) is determined to have failed. When
this occurs, the passive link automatically activates.
The FDDI frame format is similar to the
format of a Token Ring frame. This is one of the areas where FDDI
borrows heavily from earlier LAN technologies, such as Token Ring. FDDI
frames can be as large as 4,500 bytes. Figure 8-10 shows the frame
format of an FDDI data frame and token.
Figure 8-10: The FDDI frame
is similar to that of a Token Ring frame.
The following descriptions summarize
the FDDI data frame and token fields illustrated in Figure 8-10.
Copper Distributed Data Interface (CDDI)
is the implementation of FDDI protocols over twisted-pair copper wire.
Like FDDI, CDDI provides data rates of 100 Mbps and uses a
dual-ring architecture to provide redundancy. CDDI supports distances of
about 100 meters from desktop to concentrator.
CDDI is defined by the ANSI X3T9.5
Committee. The CDDI standard is officially named the Twisted-Pair
Physical Medium Dependent (TP-PMD) standard. It
is also referred to as the Twisted-Pair
Distributed Data Interface (TP-DDI), consistent with the term Fiber-Distributed
Data Interface (FDDI). CDDI is consistent with the physical and
media-access control layers defined by the ANSI standard.
The ANSI standard recognizes only two
types of cables for CDDI:
shielded twisted pair (STP) and unshielded
twisted pair (UTP). STP cabling has a 150-ohm impedance and adheres to
EIA/TIA 568 (IBM Type 1) specifications. UTP is data-grade cabling
(Category 5) consisting of four unshielded pairs using tight-pair twists
and specially developed insulating polymers in plastic jackets adhering
to EIA/TIA 568B specifications.
Figure 8-11 illustrates the CDDI
TP-PMD specification
in relation to the remaining FDDI specifications.
Figure 8-11: CDDI
TP-PMD and FDDI specifications adhere to different standards.
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