From professional translators, enterprises, web pages and freely available translation repositories.
Fiber Distributed Data Interface
The FDDI standard dates to October 1982, when the American National
Standards Institute (ANSI) Committee X3T9.5 was chartered to develope
high-speed data networking standard. The resulting effort was the devel-opment of a token-passing optical fiber media network based the
use of two fiber pairs, each operating at 100 Mbits/s.Position in the OSI Reference Model FDDI is defined as the two bottom layers of the seven-layer Internation-al Systems Organization (ISO) Open System Interconnection (OSI) Refer- ence Model: the physical and data-link layers. Figure 5.1 illustrates the relationship of FDDI, Ethernet, and Token Ring LANs to the seven-layer ISO OSI Reference Model. In examining Fig. 5.1, note that FDDI is simi- lar to Ethernet and Token Ring in that it provides a transport facility
Position in the OSI Reference Model
FDDI is defined as the two bottom layers of the seven-layer Internation-
al Systems Organization (ISO) Open System Interconnection (OSI) Refer-
ence Model: the physical and data-link layers. Figure 5.1 illustrates the
relationship of FDDI, Ethernet, and Token Ring LANs to the seven-layer
ISO OSI Reference Model. In examining Fig. 5.1, note that FDDI is simi-
lar to Ethernet and Token Ring in that it provides a transport facility
for higher-level protocols such as TCP/IP. In fact, FDDI was, and in some
locations still is, commonly used at the Internet Service Provider (ISP)
peering points that provide interconnections between ISPs.
The FDDI standard recognized the need to subdivide the physical
layer. Under the FDDI standard the physical-medium-dependent (PMD)
sublayer defines the details of the fiber-optic cable used, while the physi-cal (PHY) layer specifies encoding/decoding and clocking operations.Figure 5.2 illustrates the FDDI subdivision of the physical layer. Later LAN standards, such as Fast Ethernet and Gigabit Ethernet, can be con-sidered to use the learning curve from FDDI as they also resulted in thesubdivision of the physical layer.
The actual coding method used by FDDI results in the encoding of 4
bits with the use of a 5-bit pattern. Thus, this encoding technique is
referred to as a 4B/5B code. Because 4 bits are encoded into 5, this meansthere are 16 4-bit patterns. Those patterns, which are listed in Table 5.1,were selected to ensure that a transition is present at least twice for each 5-bit code. Because 5-bit codes are used, the remaining symbols provide special meanings or represent invalid symbols. Concerning special.
• Fiber Specifications
FDDI represents a token-passing dual-ring network. The ANSI PMD
standard specifies several optical characteristics that govern FDDI opera-tion. Those optical characteristics include the type of optical fiber used,the wavelength of the optical signal that transmits the data, and the amount of power loss in the cable; the latter is specified in terms of both a power budget expressed in decibels and cable attenuation expressed in decibels per kilometer.
FDDI 4B/5B Codes
4-bit code function 5B code Symbol Invalid-code assignments
00001 Void or halt
00010 Void or halt
01000 Void or halt
10000 Void or halt
OPTICAL FIBER SUPPORT FDDI can support 62.5/125-, 50/125-,
and 100/140-?m multimode fiber sizes; 62.5/125 multimode fiber is the
preferred medium. FDDI also supports the use of single-mode fiber,
which is commonly used for long-distance transmission that enables the
technology to extend to a metropolitan area network (MAN) of up to
100 km. When single-mode fiber is used, it has a core diameter of 8 to 10µm and a cladding diameter of 125 µm. Thus, FDDI single-mode fiber is commonly specified as 8/125, 9/125, and 10/125
OPTICAL TRANSMITTER Available transmitters and receivers
operate at 850, 1300, and 1550 nm; the 1300 nm wavelength is most com-monly used. Transmission at 850 and 1300 nm is designed for use over multimode fiber. Single-mode fiber that can support FDDI for greater transmission distances operate at 1300 and 1500 nm. Both lasers and laser diodes are used with multimode fiber; however, when single-mode fiber is used, the transmitter must be a laser diode since it has the narrow wavelength required to transmit optical signals into the narrow core of the fiber.
ATTENUATION For multimode fiber the FDDI PMD standard speci-
fies a power budget of 11.0 dB. Because the power budget of a system
represents the minimum transmitter power and the minimum receiver
sensitivity, this means that up to 11 dB of the optical signal can be lost
• In addition, the maximum cable attenuation is 1.5 dB/km at 1300 nm.
• When single-mode fiber is used, the PMD specifies a range of power
• budgets. The specified range depends on the types of transmitters use and extends from 10 to 32 dB (maximum).
• Media Interface and ST Connectors
• One of the well-thought-out portions of the ANSI standard is represent-ed by the FDDI Media Interface Connector (MIC). This connector, which is used to connect multimode fiber to an FDDIstation, consists of a keyed plug and a keyed receptacle and is polarized to ensure that an appropri ate transmitter/receiver-to-fiber association occurs, representing perhaps the first “dummyproof” optical connection FDDI also supports ST-type connectors to physically connect optical fiber to an FDDI station. Although ST connectors provide a lower-cost alternative for connecting optical fiber to an FDDI station, they do not
• provide a polarized receptacle. Thus, when using an ST-type connector, you must ensure that transmit and receive connections are not reversed.
Figure 5.3 illustrates how FDDI can compensate for a cable break. The
left portion of the figure illustrates the use of the dual-ring structure to
interconnect four workstations. Note that the FDDI backbone consists
of two separate fiber-optic rings, one of which is active and referred to as the primary ring, while the other ring can be considered to be “on hold,” waiting to be activated, and is referred to as the secondary ring. Each device directly connected to the FDDI backbone must be attached to both the primary and secondary rings, resulting in the term dual attach- ment. If a problem occurs on the primary ring, the secondary ring will be used. In fact, FDDI’s Station Management (SMT) facility built into each FDDI adapter on either side of a cable break or another impairment on a cable recognizes that communications have failed on the ring. SMT then wraps the communications from the primary ring back out onto the sec- ondary ring in the opposite direction, permitting all stations other than those in that portion of the ring that is broken to resume communica- tions. The middle portion of Figure 5.3 illustrates the occurrence of a break in the FDDI network, while the right portion of that illustration shows how SMT interconnects or wraps communications from the pri- mary ring onto the secondary ring in the opposite direction to re-form the ring. Thus, a break in an FDDI ring can be considered as similar to the battery commercial; that is, like the bunny in the commercial, the FDDI ring keeps on operating
Two station types are defined under the FDDI standard: class A and
class B. Class A stations connect to both primary and secondary rings
that make up an FDDI network and are better known as dual-attachment stations. Class B devices are indirectly connected to a ring via a class A device. Thus, a class A station acts as a wiring concentrator that connects several class B stations to the FDDI ring. As you might expect, a class B station is referred to as a single-attachment station.
Figure 5.4 illustrates the use of several single-attachment stations and
one dual-attachment station on an FDDI ring. Note that this configura-
tion is designed as an economy measure since it would have been too
expensive during the 1980s, when FDDI was developed with SMT capa-
bility for each device connected to the LAN. Thus, restricting SMT to
dual-attachment stations results in the wrapping of communications
during a cable break or another impairment supporting redundancy for
multiple stations. Because of the high degree of redundancy affordedthrough the use of FDDI, it remains a popular infrastructure at Inter-net peering points.
Port Types and Rules
To prevent the incorrect configuration of a network, ANSI FDDI stan-
dards specify certain connection rules. Those rules cover the functions of four defined port types: A, B, M, and S. Those ports govern connectivity between workstations and a concentrator as well as the connection of a concentrator to the primary and secondary rings. Figure 5.5 is a schematic diagram of the configuration of these ports in an FDDI network.
In examining Figure 5.5, it is important to note that each port con-
nector has a specific mechanical keying interface defined by the PMD
standard. For example, port A connects to the incoming primary ring
and outgoing secondary ring. In comparison, port B connects to the
outgoing primary ring and the incoming secondary ring. In compari-
son, port M provides connectivity to a single-attachment station or to
another concentrator, while port S connects a single-attachment station
or single-attachment concentrator.
FDDI Frame Format
The basic FDDI frame format is illustrated in Figure 5.6. The preamble
is variable in length, consisting of a minimum of 16 4B/5B I symbols.
The actual beginning of the frame is the Start Delimiter (SD) field.
This field consists of the 4B/5B J and K control symbols. The SD field
is followed by the Frame Control (FC) field. This 8-bit field identifies
the type of frame and is followed by the Destination Address (DA) and
Source Address (SA) fields. Each field is 48 bits in length and corre-
sponds to the MAC (Media Access Control) addresses used by Ethernet
and Token Ring LANs. Thus, the first 3 bytes represent the manufac-
turer of the FDDI adapter, while the last 3 bytes represent the specific
adapter number manufactured by the vendor. The source address is fol-
lowed by a variable Information field and a 4-byte Frame Check
Sequence (FCS) field. The FCS contains a 32-bit cyclic redundancy
check (CRC) value that provides integrity for the FC, DA, SA, and infor-
mation fields. The End Delimiter (ED) field consists of two 4B/5B T
FRAME TYPES FDDI specifies several types of frames, each of which
has a defined function. Those frame types include MAC frames, SMT
frames, and LLC frames. MAC frames carry Media Access Control data.
Such frames include claim frames used in the ring initialization process
and beacon frames; the latter are used in the ring fault-isolation process.SMT frames are used to transport FDDI management information between frames. Such frames operate, control, and maintain the FDDI ring and its stations. As indicated earlier, S
RING SCHEDULING AND SERVICES FDDI supports the opera-
tion of a timed-token protocol (TTP), which defines the means by which a station acquires access to a ring. As we will soon note, several timers gov- ern the operation of an FDDI LAN. However, before discussing the role of timers, we shall describe the two types of services supported by
ASYNCHRONOUS SERVICESIn FDDI terminology, asynchronous transmis- sion refers to the ability of a station to transmit data during periods when bandwidth is not reserved by synchronous services. Thus, we can note that in the FDDI context the term asynchronous services represents the transmission of information that is not extremely delay-sensitive (i.e.,not time-critical). FDDI: asynchronous and synchronous
SYNCHRONOUS SERVICES A second type of service specified by FDDI is synchronous transmission services. In synchronous services, each sta- tion is guaranteed a portion or slice of the 100-Mbit/s FDDI band-width. The actual amount of synchronous bandwidth allocated to a station is negotiated using an allocation procedure defined by SMT. As we will soon note, when a station does not need its guaranteed band- width for synchronous services, it can use that bandwidth for asyn-chronous services
TOKEN PASSINGIf you are familiar with the 4- or 16-Mbit/s Token
Ring LAN frame format, you are probably mystified as to the location of
the token in the FDDI frame. Unlike Token Ring, which includes a token
bit in its Frame Control field, FDDI uses a special six-symbol frame as
illustrated in Figure 5.7. The token frame consists of four fields: a pream- ble field of 16 or more I symbols, an SD field of J and K symbols, an FC field, and an ED field. Stations that have data to transmit must first acquire a token. However, unlike a Token Ring LAN where the token is part of the frame and is transmitted within the formed frame, under
FDDI the token is held by the transmitting station, a process referred to
as token absorption. The actual period of time that the token is held is
specified by a timed-token protocol (TTP). A station can transmit as much data as it has during its token hold time or until the hold time expires. Actually, each station on an FDDI ring uses three timers to regulate its operation. These timers are described in the following three paragraphs.
TOKEN ROTATION TIMER The token rotation timer (TRT) is used to time
the duration of operations permitted by a station. The value assigned to
the TRT depends on the state of the ring. For instance, during steady-
state operations the TRT will expire when the target token rotation time
(TTRT) is exceeded. Stations negotiate the value for the TTRT during the
claim process, which determines the station that will initialize the FDDI
ring. This process represents an auction in reverse, where the station with the lowest bid for the TTRT time acquires the right to initialize the ring.
TOKEN HOLDING TIMER The token holding timer (THT) controls the
period of time during which a station can operate asynchronously.
FDDI supports both asynchronous and synchronous services; the for-
mer designed to support bandwidth-intensive operations when the
station does not need the guaranteed bandwidth provided by synchro-
nous services. Thus, asynchronous frames are transmitted only when
bandwidth reserved for synchronous operations is not used. In compar-
ison, synchronous frames can be transmitted at any time as long as the
negotiated synchronous bandwidth allocation is not exceeded. The
token holding time for a station is initialized with the value corre-
sponding to the difference between the arrival of the token and the
TTRT value. To transmit asynchronous frames, a station must have a
token and the THT timer must not have expired.
VALID TRANSMISSION TIMER A third FDDI station timer is the valid
transmission timer (TVX). The function of this timer is to time the peri-
od between valid transmissions on the FDDI ring. If a token is lost,
excessive noise occurs on the ring, or another impairment results in the
TVX expiring, the station assumes a problem occurred and initiates a
ring initialization operation to restore the ring to an operational status.
FDDI represents a well-thought-out but complex fiber-optic LAN tech-
nology. While it can support a dual ring up to 100 km (62 miles), which
makes it attractive for MAN operations, its primary use is in a LAN
environment. Unfortunately, its relatively high cost because of its com-
plexity has limited its appeal in comparison to the Fast Ethernet and
Gigabit Ethernet, which benefit from lesser complexity and greater
economies of scale; the latter advantage is due to the considerably larger market for the Ethernet products. Nevertheless, FDDI continues to be used in mission-critical areas where reliability as well as the ability to transmit data near the maximum LAN operating rate is critical.
Ethernet and Fast Ethernet
Anyone who has not been in a prolonged period of isolation would
know that the battle for LAN dominance is over, with Ethernet in its
several flavors representing the victor. However, what we may not realize is the fact that fiber-optic components have a long association with Eth- ernet, dating from their use as a transmission extender for 10 Mbit/s
Ethernet to the preferred media used to construct a Gigabit Ethernet
network. In this section we will focus on the use of optical components
to extend the capabilities of different types of Ethernet networks. How-
ever, because fiber-optic media are used primarily to extend transmis-
sion distance in an Ethernet and Fast Ethernet environment, we will
begin our examination of the use of fiber by focusing on the network diameter of an Ethernet LAN.
Network Diameter Constraints
Ethernet uses a Carrier Sense Multiple Access with Collision Detection
(CSMA/CD) access protocol. Under this protocol a station with data to
transmit first listens to determine if another station is transmitting or
if the network is idle. If the network is idle, the station will transmit a
frame; otherwise, the station waits for an idle condition. Once the sta-
tion begins transmission, it will listen for a short period of time to
determine if a collision occurred. That period of time is referred to as
the Ethernet slot time Tsand is defined as the time duration required for
the transmission of 512 bits of data (64 bytes). The selection of 64 bytes
represents the minimum length frame for Ethernet and Fast Ethernet.
For 10BASE-T, the slot time is 512 bits ? 100 ns/bit or 51.2 ?s. For 100BASE-T, the slot time is one-tenth the Ethernet slot time or 5.12 ?s. For both Ethernet and Fast Ethernet, a so-called 512 rule defines the network diameter for half-duplex LAN operations. Basically, the 64-byte frame (512 bits) is the smallest packet size that can be transmitted, and the net- work diameter is determined by the time the shortest frames take to travel round trip to the farthest node in the network. If frames do not arrive by this time, the network can experience late collisions. Thus, the importance of the slot time is twofold. It not only provides time for a station to detect collisions but also specifies the amount of time delay between a transmitting station and a receiving station. Because signals propagate down a transmission medium, this, in turn, results in the slot
time governing the network diameter.
DETERMINING ONE-WAY DELAYTo determine the one-way
delay Dopermissible on an Ethernet LAN, let’s consider a two-node net-
work such as the one shown in Figure 5.8. In this example let’s assume that station A transmits a frame to station B, requiring a time of TA to reach B’s adapter. As A’s frame traverses the LAN, let’s further assume that workstation B has data to transmit. Because A’s frame has not yetn reached B’s workstation, B listens to the LAN, does not “hear” A’s frame, and begins its transmission. From a worst-case collision detection per-
spective, workstation B would begin its transmission just before the first bit in workstation A’s frame arrives. This is a worst-case scenario
because the collision induces a high voltage that requires the longest
time for the frame to flow down the cable and reach workstation A.
The high voltage, which indicates the presence of a collision, is applica-
ble only to Ethernet and Fast Ethernet and copper-based Gigabit. How-
ever, the Radio Frequency (RF) version of Ethernet, referred to as
10broad-36, uses a different mechanism to detect the presence of a colli- sion. Returning to our two-workstation example, while workstation B will
Usage Frequency: 1
Search human translated sentences
Users are now asking for help: ox (English>Nepali) | supplerende (Danish>Spanish) | agrosciences (Finnish>English) | 50-60 word essay on eduction (Hindi>English) | makroregionálním (Czech>Finnish) | factors of production (Tagalog>English) | snow in the morning later becoming rain (French>English) | utang (Tagalog>English) | escríbela de nuevo (Spanish>English) | chúc một ngày tốt lành (Vietnamese>English) | ariviyal valarchi nanmaika katturai in tamil (English>Tamil) | kasingkahulugan ng pananaw (Tagalog>English) | encrypted meaning (English>Tagalog) | pluto (English>Tagalog) | essay on mera priya shahar kolkata (English>Hindi)