Introduction
TheFiberDistributedDataInterface(FDDI)specifiesa100-Mbpstoken-passing,dual-ringLANusing
fiber-opticcable.FDDIisfrequentlyusedashigh-speedbackbonetechnologybecauseofitssupportfor
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 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,
theprimaryringisusedfordatatransmission,andthesecondaryringremainsidle.Aswillbediscussed
in detail later in this chapter, the primary purpose of the dual rings is to provide superior reliability
and
robustness. Figure 8-1 shows the counter-rotating primary and secondary FDDI rings.
Standards
FDDIwasdevelopedbytheAmericanNationalStandardsInstitute(ANSI)X3T9.5standardscommittee
inthemid-1980s.Atthetime,high-speedengineeringworkstationswerebeginningtotaxthebandwidth
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-criticalapplicationsfromlargecomputerstonetworks.FDDIwasdevelopedtofilltheseneeds.
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 Transmission Media
FDDIuses opticalfiberas theprimarytransmission medium,butit alsocanrun overcoppercabling. 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 2
km between stations using multimode fiber, and even longer distances using a single mode.
FDDIdefinestwotypesofopticalfiber:single-modeandmultimode.Amodeisarayoflightthatenters
the fiber at a particular angle. Multimode fiber uses LED as the light-generating device, while
single-mode fiber generally uses lasers.
Multimode 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 multimode fibers. For this reason, multimode fiber is generally used for
connectivity within a building or 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
fiber is capable of delivering considerably higher performance connectivity over much larger distances,
which is why it generally is used for connectivity between buildings and within environments that are
more geographically dispersed.
Figure8-2depictssingle-modefiberusingalaserlightsourceandmultimodefiberusingalightemitting
diode (LED) light source.
FDDI Specifications
FDDIspecifiesthephysicalandmedia-accessportionsoftheOSIreferencemodel.FDDIisnotactually
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) specifications.
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.ThePHYspecificationdefinesdataencoding/decodingprocedures,clockingrequirements,
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.
FDDI Station-Attachment Types
OneoftheuniquecharacteristicsofFDDIisthatmultiplewaysactuallyexistbywhichtoconnectFDDI
devices. FDDI defines four types of devices: single-attachment station (SAS), dual-attachment station
(DAS), single-attached concentrator (SAC), and dual-attached concentrator (DAC).
An SAS attaches to only one ring (the primary) through a concentrator. One of the primary advantages
ofconnectingdeviceswithSASattachmentsisthatthedeviceswillnothaveanyeffectontheFDDIring
if they are disconnected or powered off. Concentrators will be covered in more detail in the following
discussion.
EachFDDIDAShastwoports,designatedAandB.TheseportsconnecttheDAStothedualFDDIring.
Therefore,eachportprovidesaconnectionforboththeprimaryandthesecondaryrings.Asyouwillsee
in the next section, devices using DAS connections will affect the rings if they are disconnected or
poweredoff.Figure8-4showsFDDIDASAandBportswithattachmentstotheprimaryandsecondary
rings.
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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
orpower-downofanySASdoesnotbringdownthering.ThisisparticularlyusefulwhenPCs,orsimilar
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.
FDDI Fault Tolerance
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.
Dual Ring
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.
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 incapable of
communicating with each other.
Optical Bypass Switch
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 using optical mirrors that pass light from the ring directly to the
DAS device during normal operation. If a failure of the DAS device occurs, such as a power-off, the
optical bypass switch will pass the light through itself by using internal mirrors and thereby will
maintain the ring’s integrity.
Thebenefitofthiscapabilityisthattheringwillnotenterawrappedconditionincaseofadevicefailure.
Figure8-8showsthefunctionalityofanopticalbypassswitchinanFDDInetwork.WhenusingtheOB,
you will notice a tremendous digression of your network as the packets are sent through the OB unit.
Dual Homing
Critical devices, such as routers or mainframe hosts, can use a fault-tolerant technique called dual
homingtoprovideadditionalredundancyandtohelpguaranteeoperation.Indual-homingsituations,the
criticaldeviceisattachedtotwoconcentrators.Figure8-9showsadual-homedconfigurationfordevices
such as file servers and routers.
One pair of concentrator links is declared the active link; the other pair is declared passive. The
passive
link stays in backup 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.
FDDI Frame Format
The FDDIframe format issimilar tothe format ofa Token Ringframe. This isone ofthe areas inwhich
FDDIborrowsheavilyfromearlierLANtechnologies,suchasTokenRing.FDDIframescanbeaslarge
as 4,500 bytes. Figure 8-10 shows the frame format of an FDDI data frame and token.
• Preamble—Gives a unique sequence that prepares each station for an upcoming frame.
• Start delimiter—Indicates the beginning of a frame by employing a signaling pattern that
differentiates it from the rest of the frame.
• Frame control—Indicates the size of the address fields and whether the frame contains
asynchronous or synchronous data, among other control information.
• Destinationaddress—Containsaunicast(singular),multicast(group),orbroadcast(everystation)
address. As with Ethernet and Token Ring addresses, FDDI destination addresses are 6 bytes long.
• Source address—Identifies the single station that sent the frame. As with Ethernet and Token Ring
addresses, FDDI source addresses are 6 bytes long.
• Data—Contains either information destined for an upper-layer protocol or control information.
• Frame check sequence (FCS)—Is filed by the source station with a calculated cyclic redundancy
check value dependent on frame contents (as with Token Ring and Ethernet). The destination
address recalculates the value to determine whether the frame was damaged in transit. If so, the
frame is discarded.
• End delimiter—Contains unique symbols; cannot be data symbols that indicate the end of the
frame.
• Framestatus—Allowsthesourcestationtodeterminewhetheranerroroccurred;identifieswhether
the frame was recognized and copied by a receiving station.
Copper Distributed Data Interface
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 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
unshieldedtwistedpair(UTP).STPcablinghas150-ohmimpedanceandadherestoEIA/TIA568(IBM
Type1)specifications.UTPisdata-gradecabling(Category5)consistingoffourunshieldedpairsusing
tight-pair twists and specially developed insulating polymers in plastic jackets adhering to EIA/TIA
568B specifications.
Fiber Distributed Data Interface
