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 e-mail: trevor_smith@adc.com ® DECEMBER 2002 CED Fiberline FIBER OPTIC TECHNOLOGY Reprinted from CED, December 2002 by RSiCopyright. Copyright © Reed Business Information, a division of Reed Elsevier, Inc. All rights reserved. For reorders call RSiCopyright 651.582.3800. For subscription information call 303.470.4800. 1257271 12/02300981 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. . 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. 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