Tài liệu Maintaining fiber-optic cable at the demarc doc

T
o the traveler, the border crossing is
invisible: no armed guards, snarling guard
dogs, passport checks, or luggage searches.
To the signals crossing this border, it might as
well not even exist; but to your network man-
agers and technicians, it’s a critical crossing. The
border is the demarcation point, or “demarc,”
between your fiber network and that of your ser-
vice provider. On the outside, the service
provider handles maintenance; on the inside,
the responsibility is all yours.
Every installation involves tough choices.
Users need service, nobody wants to wait, and
cost is a constant issue. Faced with the compet-
ing demands for service, speed, and savings, you
may be tempted to do the job “well enough for
now,” with the rationalization that it can be fixed later. If, as a
network builder, you find yourself asking, “Why not?” The
answer is simple: “Y2K.”
The impending Y2K nightmare is a classic result of short-
term thinking. Programmers came up with what they consid-
ered a safe, simple fix to a relatively small problem. They as-
sumed that the fix would be replaced long before it, in turn,
became an issue in its own right. In fact, the “fix” stayed in
place and became a ticking time bomb that will likely cost bil-
lions of dollars before it is resolved.
In much the same way, we are now in the early days of fiber
networking. It may seem that we are installing enormous
amounts of capacity, but experience tells us that demand al-
ways grows faster than expected. Chances are excellent that

over relatively few years,
•
The bandwidth carried by existing fiber will increase more
than expected.
•
The amount of fiber cable in fiber troughs will grow faster
than we assume.
•
Much more equipment will be needed to support fiber infra-
structure.
•
The density of fiber on that equipment will increase.
•
The complexity of fiber management will grow geometrically.
It is impossible to accurately anticipate future needs, but the
responsible approach is to be conservative without skimping on
details that could jeopardize network performance.
Making an entrance: the fiber demarc
There are two basic styles of fiber demarc: those at which
inside and outside fibers are spliced together and those at
which connectors join them. Which type you use depends on
your internal architecture. If there is a main crossconnect
(MC) relatively close to the demarc, you might prefer splicing
incoming fiber at the entrance cabinet located there. Moves
and changes can be accomplished at the MC, as can rerouting
around failed fibers.
But if your architecture brings fiber to multiple locations
(telecommunications closets, for example) directly from the
demarc, the situation is very different. If a splice-based en-
trance cabinet were used, many moves, changes, or reroutings

could be accomplished only by resplicing. In this case, a con-
nector-based fiber entrance terminal would be far more prefer-
able. The demarc’s crossconnect capability allows connection
around internal or external failures, taking the place of the oth-
er architecture’s MC.
In both cases, the demarc should do more than merely ter-
minate fiber; it should also protect it from damage and prevent
excessive bending. In addition, it should provide protected
storage for excess cable to simplify access and prevent damage.
A wall-mounted unit saves space and provides flexibility in in-
Maintaining fiber-optic
cable at the demarc
installation
Who does what at the junction of the
public and private networks?
Reprinted from the April 1999 edition of CABLING INSTALLATION & MAINTENANCE
Copyright 1999 by PennWell
The network architecture influences whether you should splice or connectorize fiber
at the demarc. In networks with a main crossconnect near the demarc, splicing is
recommended (a). If the fiber goes directly from the demarc to telecommunications
closets, it is best to use connectors at the entrance facility (b).
IC
a)
TC
IC TC
MC PBXDemarc
b)
TC
PBXDemarc
Network Architectures

TC
IC - Intermediate
crossconnect
MC - Main
crossconnect
PBX - Private branch
exchange
TC - Telecommunications
closet
Joel K. Matthews / ADC Telecommunications
stallation. If the location is not secure, a lockable demarc is
preferable. Finally, it should provide features for clear routing
and labeling to help manage growth and change.
Yet another kind of demarc is a terminal, or cabinet, that
accommodates both splicing and connectorization.
The joy of splicing
By itself, splicing is relatively simple, reliable, and perma-
nent. In reality, however, splicing tends to take place in dif-
ficult places and times,
making the process much
more complicated. First of
all, it can take place in con-
fined spaces with relatively
little room to work. Sec-
ond, it tends to take place
where there are lots of oth-
er fibers, which can be dis-
turbed in the process.
Third, it can take place un-
der time pressure, either in

the course of a multifiber
installation or to remedy a
failure. There are several
ways to simplify and speed
splicing.
•
Don’t use splices if you don’t have to. You can’t remedy to-
day’s splice problems this way but you can prevent tomor-
row’s. You may not be planning to grow, change, or repair,
but Murphy’s Law suggests that you should.
•
If you must splice, use a system that is splicing-friendly, pro-
viding easy access, splice trays, work space, fiber protection,
lighting, and room to grow.
•
Give yourself a break. Leave plenty of slack for convenient
handling. Lay out your fibers so that repairing one won’t
damage others.
•
Allow enough time to do the job carefully. Rushing through a
job increases the likelihood of damage now and sets you up for
problems the next time you have to access the same splices.
Typically, splicing personnel (splicers) are skilled above and
beyond the normal premises technicians. Splicers are often sub-
contracted out at considerable cost.
Making connections: frames and panels
Both during and after installation, most of your contact with
the fiber will be at the frames and panels. Depending on the ar-
chitecture you choose, these will be in some hierarchy of MC,
intermediate crossconnects, and telecommunications closets

(TCs). These are all critical places for your fiber.
•
Connectorization allows greater flexibility than splicing, but
just as a cut or scrape provides the opportunity for infection,
a connector provides an opening for dust or dirt, which can
disrupt data-signal flow.
•
Complex routings, sharp turns, and untended slack provide
ample opportunity for bend-radius violations, which can
cause attenuation or even breaks in the fiber.
•
Frequent handling affects both the fiber being handled and
those around it.
•
Poor fiber management can increase the amount of han-
dling, resulting in slow in-
stallation or repair and in-
creasing the likelihood of
damage.
All of these problems
can be alleviated with plan-
ning. The first step is to es-
tablish fiber-specific proce-
dures. Once procedures are
established, train your
staff; many of the over-
sights that lead to prob-
lems will disappear.
Standardizing on equip-
ment helps to standardize

procedures. A single sys-
tem throughout the facili-
ty will simplify inventory and shorten the learning process.
Such a system should be modular and inclusive and should
provide a range of options such as crossconnect or intercon-
nect. It should cover all levels of facilities, from demarc to fiber
frame to TC to the desktop. An integrated system also elimi-
nates the risk of damage to fiber in transit from one piece of
equipment to another.
A system should be completely free of sharp corners and pro-
vide physical management for cable from end-to-end. Conve-
nience features like front and rear access speed up the installa-
tion process, reduce the chances of damage to fiber, and
simplify connections.
Attention to detail: connectors
Properly selected and used, connectors can provide nearly per-
fect signal transmission, along with the convenience of fast,
simple reconfiguration. To deliver high-speed, high-band-
width service, however, they must provide a near-perfect in-
terface. A small gap or misalignment can cause attenuation or
total loss of the signal. On the other hand, if the connectors
press the fiber end-surfaces together with too much force, the
glass can be crushed.
Environmental variations are another consideration when
selecting a connector and connector manufacturer. Connec-
tors must be carefully designed and manufactured to avoid
these problems, initially as well as over time and environ-
A secure fiber entrance terminal
provides for both splicing and crossconnect capability.
mental variation.

•
Fiber ends must be precisely shaped and polished to provide
proper mating of the surfaces and avoid pits and scratches.
•
Epoxies that hold the fiber in place must withstand extremes
of temperature and humidity. If they do not, the fiber can
“piston” or withdraw, leading to fiber damage or poor physi-
cal contact at the connector.
•
In the event of failure, the ability to identify the manufactur-
er’s lot number can help identify other potential problems
before they occur.
•
To help eliminate any potential problems, components must
be carefully tested using the best available equipment, which
should be recalibrated frequently.
Installation and handling also affect the performance of con-
nectors. Cleaning is critical and should be part of every installa-
tion or move. Careful handling of the installed connector as
well as adjacent fibers and connectors can improve contact of
the mating surfaces and prevent unnecessary fiber damage.
Going ’round the bend
The cabling installer who looks at
a fiber and asks, “What could
go wrong?” is not a pessimist,
but a realist. First of all,
fiber is made of glass. Sec-
ond, it can be subjected to
some very trying conditions.
Third, a fault can cause seri-

ous problems and be difficult to
isolate and repair.
A leading cause of problems is the vi-
olation of fiber’s bend radius. Many of these
problems occur long after installation but could probably have
been foreseen and prevented. Here are three examples.
1) A fiber is run loosely around a sharp, sheet-metal bend. The
natural stiffness of the fiber keeps the turn gradual and larg-
er than the minimum bend radius. Later, the addition of
other fibers adds bulk or weight, pressing the original fiber
against the sharp edge and into a tight bend, causing attenu-
ation or breakage. The only reliable preventive measure is
the absolute avoidance of sharp edges throughout the sys-
tem.
2) Connectorized fibers plugged straight into a fiber frame or
panel at a 90
o
angle are pulled sideways across the panel to
allow visibility and access. The bend just behind the connec-
tor may not violate minimum radius, but a hard pull on the
fiber or careless handling can eventually force it into a tight
right-angle bend. This design could cause signal attenuation
or contribute to jumper failure. A simple solution to this
problem is to make use of angled retainers, which reduce the
sideways pull and improve both visibility and access.
3) Unmanaged slack in cables or jumpers allows them to form
loops. Jostling of these loops can easily cause kinks that vio-
late minimum bend radius. The solution is to take up all
slack, even over short distances, with systems specifically de-
signed to control bending.

Fiber trough systems
Continuous fiber running in fiber trough systems faces a whole
different set of problems. While it is not “open” as it is at con-
nection points, it can be more exposed, traveling through pub-
lic areas of the facility. And while a fiber may go for years with-
out being touched, it is also subject to accumulated pressure as
newer cables are added to the run, increasing both weight and
physical pressure that can lead to unanticipated problems.
For example, within a fiber trough system, fibers may run
horizontally across an open “downspout” without any prob-
lem, until the weight of accumulated fiber starts pressing
down over the opening. The bottom fibers will then be
pressed against the angled edge between the horizontal and
vertical runs and eventually bend. The best
trough systems provide rounded corners
and baffles that separate nonexiting
fibers from those making the
turn.
Systems should also provide the
widest range of options: size, joints, and
materials. Even color may be an issue, for both
aesthetic and management reasons. Some less obvious issues
may also be important. For example, a system that requires
fewer supports may cost more to buy but less to install. Snap-
together systems can simplify installation, but only if they do
not sacrifice strength or require additional support that eats up
the time and cost savings.
In-house network builders face the classic dilemma: Fight al-
ligators or drain the swamp. The bad news is that even if you can
keep up, the system itself will betray you as higher data rates

make greater demands than existing systems can meet. The good
news is that better equipment costs only a little more and can
save a lot in installation and ongoing support costs. The same ar-
gument can be made for judicious overbuilding. The better news
is that the payback begins almost immediately.
Joel K. Matthews is product manager, fiber-optic products, at
ADC
Telecommunications (Minneapolis, MN). For more information,
visit www.adc.com.
Fiber trough systems such as
this one in a small telecommun-
ications closet should be
adaptable to different-size spaces.
C
MI