Look both ways before crossing the network

By Trevor Smith, Product Manager,
Fiber Optic Division, ADC
Telecommunications
V
alue Added Modules, sometimes
called splitter modules, are finding
acceptance at fiber demarcation and
handoff points, which are proliferating in to-
day’s metro buildouts. Though VAMs can
perform a variety of network functions, in-
cluding splitting, multiplexing and providing
access within fiber networks, the most pop-
ular application today is non-intrusive mon-
itoring, which lets providers proactively trou-
bleshoot their networks without forcing a dis-
ruption of service on customers.
It goes without saying that service in-
tegrity has become extremely important
to all carriers, including cable TV multi-
ple system operators. As e-mail and oth-
er forms of data services have become
critical to individuals and corporate en-
terprises, any significant signal loss or
extended downtime cannot be tolerat-
ed. Finding a problem before it becomes
a hard failure and affects customers does
more than prevent loss of business. It
gives the service provider flexibility in
choosing the best way and time to address
the problem. Thus, many service
providers are implementing proactive

maintenance practices by using test access
points on every fiber at the demarcation
point prior to handoff.
Testing at the demarcation point can
be a challenge. A fiber network is, by nature,
a closed system and not easy to tap and
test like its copper cousin. Typically, the
first indication of trouble is the failure of a
signal to show up at its intended destination
(or, upon arrival, it’s either corrupted or
attenuated). Even though the current service
quality may be adequate for customer
needs, these types of errors often forewarn
of equipment failure down the road. As a re-
sult, the operator who wishes to preclude fu-
ture disaster may be forced to conduct a la-
bor-intensive hunt to find the source with-
in a buried or otherwise inaccessible loop.
One way to proceed is to disconnect
connectors from the fiber frame at each sus-
pected point of failure and plug them into
an external test device. But this procedure
is time-consuming and requires that a sec-
tion of the network be taken out of service,
which can lead to customer complaints.
These issues support the strong argu-
ment that network operators need a passive,
non-intrusive testing solution. In fact, one of
the major trends in this area is non-intrusive
handoff contracts, which specify that if there

are non-catastrophic network test issues,
the carrier must continue transmitting.
Value Added Modules are finding ac-
ceptance at these critical fiber demarcation
points. Though VAMs can perform a variety
of network functions including splitting, mul-
tiplexing, monitoring and providing access
within fiber networks, the most popular ap-
plication today is non-intrusive monitoring.
VAMs simply slide into a fiber panel, are
locked into place and then connected to net-
work equipment with fiber patch cords.
The monitor module is equipped with sep-
arate ports for local testing. Within each
module, each transmit and receive signal
passes through a 90/10 splitter (95/05 split-
ters are also commonly used). Ninety per-
cent is allowed to proceed to its destination,
while 10 percent is routed to the local mon-
itor port for use by an external test device.
This allows local testing of either signal
without interruption of service with test
devices having access to the full optical sig-
nal–exactly what the customer is getting.
When commissioning a network, test
equipment uses this signal to generate a
known pattern, which simulates real cus-
tomer traffic. The pattern is then carried on
the network or circuit under test, then
test equipment at the receive point deter-

mines whether the transmission is suc-
cessful (without errors). At this point, the
technician knows that it is functioning
correctly and what the levels are supposed
to be. Periodic checks show if power lev-
els deviate from the norm or if bit error
rates are increasing, which indicates signal
degradation associated with a future fail-
ure of a laser, connector, splice or some
other network device.
Connector access
A differentiating point in product design
is in the access to the connectors them-
selves. Connectors are one of the most im-
portant factors in an optical network, for the
Look both ways before crossing the network
VAMs valuable for non-intrusive monitoring
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DECEMBER 2002
CED
Fiberline
FIBER OPTIC TECHNOLOGY
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simple fact that dirty connectors are the
highest cause of poor performance and most
likely failure points in the network. Periodic

cleaning of the connector is mandatory.
Connector cleaning can present some
unique challenges. Most VAMs have at least
two monitor ports, one each for the trans-
mit and receive circuits. Whenever a con-
nector is cleaned or new circuits turned
up, there is the potential for damaging dust
or dirt to enter other connectors within the
fiber frame. Network engineers should look
for adapter designs that include removable
retainers, which allow technician access to
individual connectors for service. This de-
sign lets a technician remove a connector,
clean it and put it back without exposing the
other fiber to the risk of breaking and tak-
ing down service.
Many transmission equipment vendors
build in performance monitoring functions
into their equipment. There is a mistaken
notion that this eliminates the need for ex-
ternal test equipment. Performance moni-
toring is optimized to report what is going
on at the transport rate (OC-48 or
OC-192). This works well in detecting ma-
jor situations such as a complete failure of
a transmitter or a fiber cut. However, there
are a number of failure conditions not de-
tected by performance monitoring:
• Input/output port failure. Because
this is a physical interface, it tends to be an

area with one of the highest failure rates.
•Faulty cabling or dirty fiber connections
on long haul and on tributary equipment.
• Sectionalizing or localizing trouble.
Most monitoring of path errors and alarms
is done at the path terminating points with-
in long haul or metro routes, making it
difficult to locate the exact point of the
problem. VAMs located at strategic points
in the network simplify this process.
• Ownership of only one end of the
path terminating equipment. A network
provider may only own the transmission or
reception equipment, not both ends. This
makes it impossible to test the network
from end to end. A VAM located at the op-
tical handoff solves this problem.
There are also issues related to inter-
operability between multi-vendor network
elements to consider. No regulations exist
for standardization on the testing portion
of the signal, and different manufacturers
of transmitters and receivers may look at bit
error rates in different ways. They may not
communicate test information between
themselves. There are open-platform test
boxes, which can plug into a receiver from
Brand X, or a transmitter from Brand Y,
and it will look at the overhead. But with-
out a VAM, it is still an intrusive solution

and doesn’t provide a true picture of the
network, only the specific point under test.
Cost roadblocks
Network operators don’t always see the
value of VAMs until after they experience a
serious network failure. VAMs may be per-
ceived as an operations department luxury
that doesn’t contribute to revenue genera-
tion. “A VAM costs money and doesn’t
really do anything except provide an op-
portunity to test,” the argument goes.
There are several value propositions to
counter these perceptions. One way to look
at operations cost includes asking the ques-
tion, “What is the cost of service going
down?” If a circuit fails and can be dis-
covered and repaired in minutes with a
VAM, customers probably will not be up-
set. When service is down for a day or two,
as sometimes is the situation with intrusive
methods, customers might consider switch-
ing providers in an attempt to find one
who guarantees service.
In addition, with VAMs, maintenance is
simpler to administer and maintenance costs
are reduced. With an open-platform VAM in-
stalled, the technician has only a single test
box to carry. Regardless of what brand trans-
mission equipment is on the network, a tech-
nician can plug into this VAM to obtain test

results. The technician doesn’t have to learn
the specifics of each piece of transmission
equipment, its proprietary software, different
testing protocols, and many other nuances.
The fact most operators quickly recognize
is that the combined cost of a VAM and oth-
er associated passive termination equipment
is less than one percent of the infrastructure
service cost needed to deliver a fiber circuit.
This one-time cost is miniscule compared to
the millions of dollars spent on transmission
elements. Still, the cost savings in problem
prevention and customer retention gained
through VAMs can be enormous.
Conclusion
With competition at an all-time high
in the broadband marketplace, network
reliability, efficiency and performance are
critical to acquiring and retaining cus-
tomers. Operators find themselves in a sit-
uation where 99.999 percent operability
is mandatory, even with limited people re-
sources and time.
The VAM, when used for noninstrusive
access for network testing and monitor-
ing, has proven to be a critical link in en-
suring that these valuable networks are
up, running and providing revenue. ■
Fiberline
FIBER OPTIC TECHNOLOGY

Figure 1: Typical VAM (Value Added Module) application in cable TV.