7
C H A P T E R
Fiber Distributed Data Interface
Background
豹肉The Fiber Distributed Data Interface (FDDI) standard was produced by the ANSI X3T9.5 standards
committee in the mid-1980s. During this period, high-speed engineering workstations were
beginning to tax the capabilities of existing local-area networks (LANs) (primarily Ethernet and
Token Ring). A new LAN was needed that could easily support the workstations and their new
distributed applications. At the same time, network reliability was becoming an increasingly
important issue as system managers began to migrate mission-critical applications from large
computers to networks. FDDI was developed to fill the needs.
After completing the FDDI specification, ANSI submitted FDDI to the International Organization
for Standardization (ISO). ISO has created an international version of FDDI that is completely
compatible with the ANSI standard version.
Today, although FDDI implementations are not as common as Ethernet or Token Ring, FDDI has
gained a substantial following that continues to increa as the cost of FDDI interfaces diminishes.
FDDI is frequently ud as a backbone technology as well as a means to connect high-speed
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computers in a local area.
Technology Basics
FDDI specifies a 100-Mbps, token-passing, dual-ring LAN using a fiber-optic transmission medium.
It defines the physical layer and media-access portion of the link layer, and so is roughly analogous
to IEEE 802.3 and IEEE 802.5 in its relationship to the Open System Interconnection (OSI) reference
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model.
Although it operates at faster speeds, FDDI is similar in many ways to Token Ring. The two networks
share many features, including topology (ring), media-access technique (token passing), reliability
features (redundant rings, for example), and others. For more information on Token Ring and related
technologies, refer to Chapter6, “Token Ring/IEEE 802.5.”
One of the most important characteristics of FDDI is its u of optical fiber as a transmission
medium. Optical fiber offers veral advantages over traditional copper wiring, including curity
(fiber does not emit electrical signals that can be tapped), reliability (fiber is immune to electrical
interference), and speed (optical fiber has much higher throughput potential than copper cable).
FDDI defines u of two types of fiber:single mode (sometimes called monomode) and multimode.
东风悦达起亚嘉华Modes can be thought of as bundles of light rays entering the fiber at a particular angle. Single-mode
fiber allows only one mode of light to propagate through the fiber, while multimode fiber allows
multiple modes of light to propagate through the fiber. Becau multiple modes of light propagating
through the fiber may travel different distances (depending on the entry angles), causing them to
arrive at the destination at different times (a phenomenon called modal dispersion), single-mode fiber
FDDI Specifications is capable of higher bandwidth and greater cable run distances than multimode fiber. Due to the characteristics, single-mode fiber is often ud for interbuilding connectivity, while multimode fiber is often ud for intrabuilding connectivity. Multimode fiber us light-emitting diodes (LEDs) as the light-generating devices, while single-mode fiber generally us lars.
FDDI Specifications
FDDI is defined by four parate specifications (e Figure 7-1):
•
Media Access Control (MAC)—Defines how the medium is accesd, including frame format,token handling, addressing, algorithm for calculating a cyclic redundancy check value, and error recovery mechanisms.•
Physical Layer Protocol (PHY)—Defines data encoding/decoding procedures, clocking requirements, framing, and other functions.•
Physical Layer Medium (PMD)—Defines the characteristics of the transmission medium,including the fiber-optic link, power levels, bit error rates, optical components, and connectors.•Station Management (SMT)—Defines the FDDI station configuration, ring configuration, and ring control features, including station inrtion and removal, initialization, fault isolation and recovery, scheduling, and collection of statistics.
Figure 7-1FDDI Standards
Physical Connections
FDDI specifies the u of dual rings. Traffic on the rings travels in opposite directions. Physically,the rings consist of two or more point-to-point connections between adjacent stations. On
e of the two FDDI rings is called the primary ring; the other is called the condary ring. The primary ring is ud for data transmission, while the condary ring is generally ud as a backup.
Class B or single-attachment stations (SAS) attach to one ring;Class A or dual-attachment stations (DAS) attach to both rings. SASs are attached to the primary ring through a concentrator , which provides connections for multiple SASs. The concentrator ensures that failure or power down of any given SAS does not interrupt the ring. This is particularly uful when PCs, or similar devices that frequently power on and off, connect to the ring.
Logical Link Control
Media Access Control
Physical Layer Protocol Physical Layer Medium
Station Management FDDI
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Traffic Types A typical FDDI configuration with both DASs and SASs is shown in Figure 7-2.
Figure 7-2FDDI Nodes: DAS, SAS, and Concentrator
Each FDDI DAS has two ports, designated A and B. The ports connect the station to the dual FDDI ring. Therefore, each port provides a connection for both the primary and the condary ring, as shown in Figure 7-3.
Figure 7-3FDDI DAS Ports
Traffic Types
FDDI supports real-time allocation of network bandwidth, making it ideal for a variety of different application types. FDDI provides this support by defining two types of traffic:synchronous and asynchronous . Synchronous traffic can consume a portion of the 100-Mbps total bandwidth of an FDDI network, while asynchronous traffic can consume the rest. Synchronous bandwidth is
allocated to tho stations requiring continuous transmission capability. Such capability is uful for transmitting voice and video information, for example. Other stations u the remaining bandwidth asynchronously. The FDDI SMT specification defines a distributed bidding scheme to allocate FDDI
bandwidth.
Asynchronous bandwidth is allocated using an eight-level priority scheme. Each station is assigned an asynchronous priority level. FDDI also permits extended dialogues, where stations may过敏性皮肤炎
temporarily u all asynchronous bandwidth. The FDDI priority mechanism can esntially lock out stations that cannot u synchronous bandwidth and have too low an asynchronous priority.
SAS SAS SAS
DAS FDDI
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Concentrator
Port A
Port B FDDI DAS
Secondary Primary Secondary
Primary
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Fault-Tolerant Features Fault-Tolerant Features
商业合同模板FDDI provides a number of fault-tolerant features. The 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 itlf) into a single ring, as shown in Figure 7-4. In this figure, when Station 3 fails, the dual ring is automatically wrapped in Stations 2 and 4, forming a single ring. Although Station 3 is no longer on the ring, network operation continues for the remaining stations.
Figure 7-4Station Failure, Ring Recovery Configuration
Station 4Station 3
Station 2
Station 1
Ring wrap Ring wrap B A
MAC B A平淡无奇的意思
MAC
B
A B A
MAC
Failed station
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Fault-Tolerant Features Figure 7-5 shows how FDDI compensates for a wiring failure. Stations 3 and 4 wrap the ring within themlves when wiring between them fails.
Figure 7-5Failed Wiring, Ring Recovery Configuration
As FDDI networks grow, the possibility of multiple ring failures grows. When two ring failures occur, the ring will be wrapped in both cas, effectively gmenting the ring into two parate rings that cannot communicate with each other. Subquent failures cau additional ring gmentation.Station 4Station 3
Station 2
Station 1
Ring wrap B A
MAC B A
MAC
B
A B A
MAC
MAC
Ring wrap
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