How to restore fiber-optic premises networks
As fiber-optic cables course through campus and premises networks, network providers confront the challenge of identifying, locating, repairing and restoring communications quickly and cost-effectively after an outage
the light brigade inc.
The high bandwidth of optical fiber proves useful in campus and premises network applications--mostly in backbones--where fiber`s information-handling capacity and transmission capability offset the perceived drawbacks of high cost, difficult installation and complex repairs.
Telephone companies, cable operators and communications service providers should have carefully planned routines for fiber restoration following an outage. They need such procedures because their cable spans often run in exposed aerial locations where cable is strung between utility poles or buildings, or their cables are direct-buried between structures where they can be cut by backhoes and other trench-digging equipment. Fiber-restoration practices, therefore, are critical to maintaining the operation of premises communications networks.
Premises and campus local area networks consist of intrabuilding and interbuilding links covering relatively short distances compared to the wide-area and metropolitan-area networks usually associated with long-haul optical communications. As a result, premises networks require different planning approaches and equipment so network providers can respond quickly to outages and supply the necessary restoration service.
Planning is key
During the network design stage, the network provider must establish a financial value for the type of restoration posture needed to protect the physical cabling plant. For example, for networks where high data rates are required, reliability is critical to the success of an enterprise. For other networks, security is the major issue. To others, priority users must be served, or route diversity is paramount. Route diversity means running cable over two different routes, not putting two separate cables in the same duct. This strategy, of course, may involve higher labor, materials and construction costs.
Another design issue concerns the types of failures that have occurred in the past. Because network failures tend to repeat, a restoration plan can be built around maintenance and repair records. For example, the following fiber-optic problems can cause a premises network to fail:
Broken fibers at connector joints or patch panels
Cables damaged at patch panels
Cables cut in ceilings or walls
Cables cut by outside construction
Contaminated or mis-keyed connectors
Too much or too little loss (overdriving the receiver)
Improper cable rolls
Transmission equipment failure
These problems fall into several well-defined areas that can be covered in the premises cable-plant restoration plan.
Patch-panel related--These failures are usually caused by improper dressing of the jumpers and cables, improper keying of connectors or contamination of the connection. Improper cable routing and localized damage are also possible causes.
System-related--Overdriving or underdriving the optical transmission system can cause either total or intermittent network failure.
Installation-related--Improper bend radius, overclamping the cables and improper rolls of the transmit/ receive fibers fall into this category. Installations that are made around previously installed fiber networks can also lead to failures if not enough attention is paid to dressing, termination and cable routing.
Construction-related--Problems caused by construction-related activities are generally made by backhoes and other heavy construction equipment. In premises networks, localized failures can result from poor cable identification or lack of care by technicians.
After the types and locations of probable fiber-optic network failures have been identified, network providers can specify the equipment and procedures needed to restore the premises network. It is recommended that the following equipment be available as part of the fiber-optic cable plant restoration plan.
Optical inspection microscope--This device is used to identify poor connector finishes and surface contamination. Magnification can be from 100¥ to 400¥ power, with the larger magnification recommended for working with singlemode fibers.
Fiber-optic cleaning kit--Many faults in optical systems are caused by contaminated connectors. A simple cleaning procedure can resolve these problems. Also, keep connectors clean and capped when not in use.
Optical-loss test set--Consisting of a stabilized light source and a calibrated power meter, the optical-loss test set (olts) is used for end-to-end span tests. The set should be matched to the operating wavelength and connector used in the optical transmission system. The power meter is the essential go/no-go instrument in fiber-optic cable troubleshooting. This instrument can check power levels at the transmitter, receiver and all connection points in the system. If the system is well-documented, service records can be used to isolate whether the problem is in the electro-optics equipment or in the physical plant. For some network problems, the power meter might show only a slight decrease in power level. For this reason, accurate records are essential to provide comparisons between existing and historical conditions.
Visual tracer/fault locator--This inexpensive instrument transmits visible light through a fiber. The more-powerful versions use red lasers operating in the visible spectrum and can locate breaks in many types of jumpers and buffered fibers. White-light versions are also available but do not have the power to locate internal breaks.
Optical time-domain reflectometer (otdr)--This all-purpose instrument is nearly always used first in restorations, yet it is the most application-dependent. In long-distance networks, where most outages occur away from the equipment at either end of the link, the otdr is critical for restoration. In the case of premises and local area networks, however, most problems can easily be identified without an otdr by using the other instruments.
Restore the outage
After the network problem and its location have been identified, the following restoration process checks should be performed:
Check whether the faulty cable span has retrievable slack. If so, pull the slack back to the fault and create a single repair point.
Resolve if it is easier or quicker to replace the cable segment than to repair it.
Determine if connectors or splices are needed for the repair.
In any case, protect the repair points to prevent future damage to the span. Do so by adding closures or patch panels, or by rerouting the cable to other locations.
Conclude if the system can handle the additional losses caused by adding connectors, splices and extra fiber.
If necessary, provide a temporary drop cable until a permanent restoration can be made.
If the physical fiber is identified as the point of failure, restore the optical path using a logical and safe procedure. Here, use of a troubleshooting flow chart is suggested (see figure). Begin by eliminating the transmission equipment as the source of trouble, and then isolate the problem within the physical plant using the step-by-step flow-chart procedure.
After isolating the source of the trouble, you can initiate the restoration. Two general scenarios apply in restoring the premises network. In one, retrievable cable slack is available, and in the other, it is not.
For an emergency restoration where there is retrievable slack, first locate the cable fault and pull spare cable back to the failure point. Then, confirm that the cable break is where it appears to be. Use a visual light source to check each fiber from both ends. Make sure that a second break is not located close to the first one; if so, cut it out while restoring the initial fault.
The area around the fault must then be scouted to find the best repair point and to identify the optimum method to restore the optical fiber. It may be necessary to pull the cable back to a ceiling, floor, post or other location for physical mounting. This location should be noted on available drawings. If a splice panel is added, it should also be labeled and secured. After the cable is repaired, the fiber spans should be retested for loss using the optical-loss test set.
If there is no retrievable cable slack to use during the restoration, it may be quicker to pull a new cable or segment than to repair the damaged one. Also, decide whether to splice or connectorize the new cable and how to protect the coupling.
It there is no retrievable slack, a new segment of cable could be added to the span. This segment, however, requires two termination points and twice the cost in labor, materials and parts. The new cable section must also have at least the same number of fibers as the span of cable being replaced.
The process for retermination is the same whether there is retrievable slack or not, except that two termination points instead of one, respectively, need to be addressed.
After the emergency restoration process is satisfactorily completed, the network documentation should be updated and the work evaluated, as follows:
1. Redocument and retest all splices, spans and segments.
2. Adjust the "as-built" drawings. New vaults, closures, splices and slack cable points need to be added or adjusted.
3. Conduct a meeting with all concerned parties to review all aspects of the restoration.
4. Check out these restoration issues:
The cause of the outage and its impact on network operation
An evaluation of the restoration work performance in terms of procedures, equipment, parts and staff
The solutions to all discovered problems
Improvements to the present restoration plan
The provisioning of rebuild kits and parts replacement inventory. u
Larry Johnson is president of The Light Brigade Inc. in Kent, WA. This article is extracted from his contribution to Fiber Optics Technician`s Manual, published this year by Delmar Publishers. It also appeared in the September 1996 issue of Cabling Installation & Maintenance, a PennWell publication.
Storage Loops Aid in Premises Network Modifications
Premises networks and the buildings in which they are located must accommodate many adds, moves and changes over their lifetimes. Network plans for these needs can help resolve some of the problems associated with fiber-optic cable restoration. This issue concerns retrievable slack, and how network problems can be solved by fiber-cable storage loops.
Fiber-cable spans designed with slack points enable spare cable to be pulled, and require only one termination point. Then, when there is an outage, the use of quick-connect mechanical splices or direct connectorization allows for network restoration to take place with minimal optical signal loss.
Because most fiber cables within a building are breakout or distribution types, they can be easily reterminated. The main problem with storage loops is where and how to locate the splice or connection panels or mini closures--these parts provide strain relief for the cable and physical protection for the splices or connectors. Panels or closures can be emplaced above the floor, and be either wall- or ceiling-mounted. In many networks, aesthetics and size are key location factors. For other networks, security and access are important considerations.
Basic Restoration Strategy
Premises network providers should have a basic restoration strategy in place to address the repair of a failure. This strategy should include the following:
All fiber routes should be properly documented for both optical performance and physical routing.
All patch-panel designations, signal types and interconnect routing information should be recorded.
All transmitters and receivers should be documented as to their optical transmit and power levels. Receivers should be documented for both minimum and maximum power levels.
All network spans should be documented for optical loss. This data should encompass operation at both 850 and 1300 nm for multimode applications, and at 1300 and 1550 nm for singlemode applications. The documentation should identify the fiber size and the manufacturer.
If otdr tests have been performed, copies of the traces should be included in the test reports.
For fiber cables with sequential length markings, the difference between the markings indicates the cable length in meters or feet for each segment. This information should be tabulated.
Fibers should be identified and prioritized to identify the most important fibers for immediate restoration.