alue-added modules
(VAMs), sometimes called
splitter modules, are find-
ing tremendous acceptance
at fiber demarcation or hand-
off points, which are proliferating in
today’s collocation scenarios. Although
VAMs can perform a variety of network
functions, including splitting, multi-
plexing, and providing access within
fiber networks, the most popular appli-
cation today is non-intrusive monitor-
ing, which lets providers proactively
troubleshoot their networks without
forcing a disruption of service on cus-
tomers.
The driver behind this trend is collo-
cation. Competition has never been
keener in the telecommunications mar-
ketplace. In this era of deregulation
and nonstop technology advances,
competitive local-exchange carriers
(CLECs) are pushing incumbent local-
exchange carriers (ILECs) to provide
better service, lower rates, and
enhanced offerings. But CLECs still
rely on incumbents. Because of high
startup costs, staffing, and logistical
issues, many CLECs collocate equip-
ment and hook into local-exchange
fiber networks from an ILEC’s central

Collocation Cage
Photo 1. A value-added module
housed in a fiber frame.
Reprinted with revisions, from the October 2000 edition of LIGHTWAVE
Copyright 2000 by PennWell Corporation
of cages separating the equipment and
providing security, carriers swap back
and forth a multitude of different sig-
nals and services through this single
office. In theory, cooperation should be
improved since everybody is still just
leasing space and more or less on
equal footing, but ownership of net-
work problems can still lead to territo-
rial disputes.
Point of demarcation
In either environment, a service
provider selling a DS-1 (1.544
Mbits/sec), a DS-3 (44.736 Mbits/sec),
or any SONET rate to a customer is
responsible for that circuit up to the
agreed-on demarcation point. Handoff
in the optical domain can be accom-
plished in a number of ways.
Generally, fiber-optic patch cords are
simply connected to fiber panels or
frames, with one provider granted
access to the rear, the other to the front.
For example, an ILEC hands off OC-12
(622-Mbit/sec) optical bandwidth to

works provides some
unique challenges. A fiber
network is by nature a closed system
and, unlike its copper cousin, not easy
to tap. The first indications of trouble
are typically the failure of a signal to
show up at its intended destination or
when its arrival is either
corrupted or attenuated.
Even though the current
service quality may still
be adequate for cus-
tomer needs, these types
of errors often are a fore-
warning of equipment
failure down the road.
As a result, an operator
may be forced to con-
duct a labor-intensive
hunt to find the source
within a buried or oth-
erwise inaccessible loop
to avoid future prob-
lems.
One approach is to
disconnect the connec-
tors from the fiber frame
at each suspected point
of failure and plug them
into an external test

acceptance at these critical fiber
demarcation points. The modules
Photo 2. A value-added module with removable retainers
allows a technician to access individual connectors for
service.
Photo 3. If an open-platform value-added module (VAM) is
installed, the technician requires only a single test box.
Regardless of what brand of transmission equipment is on
the network, a technician can plug into the VAM to obtain
test results.
slide into the fiber frame, and fiber
patch cords are installed from each to
the network equipment (see Figure 1
and Photo 1). The VAM is equipped
with separate ports for local testing.
Within the module, each transmit and
receive signal passes through a 90/10
splitter; 90/05 splitters are also com-
monly used. While 90% of the signal
is allowed to proceed to its destina-
tion, 10% is routed to the local moni-
tor port for use by an external test
device. This routing allows local test-
ing of either signal without interrup-
tion of service, with test devices hav-
ing access to the full optical signal—
exactly what the customer is getting
(see Figure 2).
When commissioning a network,
test equipment uses this signal to gen-

used to manufacture with precision are
difficult to master.
On top of that, significant pressure
exists for fiber component vendors to
bring products to market as quickly as
possible in high volume. Because of
time and technology constraints, many
systems vendors do not have the luxu-
ry to develop capabilities in-house and
are willing to purchase from outside
sources or co-develop components
with established vendors. As a result,
these vendors sacrifice internal control
over quality and are sometimes subject
to the irregular manufacturing sched-
ules of their suppliers.
Vendors such as ADC that develop
the technology and perform produc-
tion in-house generally are immune to
these types of problems. Producing
millions of couplers a year allows an
in-house vendor to fine-tune the man-
ufacturing equipment, software, and
processes to guarantee very high yield,
quality, and high reliability in field
environments. Compliance with in-
dustry standards, for high tempera-
ture, high humidity, and accelerated
aging, such as required by Telcordia
Technologies (Bellcore) GR-1209 and

service. In some VAM designs, the
adapters are fixed to the sheet metal
of the fiber frame, and the only way
to get to the connectors is by remov-
ing the cover and exposing all circuits
to contaminants and potential fiber
damage.
Performance-monitoring limitations
Many vendors build performance-
monitoring functions into transmis-
sion equipment. However, there is a
mistaken notion that this function
eliminates the need for external test
equipment. Performance monitoring
is optimized to report what is going
on at the transport rate, for example,
OC-48 (2.5 Gbits/sec) or OC-192 (10
Gbits/sec). It is very good at detecting
major problems such as the complete
failure of a transmitter or a fiber cut.
But there are a number of failure con-
Figure 2. The value-added module is equipped with separate ports for local testing. Each
transmit and receive signal passes through a 90/10 splitter—90% is allowed to proceed to its
destination, while 10% is routed to the local monitor port for use by an external test device.
Local Testing of Signals
Customer
receives 90%
Demarcation point
Service-provider transmits signal
10% routed

at the optical handoff solves this
problem.
There are also interoperability issues
between multivendor equipment to
consider. No regulations exist for stan-
dardization on the testing portion of
the signal. Therefore, manufacturers
may look at bit-error rates in different
ways and not communicate this test
information to other vendors.
Open-platform test boxes, which can
plug into a receiver from brand X or a
transmitter from brand Y and look at
the overhead, are available. But with-
out a VAM, it is still an intrusive solu-
tion and doesn’t provide a true picture
of the network, only the specific point
under test.
There is also a notion that testing can
be done using the digital-crossconnect
systems (DCSs) employed by some car-
riers. While DCS equipment does
include some testing features, these
boxes are geared to the telephony
switched network and optimized for
DS-1, DS-3, and other electrical signals.
Even though the DCS may have an
optical input and output, it still only
tests a signal comprising electrical sig-
nals, and all-optical crossconnect sys-

installed, the technician has to carry
only a single test box. Regardless of
what brand of transmission equipment
is on the network, a technician can
plug into the VAM to obtain test
results (see Photo 3).
Moreover, the technician
doesn’t have to learn the
specifics of each piece of
transmission equip-
ment—its proprietary
software, different testing
protocols, etc.
Keeping technicians
trained and up-to-date on
a variety of vendor-specific equipment
is expensive. Once technicians are
well-trained, their market value
increases and retention becomes an
issue. As a result, many carriers are
adopting modular products that are
easy to learn and easy to service. This
approach not only brings down opera-
tional training costs, it makes it possi-
ble to hire less-skilled personnel to
work on these networks.
The bottom line is that the combined
cost of a VAM and other associated
passive termination equipment is less
than 1% of the infrastructure service