Taut-sheath optical splicing in the aerial and underground plant
Taut-sheath splicing is common in copper networks. Recent advances make it a viable option for optical cables as well?if certain conditions are met.
Gabor Kiss Bell Communications Research
Taut-sheath splicing is a common practice for copper cables but has only recently become an option for optical cables. This lag is a consequence of two profound differences between copper and fiber: (1) copper is easy to splice, with negligible loss and 100% yield, and (2) bending causes attenuation in fiber but not in copper. Thus, taut-sheath splicing of fiber has had to wait until splicing techniques became robust enough to be performed in the field, and for the development of methods to deal with slack storage and cable termination issues.
Taut-sheath splicing for fiber-optic cable provides direct cost benefits compared to traditional splicing methods. Another driving force in the development of the technique is a desire to "make fiber just like copper." There is an undeniable legacy of experience with copper plant that can be an obstacle to working with fiber. However, many skills can be carried over from the copper-cable industry if the products are amenable. Indeed, it is unlikely that engineering and construction staffs can consist of both twisted-pair copper and fiber specialists; hence, the ability to rapidly shift between these media (and possibly also to coaxial cable) is important. The design of fiber cable, closure, and splicing products and the practices they require affect how successful this transfer of skills can be.
Evolution to taut-sheath splicing
A taut-sheath splice in an aerial application is similar to a balloon splice in that splicing is performed in an uncontrolled or poorly controlled environment in the air, rather than inside a vehicle. As with balloon splicing, the elimination of slack loops gives immediate and quantifiable benefits over traditional place-and-loop construction. The major additional benefit is that a splice point can be created anywhere, rather than at predetermined points. Thus, the splice point can be dictated by location and logistical parameters that may not be known by planning and engineering staff. A branch may be created at any point at any time, reducing the pressure to make accurate and conservative predictions. In addition, taut-sheath splicing allows the use of self-support (figure-8) cable, which is not possible with balloon splicing.
Issues commonly encountered for balloon splicing that must be re-examined for taut-sheath splicing include
marking the reverse oscillation lay
(rol) point on the sheath of loose-tube cable,
preventing temperature-induced cable
loss by proper termination of the cable`s central member,
the yield of splicing operations.
In addition, a new issue is encountered in that taut-sheath closures tend to be free-breathing. A taut-sheath hermetic closure may be a desirable product class, yet the deployment of splices and possibly connectors in free-breathing closures introduces condensation and biohazards into the splicing operation.
The most important cable issue relevant to taut-sheath splicing is the rol point in loose-tube cables. In such cables, the buffer tubes are spiraled around a central member in one direction for a distance, and then the direction is reversed. This is necessary to give the final cable the required flexibility. The distance between the reversal points is the rol distance, which is determined by the cable manufacturer based on buffer tube diameter, CM diameter, and other factors. It is typically about 20 inches. The rol point is shown in the photo.
In order to accomplish taut-sheath splicing, the rol point must be identified, and the desired buffer tube unwound. This can only be avoided if it is permissible to cut an entire buffer tube and all of the fibers contained within. This in turn is determined in part by the number of fibers within a buffer tube. For example, if only four fibers from a 12-fiber tube are being dropped off to an optical network unit (onu), then the tube cannot simply be cut. It must be unwound and shaved and the desired four fibers cut. If the tube contains only the four fibers needed for the onu, then the entire tube can be cut (at an extreme or in the middle), regardless of where the rol happens to be.
The rol points can be marked on the cable sheath. Whether this is done or not is a question of cost. Since the rol point is determined well upstream in the cable manufacturing process from the sheath marking point, the manufacturing line must be extremely well controlled. Not all cables have marked rol points, and specifying this feature may carry a cost premium.
If the rol points are marked, then locating the closure is trivial. If they are not, then the sheath must be opened and stripped back in one direction until the rol point is found. In some cases, this will not occur until the end of the closure opening. Since the sheath opening must be extended some distance beyond the rol point to permit unwinding of the buffer tubes, two closures will then need to be ganged. The ability to be ganged and still retain all of the functionality of a single closure is vital to the success of a taut-sheath closure product.
The rol issue becomes more complicated and critical when dealing with high-fiber-count cables. For example, a 288-fiber cable will contain 24 tubes of 12 fibers each. These will typically be laid as an inner layer of 9 tubes and an outer layer of 15 tubes. The rol of the inner tubes will still be 20 or so inches, but that of the outer layer will be on the order of 36 inches. This makes marking the rol points absolutely imperative for taut-sheath applications, since the possibility of a sheath opening of more than 36 inches is unacceptable. (It would require ridiculously large and expensive closures or the ganging of three and four ordinary closures.)
Knowing the location of the rol points is even more important when accessing the inner layer of dual-layer cables, since this can only be done if the outer layer is unwound far enough to expose the rol point of the inner layer. The best situation for taut-sheath splicing would be if the inner layer rol were exactly half the length of the outer layer rol, and the outer layer rol points were marked, so that locating an outer rol point would automatically lead to locating an inner rol point. However, this strategy does not appear to be practiced.
The case of a cable with four fibers per tube cable is worthy of discussion. On the one hand, having four fibers per tube allows one to cut the entire tube when splicing to an onu drop (assuming there are two "working" and two "protect" fibers). In this case, the rol can be anywhere relative to the closure opening. This works with up to 24-fiber cables. Beyond that number, two layers of buffer tubes are needed (presumably up to a maximum of 96 fibers), and now the rol of the outer layer must be marked to give access to the inner layer. However, the location of the inner rol relative to the outer rol is unimportant.
Temperature-induced cable loss
The sheath of a fiber-optic cable is typically made of medium-density polyethylene, which tends to shrink with time as it ages at high temperature. This is a one-time irreversible aging phenomenon and is distinct from thermal expansion/contraction (which is reversible). If sheath shrinkage occurs to the point that the sheath shrinks out of the closure, water can enter unimpeded. To prevent this, a cable sheath termination mechanism must be part of the closure product. The mechanism must prevent cable pullout up to 100 lb, according to criterion R5-4 of GR-771. Since taut-sheath closures tend to be free-breathing and therefore less robust, this requirement may be more difficult to achieve.
A similar aging phenomenon has also been found to occur in the buffer tubes. With exposure to high temperature, the buffer tubes tend to shrink gradually. Then, upon exposure to cold temperatures, thermal shrinkage is superimposed, leading in some cases to contacts between the fibers and the inner wall of the buffer tubes and to large bend-induced attenuation. This phenomenon, known as temperature-induced cable loss (ticl), has been extensively documented by Bell Communications Research (Bellcore). ticl is a possibility within the expressed buffer tubes in balloon splices (see "Fiber retraction in balloon or taut-sheath splices" on page 64). That possibility remains with taut-sheath splices, except that since the total length involved is considerably shorter, the total attenuation produced by this mechanism would be smaller (though the attenuation magnitudes involved are so high that even a fraction could represent a significant problem).
As with balloon splices, correct termination of the central member is important to prevent ticl within the cable, outside of the closures. The difference between the two cases is that correct termination is not difficult for balloon splices, but it may be extremely inconvenient or impossible for a taut-sheath splice. Consider that in order to produce a correct termination, using either the hardware built into a closure or an external integrated sheath/central member clamp, one must first cut the central member (see Fig. 1). However, in a taut-sheath scenario the desire is to locate the rol point and unwind a single buffer tube. In this case the central member is never accessible, so correct termination is impossible. The only solution is to use cable that is resistant to ticl by virtue of a short coupling length. Similarly, a correct termination technique that minimizes the exposed length of the CM is made more difficult by the need to route a large number of buffer tubes around the clamping hardware.
Suppose that the coupling between the buffer tubes and the dimensionally stable CM is more indirect and less effective for the outer layer than the inner layer. While there is no evidence yet to support the supposition, it seems reasonable that the outer layers in a loose-tube cable will experience ticl under more benign conditions than the inner layer. An interesting observation in the construction of dual-layer loose-tube (high-fiber-count) cables is that in order to minimize the need to access inner tubes, the outer layer is filled preferentially, and the inner layer has a combination of buffer tubes and filler rods. For example, a 192-fiber cable may have 15 tubes of 12 fibers each in the outer layer, and one tube plus eight filler rods in the inner layer. This effectively maximizes the number of the most vulnerable fibers, in the context of ticl.
The fundamental solution is to produce cables with inherently dimensionally stable buffer tubes. This question is an area of ongoing research, particularly since the changes that are being made in buffer tubes recently are driven by ease-of-use considerations rather than dimensional stability.
Before leaving the subject of ticl, there is a final issue that is relevant to taut-sheath splicing--remediation. Inquiries in the lab and during post mortems of ticl occurrences reveal that the length of cable affected is quite small, on the order of 10 to 30 ft. Our recommended remediation has in some cases been simply to cut off 10 to 30 ft of slack, re-splice, and re-terminate the cable correctly. Since the whole point of taut-sheath splicing is to eliminate slack loops, this avenue would no longer be available. Thus, not only does taut-sheath splicing increase the likelihood of ticl occurrence, it also makes it more difficult to recover from such an occurrence. The only recourse left would be to splice in a short section of cable, creating an additional splice point.
Balloon splicing would eliminate the slack needed for remediation also. Therefore, anyone who is contemplating balloon and taut-sheath splicing on loose-tube cable would do well to get educated about ticl, specify cables with a coupling length of less than 12 ft, and support Bellcore`s efforts to catalyze the development of inherently ticl-proof buffer tube.
Yield of splicing operations
As with balloon splicing, taut-sheath splicing requires the ability to join fibers in uncontrolled or poorly controlled environments. A major difference, however, is that the penalty for failing to produce a good splice is far greater in the taut-sheath environment. In a balloon splice there is plenty of slack for an additional attempt, or two, or three. Productivity, of course, would suffer and the cost of the eventual splice would be high, but the splice would be accomplished.
On the other hand, failure to make a successful splice when there is only a foot or so of slack to work with could be disastrous. Indeed, the amount of slack available depends on the outcome of a fundamental decision: whether or not to darken the downstream fiber. If this is permitted, then the amount of slack available to make the splice is the entire length of the closure opening or about 20 inches of sufficient but not generous slack (and of course there is unlimited slack in the onu drop or branch cable that is being spliced in). This abandons the fiber on the downstream side, unless a second taut-sheath splice is made downstream. If, however, the engineering decision is made to use the downstream fiber--for example, in a fiber ring build--then the cut must be made in the middle of the closure. This leaves very little slack (about 10 inches) to splice in the onu drop and also to splice onto the fibers that continue past the access point (see Fig. 2).
When working with 10 inches of slack, seemingly trivial considerations suddenly become paramount. Does the fiber need to be re-cleaved to make a second attempt? How much fiber must be stripped in order to cleave? What fraction of cleaves gives acceptable angle? Does the wind guard open toward or away from the craftsperson (see Fig. 3)? The penalty for failing to make an acceptable splice using the 10 inches of slack is that the sheath must be re-opened, a second closure ganged, and so on--in other words, a real mess.
Fusion splices by definition cannot be remade without breaking and re-cleaving the fiber. Mechanical splices tolerate multiple matings to different extents. In addition, cleave angle is much less critical for mechanical than for fusion splices. Indeed, an angled cleave up to about 10 to 15 improves the reflectance performance of mechanical splices.
Another point to consider when selecting a fiber-joining method is that when working in tight quarters with little slack, it is sometimes difficult to bring the fiber ends together for a multistep joining operation. With a field-installable connector or hybrid splice/connector product, all operations are performed on one end only, until the very last step, when the fiber ends are brought together.
Condensation and biohazards
One of the cost advantages of taut-sheath splicing is the use of free-breathing closures. Although there is no inherent link between the two concepts, they seem to correlate, and most, if not all, taut-sheath aerial closures are also free-breathing.
Due to the relatively free exchange of air and humidity, there is every reason to expect condensation to form on the fiber joint. The effect of this exposure to liquid water on the strength of the fiber is critical. A mechanical splice or a fusion-splice protector may not be effective in isolating the fiber from liquid water. A combination of water and high temperature will enable cracks to propagate, particularly if the fiber is under stress, possibly leading to eventual delayed fracture. It is worth noting that some mechanical splice designs do not isolate the bare fiber from stresses applied to the coatings: for example, torsional stress resulting from looping the fiber during slack storage. Fusion-splice protectors vary widely in their ability to withstand exposure to high temperature and humidity.
Another issue with high humidity and condensation is water`s ability to penetrate cracks. The effective vapor pressure of water decreases when it is trapped in a crevice, so it can remain for extended periods. When the temperature drops below freezing, the trapped water expands, forcing the crevice open. This is how rocks become dirt. The same process may operate within mechanical splices, interfering with the fiber alignment that must be maintained indefinitely.
The issue of biohazards arises since any microbes present in the atmosphere would penetrate a free-breathing closure easily. The organic materials used in splices and even the fiber coatings may be affected. A first attempt to deal with this issue is the requirement in Bellcore`s Generic Requirements documents--gr-326-core, gr-765-core, gr-1095-core, and gr-2919-core--that products intended to be deployed in free-breathing closures be tested in the bacteria exposure test defined in those documents.
Underground plant
Having discussed taut-sheath splicing in aerial applications, it is worth considering the concept of taut-sheath splicing in the underground plant. Typically, underground plant access is by means of a slack loop long enough to reach a van parked nearby. This is dictated by the reluctance of most telephone companies to fusion-splice underground. However, due to the value of space in a crowded manhole, this would be an ideal place to consider taut-sheath splicing. Of course, if access to an underground cable passing through a manhole with no slack loops is unexpectedly required, taut-sheath splicing is the only option.
The key issue driving the consideration of taut-sheath splicing in the underground plant is the opportunity to accommodate unanticipated entry (from a new building, for example) to backbone cables at an arbitrary manhole. In the absence of a taut-sheath option, any junction to a backbone cable must occur at manholes where slack coils are positioned, so that the closure can be pulled into a splicing vehicle. In order to make this junction, cable from the new building must be placed from the nearest manhole to the manhole where the slack is located. In the best case, duct space will be available and the ducts will be clean and straight. In the worst case, ducts will be overcrowded or impassable. If this happens, all of the options are poor: Build new duct, lease duct space, or retire the plant occupying the duct. How much easier it would be to simply make the splice in the nearest manhole!
In addition to the costs associated with duct space, other savings accrue from taut-sheath splicing underground. The cost of the extra cable needed to span the manholes is saved, as is the cost of the cable in the slack loops. Although it is difficult to directly associate a cost, the opportunity to save space within crowded manholes is also a positive factor.
A technique available in the underground plant which somewhat eases the constraints of taut-sheath splicing is that of "lofting," or moving small amounts of cable slack between manholes. Such slack is present as a matter of course for cables residing in ducts, and one can generally obtain several feet of slack in the cable between two manholes separated by 1000 ft. This alternative of "loose-sheath" splicing produces a somewhat similar result to a balloon splice, since the sheath opening is greater than the closure dimension and expressed fibers are stored as slack.
The technology that needs to be in place for underground taut-sheath splicing includes hermetic taut-sheath closure. The probability that the closure itself will be exposed to deleterious environments is very high. Many manholes experience periodic flooding with groundwater, fuels from leaking tanks, steam from leaking pipes, etc. The decision to deploy more environmentally sensitive mechanical splices depends on the confidence one has in the closure product and the installation procedures. This confidence can be enhanced by the deployment of monitoring systems either within the manhole or closures.
Also, underground splicing would be required. Mechanical splicing is a good option, but then the splice must be carefully protected against groundwater incursion by the correct application of the proper closure. Fusion splicing has less stringent requirements for protection, but the policy of some telephone companies is to forbid fusion splicing underground. Bellcore has determined that this practice should be permissible if air pumping is performed and gas monitoring is in place. gr-765-core incorporates criteria intended to stimulate the industry to think in this direction--for example, using filament heating, inert gas flush, monitors, interlocks, or sealed switches.
Another aspect of the underground splicing question is that of local work rules regarding manhole guards. In some locations a guard is required to protect the craftsperson underground from the possibility of experiencing a problem which would prevent him or her from exiting. Obvious examples are sudden illness or an accident. There is also the possibility of unbreathable "dead air" and even helium from leak testing. If underground splicing requires two-person teams, then productivity is higher if the closure is brought into a splicing vehicle where a single person can do the job.
Numerous other issues need to be explored before a decision to splice underground can be made. The condition of the manhole will vary widely, from moderately unpleasant to intolerable. Productivity in difficult environments is bound to be low. Many other operations need to be considered, such as the need to disassemble the cable before splices can be accessed. All of the rol issues discussed earlier also come into play for loose-tube cables.
Conclusion
Taut-sheath splicing is emerging as a major enabling technology to continue driving fiber farther into the physical plant. Fiber is already the medium of choice for inter-office and feeder plant, and it will become the medium of choice for distribution also. It is a new but already practiced technology in the aerial plant. It has immediate quantifiable benefits over traditional place-and-loop construction due to the elimination of slack loops. It has other, even larger, benefits, such as the opportunity to enter a cable at an arbitrary point or to effect a repair without splicing in a cable section. These benefits are difficult to quantify since they would vary over a wide range from situation to situation, but are nevertheless real. The aesthetic benefit is also difficult to quantify but can be a strong motivator when local codes forbid storage of slack.
In the underground plant, taut-sheath splicing is in its infancy, faced both with a strong motivator (duct space) and a formidable obstacle (splicing underground).
It is especially important during the transition period from copper to fiber that the discontinuity between the two media be made as transparent as possible to engineering and construction staffs--to allow fiber to be handled "just like copper." The technologies that evolve to allow this will continue to be useful in all-fiber installations.
Key factors that must be in place for success are the marking of rol points for loose-tube cable and the coordination of rol points for dual-layer cable, and robust splicing techniques that produce low-loss splices and high yield in situ, whether in the air or underground. u
Gabor Kiss is a senior engineer at Bell Communications Research (Morristown, NJ). He can be reached at tel: (973) 829-4952, fax: (973) 829-5965, and email: [email protected].