Today’s fiber networks carry an ever-increasing percentage of the network traffic, making ready access to the fiber for testing and troubleshooting even more critical. While traditional fiber-management options are designed to manage and protect fibers, advanced fiber-management approaches that incorporate optical switching capabilities are necessary to match the economic and performance needs of ever-evolving networks, including fiber-to-the-premises (FTTP) architectures.
FTTP PONs are point-to-point systems with no redundant paths. As such, any maintenance involves taking the network out of service until the problem is resolved. Network operators, therefore, need the fastest possible access to the fiber to isolate and repair problems quickly and minimize the network’s downtime.
Electronics at each end of the PON-the optical line terminal (OLT) and the optical network terminals (ONTs)-communicate with each other and allow for some segmentation of network faults. If an OLT loses contact with a single ONT, for example, it is logical to assume the problem is somewhere between the splitter output and the ONT. If the OLT loses contact with all the ONTs off one splitter, the problem likely is located between the central office (CO) and the splitter. However, the electronics at each end of the PON cannot identify the exact location of the problem.
The only way to determine the exact location of a fault in the fiber network is to access the fiber itself and perform optical tests, including an optical time-domain reflectometer (OTDR) trace. Although there have been many advances in terms of management, density, and ease of access, technicians still face challenges when attempting to access the network for troubleshooting, particularly in FTTP architectures.
Purchasing and deploying the outside plant (OSP) fiber network is a significant investment for any carrier, and maximizing that investment is critical. Traditional fiber-management systems, such as fiber distribution frames (FDFs), play an important role in realizing that value. FDFs enable easy access to the fiber network for customer churn (adds, moves, and changes) and centralized test points for troubleshooting.
To perform optical testing, the technician must remove a fiber connection from the network to insert the test signal, which can introduce several potential problems. The technician may be unable to find the right connection. He or she may pull the wrong connection or damage the adjacent circuits when accessing the desired connection. These issues can be magnified when working with a PON because there is no backup system in place.Many network outages can be traced back to technician error, and many of those errors are attributed to poor record keeping. Record keeping in our industry is often a neglected area. For instance, knowing where every patch cord is routed requires records that are only as good as the last person who touched them. How important is it to know which patch cord to disconnect for troubleshooting a fiber? Just ask the customer who lost service when the wrong connection was broken.
Combine faulty record keeping with greater termination density, and the possibility for errors increases even further. While many high-density fiber-management systems are designed to allow easy port identification and port access, the greater number of terminations in a smaller space increases the possibility of human error.
The optimal solution for preventing technician error is one that minimizes the need for technicians to access the network. Advanced FDFs with optical switching capabilities address the aforementioned challenges and, in the process, increase service providers’ profitability, reduce maintenance costs, and enable improved customer service.
One way to improve FDF functionality is to incorporate optical splitters, which are used in combining signals, multicasting, and providing test access. When used within the FDF system, optical splitters can provide an unintrusive “window” into the fiber for testing and troubleshooting.
With a splitter module, technicians avoid taking a signal out of service when they pull the patch cord off the frame to plug into an OTDR. Instead, they can actually tap into the network via the splitter to monitor the signal without interrupting service. This eliminates the possibility of taking the wrong circuit out of service, and it is much more forgiving of record-keeping errors.
While placing these value-added splitter modules (VAMs) into the FDF system to provide easier fiber access, a technician is still required to go to the frame, locate the port, set up equipment, and run the tests. So even though the VAM provides that critical window into the fiber network, this process remains both time-consuming and labor-intensive.
A more advanced approach eliminates the need to dispatch the technician to the FDF to perform testing. This is accomplished using the splitter modules previously discussed but incorporating an optical switch to connect the test equipment to the splitter ports. The test equipment would be connected to one side of the optical switch.
Taking the same monitoring window provided by the splitter VAM, the test equipment-OTDRs, power meters, OLT test equipment, OC-X test equipment or any other desired test equipment-is mounted permanently into racks within the CO. The test equipment is connected to the optical switch located in the FDF; all the monitor ports also are connected to the switch (see Figure 1). With this configuration, a technician can connect any window port to any piece of test equipment from a computer in the CO or network operation center (NOC).
When circuit testing is required, the switch makes the desired connection. The technician then runs an OTDR trace, analyzes it, and determines whether additional tests are required. This remote testing concept enables dramatic time savings in fault isolation-decreasing the process from hours to minutes. Not only is troubleshooting time reduced, but customer service is improved. Instead of sending a technician into the field to troubleshoot a problem, which requires a truck roll and lots of remote equipment, the technician now is sent out to fix the problem. The troubleshooting phase is already completed.
Advanced fiber-management approaches that combine traditional FDF functionalities with optical switching capabilities offer service providers extensive value in both traditional network and FTTP applications. This combined functionality can prove invaluable in locating faults and rapidly restoring customer service.
Today, when a fault is located in a PON, a truck must be dispatched into the field, equipped with a range of test equipment and a skilled technician. Generally, the technician first is sent to the fiber distribution hub (FDH) or splitter cabinet. At the cabinet, the technician will start by setting up the OTDRs, power meters, and other necessary equipment to test individual fibers to locate the fault. Of course, the technician must be skilled enough to use and read the test equipment accurately. Moreover, he or she must have the necessary equipment on the truck or the results of these activities can prove less than successful. A high-cost procedure, it will likely take hours to restore subscriber services.
A better alternative is to perform the fault isolation from the CO. This involves designating a spare fiber between the CO and FDH as a test fiber with a dedicated port in the FDH cabinet. This dedicated test fiber provides a path to test the fiber network between the FDF and the subscriber.
Now when a problem occurs between the FDH and a subscriber, a technician still responds to the cabinet. But that technician does not have to be highly trained on an extensive range of test equipment. The technician in the field will simply route a temporary patch cord to connect the maintenance port to the subscriber fiber. Back at the CO or NOC, a more highly trained technician, using the permanently mounted test equipment and the advanced fiber-management system, runs all the tests, identifies the problem, and relays the “fix” back to the technician at the cabinet (see Figure 2).
In this PON scenario, one highly trained technician can perform any test from the CO, decreasing the cost of truck rolls. In fact, test equipment costs are greatly reduced since the need for equipment in every truck is eliminated; the test equipment in the CO is available for any fiber.
This test configuration also provides easier fault isolation when the problem is located between the CO and the FDH cabinet. Traditionally, a technician was sent to the frame and he or she located the patch panel and port, pulled the patch cord, and hooked up the appropriate test equipment. Again, this process can increase the likelihood of technician errors, poor record keeping, or cable congestion-and it takes time. Using the process provided by advanced fiber management and an optical switch, the technician in the CO can easily test the network between the CO and the FDF without taking the network out of service or disconnecting any network terminations.
Another benefit of this configuration occurs during service turn-up. To speed final link testing, a technician can simply hook up the ONT at the subscriber premises and plug it into the maintenance link at the FDH to perform initial testing without tapping into the live network. Once the link is tested and verified from the CO, the FDH connection can be swapped from the maintenance port to the optical splitter.
There are other applications in which these advanced fiber-management features and functionality can add significant value to a service provider’s network. One such application is monitoring a service or traffic handoff point or managing critical circuits. A service provider may have several high-value customers with service level agreements that guarantee reliability and availability. Maintaining this premium service requires the provider to identify any network issues before they become network outages. With monitoring ports and test equipment permanently set up and connected to the switch, these circuits can be cycled through a testing scenario on a regular basis. For instance, OTDR traces can be gathered at regular intervals and compared with an original trace to determine any potential issues.
In summary, advanced FDFs with optical switching capabilities enable the early identification of network problems before service is affected. Such systems incorporate the latest techniques for optimizing access networks in terms of accessibility and the time required to configure, perform maintenance, and troubleshoot problems.Patrick Thompson is program manager in the Fiber Cable Management Solutions Division of ADC (Minneapolis, MN). He may be reached via the company’s web site at www.adc.com.
Ron Mackey is chief marketing officer of Calient Networks (San Jose, CA) and may be reach via his company’s web site at www.calient.com.