Look both ways before crossing the network

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-intrusiv

By Trevor Smith, Product Manager,

Fiber Optic Division, ADC

Telecommunications

Value 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 soth-ervices 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 with-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

e-mail: trevor_smith@adc.com

®

D E C E M B E R 2 0 0 2

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Fiber line F I B E R O P T I C T E C H N O L O G Y

Reprinted from CED, December 2002 by RSiCopyright.

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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, makwith-ing 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 toth-ermination 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 ■

Fiber line F I B E R O P T I C T E C H N O L O G Y

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