The Comparison Between Air Blown Fiber Systems and Conventional Cabled Optical Fiber Systems
:: What Is Air Blown Fiber?
Air Blow Fiber (ABF) systems claim to offer reduced cost, increased design flexibility and other advantages that cannot be matched by conventional optical fiber cables.
ABF systems originated in 1982 at British Telecom. The intent of the design was to easily accommodate moves, adds, and changes with minimal disruption. The ABF system was developed to allow switching between fiber types as they evolved.
ABF systems consist of a network of tube-cables installed between locations, with fiber blown into the tubes only as needed. ABF systems claim to offer cost advantages over conventional cabling systems in two ways. First, the decision to purchase fiber can be postponed until the fiber is actually needed. Second, it eliminates the need for splicing and interconnection points.
:: Components of the ABF System
There are four components to the ABF system: the tube-cables, the blowing apparatus, the optical fiber bundles and the connecting/termination hardware.
The tube-cables consist of multiple individual tubes, which are bundled together in a single sheath. If installed indoors, the tube-cables must comply with the appropriate flame rating for the area in which they are installed – the same way innerduct must comply with appropriate flame ratings.
To install a fiber between two locations, there must be an available tube in a tube-cable between those locations. The tube-cable versions can range in size from 5mm for a single-tube to 53mm for a 19-tube model.
Naturally, more tubes increase the size of the tube-cable, which increases the corresponding bend radius of the tube-cable.
Special connecting hardware called tube distribution cabinets is required everywhere the tube-cable branches to another location and at the end of every length of tube-cable.
Additionally, special push/pull connectors are needed to connect tubes from one tube-cable to another tube-cable.
Once the tube-cables are in place, the installer must test them to ensure integrity and continuity. These checks are conducted as tube-cable installation acceptance criteria, but must also be conducted prior to installing the fiber.
The integrity check verifies that the tube-cables are able to hold the pressurized nitrogen or compressed air used to propel the optical fiber through a tube.
To check for integrity, nitrogen is pumped into each tube, the tube is capped, and 2-man teams with pressure gauges check the pressure on both ends of each tube. If the pressure is the same at both ends and there are no other indicators of a problem (hissing, etc.), the integrity of the tube is acceptable. If the tube will not hold pressure, either the problem must be fixed or the tube must be capped and labeled to prevent it from being used.
The continuity check also ensures that connections between tubes actually lead to the desired location. Most ABF system supplies recommend the continuity check be conducted by blowing a small pellet from one location to another.
If the pellet is not received, it means one of three things.
- The tube is kinked and will not allow the pellet to pass
- The tubes have been incorrectly patched and the pellet exited in the wrong location
- Dirt, insects, rodents or other foreign material blocked the empty tube
Regardless, the problem must be fixed or the tube must be capped to prevent it from being used.
Blowing the Fibers
When integrity and continuity checks are completed, the installer is ready to install the fibers. During installation, compressed air or nitrogen is used to propel fibers from one location to another.
To make this possible the 2-, 4-, 6-, 12- or 18-fiber (typical) bundles are manufactured with a plastic skin that has special friction properties. A blowing tip is attached to the fiber bundle to aid in the installation. The blowing tip (an additional expense) is placed into a viable tube, and the specialized blowing apparatus creates a wave of air that carries the fiber from one location to the desired location.
Published commercially available lengths for the fibers and fiber bundles range from 2.1 to 4.2 km, depending upon single-mode or multimode fiber selection. Therefore, depending upon the route length, fibers may need to spliced along the route. This requires the use of more hardware and the use of a fusion splicer. At each end, the fibers must be broken out of the skin and furcated to prepare for connectorization.
Finally, the fiber are terminated and placed into termination hardware, and the link is tested to determine the loss in the link.
:: Conventional Fiber Optic Cable
Optical fiber cables are installed from one point to another. Even cables with up to 864 fibers have diameters less than 25.4mm (1 inch) and are available in lengths from 4 to 12 km depending on whether the cable is indoor or outdoor and whether it is single-mode or multimode fiber.
In contrast, an ABF cable with 288 fibers will have a diameter of 43.2mm (1.7 inch) and tube-cable technology cannot even support higher fiber counts.
The optical fibers are already installed in fiber cable, and the cable can be loose tube, stranded single-tube, ribbon or tight-buffered indoor cable, depending on the customer’s preference.
Once the cable is pulled throughout the pathway, the fibers are furcated, terminated and inserted into passive hardware. The fiber is then tested for end-to-end attenuation, and Optical Time Domain Reflectometer (OTDR) traces are taken to ensure the cable and fiber optic joining points were installed properly. After any problems are identified and repaired, the conventional fiber optic cable system is ready to be used.
If moves, adds or changes are necessary, the cabling system is reconfigured at the cross-connect hardware. If a cable is not already at the new location, a new cable is simply installed either from a consolidation point, a multi-user telecommunications outlet assembly (MUTOA) or the nearest telecommunication room. The system for the new link is tested for end-to-end attenuation and OTDR traces to ensure the system was installed properly.
Accounting for moves, adds and changes with ABF is far more complex, expensive and time-consuming.
:: Cost Comparison between ABF and Conventional Fiber Cables
Following is an example comparing the cost of ABF to conventional cabling using 1000 feet in riser space – an ABF tube-cable with four tubes, each with six 62.5/125um fibers to meet current needs compared to a conventional OFNR optical fiber cable with 24-fiber 62.5/125um as a future-proof overbuild. The cost breakout is below:
Air Blown Fiber Cost
1. Materials – 4-cell tube-cable, tube-cable/tube connectors, 12 blown fibers and termination materials (24 each) $3,922
2. Labor – Installing a 4-cell tube-cable, fiber installation, termination and testing $1,654
3. Total: $5,576
4. Total cost per installer fiber: $464.67
Conventional Fiber Optic Cable Cost
1. Materials – 24-fiber 62.5/125um OFNR cable, termination materials (48 each) $3,547
2. Labor – Installing cable, fiber termination and testing $1,940
3. Total $4,987
4. Total cost per installed fiber $207.79
The indoor cable example illustrates that the conventional cable installation cost is significantly less per fiber, considering the fact that 24 (not 12) fibers have been installed and terminated. The cost per fiber of the ABF system is $464 versus $208 for a conventional cable system.
The installed and terminated conventional cable is ready for use when the system needs to be expanded, whereas the ABF system would require a crew to perform the tube integrity and continuity test to ensure that the tubes have not become blocked or kinked. Once the tube is validated for integrity and continuity, additional fiber may be blown in to expand the system.
The additional testing and fiber installation will likely extend the completion time of the move, add or change to the network, and each time a network change must be made, a crew has to be called in to perform all these steps for the ABF network.
Not only are all of these steps necessary, there is the additional risk of network disruption each time someone is testing or blowing fiber in if the existing fiber, tube or tube-cable is disturbed during testing or installation.
:: Outside Plant Considerations
In recent years, ABF manufacturers have developed outdoor tube-cables. The outdoor tube-cables have waterblocking materials between the tube-cable core and outer jacket.
However, there is no waterblocking technology inside the individual tubes. So the tube-cable would not pass the fluid penetration requirements specified in ICEA-640, when tested to Fiber Optic Test Procedure (FOTP) 82, “Fluid Penetration Test for Fluid-Blocked Fiber Optic Cable.”
This requirement requires a 1-meter length of cable to withstand a 1-meter static head or equivalent continuous pressure of water for 24 hours without leakage through the open cable end.Conventional outside plant fiber optic cables are designed and meet this requirement.
Additional problems may be encountered over the lifetime of the ABF cable. For example, since the empty tubes are hollow, insects may easily migrate from the tube distribution cabinets into the tubes and block the future ABF pathways.
Other problems relating to the continued reliability and integrity of the tube-cable system may include loose tube connectors and kinked tubes.
:: Other ABF Viewpoints
ABF proponents claim that the flexibility of this technology is unmatched. Unfortunately, any flexibility in the system is a direct function of the up-front planning and spending. If a tube-cable is not installed between points, fiber cannot be installed between desired locations.
One stated benefit of ABF systems is the ability to blow out previously installed optical fiber and blow in new fiber(s) if necessary while reusing the original fiber bundle. While the ability to blow out existing fiber appears to be an attractive option, it is difficult to imagine this being practical.
Anticipating growth, the designers most likely included spare tubes to each location. If fiber is blown out, it must be reinstalled in a location that is shorter than the original location. Even if this criterion is met, an obvious disconnect remains, which is using fiber in a new location that was deemed unsuitable for its original location.
The final flexibility argument revolves around hub placement. ABF designers and installers indicate that in order to comply with the TIA/EIA-568-B.1, all cables must originate from one central point of administration. In the ABF system, if another site makes a more logical hub, relocate the hub. The implication is that this is not possible with optical fiber cabling systems.
In reality, this is a greater strength with conventional cabling than with ABF. If a hub is more suited to another location, place it there. This can be done at any time and for the minimal expense of the cable installation. There is no need to plan the potential of a new hub and pre-install expensive hardware and pathway material.
Nagging questions remain regarding ABF technology. Specifically, there are standards questions, reliability questions and sourcing questions that need to be answered. ABF installations rely on a technology that is not addressed in any domestic industry standards.
The tube-cables with the installed fiber do not meet the cable requirements of TIA/EIA-568-B.1. (TIA/EIA-568-B.1 refers to Insulated Cable Engineer Association (ICEA) S-83-596, “Standard For Fiber Optic Premises Distribution Cable,” and ICEA S-87-640, “Optical Fiber Outside Plant Communications Cable.”) Field fabricated cables are neither recognized by TIA/EIA-568-B.1 nor by any of the cable standards recognized therein.
With no standards in place to govern the performance of the cable, reliability questions remain unanswered. There are no test procedures defined to test the long-term reliability of the tube-cable, tube or installed fiber. With that in mind, end-users must determine in advance exactly who is responsible for warranting the use of the pipe system after the installation is complete.
Finally, there are only a few manufacturers offering ABF systems in the United States today. An end-user purchasing an ABF system is now dependent upon their proprietary source for product, pricing and performance. The up-front investment in an ABF system makes it difficult to justify a return to conventional cabling later, and since it is a proprietary (often sole-source) solution, there is no incentive to drive competitive pricing.
This lack of competitive pricing not only applies to the supply of cabling products and hardware but also to the cost for installation services since there are virtually only a few installers nationwide who are certified installers and offer an extended manufacturers warranty on the installed product.
From 1997 to 2002, the ABF fiber pricing has decreased by 36% in contrast to the conventional cabled fiber price decrease of 60%. The reason for this large difference is clearly the result of increased competition and pricing pressure based on free market economics at work on the conventional fiber cable technology compared to the lack of such free market competition on ABF technology.
:: What Is the Conclusion?
With the above questions hovering unanswered and with serious issues surrounding claims of reduced cost, increased flexibility and reliability, ABF is certainly not an approach to be chosen lightly.
Conventional optical fiber cabling is a proven, standards based design that can easily meet the requirements of the most demanding customers, at a significantly lower cost, well into the future.