installation
The loose-tube advantage
for data center applications
A
The traditional round cable goes
head-to-head with ribbon fiber in
terms of value and performance.
data center is at the heart of
many successful enterprise business processes. As such, it needs to operate at
100% efficiency. Dubbed the “factory floor of the Information Age,” the data center can house one company’s
data storage or host multiple Web sites and co-locations for many companies’ networks.
Utmost consideration must be given to the detailed planning and designing of an efficient data center for
reliability and longevity, whether the scenario is large or
small. The TIA-942 Telecommunications Infrastructure
Standard for Data Centers has recognized nine core elements of a data center, seven of which are
defined spaces (computer
room, telecom
room, entrance room,
main distribution area, horizontal distribution area, zone
distribution area, and equipment distribution area) and
two of which are the backbone
and horizontal cabling subsystems that tie the spaces together. From outside services in, the
backbone cabling subsystem
provides the interconnection between the entrance room,
computer room, and telecom room out to the main distribution area. Backbone cabling also connects the switches in the main distribution area to the local area network
(LAN) switches in the horizontal distribution areas.
The cabling infrastructure must be capable of servicing
present and future applications in the data center. Fiber-
optic cable is the preferred medium for the data center
backbone due to channel capacity and scalability, total immunity to electromagnetic interference (EMI) and radio
frequency interference (RFI), elimination of the crosstalk
inherent in copper cabling, smaller diameter, and ease of
installation, particularly when using pre-terminated modular trunk cables. Furthermore, plenum-rated cables are
commonly specified for maximized flame
and smoke protection
within the data center.
Selecting the type of
fiber-optic cable boils down
to the applications and the installation.
Cable construction choices are ribbon, loose tube, or
tight buffer. Where ribbon cable has found its niche because of its high-density configuration, specifiers are now
taking a look at other constructions to fit unique and
stringent data center requirements as well as industry
standards. These additional considerations address issues
that include higher mechanical robustness, improved
cooling efficiency, fire loading, as well as flexibility and
modularity for easy moves, adds, and changes (MACs).
The Adventum Dual-Unilite cable from Berk-Tek,
designed specifically for
data centers, uses two
12-fiber cables bonded
with Siamese-style jackets
for easy separation and
termination.
Making connections
CAROL EVERETT OLIVER, RCDD, is the marketing analyst for Berk-Tek,
a Nexans Company (www.berktek.com) and author of numerous articles
on cabling topics. BENI BLELL, RCDD, is the fiber-optic product business
manager for Berk-Tek with more than 13 years’ experience in fiber optics.
Everything begins and ends with connectors. Installers
agree that when it comes to fiber terminations, the modular MPO/MTP pre-terminated multifiber has become
popular in the data center because of its ease of installation and its reliability. The MPO or MTP (“multiple terminations push-pull” or “mechanical transfer pull”) is a
linear multi-array connector containing 12 fibers in a ferrule with an easy push-on and pull-off design.
The most common single point of cabling-system failure is the connector interface. Pre-terminated assemblies
are the most reliable cable and connector combination to
address this issue. The cable is pre-terminated in the fac-
Reprinted with revisions to format, from the December 2006 edition of CABLING INSTALLATION & MAINTENANCE
Copyright 2006 by PennWell Corporation
tory to allow quick deployments, and is pre-tested to ensure reliability. Pre-calculated lengths of pre-terminated, multi-fiber
MTP/MPO trunk cable assemblies have been proven to substantially reduce installation times and provide better reliability.
The factory-terminated ends arrive on site with protective
case, the loose-tube cables are already in loose form and do
not require a “de-ribbonizing” process whereby ribbon fibers
must be separated from the ribbon matrix before they can
be terminated with LC-type connectors. The separation of
12 fibers from a ribbon matrix can further damage the fibers
if this process is not performed
Because the fibers are loose and not bonded in a ribbon correctly.
Loose-tube cables prove to be
matrix, they do not exhibit any preferential bend and can
much more flexible over the tight
bends in the concentrated pathbe flexed in any direction with significantly reduced risk of ways
found in the data center
environment.
For example, trydamage relative to the ribbon cables.
ing to bend a 48-fiber ribbon
encasements and pulling eyes.
cable around tight corners, such as from overhead baskets
The MPO connector originally was designed specifically to
into racks or cabinets, may be difficult as the typical bend
accept a ribbon array of fibers, as the fibers must line up with
radius is 7.8 inches. The 6.9-inch bend radius of a more comthe flat connector housing; however, termination technologies
pact loose-tube construction can be twisted and turned withhave evolved to the point where it is no longer necessary to
out affecting the fiber, resulting in a more “forgiving” cable.
have a flat ribbon to terminate to MPO connectors. A popuThe tighter bend radius of loose-tube cables has additional
lar option is to use ribbonized optical fibers from a loose-tube
benefits for slack loops. Slack loops for future applications are
cable. This ribbonizing process is performed by the cable
wound in fiber distribution panels. Ribbon cables have largassembly manufacturer, and the result is an assembly with
er slack-loop diameters.
greatly enhanced functionality.
The ribbon cable is aptly named for its flat, ribbon-like
Size and toughness
structure, which houses fibers side-by-side. Twelve fibers in
Ribbon cables were designed for high-density applications,
a row are bonded and held together by an ultraviolet acrylate
but their overall diameter is actually larger than a loose-tube
matrix. Because the fibers are first bonded in rows
cable with the same fiber counts. The loose-tube cable conof 12 and then stacked in layers of ribbons (for
tains subsets of 12 fibers totaling a maximum of 18 subsets—
counts greater than 12), the ribbon-making and
or 216 fibers total—under one jacket. Compared to a ribbon
cable-making
process
becomes
cable, the loose-tube cable is less than half the weight and
expensive. Depending on the fiber type and total fiber count,
approximately 25% smaller in diameter.
a loose-tube construction can save up to 40% on the initial
Smaller optical cables can help prevent pathway and cabicable price.
net congestion, while enhancing airflow and facilitating more
Furthermore, the flat nature of the ribbons can become an
efficient cooling. Thicker cables that impede airflow can also
obstacle, with some installers reporting that high-count
introduce a fire-loading problem in data centers.
ribbon cables become stiff. Because of the stacked-maRibbon cable is specifically designed for the controlled envitrix makeup, the cable has an inherent preferential bend,
ronment of indoor applications. When data center cabling runs
which means it will bend in one preferred axis. If twistunderneath an access floor, it becomes susceptible to crushed or pulled in the opposite direction of the axis, the
ing and subsequent cable degradation. The Insulated Cable
fibers could become damaged. For this reason, manufacturers
Engineers Association’s (www.icea.net) ICEA S-83-596 Stanare working on alternative connecting methods that do not
dard for Optical Fiber Premises Distribution Cable defines a
use ribbon-type cables.
crush- and tensile-performance rating for an indoor distribuThe alternative to a ribbon cable is a loose-tube cable, which
tion style cable. One way to mitigate the crush factor is through
allows the fibers to lie freely within dry buffered tubes inside
armoring. Interlocking armor is usually a spirally wrapped
the jacket. Because the fibers are loose and not bonded in a
aluminum tape wound around the cable under the outer
ribbon matrix, they do not exhibit any preferential bend and
jacket. Armored cables are heavier, thicker, and more costly
can be flexed in any direction with significantly reduced risk
than those without armor.
of damage relative to the ribbon cables.
An indoor/outdoor-rated cable offers the mechanical robustDuring the factory termination process, the loose fibers are
ness of an outdoor cable with the flexible, smaller diameter of
first ribbonized and then terminated with an MPO-style conan indoor cable. It has an outdoor crush rating typically twice
nector. Another benefit of loose-tube trunk cables becomes
that of an indoor distribution cable, along with a robust tenevident when MPO-to-LC terminations are required. In this
sile strength rating of 600 lbs. In installations in which the
backbone cable also services the enterprise network outside of
the data center spaces—such as between facilities—an indoor/
outdoor cable typically has a far better operating temperature
range, including a dramatically lower operating-range floor
(~ -40º C) than a ribbon cable (~ 0º C). Because the cable can
be installed both indoors and outdoors, there is no need for a
splice termination point within 50 feet of building entrance.
The data center is evolving as new applications or blade
servers are added, particularly in shared co-locations. Upgrading a ribbon-style cable or changing out connectivity is not a simple procedure. To remove the ribbon cable
from the MPO connector includes “de-ribbonizing,” which
sometimes can result in fiber damage. With a loose-tube
construction, the MPO can be cut off and the cable can be
re-terminated by fusion splicing a pigtail or via traditional
field termination.
Ease of installation and termination yield signifcant advantages when time is critical for new construction, data-center
expansion, or MACs. Data centers should be planned with scalability in mind, and the type of cabling should be carefully specified for today’s applications, and for easy transition during
future upgrades.