fiber dicriptoin

8/7/2019 fiber dicriptoin

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Fiber Type and Performance

As fiber becomes more widely deployed in premises applica-

tions, a system designer should evaluate both multimode and

single-mode optical fiber to ensure the system meets present

requirements and those of future upgrades. Data ratesincrease as new applications are being created. The system

designer can allow for bandwidth scalability by installing

optical fiber instead of other media. Use of optical fiber maxi-

mizes the prospects of ensuring compatibility with all future

applications.

The purpose of this chapter is to familiarize the reader with

fiber types and performance requirements needed to support

local area network (LAN) and storage area network (SAN)

applications commonly used in premises networks and data

centers and to describe considerations necessary to ensure

bandwidth scalability for future upgrades.

Applications

Six primary network applications are in use today. Each one

operates somewhat differently from the others and some are

interrelated.The systems are Ethernet, Token Ring, Fiber

Distributed Data Interface (FDDI), Fibre Channel,

Asynchronous Transfer Mode (ATM), and Synchronous Optical

Network (SONET). Some of these are designed for data trans-

mission only. Others can carry voice, data and video signals

simultaneously despite the huge difference in the transmis-

sion rates for these three types of signals. This chapter will

cover the transmission requirements for each application.

Ethernet

Ethernet is used primarily for data transmission. It originally

began as a bus-based application with coaxial cable as the

primary bus medium, but fiber replaced coax to extend

usable distance. Ethernet is now predominantly deployed in

switch networks. Ethernet versions using fiber are 10BASE-F

(10 Mb/s), 100BASE-F (100 Mb/s), 100BASE-5 (100 Mb/s),

1000BASE-S (1000 Mb/s), 1000BASE-L (1000 Mb/s), 10GBASE-S(10 Gb/s), 10GBASE-L (10 Gb/s), 10GBASE-LX4 (10 Gb/s),

10GBASE-E (10 Gb/s). New intrabuilding fiber installations

usually operate over multimode fiber at

1000 Mb/s (gigabit) with 850 nm transceivers. With such

installations, scalability to 10 Gb/s should be considered

desirable. Campus backbone applications running Gigabit

Ethernet over multimode fiber may achieve link lengths up to

2006 Corning Cable Systems 3.1 Design G uide

1000 m; however, link lengths greater than about

550 m will require single-mode fiber to provide

10 Gb/s scalability. Ethernet systems are inclu-

sively standardized as IEEE 802.3. To date there isonly one copper media solution for 10 Gigabit

Ethernet, 10GBASE-CX4, which requires a factory-

terminated twin-axial cable for a maximum 15 m

distance. Twin-axial cable consists of a thick bun-

dle of eight separately shielded twin-axial cable

pairs.

Token Ring

Token Ring is a ring-based network application

used for data transmission. It operates at either

4 Mb/s or 16 Mb/s at the 850 nm operatingwavelength. Token Ring uses a token to pass

data between stations. Only the station that has

the token can transmit data. It uses twisted cop-

per pairs (shielded and unshielded) or optical

fiber as the transmission medium. Token Ring is

based on the IEEE 802.5 Standard.

FDDI

Fiber Distributed Data Interface (FDDI) is a dual-

ring (counter-rotating), token-based network

application for data and digital video transmis-

sions. It was designed to accommodate higher

data rates over longer distances with increased

reliability over previous applications. It operates

at 100 Mb/s using two rings; one ring for the sig-

nal and one ring as a backup in case of node or

cable failure. It operates at 1300 nm and was

originally written for 62.5/125 m multimode

fiber, but 50/125 m multimode fiber can also be

used.

Fibre Channel

Fibre Channel is a high-performance serial link

application with data rates of 1 Gb/s, 2 Gb/s,

4 Gb/s and 10 Gb/s. The standard specifies multi-

mode fiber and single-mode fiber as the primary

media type. The fiber type recommended

depends on the desired distance and data rate.


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The primary application is for data center SANs over multi-

mode fiber operating at 850 nm, such as laser-optimized

50/125 m multimode fiber. Links between buildings may

require single-mode fiber.

ATM

Asynchronous Transfer Mode (ATM) is designed to allow the

efficient transmission of data between networks. If a user

multiplexes voice (low-rate), data (medium-rate), and video

(high-rate) signals over the same system, the system must be

capable of handling the signal that requires the highest

information rates (probably video). ATM efficiently uses the

available bandwidth by packaging the inputs from voice, data

and video sources into a series of 53-byte packets (5 bytes for

addressing, 48 bytes of information) for transmission andswitching at a rate that is compatible with the connecting

network. ATM can operate at different speeds using the same

packet system and automatically adjusts to the network

speed of the addressee. As system requirements change, so

can the data rate to meet those requirements.The data rates

range from 52 Mb/s to

2.5 Gb/s.

SONET

Synchronous Optical Network (SONET) is an optical multi-

plexing hierarchy for the transmission of voice, data and/or

video over single-mode fiber. SONET uses a base rate of 51.84

Mb/s with higher data rates in multiples of the base rate.

SONET is not a network application in and of itself, but rather

a system for coordinating and integrating different applica-

tions and networks over wide areas. SONET takes an incom-

ing multiplexed signal and reformats it to an electrical signal

called a Synchronous Transport Signal (STS). The

electrical signal is then converted to an Optical Carrier (OC)

signal. For example, an STS-1 electrical signal would be con-

verted to an OC-1 optical signal. The OC signal has the same

rate, format and functions as the STS signal. The SONET sig-nal can assume the same format as another application such

as ATM, Ethernet or FDDI.

Many of the developing high-data-rate applications are bas-

ing their transmission criteria on the SONET transmission

scheme.

2006 Corning Cable Systems3.2Design Guide


End Equipment

Span length, application and data rate are the

determining factors in the selection of fiber typeand end equipment. All must be considered in

order to make the best overall selection.

Multimode fiber is appropriate for the majority

of premises applications, as the associated opto-

electronic transmission equipment is usually

more economical than that for single-mode sys-

tems. Analysis of a specific system design will

lead to the selection of the most suitable fiber

type and end equipment, after which detailed

consideration of the optical parameters for both

fiber and system is necessary.

The following is a discussion of the nature and

meaning of those optical parameters with which

the designer should be familiar.

Transmitters

The transmitter is an electronic device that

receives an electrical signal, converts it into a

light signal and launches the signal into a fiber.

The transmitter can be a light emitting diode

(LED) or a laser.The common characteristics ofthese light sources influencing fiber selection are

center wavelength and spectral width.

LEDs are inexpensive when compared to most

lasers and are primarily used with multimode

fiber because they emit light in a broad cone

that can only be captured efficiently by the large

numerical aperture of multimode fiber. LEDs

have a maximum modulation rate of 300 MHz,

which translates to a 655 Mb/s data rate. For sys-

tems operating at > 655 Mb/s, lasers must be

used. A typical output power for an LED source is-12 dBm.

Fabry perot (FP) lasers and distributed feedback

(DFB) lasers emit light in a very narrow beam,

making them ideal for use with the small

numerical aperture of single-mode fiber.These

may also be used for multimode systems operat-

ing at 1300 nm.


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Figure 3.1 Spectral Profile Comparison, Laser and LEDfor 850 nm Transmission


Vertical cavity surface emitting lasers (VCSELs) emit light in a

larger spot size than FP or DFB lasers but in a much smaller

beam than an LED. Because their cost is closer to that of an

LED,VCSELs provide the optimum solution for high-bit-rate (1 Gb/s) 850 nm serial operation over 50/125 m multimode

fiber.

Center Wavelength

Optical fiber transmitters are characterized by the wave-

length at which they emit light. The nominal emission wave-

length is called the center wavelength of the transmitter,

although the transmitted signal is actually a collection of

wavelengths around this nominal value. The center wave-

length is primarily a function of the type and configuration of

the materials used to fabricate the transmitter. It is usuallyexpressed in nanometers (nm). LEDs with center wavelengths

at 850 nm or 1300 nm have been in wide use for many years

and the transmission specifications for multimode fiber are

given at these two wavelengths. Laser transmitters for single-

mode systems operate at center wavelengths of 1310 nm or

1550 nm; thus single-mode fibers carry specifications for

transmission at these two wavelengths. VCSELs operate at a

center wavelength of 850 nm over multimode fiber.

Spectral Width

The total power produced by an optical transmitter is not

confined to just the center wavelength. It is distributed over a

range of wavelengths spread about the center wavelength.

This range is quantified as the spectral width, Dl, measured

in nanometers (nm), and it impacts the overall transmission

capacity of a fiber optic link (Figure 3.1). Spectral width is usu-

ally expressed as a full-width, half-maximum (FWHM) value

(Figure 3.2). Transmitter specifications include a specification

for spectral width. For LEDs, typical FWHM values for

spectral width will be 30-50 nm; while for VCSELs, it would

typically be 0.2-0.4 nm; and for FP lasers, it would typically be

1-3 nm. Figure 3.3 shows characteristics of a VCSEL.

Receivers

As with transmitters, each piece of optical fiber transmission

equipment contains a receiver. Nearly all types of receivers

used in optical fiber systems incorporate a photodetector

such as a photodiode to convert the incoming optical signal

back to an electrical signal. The operating wavelength of the


VCSEL

LED

Wavelength (nm)

Intensity

One-HalfMaximum

Intensity

Intensity

Full-Width, Half-Maximum(FWHM) Spectral Width

Wavelength (nm)

Maximum

Intensit

Figure 3.2 Pulse Width of a Light Source Showing Full-Width,Half-Maximum (FWHM)


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receiver matches that of the transmitter. A receiver designed

for 1300 nm operation is not suitable for use at 850 nm.

Data Rate

The data rate is the maximum number of bits per second

that can be transmitted and received with a bit error rate

(BER) below a certain level. A typical BER is one error in 1012

pulses.

The typical sources used in premises applications are directly

modulated (DM).These sources can be LEDs or lasers, such as

a VCSEL. They are called directly modulated because the

source itself turns on and off. Contrast this to an external

modulated (EM) laser which is always on and the light is

modulated by an external source. EM lasers perform betterthan DM sources but also cost much more. The performance

of DM sources is suitable for most premises applications.

These lasers are used because they perform adequately and

cost less. For directly modulated sources, the limiting factors

are the time required for a light to turn on (rise time) and

turn off (fall time) for each pulse. The rise time is typically thetime required for the light output to rise from 10 to 90 per-

cent of the maximum level.The fall time is the reverse. Often

the rise and fall times are the same; however, the longer of

these two quantities is considered the response time. A typi-

cal value might be a few nanoseconds.

The receiver also has a rise and fall time that can limit the

data rate. Photodetectors take a finite time to respond to

changes in light levels (on and off pulses) and generate an



electrical current. The magnitude of this time

depends on the material and design of the pho-

todetector. The longer the response time, the

lower the data rate that can be successfullytransmitted. (Figure 3.4).

Another factor is the relationship between the

size of the photodetector and the response time

of that detector. The larger the photodetector,

the more light from the optical fiber it will cap-

ture, making alignment less critical. A larger pho-

todetector has a slower response time, however.

The numerical aperture of the receiver should be

properly matched to the numerical aperture of

the optical fiber to obtain optimal performance.

The two main types of receivers are the PIN and

the avalanche photodetector.The PIN is the mostwidely used and economical solution.

Dynamic Range

Bit errors can also occur when too much or too

little light strikes the photodetector.The

response of a photodetector is linear only within

a certain range of power levels. This is called the

dynamic range. Exceeding the linear response

area (dynamic range) for a given photodetector

causes it to generate a non-proportional amount

of electrical current. If the dynamic range is

exceeded, the receiver is saturated. An optical

attenuator can be placed at the receiver in line

with the optical fiber to reduce the amount of

received light power. The receiver sensitivity

specifies the minimum power level required. A

typical value would be -17 dBm for 1 Gb/s

Ethernet operation at

850 nm.

Operating Wavelength

Operating wavelength is another important

parameter in system design. Multimode fiber is

optimized for operation in two windows:

850 nm

1300 nm

Attenuation is lower at 1300 nm than at 850 nm.

Legacy 62.5/125 m multimode fiber was opti-

Spectral Width (Dl)

D

ataRate

LongShort

Narrow Wide

Low

High

Response Time

Figure 3.4 Relationship Between Spectral Width,Response Time and Data Rate


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mized with respect to bandwidth at 1300 nm to take advan-

tage of this lower attenuation. As data rates have increased,

multimode systems are now less likely to be attenuation-lim-

ited and more likely to be bandwidth-limited due to modaldispersion. VCSELs which operate at 850 nm have prompted

the development of laser-optimized 50/125 m multimode

fiber which can extend the achievable distance at high-data-

rates while still taking advantage of the overall lower system

costs associated with multimode fiber.

Dispersion unshifted single-mode fiber having a low attenua-

tion in the water peak region as specified in ITU-T G.652.D

and TIA/EIA-492-CAAB is designed for operation in the 1310

nm and 1550 nm regions; however, there is a tradeoff at each

wavelength region. The attenuation at 1550 nm is generally

lower than that at 1310 nm. The chromatic dispersion, howev-er, is much higher at 1550 nm than at 1310 nm. For premises

applications, TIA/EIA-568-B.1, Commercial Building

Telecommunications Cabling Standard, and IEC 11801, Generic

Cabling for Customer Premises, recommend the use of disper-

sion unshifted single-mode fiber because premises communi-

cation standards are designed for operation at 1310 nm. The

10 Gb/s Ethernet Standard specifies operation at 1310 nm and

1550 nm with dispersion unshifted single-mode fiber having

a low water peak.

Optical Fiber Specifications

There are two major classifications of fiber. In general, multi-

mode fiber is best suited for premises applications, where

links are short and there are many connectors. The higher

numerical aperture of multimode fiber allows the use of rela-

tively inexpensive LED and VCSEL transmitters. Single-mode

fiber is best suited for long distance systems.

The standard types of multimode fiber in North America are

50/125 m and 62.5/125 m optical fiber. These fiber types are

recognized by TIA/EIA-568-B.3 and IEC 11801. TIA/EIA-568-B.3

also recognizes single-mode optical fiber for backbonecabling. TIA/EIA-492AAAA,TIA/EIA-492AAAB, and TIA/EIA-

492CAAB specify mechanical, geometrical and optical charac-

teristics for 62.5/125 m, 50/125 m and single-mode fibers

respectively.

Laser-optimized 50/125 m multimode fiber is designed for

850 nm operation at 1 Gb/s and higher. The fiber supports

10 GbE and 10 Gb/s Fibre Channel system operation at

850 nm for distances up to 550 meters. TIA/EIA-492-AAAC


specifies mechanical, geometric and optical char-

acteristics for laser-optimized 50/125 m multi-

mode fiber.The fiber has been fully adopted into

TIA/EIA-568-B.3 and IEC-11801.

Selection of the appropriate multimode fiber

type for a given application should be made on

the basis of current and anticipated future band-

width and link length requirements. Future link

lengths may increase over those of the initial

installation due to cable plant expansions or

equipment moves, adds or changes. There are

currently multiple bandwidth measures used to

predict multimode fiber system performance.

Until recently, 62.5/125 m multimode fiber has

been the dominant fiber type used in LAN instal-lations. These legacy multimode systems were

designed for use with LED sources which create

an overfilled launch (OFL) condition. The system

performance of fiber operating with LED sources

is best characterized by the OFL bandwidth test

method described in TIA/EIA-455-204. With the

migration toward higher-data-rate systems oper-

ating with laser light sources, fiber bandwidth

measurements techniques have evolved and

have been adopted into standards which better

characterize system performance under laser

launch conditions. TIA/EIA-455-204 and IEC

60793-1-41 specify a bandwidth test method

using restricted mode launch (RML) conditions

characteristic of VCSEL sources. This method has

been shown to provide a suitable bandwidth

measure for systems operating at 1 Gb/s.

For systems operating at data rates greater than

1 Gb/s, TIA/EIA-455-220 and IEC 60793-1-49 band-

width test methods are used which include a

series of small spot size launches (approximately

5 m) indexed across the fiber core. Measurementsare made of the output pulse time delay and

mode coupling power of the fiber as a function

of radial position (Figure 3.5). These measure-

ments are referred to as differential mode delay

(DMD) measurements. Data from these measure-

ments can be analyzed by two methods to deter-

mine whether the fiber meets the effective modal

bandwidth (EMB) requirement of a specific appli-

cation.



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Application Wavelength (nm) Data Rate Max Distance (m)

Gigabit Ethernet 850 1000 Mb/s 600


Serial 10 Gigabit Ethernet 850 10 Gb/s 82CWDM 10 Gigabit Ethernet 1300 10 Gb/s 300

Fibre Channel 850 1 Gb/s 500




FDDI 1300 100 Mb/s 2000

ATM 1300 622 Mb/s 300

Figure 3.6 Data Rate/Length Capabilities of LANscape Solutions Cabled Standard 50/125 m Multimode Optical Fiber




Serial 10 Gigabit Ethernet 850 10 Gb/s 300CWDM 10 Gigabit Ethernet 1300 10 Gb/s 300





FDDI 1300 100 Mb/s 2000

ATM 1300 622 Mb/s 300

Figure 3.7 Data Rate/Length Capabilities of LANscape Solutions Cabled Laser-Optimized 50/125 m Multimode -300 Optical Fiber

Fiber Type RecommendationsSummary

Corning Cable Systems recommends the use of 50/125 mfiber for building backbone, campus backbone, horizontal

cabling, centralized cabling and data centers. This allows the

user to operate at slower speeds initially but to move to

higher-data-rate laser-based systems as bandwidth demands

increase. Depending upon fiber grade selection, this

approach will provide an upgrade path to 1 Gb/s for a dis-

tance of 600 to 1000 m and to 10 Gb/s for distances of at

least 82 to 550 m.

Where fiber is to be added to extend the length or connectiv-

ity of legacy fiber links, the added fiber should be of the

same core size as the legacy fiber, rather than mixing fiber

types. Where partially populated switches are connected to

62.5/125 m fiber, additional ports may be populated with

50/125 m fiber.


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Application Wavelength (nm) Data Rate Max Distance (km)Gigabit Ethernet 1310 1000 Mb/s 5

Serial 10 Gigabit Ethernet 1310 1000 Mb/s 10

Serial 10 Gigabit Ethernet 1550 10 Gb/s 40

CWDM 10 Gigabit Ethernet 1300 10 Gb/s 10





FDDI 1300 100 Mb/s 40

ATM 1310 622 Mb/s 15

ATM 1310 2.5 Gb/s 40

ATM 1550 2.5 Gb/s 80

Figure 3.10 Data Rate/Length Capabilities of LANscape Solutions Cabled Single-Mode Optical Fiber










FDDI 1300 100 Mb/s 2000

ATM 1300 622 Mb/s 300

Figure 3.8 Data Rate/Length Capabilities of LANscape Solutions Cabled Laser-Optimized 50/125 m Multimode -550 Optical Fiber










FDDI 1300 100 Mb/s 2000

ATM 1300 622 Mb/s 300

Figure 3.9 Data Rate/Length Capabilities of LANscape Solutions Cabled Standard 62.5/125 m Multimode Optical Fiber

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