WHITE PAPER
It's Happening in the Hub
It’s Happening in the Hub
Fiber Distribution Hubs (FDH) continue to play a vital role in supporting rapid
deployment and connection in FTTP networks. Innovation in FDH design occurs
at a rapid rate and next generation features 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
• Highest performance optical connectors and splitters available; all the
optical components and enclosures have completed a rigorous regiment
of independent testing far beyond any test program seen in the industry
to date.
As a result, FDH products have been widely accepted in FTTP networks. FTTP
is now seeing large-scale deployment and FTTP deployment is definitely still
happening at the hub.
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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
architecture has been adopted as a standard in ITU-
T G.983.x that defines the protocols, data rates and
operating wavelengths necessary to support 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

additions, service and maintenance.
PON architecture includes Optical Network Terminal (ONT)
equipment at the premises for resolution of voice, data
and video services. Standardization 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 construction
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
typically 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 provide
easy access and flexibility. The advantage of configuring
the network with connectors is to allow flexibility for
service provisioning and for network testing.
It's Happening in the Hub
Fiber-to-the-Business
ONT
FDH
CO/HE
OLT
Optical Distribution Network
Fiber-to-the-Home
Fiber-to-the

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
(usually mass splicing) with splices stored in standard
splice closures.
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.
FDH Pad and Pole
Central Office/Headend
Underground Distribution
Aerial Distribution
Fiber Management Parking Adapter
Termination Splice Shelf and Trays
Splitter Shelf
and Modules
FDH Network Function
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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 (144-, 216-, 432-, 576-,

(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 increasing termination density and
also increasing the fiber handling capacity 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 required 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 enclosure’s
direct termination needs. Network planners, however,
considering newer greenfield developments, look for
ways to defer cost and match the FTTP build to the
pace of the development’s build. 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

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 splitter 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.
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 splitter module, allowing
the splitter module to be reduced in size. One design
includes hinged parking that allows 50 percent reduction
in splitter module size, and hence FDH enclosures that
are 50 percent smaller.
Today, most carriers take an incremental approach to
adding splitter modules – deploying FDH enclosures
initially with just the splitter modules required 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

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 module connector to
mate with the backplane connector. This “blind-mate”
approach using a common backplane technology
improves efficiency in future expansion activities.
It's Happening in the Hub
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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 additional 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 preterminated 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

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 existing standardized tests with existing
procedures. In some cases, new test procedures have
been developed and refined to support the new
configurations and new technologies. The overall
program is composed primarily of testing regiments
drawn from Telcordia Generic Requirements. First and
foremost, the qualification program involves testing
optical connectors to GR-326-CORE, Issue 3. All
connectors utilized in the FDH enclosure are subject to
the complete outdoor service life requirements and to the
full spectrum of long-term reliability tests. In addition to
testing at 1310nm and 1550nm as required in GR-326,
the test programs included additional test wavelengths of
1490nm and 1625nm 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 directivity. Splitter
qualification is conducted at the full operation spectrum
of four wavelengths including 1310, 1490, 1550 and