Termination of installed optical fiber cables has always been perceived as a difficult, expensive, time-consuming process that discouraged some contractors from developing in-house capability for fiber installation. The myth that fiber is fragile because it’s made of glass and difficult to install has persisted, even though fiber is the most widely used communications medium and many contractors already have added it to their capabilities.
Recently, new high-speed systems have intensified some contractors’ fear of fiber. Multimode fiber networks for 40 and 100 gigabits per second (Gbps) use parallel transmission with eight or 20 fibers per link using 12-fiber MTP/MPO connectors that are not easily terminated in the field. The new generation of optical local area networks (LANs) based on fiber to the home (FTTH) technology uses single-mode fiber, not multimode. That has added to the concern over installation. The smaller single-mode fiber seems scarier because most contractors are unfamiliar with its installation.
What’s a contractor to do?
Looking for solutions
Some contractors choose to install just the cable and hire subcontractors to do the termination work. Others have discovered that cable manufacturers would cut the cable to length, terminate it, pack it in a box and ship it to the job site ready for installation, so they took that route, thinking it would be simpler.
A prefab cable system, often called “plug-and-play” cabling, seems like an easy installation. Unpack the complete cable assembly, lay the cables, plug connectors into patch panels, and the job is done. Some companies offer backbone cables terminated in multifiber MTP/MPO connectors and breakout modules that fit into racks to form patch panels for single fiber connectors. Instead of one connector per fiber, the backbone cables have one connector per 12 fibers; each 12-connector module connects to one of the MTP/MPO connectors.
On the surface, choosing between on-site termination or prefab assemblies seems like a black-and-white issue, a tradeoff between training and time and how it affects installation cost. But it turns out the choice is really shades of gray. The decision to go prefab or full installation often hinges on some very specific details.
What is a “prefab cable system?”
A contractor can order a cabling system that will be manufactured in a factory to exact specifications, cut to exact length, terminated with the proper connectors, tested and packed in a box to be shipped to the work site. The cabling will use the fiber and cable type specified for the job and be ready for installation. Connectors are typically enclosed in a protective pulling cover that acts as a grip for connecting to pull ropes.
Until recently, these systems were limited to premises cabling, but FTTH has led to the development of outside plant versions that use outdoor or indoor/outdoor cables and even ruggedized, sealed connectors. The most popular use of these is for drop cables from the street to the home in FTTH installations.
Most prefab cabling now uses cables made with bend-insensitive (BI) fibers that, as the name implies, are less sensitive to the bending of the cable during and after installation. BI fibers also allow for cables to be made smaller in diameter, which eases installation. Cables the same size or even smaller than the traditional 3-millimeter simplex cable now can contain up to 12 fibers. Some manufacturers offer cables with up to 144 fibers, even armored for protection when installed under raised floors, but the protective covers over the connectors can become quite bulky.
Factory-terminated cables generally are made with high-quality connectors terminated using epoxy/polish techniques, the most reliable and lowest loss connector technology. The cabling system’s performance will be specified, tested at the factory and guaranteed.
What could go wrong?
Things to consider
Prefab cabling systems allow faster installation and comparable costs to field termination but are not without their problems. Designing the cabling system is the first problem. It must be accurately designed so cables can be built close to the exact length: Too short and the cabling won’t work; too long and it requires finding locations for storing excess cable in service loops. One manufacturer described the problems encountered in one installation where approximately 50 meters of excess cabling was stored in figure-eight coils under a raised floor.
Most manufacturers offer assistance in designing the prefab cable plant, which, of course, is easier in new construction where CAD drawings are available. But the contractor must know the building where the cables will be installed well enough to plan for the cable to be the proper length to end up where it is supposed to while avoiding all the obstacles in between. It can be a big problem getting exact lengths for the cables and finding storage for excess cables in older buildings. It also is the contractor’s responsibility to find out if the prefab cable requires special sizes of conduit, cable trays or unusual firestopping.
Installation of a prefab cabling systems may be more difficult than bare cable since the ends with connectors, even inside protective covers, tend to be bulky and require more care to handle. As a result, running cables inside walls or pulling in conduit may be more difficult. Manufacturers can design cables that fit in the common fiber optic innerducts as long as the contractor specifies this in the order.
Installers still need some training to install prefab cabling systems. They need instruction on how cables have to be handled (that is, with care to prevent damage due to pulling tension, kinking or pulling around sharp corners). Cables must be laid so they are not subjected to excess cable weight pressing on them in cable trays.
Care must also be taken to not harm the connectors on the ends. The installers must know how to handle the ends during pulling and how to remove the pulling grips without damaging the ends. And every installer making connections with the cables, either to cable-breakout modules in a patch panel or to other cables, must know how to clean the connectors and handle the connections properly.
Most cabling manufacturers that offer prefab cabling systems will offer a warranty on the cable plant only if it is installed by one of their certified installers. Contractors may be forced to have their crews trained by the manufacturer to offer the customer the cabling warranty, or the manufacturer may recognize third-party certifications.
Finally, there is the issue of testing. For documentation to present to the customer, the installer still has to test the installed cables and record the results. That brings up the issue of correlating the field test data with the factory test reports. Test results can be a big issue because of the differences in test equipment used by the manufacturer and the contractor. In addition, there are inherent uncertainties of fiber optic measurements, especially with multimode BI fiber. Methods for testing multimode BI fiber are still being developed, so it is important to follow the manufacturer’s directions for testing. Plus, there may be a lot of finger pointing if a cable or connection tests as unacceptable. Was it shipped from the factory that way, damaged in transit or damaged during installation? Should the contractor have tested it in the box before installing it?
How do you make a choice?
All of these issues have led many contractors to simply get personnel trained to design and install fiber optics and equip their crews properly. Advances in termination methods—including much better and faster termination techniques, such as the prepolished/splice style connectors—have simplified the installation process considerably. As a result, fiber optic installation work can be highly profitable.
In the past, most contractors looked to prefab cabling for single-mode installations because direct termination was more difficult than multimode. Single-mode termination requires tighter tolerances and different polishing processes. The traditional alternative—fusion splicing on pigtails—required a very expensive fusion splicer and a trained operator. But the millions of FTTH installations and rapidly growing number of premises single-mode networks have led to major performance advances in prepolished/splice connectors, making them cheaper, faster and more reliable. Now an installer with a termination kit and a few hours of training can do perfectly acceptable single-mode terminations in a couple of minutes.
But multimode installation, usually considered easy by most installers, has a new variation that changes the situation. LANs using multimode fiber at speeds of 40 and 100 Gbps are now using parallel transmission, with multiple links of 10 Gbps per fiber in each direction. Thus, a 40 10 Gbps link will use eight fibers and a 100 10 Gbps link needs 20 fibers. These parallel links use 12-fiber MTP/MPO connectors that are not generally considered field-installable. When a customer wants a network for these systems, a prefab cabling system may be the only logical choice.
Other applications are also commonly installed with prefab systems where installation is difficult, or time is a big issue. For example, cellular systems are expanding from fiber backhaul to the cell tower then up the tower to the antennas. Fiber to the antenna almost always uses prefab cables or at least pigtails, which are plugged into the antennas on the tower end and terminated inside enclosures on the ground.
With the prevalence of fiber optics in today’s networks, contractors doing low-voltage work need to become competent in fiber optics. This involves not only getting employees trained in installation, but also having staff who understand fiber optic network design and who keep up with the state-of-the-art technology, products and applications that affect your contracting decisions.
Thanks to Eric Klaiber of Corning and Loni Le Van-Etter and Dave Cook of 3M for their contributions.