-
The Book on
The essential information you need to know when deploying FTTX,
from the central office to
the outside plant to the customer premises
Foreward by Jason Meyers Managing Director, Penton Custom
MediaPenton Media is the publisher of Telephony Magazine
The eagerly awaited follow-up to ADCs
The Book on FTTX
-
ADC Telecommunications, Inc., P.O. Box 1101, Minneapolis,
Minnesota USA 55440-1101Specifications published here are current
as of the date of publication of this document. Because we are
continuously improving our products, ADC reserves the right to
change specifications without prior notice. At any time, you may
verify product specifications by contacting our headquarters office
in Minneapolis. ADC Telecommunications, Inc. views its patent
portfolio as an important corporate asset and vigorously enforces
its patents. Products or features contained herein may be covered
by one or more U.S. or foreign patents.
104918 1/08 Original 2008 ADC Telecommunications, Inc. All
Rights Reserved
The essential information you need to know when deploying FTTX,
from the
Central Office to the Outside Plant to the Customer Premises
The Book on Next Gen Networks
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The Book on Next Generation Networksii
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iiiThe Book on Next Generation Networks
Foreward
The Problem with Innovation
By Jason Meyers, Managing Director, Penton Custom Media Penton
Media is the publisher of Telephony Magazine
The above is a title most people probably would not expect to
see on a foreword to a book about next generation networks. But
there is a reason behind it and a point to it, both of which I will
get to in a moment.
First, though, what is that problem? What could be problematic
about innovationin particular, about the network technology
innovation that drives communication networks into the next
generation, driven by the need and demand for advanced services and
increasingly ubiquitous and continuous and instantaneous
communications capabilities?
The problem can be summed up in two words: expectation and
execution.
Innovation creates expectation in droves. Industries like
telecom live and die by the expectation that is created by
innovation. Companies get put on the map because of it. Whole
market sectors are created around that innovation and the
accompanying marketing buzz it generates. Its electric. Industry
associations and alliances are formed around those expectations.
The media thrives on the expectation and multiplies it. (Some might
say its the medias fault.) Promises are made.
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Foreward
Then comes the executionor lack of it. This is where the rubber
hits the road (or skids off into the ditch). Its one thing to make
promises, to build up expectations. Its another to deliver on the
expectations created, regardless of how technologically promising
the innovation may be. Those markets and buzz created by the
expectation? Without proper execution, they are more than likely to
fizzle.
So the problem with innovation, quite simply, is one of
follow-through. The problem is an inadequate attention to the
detail required to turn innovation into a market.
So why did I choose this phrase as a title to the foreword of
The Book on Next Gen Networks? Because I contend that this book
goes a long way toward solving the problem. This is a book about
executionnamely, the execution required to leverage next generation
network innovation and use it to build markets.
How does one volume accomplish that which whole market sectors
have at times tried and failed to accomplish? By concentrating on
the details. This book doesnt speak in broad strokes about what
various technologies can potentially accomplish, the services they
can potentially enable or how competitively important it is to
deploy those technologies in your networks. Instead, this book is a
practical exploration and application of specifics.
The Book on Next Gen Networks goes deep, into the central
office, to the distribution hub, the access network and into the
customer premises. It explores, for example, why a proper fiber
cable management system is so critical to network performancenot
only right now, but also in the not-so-distant future, when todays
will be carrying applications no one has yet thought of, and
expanding because of it. Or where (and why) splitters should be
deployed in a PON environment, and how a decision like that can
help a network accommodate new services.
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vThe Book on Next Generation Networks
This book analyzes the performance and cost issues that can
occur if the wrong moves are made, and the benefits that can be
realized by making the right ones. To that end, this is a book
about preparing for the future. In fact, it attemptsas much as is
possible in this ever-adapting network environmentto actually
predict the future: What could the long-term consequences of a
deployment decision or process be? How will the role of the network
technicians who deploy the networks evolve, and what training will
be required of them? How will new construction and the changing
architecture of buildings impact how FTTP will be deployed?
The Book on Next Gen Networks is conceived and written to help
those who consume it bridge the gap between expectation and
execution. Read it, apply it, repeat it. Industry associations and
alliances and alliances are formed around that expectation. It will
help you deliver on the promise of innovation.
Enjoy!
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viiThe Book on Next Generation Networks
Table of Contents
Introduction: The Motivation for GPON Migration
........................................... 3
Central Office
Chapter 1 The Elements of Fiber Cable Management
................................. 11
Chapter 2 Effective Integration of Reduced Bend Radius Fiber
into the Network
........................................................................
19
Chapter 3 Incorporating Passive CWDM Technology vs. Deploying
Additional Optical Fiber
..............................................................
25
Chapter 4 Adding New Video Services Warrants New Central Office
Considerations
.................................................................
31
Distribution
Chapter 5 Its Happening in the Hub
........................................................... 39
Chapter 6 Extreme-Environment Performance Considerations for
FTTX Splitter Modules
........................................................... 51
Chapter 7 Plug and Play Splitter Architectures Drive Operational
Savings .... 61
Chapter 8 The Economics of FTTN vs. FTTP
................................................. 65
Chapter 9 Resectionalizing the Distribution Area
.......................................... 71
Access
Chapter 10 Creating a Cost-Effective Plug and Play FTTX
Architecture .......... 79
Chapter 11 Innovative Installation Techniques for Fiber Drop
Terminals ......... 83
Chapter 12 Above vs. Below Ground Drop Splicing: Considerations
for Drop Cable Connections in the FTTX Network
...................... 89
Chapter 13 Outside Plant Connections You Can Rely On
.............................. 93
Chapter 14 Cost Optimizing Outside Plant Cable Assemblies
...................... 105
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viii Table of Contents
Customer Premises
Chapter 15 Multiple Solutions for Connecting Multiple Dwelling
Units (MDUs)
..............................................................
113
Chapter 16 Deploying Reduced Bend Radius Fiber in MDU
Environments... 125
The Technician
Chapter 17 Properly Training Next-Generation Technicians on
Next-Generation Products
.................................................... 133
Chapter 18 The Technicians Perspective on Reduced Bend Radius
Fiber ................................................. 137
Glossary
.................................................................................................
143
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The Book on Next Generation Networks
Introduction
High-Rise MDU Medium-Rise MDU
Horizontal MDU
Low Rise/Garden MDU
Residential
Residential
FeederDistributionDrop
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3The Book on Next Generation Networks
Introduction
The Motivation for GPON Migration
By December 2007, approximately eight million homes had been
passed with fiber for Fiber-to-the-Home (FTTH) or
Fiber-to-the-Premises (FTTP) applications. Included in these
numbers are an astonishing five hundred communities that have
chosen fiber as a means of delivering broadband applications to
homes and businesses. Of these numbers, it is estimated that almost
half, or around 3.5-million of these homes and businesses are
connected using Broadband Passive Optical Networking (BPON),
Ethernet Passive Optical Networking (EPON) or
Ethernet-in-the-First-Mile (EFM)1.
Predicting the telecom future is never easyand it follows that
building an access network that is future-proofed against rising
bandwidth demand and next-generation technologies is a major
challenge for todays service providers. But that doesnt mean
decisions have to be based on a coin flip either. There are many
practical considerations that can be examined when selecting an
FTTP infrastructure that will not only meet current demand, but
also provide the flexibility for a smooth migration to
next-generation demands.
This is particularly true of the passive optical network (PON)
portion of the network. A close look at several practical
considerations, based on informed decision making, will provide a
firm foundation for designing a network that can cost-effectively
transition between legacy and future access technologies. Our own
telecommunication history provides many troubling examples of
networks that were built without giving thought to future
innovation. Building telephone networks with copper, our
predecessors could not have predicted todays broadband
revolutioneven though we seem to have made the most of this legacy
infrastructure with xDSL technologies.
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Introduction
However, through the unpredictable performance of xDSL and the
overall condition of the legacy copper networknot to mention some
very costly lessons learnedservice providers have realized the
importance of network flexibility. FTTP offers service providers a
clean slate for deploying todays new services to their
bandwidth-hungry subscribersand it all begins with designing the
proper PON architecture.
For the access protocols and the movement to Gigabit PON (GPON)
migration, some additional concepts may need to be considered:
GPON is the next generation of PON electronics currently
being
introduced to the marketplace.
GPON will NOT be the final technology deployed.
The network design should accommodate flexibility for the
current migration and beyond.
In theory, the passive connectivity infrastructure must be
agnostic to the service delivery technology.
GPON is making it easier for PON networks to move to an all-IP
format where the external interfaces to the core are moving to an
all Gigabit Ethernet network creating a movement away from the
traditional ATM transport to a pure IP transport. GPON is
IP-centric while allowing the traditional services of voice and
video, yet acknowledges the strengths of the service provider to
differentiate themselves on quality of service (QoS) issues.
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5The Book on Next Generation Networks
GPON continues to have the long reach that effectively
eliminates active components in the access network with little or
no significant changes to the physical architecture that has
already been built for BPON and EPON. Architecture designs should
account for a smooth transition between technologies by
accommodating practical considerations for future architectures. We
do not have a true crystal ball as to what these technologies will
become. If we did, we would simply build for the future. However,
isnt this exactly what we should be doing--building for the
future?
Wheres the motivation?
As predicted, GPON, a culmination of the best in BPON and EPON,
is poised to dominate the access market by offering a much-needed
bandwidth boost. We can all agree that eventually everythingvoice,
video, and datawill be moving to IP and the quadruple-play
applications, including network appliances, security,
videosurveillance, etc. The advantages of GPON are a key driver for
gaining the commitment of the large-volume carriers toward the GPON
standard.
GPON is emerging on queue with higher split ratios that can deal
with the challenges of delivering high-speed, high-bandwidth
packaged services to business and residential customers. This is
putting pressure on service providers to make decisions for ramping
up their networks for GPON from the central office (CO) to the
outside plant (OSP).
Ensuring FTTP networks can easily migrate to GPON promises to
pay huge dividends to service providers in the coming years. As
GPON develops as the standard of choice for FTTP networks, both
cost reductions and interoperability will be accelerated. Those
providers who make informed choices in deploying flexible,
interoperable, reconfigurable networks will reap substantial
benefits in the move to GPON and beyond. They will be able to
quickly offer new and improved services as they evolve, without the
need for major network overhauls.
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Introduction
Standards bodies
If service providers arent already convinced by GPONs ability to
provide future enhanced services, maximize interoperability,
utilize enhancement bands, and provide increased capacity with the
promise of higher split ratios, the International Telecommunication
Union (ITU) provides further motivation. The ITU points out that we
can expect a significant increase in demand for dedicated Gigabit
Ethernet (GigE) and 10GigE services to both businesses and
residential customers.
This means every service provider must decide how to best
integrate all types of services onto a single backhaul fiber
network. A smooth and easy migration capability to GPON is the most
viable solution. GPON enables PON networks to easily move to an
all-IP format while external interfaces to the core move to an
all-gigabit ethernet formata movement away from the traditional ATM
transport to pure IP transport.
The ITUs ratification of the GPON standard in 2003 has also
helped put electronics vendors on the same page in terms of getting
behind one standard. This standard will enable the major cost
challenges associated with optical network terminals (ONTs) at the
customer premise to be addressed and, in time, will bring those
costs down significantly.
GPON combines the best of BPONs quality-of-service attributes
with the best of EPONs ability to transport and interface on an
all-IP network. It also addresses the higher application bandwidth
needs by providing 2.4 Gbits/sec downstream and 1.2 Gbits/sec
upstream.
The transition to GPON
Making the move from BPON or EPON to GPON involves three key
architectural components. Addressing the fibers loss
characteristics in terms of spectral attenuation, using the
appropriate class of optics, and considering the advantages offered
by greater split ratio capability will all
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7The Book on Next Generation Networks
affect the networks migration to GPON. Each of these
considerations will be addressed in greater detail within this
book.
Connectorization also plays a role in creating a migration-ready
FTTP network, particularly when considering the single fiber
requirements of next-generation video applications in GPON
architectures. The use of APC connectors that offer the lowest
return loss characteristics of all current connectors will optimize
high bandwidth and allow for longer reach.
Splitter configuration in the optical distribution portion of
the networkbetween customers and the COhas been a hot topic over
the last few years. We believe a centralized splitter approach
offers the best flexibility advantages. It maximizes the efficiency
of OLT PON ports, and unlike the cascaded approach, does not risk
stranding unused ports in areas of low take rates. There will also
be further advantages when it comes to testing and troubleshooting
the network.
With the GPON standard already revolving around centralized 1x32
splitter architectures in the OSP, GPONs promise of a 1x64 splitter
ratio offers even more incentive to service providers by doubling
the number of homes serviced from a single splitter.
Moving to the CO, flexibility becomes the pathway to easy
migration capability. A network must always be built as a flexible
long-term entity that adapts to inevitable changes in both
equipment and technology. A cross-connect network offers excellent
flexibility for configuration points and should include
high-quality APC connectors for handling the higher power necessary
for any analog video application.
Cable management in the CO is also an issue worth consideration.
In fact, the considerations for GPON within the CO can be summed up
in just three wordsflexibility, quality, and protection.
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8 Introduction
A final word
Weve covered a lot of ground in a short time, but these and
other topics are covered in greater detail as you read through this
book. Suffice it to say that network architects owe it to
themselves to carefully plan ahead to avoid having to re-build the
network to accommodate each new application or technology.
Summing it all up, the inevitable need to migrate to GPON is
already upon us, and the future generations of PON are already on
the drawing board. Making informed network decisions today will not
only make a migration process less painful, but it is also good
business sense. GPON not only supports TDM voice today, it has a
true migration platform to an all-IP network. But most importantly,
it guarantees that existing architectures will migrate to future
technologies without requiring forklift upgrades.
I hope youll see this latest edition of The Book on Next
Generation Networks as a tool for helping you make good decisions
for upgrading your access network. It represents the experience and
know-how of many fine architects, planners, and design technicians.
I wish you the best of luck in meeting the unique challenges of
your network and hope youll consider our ADC team as you work
towards making your network plans a reality.
Enjoy!
Patrick J. Simms, RCDD Principal Systems Engineer, ADC
[email protected]
1. Source: RVA LLC, Market Research & Consulting, Fiber to
the Home: Advanced Broadband 2007
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Central Office
The Book on Next Generation Networks
High-Rise MDU Medium-Rise MDU
Horizontal MDU
Low Rise/Garden MDU
Residential
Residential
FeederDistributionDrop
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11The Book on Next Generation Networks
Chapter 1
The Elements of Cable Management
As service providers continue upgrading their networks to
transport high-bandwidth broadband services, an increase in fiber
usage is essential to meet both bandwidth and cost requirements.
But just deploying this ad-ditional fiber is not enougha
successful, well-built network must also be based on a strong fiber
cable management system.
Proper fiber management has a direct impact on the networks
reliabil-ity, performance, and cost. Additionally, it affects
network maintenance, operations, expansion, restoration, and the
rapid implementation of new services. A strong fiber cable
management system provides bend radius protection, cable routing
paths, cable accessibility, and physical protection of the fiber
network. Executing these concepts correctly will enable the network
to realize its full competitive potential.
Introduction
With demand steadily increasing for broadband services that will
include several bandwidth-hungry technologies like high-definition
television (HDTV) and higher Internet speeds for handling larger
file sharing re-quirements, fiber is being pushed closer and closer
to the customer premises. This, in turn, creates a need for both
additional fiber in the central office /data center and the active
equipment that must be managed to accommodate future network
growth.
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Central Office
Any new broadband network infrastructure must have the inherent
capability to easily migrate to the next generation of technologies
and services. This is a key consideration for service providers
beginning to deploy triple-play broadband serviceswhether its from
a multiple service operator (MSO) headend, a central office (CO),
or wireless mobile switching center (MSC). As the amount of fiber
dramatically increases, the importance of properly managing the
fiber cables becomes a more cru-cial issue.
The manner in which fiber cables are connected, terminated,
routed, spliced, stored, and handled will directly and
substantially impact the networks per-formance and, more
importantly, its profitability. New technologies and products have
been developed in the last few years to improve bend radius
protection, cable routing paths, accessibility, and physical
protection.
Bend radius protection
There are two types of bends in fibermicrobends and
macrobendsthat can affect the fiber networks long-term reliability
and performance.
The microbend is a small, microscopic bend that may be caused by
the cabling process itself, packaging, installation, or mechanical
stress due to water in the cable during repeated freeze and thaw
cycles. External forc-es are also a source of microbends. An
external force deforms the cabled jacket surrounding the fiber, but
causes only a small bend in the fiber. A microbend typically
changes the path that propagating modes take, result-ing in loss
from increased attenuation as low-order modes become coupled with
high-order modes that are naturally lossy.
A macrobend is a larger cable bend that can be seen with the
unaided eye and is often reversible. As the macrobend occurs, the
radius can become too small and allow light to escape the core and
enter the cladding. The result is insertion loss at best and, in
worse cases, the signal is decreased
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13The Book on Next Generation Networks
or completely lost. Both microbends and macrobends can, however,
be re-duced and even prevented through proper fiber handling and
routing.
The minimum bend radius will vary depending on the specific
fiber cable. However, in general, the minimum bend radius of a
fiber should not be less than ten times its outer diameter. Thus, a
3 mm cable should not have any bends less than 30 mm in radius.
Telcordia recommends a minimum 38 mm bend radius for 3 mm patch
cords. Also, if a tensile load is applied to a fiber cable, such as
the weight of a cable in a long vertical run or a cable pulled
tightly between two points, the minimum bend radius is increased
due to the added stress.
The advent of bend insensitive or reduced bend radius fiber is
an example of how technology has addressed the bend radius issue.
Whereas the mini-mum bend radius should not be less than ten times
the outer diameter of the fiber cable in typical fiber, reduced
bend radius fiber provides more leeway. However, service providers
must understand that these new fibers do not diminish the need for
solid fiber cable management. On the con-trary, the increase in the
sheer number of fibers being added to the system to accommodate
broadband upgrades makes bend radius protection as important as
ever.
As fibers are added on top of installed fibers, macrobends can
be induced on the installed fibers if they are routed over an
unprotected bend. A fiber that had been working fine for many years
can suddenly have an increased level of attenuation, as well as a
potentially shorter service life. The impor-tance of bend radius
protection is critical to avoid operational problems in the
network.
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Cable routing paths
The second element of fiber cable management is cable routing
paths and is related to bend radius protection. Improper routing of
fibers by techni-cians is one of the major causes of bend radius
violations. Wherever fiber is used, routing paths must be clearly
defined and easy to followto the point where the technician has no
other option than to route the cables properly. Leaving cable
routing to the technicians imagination leads to an inconsistently
routed, difficult-to-manage fiber network.
The quality of the cable routing paths, particularly within a
fiber distribution frame system, can be the difference between
congested chaos and neatly placed, easily accessible patch cords.
Its often said that the best teacher in fiber routing techniques is
the first technician to route it properly. Con-versely, the worst
teacher is the first to use improper techniques, since sub-sequent
technicians are likely to follow his lead.
Well-defined routing paths, therefore, reduce technician
training time, in-crease the uniformity of the work done, and
ensure and maintain bend radius requirements at all points, thus
improving overall network reliability. It is important to note
that, again, the use of bend insensitive fiber does not diminish
the need for clear cable routing pathsthere are benefits that go
beyond bend radius protection.
Having defined routing paths makes accessing individual fibers
easier, quicker, and saferreducing the time required for
reconfigurations. Fi-ber twists are reduced to make tracing a
particular fiber for rerouting much easier. Even with new
technologies, such as the use of LEDs at both ends of patch cords
for easy identification, well-defined cable routing paths still
greatly reduce the time required to route and reroute patch cords.
All of this directly affects network operating costs and the time
re-quired to turn up or restore service.
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15The Book on Next Generation Networks
Cable access
Cable access is the third element to good fiber cable management
and refers to the accessibility of the installed fibers. As the
number of fibers increases dramatically in both the distribution
frame and the active equip-ment, cable access becomes an
increasingly important issue for broadband service providers. In
the past, an active equipment rack might have had about 50 fibers
exiting, and managing those fibers was much less of an is-sue. But
as that same rack is fitted for next generation broadband services,
there may be up to 500 fibers involved, making proper management
and accessibility a vitally important matter.
With huge amounts of dataas well as revenuemoving across those
fi-bers, the ability for technicians to have quick and easy access
is critical. When there are service level agreements in place,
particularly for customers with high priority traffic, the last
thing any service provider wants is service interruptions caused by
mishandling one fiber to gain access to another.
As previously mentioned, there are patch cords designed today
with LEDs at both ends to help technicians identify particular
cable runs with no chance of error. These innovations can be
implemented into a good cable man-agement system to help minimize
problems caused by disconnecting the wrong patch cord. There are
many other tools and techniques for ensuring that every fiber can
be installed or removed without bending or disturbing an adjacent
fiber.
The accessibility of the fibers in the fiber cable management
system can mean the difference between a network reconfiguration
time of 20 minutes per fiber and one of over 90 minutes per fiber.
Since accessibility is most critical during network reconfiguration
operations, proper cable ac-cess directly impacts operational costs
and network reliability.
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Physical fiber protection
The last element of a fiber cable management system addresses
the physi-cal protection of the installed fibers. Every fiber
throughout the network must be protected against accidental damage
by technicians or equipment. Fibers traversing from one piece of
equipment to another must be routed with physical protection in
mind, such as using raceway systems that pro-tect from outside
disturbances.
Without proper physical protection, fibers are susceptible to
damage that can critically affect network reliability. The fiber
cable management system should always include attention to ensuring
every fiber is protected from physical damage.
A final wordplanning
Finally, since many service providers are in the processor soon
will beof upgrading networks for delivering high-bandwidth
broadband services, it is important to stress the need for planning
in terms of cable management. Todays network is a living and
growing entityand what is enough today will almost certainly be too
little tomorrow. With that in mind, future-proof-ing the network
wherever possible should be a major considerationand fiber cable
management is no different.
For example, the current upgrades to broadband service delivery
taking place in COs, MSOs, or MSCs require more fiber deployment.
Four- and six-inch fiber raceway systems are already becoming
inadequate to properly manage these larger amounts of fiber.
Service providers must plan ahead for a centralized, high-density
fiber distribution frame lineup using 24-inch raceways that can
accommodate not only todays fiber requirements, but also those
expected in the future.
Although installing a 24-inch raceway system is more expensive
today, hav-ing to go back in and retrofit the system in a few years
represents a much higher cost and significant risk to the fiber.
Ignoring future growth, particu-
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17The Book on Next Generation Networks
larly in terms of fiber, will result in higher long-term
operational costs result-ing from poor network performance or a
requirement to retrofit products that can no longer accommodate
network demand.
Another consideration in planning for good fiber cable
management con-cerns the active equipment rack. Most manufacturers
have traditionally overlooked the need for providing cable
management within their equip-ment. Before purchasing, service
providers should insist that cable manage-ment is included within
every piece of active equipment to ensure their investment will
operate at peak efficiency over time.
All four elements of a fiber cable management systembend radius
protec-tion, cable routing paths, cable access, and physical
protectionstrengthen the networks reliability and functionality
while lowering operational costs and ensuring smooth upgrades when
necessary.
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19The Book on Next Generation Networks
Chapter 2
Effective Integration of Reduced Bend Radius Fiber into the
Network
Introduction
Bending of singlemode fiber has everyone talking these days. The
idea that you can bend a fiber around a pencil without a dramatic
increase in attenuation is a concept that has everyone considering
new fiber applications and design possibilities.
Today, industry standards for traditional singlemode fiber
typically specify a minimum bend radius of ten times the outside
diameter of the jacketed cable or 1.5-inches (38 mm), whichever is
greater. This new breed of flex-ible singlemode optical fiber has
the potential to significantly reduce these minimum bend radius
requirements to values as low as 0.6-inches (15 mm), depending on
the cable configuration, without increasing attenuation.
There are many names for optical fiber that can endure a tighter
bend radius bend insensitive, bend resistant and bend optimized are
sever-al that come to mind. However, some of these terms can be
somewhat misleading. Designers and installers may believe reduced
bend radius optical fiber is impervious to all the forces that can
increase attenuation and cause failure on an optical fiber link.
Staff and contract technicians can make false assumptions on its
durability and performance capabilities as well. Such beliefs can
have a serious impact on network performance.
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Central Office
For purposes of accuracy, ADC uses the term reduced bend radius,
be-cause this title best describes what the product actually
delivers. As with any optical fiber, attention must be paid to how
the cable is deployed and handled throughout the lifetime of the
network, in order to ensure optimal performance.
What is reduced bend radius optical fiber?
As mentioned above, reduced bend radius fiber is able to
withstand tight-er bends within frames, panels, and pathways. To
understand how this is achieved, it is important to understand that
all fiber types rely on principles of Total Internal Reflection,
which allows light signal to travel from one end of the fiber to
another (see Figure 1). By improving the bend radius of optical
fiber, light entering the core is effectively reflected by the
clad-ding back into the core. Instead of using a matched clad
profile, some con-structions of reduced bend radius optical fiber
use a depressed clad profile with a lower index of refraction than
the core, causing light to stay within this core.
n1
n2
Refracted
Reflected
Cladding
Core
Figure 1Principle of Total Internal Reflection for Optical
Fibers
Fiber cladding has a lower Index of Refraction (IOR) than the
core, causing light to stay within the core. Depression of the
cladding
profile promotes Total Internal Reflection
To achieve tighter bend radii, some constructions change the
mode field diameter (MFD)the area across the core of the fiber that
fills with light. Typical MFD for standard singlemode optical fiber
is about 10.4m; reduced bend radius optical fiber may exhibit MFD
of between 8.9m and 10.3m.
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21The Book on Next Generation Networks
Regardless of the type of construction, all reduced bend radius
fiber prod-ucts do one thing very wellthey can perform under a
tighter bend radius where macrobends occur. Examples include a
central office application, where fiber passes from a panel into a
vertical cable route or in an FTTX deployment within the confines
of an optical network terminal (ONT).
The fibers performance is definitely impressive. For example, in
ADC tests a standard singlemode optical fiber with one turn around
a 1.26-inch (32 mm) diameter mandrel shows induced attenuation of
less than 0.50 dB at 1550 nm. This same test on a reduced bend
radius singlemode 1550 nm optical fiber shows less than 0.02 dB of
attenuation.
In general, reduced bend radius optical fiber is designed to
perform with low loss across the spectrum of wavelengths, from 1285
nm to 1650 nm, using all the channels available on those
wavelengths to maximize bandwidth. Current designs include low
water peak or zero water peak so that high attenuation is avoided
at 1383 nm. Many re-duced bend radius optical fiber products meet
ITU-T Recommendation G.657, meaning they work well at 1550 nm for
long distance and voice applications and at 1625 nm for video
applications.
Does it improve performance?
Despite the improved bend radius, the reality of this fiber is
that bend ra-dius protection is still a concernjust not to the
extent that it is in standard fiber. There is still a mechanical
limit on how tightly any optical fiber can be routed before the
structural integrity of the glass is violated.
The assumptions about improved performance are not accurate
either, at least beyond the exceptional bend radius performance. In
reality, the perfor-mance of reduced bend radius optical fiberor
any optical fiberdepends upon many factors, not just bend radius
properties.
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By itself, reduced bend radius optical fiber does not offer
improvements in attenuation. True, it bends more tightly without
causing additional attenu-ation. Yet laid out on a long, straight
run next to a standard optical fiber, there is no difference in
performance that can be attributed to the cables construction. It
is inaccurate to believe that reduced bend radius optical fiber is
the end-all solution when, in fact, there are many other factors
that determine optical fiber link performance.
Durability Reduced bend radius optical fiber offers the same
crush resis-tance and tensile strength as the same cable with
standard singlemode fi-ber. As with standard optical fiber,
excessive weight will crush reduced bend radius optical fiber and
excessive pulling tension will damage the cable, both of which
affect attenuation.
Connector pull-off resistance Cable assemblies and connectors
must meet Telcordia (GR-326) requirements for strength of the fiber
termination connector. Reduced bend radius optical fiber does not
improve connector pull-off resistance. Connectors that are easily
loosened or disconnected in-crease attenuation and cause
failures.
Connector performance When it comes to connector performance,
endface characteristics determines loss from the connector. Reduced
bend radius optical fiber does not impact insertion loss from
connectors, making termination and quality of connectors an
important consideration in link performance.
Proper applications for reduced bend radius optical fiber
Singlemode reduced bend radius optical fiber offers benefits for
applications that including the central office, FTTX deployments,
data cen-ter, and OEM solutions. Singlemode reduced bend radius
optical fiber is best suited for environments where little or no
bend radius protection is available. It is also ideal for
applications where space is an issue. Specific ap-plications that
make sense for this type of fiber include places in which:
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23The Book on Next Generation Networks
Space is tight For drop cable or termination of pigtails in
multiple dwell-ing unit (MDU) and optical network terminal (ONT)
boxes for FTTX deploy-mentswhere there is no space and often no
cable managementreduced bend radius optical fiber offers less
chance of increased attenuation during field installation and
maintenance.
No fiber management is available The front of frames and
routerswhere moves/adds/changes occuris ideal for use of reduced
bend ra-dius patch cords and multifiber breakout assemblies. Many
OEM active components do not have bend radius limiters or
protection on the front of the equipment.
Space is at a premium Patch cords and multifiber breakout
assemblies that can bend more tightly enable increasing density of
active equipment in racks and cabinets without sacrificing access.
For manufacturers of ac-tive equipment, reduced bend radius optical
fiber can help reduce size of electronics, improving density and
airflow. However, in these applications, even more consideration
must be paid to the elements of proper cable management. Tighter
bend radius also offers OEMs the chance to increase the
functionality of active equipment by utilizing less chassis
space.
Of course, a key advantage of reduced bend radius optical fiber
is use in high bandwidth applications. For standard optical fiber,
the 1625 nm to 1550 nm wavelengths are the first to go when the
cable is wrapped around a mandrel. Preserving these wavelengths
around tighter bends offers ben-efits for OEMs seeking to improve
functionality of network equipment or network managers looking for
the efficiency of having all wavelengths available on a given
optical link.
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Conclusion
Singlemode reduced bend radius optical fiber has generated quite
a buzz, and it is a great step forward in optical fiber
construction. It makes much-handled patch cords and multifiber
assemblies less susceptible to macrobends that affect attenuation
and limit bandwidth of optical fiber links.
It is crucial for the health and performance of the network to
be aware that reduced bend radius fiber does not, in any case, mean
that the fundamen-tals of proper fiber management are to be
ignored. In fact, as this fiber is used in higher density
applications, factors such as connector access and cable routing
paths become even more crucial. Reduced bend radius optical fiber
is just one aspect of a complete strategy for efficient,
future-proofed network management.
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25The Book on Next Generation Networks
Chapter 3
Incorporating Passive CWDM Technology vs. Deploying Additional
Optical Fiber
The recent advancement in telecommunication applications for
voice, video and data places additional demands on fiber optic
networks. Adding additional fiber to existing networks can be very
costly to service providers. In most cases, a far betterand less
costlyoption is found in coarse wavelength division multiplexing
(CWDM) technology.
CWDM technology adds greater fiber bandwidth while increasing
the flex-ibility, accessibility, adaptability, manageability and
protection of the net-work for applications up to 60 km.
What is CWDM?
CWDM can be viewed as a third generation of WDM technology. WDM
was developed as a fiber exhaust solution and traditionally
employed the 1310 nm and 1550 nm wavelength signals. In most WDM
scenarios, providers with a fixed number of fibers had run short of
bandwidth due to rapid growth and/or unforeseen demand. By
multiplexing a signal on top of the existing 1310 nm wavelength,
they could create additional channels through a single fiber to
increase the networks capacity.
However, demand continued to increase dramatically with new
inno-vations and applications such as the internet, text messaging
and other high bandwidth requirements. This created the need for
very fine channel spacing to add even more wavelengths or channels
to each fiber. Dense WDM (DWDM) was a major breakthrough as
equipment provid-
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ers pushed to offer new equipment, promising nearly unlimited
bandwidth potential. However, while DWDM was quickly adopted for
long-haul and transoceanic optical networking, its use in regional,
metropolitan, and cam-pus environments was, in most cases, cost
prohibitive.
A more targeted and cost-effective solution followed with CWDM,
a more recent standard of channel spacing developed by the
International Telecommunication Union (ITU) organization in 2002.
This standard calls for a 20 nm channel spacing grid using
wavelengths between 1270 nm and 1610 nm (see Figure 1). The cost of
deploying CWDM architectures today is significantly lower than its
DWDM predecessors.
Prior to ITU standardization, CWDM was fairly generic and meant
a number of things. For instance, the fact that the choice of
channel spac-ing and frequency stability was such that erbium-doped
fiber amplifiers (EDFAs) could not be used was a common thread. One
typical definition for CWDM was two or more signals multiplexed
onto a single fiber, one in the 1550 nm band and the other in the
1310 nm bandbasically, the original definition for early WDM.
1200 1300
O-band1260-1360
E-band1360-1460
Wavelength (nm)
Fibe
r at
tenu
atio
n (d
B/km
)
S-band1460-1530
C-band1530-1565
L-band1565-1625
1400 1500 1600
2
1.5
1
0.5
0ITU-T G.652 fiber
Waterpeak
1270 1290 1310 1330 1350 13701390
14101430 1450 1470 1490 1510 1530 1550 1570 1590
1610
Figure 1: CWDM wavelength grid as specified by ITU-T G.694.2
Todays standardized CWDM is better defined as a cost-effective
solution
for building a metropolitan access network that promises all the
key characteristics of a network architecture service providers
dream
aboutoffering transparency, scalability, and low cost.
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27The Book on Next Generation Networks
New developments
Even though the ITUs 20 nm channel spacing offers 20 wavelengths
for CWDM, the reality is that wavelengths below 1470 nm are
con-sidered unusable on older G.625 specification fibers due to the
in-creased attenuation in the 1310-1470 nm bands. However, new
fibers that conform to the G.652.C and G.652.D standards, such as
Corning SMF-28e and Samsung Widepass, nearly eliminate the wa-ter
peak attenuation peak to allow for full operation of all ITU CWDM
channels in metropolitan and regional networks.
This enables a CWDM system to operate effectively at the low end
of the ITU grid where attenuation was problematic for earlier
fibers. For example, an Ethernet LX-4 physical layer uses a CWDM
consisting of four wavelengths near the 1310 nm wavelength, each
carrying a 3.125 Gbits/second data stream. Together, the four
wavelengths can carry 10 Gbits/second of aggregated data across a
single fiber.
As mentioned earlier, a major characteristic of the recent ITU
CWDM standard is that the signals are not spaced appropriately for
amplification by EDFAs. This limits the total CWDM optical span to
somewhere near 60 km of reach for a 2.5 Gbits/second signal.
However, this distance is suitable for use in metropolitan
applications. The relaxed optical frequen-cy stabilization
requirements also allow the associated costs of CWDM to approach
those of non-WDM optical components.
Basic implementation
As stated earlier, CWDMs appeal is firmly rooted in meeting the
additional demands being placed on fiber networks by a steady
stream of new, bandwidth-hungry applications. Adding more fiber is
one solution, but there are many possible obstacles that will
likely make this solution cost prohibitive. Although every
situation is different and brings unique considerations to the
table, nearly any fiber deployment includes rights-of-way,
trenching costs, additional equip-ment, manpower, and considerable
time.
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Market studies have indicated accrued costs between $10,000 and
$70,000 per mile to deploy new fiber cable. The large disparity is
due to different situationsfor example, it costs far more to tear
up a city street than to simply trench fiber in a rural setting.
But the key issue is that network archi-tects can incorporate a
CWDM system for much less cost and still achieve the bandwidth
increases necessary to meet demand today and well into the
foreseeable future.
Basically, a CWDM implementation involves placing passive
devices, trans-mitters, and receivers at each end of the network
segment. CWDM per-forms two functions. First, they filter the light
to ensure only the desired combination of wavelengths is used. The
second function involves multi-plexing and demultiplexing the
signal across a single fiber link. In the multi-plex operation, the
multiple wavelength bands are combined onto a single fiber for
transport. In the demultiplex operation, the multiple wavelength
bands are separated from the single fiber to multiple outputs. (See
Figures 2 and 3)
ADCs passive network solution adds value by using the
value-added module (VAM) platform to multiplex and demultiplex.
These VAMs can easily be incorporated into central office (CO),
multiple service operator (MSO), and mobile switching center (MSC)
environments for leveraging the benefits of CWDM. The MSC uses CWDM
to multiplex the different hosts on a wireless coverage system to
multiple remotes using minimal fiber strands. Even a single fiber
can service four, six, or eight different re-mote units. From
there, an antenna is attached to each device to enable indoor
wireless coverage.
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29The Book on Next Generation Networks
Metro Transport RingUsing CWDM
HEADEND
RESIDENTIAL
OpticalNode
Hub
Hub
Hub
Hub
OpticalNode
OpticalNode
OpticalNode
INDUSTRIAL
WIRELESSHANDOFF
HIGH-RISE MDU/BUSINESS
SatelliteUplink
Fiber L
ine
Wa
velengths
Figure 2: CWDMs in useFor example, MSOs can install a band
system at the head-end that will drop one wavelength to each node
along a particular ring configura-tion. This ring can be utilized
as a single fiber. Each CWDM device is packaged into the VAM
platformconnectorized and labeledfor integration into the fiber
panel or cross-connect to save floor space and eliminate extra
patch cords.
Designated, dedicated wavelengths
CWDM also offers the benefit of individual wavelengths for
allocat-ing specific functions and applications. Out-of-band
testing capability is achieved by simply dedicating a separate
wavelength or channel for nonin-trusive testing and monitoring. In
fact, any number of different applications can be applied to
specific wavelengths. For example, a particular wave-length might
be allocated specifically for running overhead or management
software systems.
This is a common practice in using CWDM for cable television
net-works, where different wavelengths are dedicated for downstream
and upstream signals. It should be noted that the downstream and
up-stream wavelengths are usually widely separated. For instance,
the
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downstream signal might be at 1310 nm while the upstream signal
is at 1550 nm. Another recent development in CWDM is the creation
of small-form-factor pluggable (SFP) transceivers that use
standardized CWDM wavelengths. These devices enable a nearly
seamless upgrade in even legacy systems that support SFP
interfaces, making the migration to CWDM more cost effective than
ever before. A legacy system is easily con-verted to allow
wavelength multiplexed transport over one fiber by simply choosing
specific transceiver wavelengths, combined with an inexpensive
passive optical multiplexing device.
Conclusion
ADC views the emergence of CWDM as the most cost-effective means
of moving ever-increasing amounts of information across
metropolitan access networks. For most providers, deploying new
fiber as a means of combat-ing fiber exhaust is not a viable
option. There are too many high costs involved with trenching the
fiber cable, and obtaining rights-of-way can be an intensely
complex issue.
CWDM simply makes sense, particularly with the technological
advancements in todays fiber and transceiver options, including VAM
sys-tems. CWDM achieves the critical goals of transparency,
scalability, and low cost that providers seek in todays highly
competitive industryan industry where new applications and
increasing demand dictate the pace for mod-ern telecommunication
networks.
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31The Book on Next Generation Networks
Chapter 4
Adding New Video Services Warrants New Central Office
Considerations
Although its fair to say the distribution and access elements
within the outside plant (OSP) portion of the Fiber-to-the-Premises
(FTTP) network de-mand the majority of attention during deployment,
its still important not to overlook implications to the central
office (CO). Any FTTP network requires the same flexibility as the
transport networkand it all begins in the CO.
The addition of video services to FTTP network presents
challenges to the CO requiring special consideration.
First, a review
Before discussing the unique challenges of video, its important
to briefly review the overall implications that FTTP has on the CO
architectureand the importance of making informed decisions in the
early stages. The goal of network planners is always to minimize
capital expenses and long-term operational expenses, while
achieving the highest possible level of flexibility in the
network.
Architectural decisions involve connection strategies between
optical line terminal (OLT) equipment and OSP fibers, flexibility
in terms of test access points, and WDM positioning. A key
requirement for providing flexibility evolves from ensuring full
cross-connect capability. With all OLTs, as well as OSP fibers,
connected at the optical distribution frame (ODF), easy ac-cess and
significant long-term network flexibility is achieved, enabling
easy adds, moves, and changes to the ODF. Since the one constant in
telecom-munications has always been change, any assumption that the
network will remain static can result in significant long-term
capital expense and flexibility issues.
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The second critical architectural decision involves placement of
the video WDM within the CO environment. The video WDM combines the
voice and data signals with video signals onto a single fibera key
element of FTTP deployment. Again, with expense and flexibility in
mind, ADC concludes that placing the video WDM in the cross-connect
ODF lineup is the best option.
This is done by using patch cords to connect the OLT equipment
to the inputs of the video WDM. A cross-connect patch cord connects
the video WDM common port to the designated OSP port, providing an
immedi-ate advantage of requiring just three connector pairs while
still maintaining maximum flexibility. With the video WDM located
at the ODF and all OLT patch cords routed directly to the ODF, even
greater flexibility is provided regarding how the OLTs are combined
and configured. Any OLT is easily combined with any other OLT,
regardless of CO location.
Factoring in the video
The addition of video signals now presents new challenges to the
con-figuration of the CO in order to maintain the same flexibility
and price points desired in deploying FTTP. The video overlay onto
the FTTP net-work adds additional fiber cable management
requirements. Also, in or-der to split the video feed to multiple
PONs, additional optical splitting is necessary. Optical path
protection switches are also incorporated where the video signal
enters the service office from the video serving office.
From the video OLT, video signals will pass through several
erbium-doped fiber amplifiers (EDFAs) used to amplify and split the
signal. Each EDFA output will be further split by additional
optical splitters to maximize the video output, allowing the most
PONs to be served using the fewest number of EDFAs. Each EDFA can
have up to four outputs, each with its own optical splitter,
depending on signal strength.
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33The Book on Next Generation Networks
The use of optical splitters is critical, but there are several
placement options. For instance, the splitters could reside in
either the OLT equip-ment frame or the fiber frame. Placing the
optical splitter in the fiber frame enables even more flexibility.
For instance, if a particular PON is located a considerable
distance away, a stronger video signal would be required and the
signal should not be split. By having the optical splitter in the
fiber frame, a patch cord can be run from the EDFA to the fiber
frame, thus bypassing the optical splitter and allowing a stronger
video signal to go to that PON. This flexibility allows video
signals of various power levels to reach PONs at various distances.
These optical splitters would reside in the fiber frame in a
chassis very close to the WDM chassis on the 1550 nm input
side.
Assuming the office providing the video service is not the same
office in which the video signal originates, optical protection
switching is also a consideration. Through diverse path routing,
both a primary and protect video feed enters the optical protection
switch in the video OLT equip-ment frame. The primary video feed
throughputs to the video OLT, but should that signal drop below a
preset power threshold, the system automatically switches to the
redundant path (or protect) video feed. The diverse path routing
takes place at the transmission side where a 1x2 splitter creates
two diverse signals. This basically provides SONET-like protection
without all the electronics by using a splitter and an optical
switchmuch more cost effective.
Several important cable management considerations that apply in
general to the FTTP network architecture will apply to a very great
extent when it comes to video signals. Since video signals are
usually high-power analog, they require considerations for the use
of angled polish connectors, con-nector-cleaning techniques, and
other cable management practices that contribute to signal
quality.
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Every network designer wants to get the most out of existing
electronics. In FTTP, that equates to getting the most PONs served
and achieving the highest network flexibility for the least amount
of expense. But the con-stantly-changing network still requires
everyone to not only peer into the future, but to also design
todays FTTP networks with the ability to adapt to the future.
Test access for the future
Testing the FTTP network is a serious challenge for service
providers. Ad-vanced ODF solutions are being adopted that enable
remote test and monitoring functionality. With traditional ODF
functionality, performing tests or troubleshooting problems
requires breaking into a patch and basically taking the network out
of service. But monitoring and testing ca-pabilities can be
incorporated into advanced ODF solutions that will enable remote
monitoring and traffic identification, as well as reduce
troubleshoot-ing and fault isolation time. The net result is more
efficiency, reliability, and cost savings.
By placing an optical NxN switch between the test equipment and
the access port on the fibers, any fiber can be tested with any
test equip-ment from the network operations center (NOC). For
example, if contact is lost with several optical network terminals
(ONTs), an optical time do-main reflectometer (OTDR) trace can be
performed over the particular fiber to isolate the fault.
Performance monitoring tests can also be accomplished without
having to dispatch a technician to the frame to man-ually perform
testing.
Built-in diagnostics can identify problems within the electronic
equipment, but to see whats happening within the fiber requires
specific test equip-ment and non-intrusive access points. In any
FTTP network, its a point-to-point connection from the OLT to the
customer. If there is a failure in that network, the customer is
out of servicethere is no redundant path available. Therefore, the
ability to restore the network quickly and easily is
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35The Book on Next Generation Networks
absolutely critical. The addition of this single switch provides
technicians with quick, easy, and reliable access to the networkall
of which greatly reduces network outage time and saves money.
Designing the CO to accommodate FTTP requires similar, if not
more strin-gent, cable management and architectural attributes as
any transport net-work. The video overlay makes even more demands
on the CO in terms of efficiency, flexibility, and accessibility.
Decisions made by service providers today will significantly impact
the future reliabilityand profitabilityof their FTTP network. But
with careful planning, future-proofing the CO is a good way to
begin.
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The Book on Next Generation Networks
Distribution
High-Rise MDU Medium-Rise MDU
Horizontal MDU
Low Rise/Garden MDU
Residential
Residential
FeederDistributionDrop
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39The Book on Next Generation Networks
Chapter 5
Its Happening in the Hub
The Fiber Distribution Hub (FDH) continues to play a vital role
in supporting rapid deployment and connection in
Fiber-to-the-Premises (FTTP) networks. Innovation in FDH design
occurs at a rapid rate and next generation fea-tures appear in
newer FDH enclosures. Key innovations include:
Miniaturized splitter modules with plug-in installation that
allow easy additions and upgrades
High-density termination fields with connectorized harnesses
allowing modular growth and flexible rearrangement
A wide range of sizes and mounting configurations that retain
craft-friendly fiber management and maintenance features
Performance enhancements to optical connectors and splitters due
to the rigorous requirements of independent testing of all optical
components and enclosures
Time- and space-saving parking lots providing cross-connect
function-ality at interconnect loss and space levels
As a result, FDH products have been widely accepted in FTTP
networks. FTTP is now seeing large-scale deployment and FTTP
deployment is defi-nitely still happening at the hub.
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Distribution
Network architectures
Fiber-to-the-Business
ONT
FDHCO/HE
OLT
Optical Distribution Network Fiber-to-the-Home
Fiber-to-the Multi-Dwelling Unit
After years of research and experimentation with access
networks, many network providers have settled on passive optical
network (PON) architectures as the direction for future subscriber
access. The PON ar-chitecture has been adopted as a standard in
ITU-T G.983.x that defines the protocols, data rates, and operating
wavelengths necessary to sup-port network services. At the same
time, the standards have established power budgets and parameters
for the fiber optic plant to ensure reliable transport all the way
to the home. The technology of high-speed PON equipment, combined
with broadband fiber offers the potential for con-necting high
bandwidth services directly to the home. The standards ensure
interoperability of equipment and therefore have driven down the
cost of deploying all optical networks. When adding in the cost
savings associated with operating an all-passive optical plant, PON
networks are attractive for overbuild as well as new network
construction.
The initiative to build PON networks is often referred to as
Fiber-to-the-Premises (FTTP), to emphasize the vision of connecting
fiber from the central office/headend (CO/HE) all the way to the
premises. PON architecture includes optical line terminal (OLT)
equipment at the CO/HE that bundles voice and data services. OLT
equipment utilizes wavelength
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41The Book on Next Generation Networks
division multiplexing (WDM) technology to provide bidirectional
voice and data services (1310 nm/1490 nm) over a single fiber.
Additional WDM components at the CO/HE allow integration of video
services onto the same fiber at the 1550 nm wavelength.
OLT equipment ports are connected through optical splitters,
allowing a single port to serve multiple subscribers. The split
ratio in PON networks can vary, but typically networks are planned
with 32- or 16-way splits. The architecture may be configured by
concatenating the splitters at a single point. Most networks are
planned with 1x32 splitters centrally located for easy access for
additions, service, and maintenance.
PON architecture includes optical network terminal (ONT)
equipment at the premises for resolution of voice, data, and video
services. Standardiza-tion of ONT equipment allows the same
equipment to provide services for Fiber-to-the-Home (FTTH),
Fiber-to-the-Business (FTTB), and Fiber-to- Multiple-Dwelling Units
(MDU) applications. Combining these applications into the FTTP
network architecture provides economies of scale for con-struction
and service deployment.
The optical distribution network provides physical connection
between the CO/HE and the premises and includes various cabling
segments including feeder, distribution, and drop. These various
segments are typi-cally joined together by connectors and splices.
The fiber distribution hub (FDH) is one of the key elements located
between the feeder and distribution segments and contains optical
connectors and splitters to pro-vide easy access and flexibility.
The advantage of configuring the network with connectors is to
allow flexibility for service provisioning and for net-work
testing.
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FDH network function
FDH Pad and Pole
Central Office/HeadendUnderground Distribution
Aerial Distribution
The FDH is a key interface between feeder cables extending from
the cen-tral office to distribution fibers routed to subscribers.
The FDH serves an analogous function to serving area cabinets (SAC)
used in copper-based networks to interconnect the feeder and
distribution segments of the net-work. The hub becomes a primary
point of flexibility in the network to con-nect subscriber
circuits. As service is required, technicians access the FDH
enclosure to route connections to complete subscriber circuits. The
FDH also serves as a central location for fiber optic splitters.
This is where the PON network differs significantly from a copper
network.
The optical splitters allow the PON OLT port to be shared among
multiple subscribers via the 1xn split, thus defraying the cost of
the OLT. By locating the splitters in the outside plant close to
the serving area, the cost of feeder fiber is also significantly
reduced. For instance, when a 1x32 splitter is placed in the FDH,
one feeder fiber may be routed into a neighborhood and provide
service connection to 32 subscribers. Another reason to locate
splitters in the FDH is that splitters can be deferred until they
are needed to satisfy service requirements. The FDH can be accessed
to add splitters as service demands grow. Newer hub designs accept
modular splitters that quickly plug into the FDH to allow capacity
to be expanded within a few minutes.
Typically, the FDH is equipped with one stub cable that is
spliced into a feeder cable and another stub cable that is spliced
to a distribution cable. Construction is usually completed using
standard splicing techniques (usu-ally mass splicing) with splices
stored in standard splice closures. Some FDHs are even equipped to
handle the splicing inside the cabinet.
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43The Book on Next Generation Networks
Key FDH capabilities and innovations
The FDH enclosure provides a crucial craft interface in the
outside plant environment. Therefore each major function of the hub
supports easy craft access for service and maintenance.
Fiber Management
Termination Splice Shelf and Trays
Splitter Shelf and Modules
Termination field
The termination field provides a location for terminating fiber
distribution cable on optical connectors and adapters. The
termination field is sized to support the number of subscribers
located in the distribution serving area downstream from the FDH.
FDH enclosures support a range of termination field sizes.
The termination field provides easy access to both sides of the
adapt-er to facilitate cleaning and maintenance. ADC FDH enclosures
feature a unique swing frame design, a hinged chassis containing
all the key optical components including splitters, connectors, and
splices. The de-sign allows easy access to optical components from
the front and rear for cleaning and troubleshooting and is
especially valuable in installations where access is limited to the
front of the cabinet only, for example, in pole mounted
applications. Large cabinets deployed in ground mount applica-tions
feature doors on the front and rear to allow full access to
connectors and splitters from the front and back.
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Terminations in the field are clearly marked to provide accurate
identifica-tion of each subscriber termination. The termination
field provides organi-zation and protection for fiber jumper
connections as they transition into the fiber management section of
the enclosure.
Recent FDH innovations include high-density component packaging
resulting in significant reduction of enclosure sizes. High-density
termi-nation fields with connectorized harnesses allow modular
growth and flexible arrangements.
High-density termination Early FDH termination requirements were
often matched exactly to the requirements for subtending living
units in the immediate fiber serving area. For instance, a 216
fiber hub was speci-fied to support a fiber serving area of
approximately 200 subscribers, pro-viding a small (approximately
five to ten percent) portion of spare fibers routed into the
serving neighborhoods. With more experience, planners realized that
additional fiber capacity downstream could be required for
unforeseen changes in the network or in services supplied. However,
while specifying increased numbers of spare fibers, resulting in
increased fiber termination requirements, users were reluctant to
increase the overall size of the enclosures. Therefore, fiber
termination fields had to handle the increased capacity within
already defined enclosure sizes. This involved in-creasing
termination density and also increasing the fiber handling
capac-ity for a particular enclosure. For example, enclosures
previously handling 216 fibers were upgraded to terminate 288
fibers. This increase in density provides the desired fiber counts
along with the spare growth capacity re-quired for typical fiber
serving areas, while maintaining the overall size of the
enclosure.
Modular, scalable distribution In overbuild scenarios, the
termination field on the distribution side is fully populated with
connectors at the initial installation, and the enclosure is
provided with fully-terminated stub cables sized for the enclosures
direct termination needs. Network planners, how-ever, considering
newer greenfield developments, look for ways to defer cost and
match the FTTP build to the pace of the developments build.
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45The Book on Next Generation Networks
A new development, constructed in phases over a period of years,
may not initially require an FDH with a fully-populated termination
field. This situation may be better served by gradually deploying
terminations as needed. To satisfy this requirement, the FDH
enclosure includes modular blocks that allow terminations to be
added as required. The modular termi-nation block allows upgrades
to the FDH to match the requirements of the FTTP network
deployment, thus deferring hardware costs.
Improved overall performance Advances in planar splitter
technology have dramatically decreased the amount of signal loss
when a single fiber is split into several outputs. Innovation in
component performance has re-sulted in lower loss connections, in
both the termination fields and the split-ters. Improved connector
performance for the widely used SC components, allows
connectorization to replace splicing on both feeder and
distribution fibers while still meeting the overall loss limits
within the FDH. Using con-nectorization for input fibers and
distribution panels greatly reduces the amount of time required to
install and upgrade an FDH.
Splitter field
Splitter modules are designed to snap-in to the splitter field
and can be added as required by service demands. The splitter field
protects, organizes, and routes both the input and output fibers.
The optical splitter modules provide up to 32 connectorized pigtail
outputs and one pigtail input.
Early generations of FDH were deployed fully loaded with
splitter modules that featured storage ports, sometimes referred to
as parking lots, located on the front of the module to stage
splitter output pigtails temporarily until they were connected into
service. The splitter module assembly included modular parking
adapters, each holding 16 or 32 connectors. As a split-ter module
was installed, the fibers were fed into the fiber management trough
and the parking adapters were snapped into place in the parking
area. Individual connectors were then easily separated from the
parking adapter and routed to the termination field during service
turn-up.
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Recently, the parking lots have been relocated to a spot in the
FDH away from the splitter modules. The parking adapters are
removed from the split-ter module, allowing the splitter module to
be reduced in size.
Today, most carriers take an incremental approach to adding
splitter mod-ulesdeploying FDH enclosures initially with just the
splitter modules re-quired to begin service connections. This
reduces the number of parking lots required for pigtail outputs. In
essence, splitter outputs time share parking lots; as the outputs
of the initial splitter modules are placed into service, the
parking lots associated with those outputs become available for
parking subsequent splitter module outputs This allows a
significant reduc-tion in the size of the parking lot, and
consequently, a reduction in the size of the FDH.
Blind-mate connections New miniaturized splitter modules feature
planar optical splitters and are 75 percent smaller, another
contributing factor in the reduction of the FDHs size.
Additionally, innovation has im-proved the way splitter modules are
installed into the enclosure. First gen-eration modules were
designed with the splitter module input extended as a pigtail,
which was spliced to feeder fibers. As each subsequent splitter was
installed, it was spliced to feeder fibers staged in splice trays.
Splic-ing consumes valuable time, and adds costs to service
turn-up. Earlier improvements included connectors on the feeder
fibers that allow quick connection during splitter module
installation, or a connector on the pig-tailed input and a
connector on the feeder fibers mated at a connector panel in the
enclosure. This approach provides a simple, much improved method
for quickly installing splitters. Connectorization of the feeder
fi-bers at the FDH also allows testing on the feeder from the FDH
if required. However, connectorization of the feeder fiber also
raised a safety concern regarding high power when analog video is
transmitted over the path. To address this concern, connectors can
be angled or adapters with shutters provided to prevent a
technician from accidentally looking into the high-powered
termination.
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47The Book on Next Generation Networks
Further innovations have resulted in a backplane connector
system for in-stalling splitter modules. In this configuration,
feeder fibers are terminated with a standard connector
pre-positioned on the backplane to receive a plug-in splitter
module with a mating connector. The backplane connector is
shuttered for safety so that a technician cannot accidentally look
into an unmated splitter module. As a splitter module is inserted
into the backplane receptacle, the module presses open the shutter
to allow the splitter mod-ule connector to mate with the backplane
connector. This blind-mate ap-proach using a common backplane
technology improves efficiency in future expansion activities.
Splice area
The FDH features a splice area to connect feeder fibers or other
cables routed into the enclosure. One use for this area is the
splicing of addition-al splitter modules to feeder fibers as the
modules are added to the FDH enclosure. An alternative to splicing
the input is to include a connector at this location.
Factory pretermination FDH enclosures typically include two
pretermi-nated stub cables. One stub cable is pre-connected to the
optical splitter module input so that it can be field-spliced to
the feeder cable. The other stub cable is pre-connected to the
termination field, so that it can be field-spliced to the
distribution cable. These cables attach to the enclosure using
standard grip clamps and liquid-tight compression fittings seal the
cables at the enclosure entrance. Orientation of the enclosure stub
cables varies, depending on the FDHs mounting method.
Craft-friendly fiber management
The FDH provides total fiber management using a unique front
facing cross-connect design. The front fiber management allows
splitter module outputs to be routed and staged within the
enclosure for efficient connec-tion into service at a later
date.
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Vertical channels using storage loops manage excess fiber slack.
The entire cabinet can be interconnected without congestion.
Connectorized pigtail ends are stored on bulkhead adapters on the
front of the module so that connector ends can be identified
quickly and connected into service. Fiber strain relief and radius
control is provided through the enclosure.
Indoor configurations
As FTTH moves into densely populated areas, the use of indoor
fiber distribution hubs becomes popular due to the number of units
within a particular building, as well as space restrictions outside
the buildings. Indoor FDHs provide all the same features as an
outdoor FDH, but are typically smaller and lighter. They do not
need to meet the same harsh environmental requirements as the
outdoor FDHs. Fiber count capac-ity ranges from 72 fibers to 432
fibers, accommodating small to large high-density buildings.
Below-grade configurations
Another option for high-density areas, as well as areas that do
not allow above ground enclosures for zoning reasons, are
below-grade fiber distri-bution hubs. These compact enclosures are
stored in below-grade vaults when not being accessed for service
configurations.
Qualification
A complete FDH qualification program draws from a wide array of
exist-ing standardized tests with existing procedures. In some
cases, new test procedures have been developed and refined to
support the new con-figurations and new technologies. The overall
program is composed pri-marily of testing regiments drawn from
Telcordia Generic Requirements.
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49The Book on Next Generation Networks
First and foremost, the qualification program involves testing
optical con-nectors to GR-326-CORE, Issue 3. All connectors
utilized in the FDH en-closure are subject to the complete outdoor
service life requirements and to the full spectrum of long-term
reliability tests. In addition to testing at 1310 nm and 1550 nm as
required in GR-326, the test programs include additional test
wavelengths of 1490 nm and 1625 nm to assure users that all
operating wavelengths and all potential maintenance channels would
function under the harshest conditions.
Optical splitters are fully tested to ensure trouble free
performance over the life of the network. The splitters use planar
technology and follow a qualification program aligned with service
life testing in GR-1209-CORE and long-term reliability testing in
GR-1221-CORE. Because of the nature of testing very large devices
(1x32 ports), special sampling techniques were developed for
optical measurement characteristics such as directiv-ity. Splitter
qualification is conducted at the full operation spectrum of four
wavelengths including 1310, 1490, 1550 and 1625 nm. All testing is
done in the format of the optical module that plugs into the FDH
enclo-sure, representing the exact configuration deployed in the
field. Tests for the new enclosures include a full range of
environmental and mechanical tests. Optical characterization is
conducted at the same four wavelengths as the connectors and
splitters. Additionally, several of the tests such as thermal
cycling and seismic qualification are optically monitored during
the test at 1625 nm, which represents the worst-case scenario from
a fiber integrity perspective.
Independent testing of the qualification program demonstrated
the FDHs reliability, assuring a performance level and longevity
expected in an FTTP network. Successful testing of all aspects of
the enclosures, including per-formance of optical connectors and
splitters, have given users the evidence and confidence to support
wide scale deployment of FDH enclosures in the distribution portion
of FTTP networks.
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Chapter 6
Extreme-Environment Performance Considerations for FTTX Splitter
Modules
Optical splitter modules used in FTTX networks contain the
splitters that make passive optical networks possible. The module
physically pro-tects the splitter and provides a means to
connectorize the splitter inputs and outputs.
Figure 1: Typical FTTX Splitter Module
Module housing (1xN splitter inside)
Bending Strain Relief
Input
Connectors
2 mm Furcation tube
A housing, constructed of plastic or metal, holds the splitter
and provides a means to up-jacket the splitter fibers with 2mm
furcation tube for connec-torization. A certain number of outputs
are connectorized. The input fiber may be connectorized, can be a
pigtail, or can be attached to the module by means of a
backplane.
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Industry standards
Telcordia GR-1209 and GR-1221 standards define the operating
requirements for splitter modules in North America. GR-1209 defines
ba-sic optical performance requirements such as insertion and
return loss, polarization-dependent loss (PDL), and uniformity.
GR-1209 also de-fines short-term environmental and mechanical
requirements such as input and output proof strength and side
loading, and a temperature and humidity profile. GR-1221 defines
the splitter modules long-term reliability requirements. GR-1221
requires splitters to go through 2,000 hours of high temperature
aging, low-temperature aging, thermal cycling, and humidity aging.
GR-1221 also subjects samples to impact and vibration testing.
The operating extremes defined in GR-1209 and GR-1221 are -40C
to +85C and up to 95% relative humidity. GR-1209 and GR-1221 will
typically be called out by North American service providers
deploying passive optical networks. Some service providers may
require their network to function at lower temperatures. In these
cases, military specifications (MIL SPECs) requiring -55C minimum
operating temperatures may be called out.
These operating extremes present challenges when designing
split-ter modules. Before large-scale North American deployment of
FTTX in 2004, most modules containing splitters and connectors were
used in central offices. Splitter modules saw stable environments
and were there-fore not extensively tested. Testing to extreme
conditions and deploy-ment in outside plant environments forced
service providers and equip-ment manufacturers to re-evaluate the
requirements of splitter modules. GR-1209 and GR-1221 do not
consider many characteristics that are important for devices
deployed in the OSP. For example, GR-1209 and GR-1221 do not define
material properties such as chemical resistance or installation
considerations such as the handling of furcation tubes at extreme
temperatures.
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53The Book on Next Generation Networks
Furcation tubing
Furcation tubing is the material slipped over the splitter
inputs and outputs. The furcation tube protects the fiber from
physical damage and makes con-nectorization possible. The furcation
tube is usually identical in construction to a 2mm simplex jumper,
but the .900mm tight buffered fiber is replaced by a hollow tube.
The hollow tube has a .900mm outside diameter and the inside
diameter is larger so that a fiber can be inserted. Once the fiber
is inserted into the inner tube, a connector can be terminated to
the ends.
2 mm Outer Jacket
Inner .900 mm Tube
Aramid StrengthMembers
Splitter Input and Output Fibers
Inserted Into This Space
Figure 2: Furcation Tube Construction
2mm simplex jumpers are typically used in controlled
environments. They are not required to meet the more stringent
requirements for outside de-ployment. It would be risky to choose a
furcation tube made out of materi-als used for controlled
environment jumpers that are only rated to -20C. Some specific
requirements of furcation tubing that arent explicitly called out
in GR-1209 or GR-1221 include cold-temperature handling and cable
routing, and thermal expansion and contraction.
Cold-temperature handling and cable routing The outer 2mm
jacketing of furcation tube is made of thermoplastic materials. The
tub-ing can become very stiff at cold temperatures. This is no
issue in a static situation. However, if new service is turned on
at cold temperatures, a technician will have to re-route the
up-jacketed splitter outputs in the fiber distribution hub (FDH).
If the furcation tube is too stiff because of the cold temperature,
routing becomes difficult and bending can occur, causing high
insertion loss.
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Standard jumper jacketing materials such as PVC become very
stiff at tem-peratures lower than -20C. Proper design requires that
furcation tubes be made of different materials. Polyurethane is one
possible choice for the outer jacket. This material remains
relatively flexible to temperatures as low as -60C and is resistant
to chemicals commonly used in tele-communications and to fungus.
Some types of PVC outer jacketing can also become permanently stiff
if exposed to high tempera-tures for extended periods of time. As
the PVC ages, plasticizers in the cable degrade causing the jacket
to stiffen. Polyurethane is also resistant to this phenomenon,
making it suitable for both very hot and extremely cold
environments.
Cold-temperature handling of furcation tube can be evaluated
several ways. First, the furcation tube should be tested to
FOTP-104 (Fiber Optical Cable Cyclic Flexing Test), but performed
at -40C. It could also be tested to FOTP-37 (Low or High
Temperature Bend Test for Fiber Optic Cable). There should be no
evidence of cracking of the outer jacket after the tests
are completed. Second, the ability to re-route furcation tube
within a cable management system must be evaluated. There are no
exist-ing industry standards to evaluate this property. However,
this prop-erty can still be subjectively tested by simulating cable
routing at cold temperatures.
A test was performed where furcation tube made of PVC and
polyurethane were wrapped around a small mandrel and aged at -40C
for 2 hours (see Figure 3). The mandrel was removed and the cables
were allowed to uncoil themselves using only the weight of the
connector (see Figure 4). The poly-urethane furcation tube was much
more flexible at -40C than PVC. This property makes polyurethane an
ideal choice for furcation tube jacketing because bending losses
are less likely to occur when an installation take place at cold
temperatures.
Figure 3: Test sample on Mandrel at -40C
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55The Book on Next Generation Networks
Thermal expansion and con-traction All furcation tubes are made
of thermoplastics. Plastics tend to expand at high tempera-tures
and contract at low tempera-tures. However, the optical fiber will
remain the same length over these temperature extremes. If the
expansion and contraction of the plastic materials over the fiber
are not accounted for, fiber bend-ing and high insertion loss could
occur.
Thermal affects usually cause inser-tion loss p