Air Blown Optical Filter Network Delivers ÒFuture ProofÓ Supervisory Control System
Planning and building optical fiber local area networks (LANs) capable of accommodating unknown future capacity and routing requirements have long vexed information systems and information technology managers. The primary cause of this vexation is the high labor cost to construct or modify conventional systems. Cost components outside the optical fiber itself include installing underground conduit, installing the innerduct that carries fiber throughout the building envelope, pulling the fiber optic cable(s) through the conduit and innerduct, then splicing the fibers to form the network.
The Metropolitan King County West Point Wastewater Treatment Plant used a new approach to install its optical fiber network during an upgrade of the primary treatment plant coincident with a $574 million modernization program. A technology called FutureFlex® Air Blown Fiber (ABF) allowed the installation of a fiber capacity to meet present requirements, while giving the flexibility to add to or modify the network in the future. (See the sidebar article titled, "ABF System Components.")
The installation at the 32-acre site on Puget Sound was completed by The Light Brigade, based in Kent, WA.
Options Until the availability of ABF, administrators designing or upgrading LANs generally weighed the relative merits of copper wire (generally category 5) and optical fiber. For the West Point plant, copper was ruled out based on previous experience (i.e., systems being taken out by single lightning strikes traveling through wire to other plant areas).
The installation of conventional loose tube fiber optic cables was also judged unacceptable after an in-depth study by our engineering staff. This was because sections of available conduit at our existing facility were undersized and unable to accommodate additional fiber, while the expanded plant had no conduit installed prior to our setting the contract. It was estimated that installation costs of conduit in a redundant configuration would be $550,000. This did not include the cost of installing loose tube fiber, innerduct and repeater stations. The cost saving by using ABF in eliminating the conduit runs was enough to pay for the contract. Additional hardware installations (repeaters, power sources and innerduct) would have extended costs beyond the budget for the project. In addition, our engineers determined that the "standard proven technology" of a loose tube fiber optic installation would add to the complexity of the network, increase its maintenance costs and reduce its reliability.
ABF also gives the flexibility to accommodate future requirements with relative ease. Conventional systems use innerduct in the conduit. When using innerduct, care must be taken not to exceed its fill ratio. Because of this, the general practice is to overbuild a system in order to avoid installing additional fiber later. However, this means that fibers may be unused. Furthermore, even if these fibers are called into duty, their reliability can be questionable because of degradation over time or unknown damage during installation. Finally, any modification to a conventional fiber system is a time-intensive operation that frequently calls for a system shut-down or other interruptions to the workplace.
ABF avoids these concerns by permitting users to install the fiber that is needed, when it is needed, simply by installing tube cable with spare tube cells. When expansions to the network are required, new fiber is blown in. These activities can be accomplished without interrupting plant operations.
Network Overview The Supervisory Control System (SCS) is the computerized communications system of the West Point Wastewater Treatment Plant that allows control stations distributed throughout the facility to communicate process status information to operations personnel.
The optical fiber network carries continuous data among 2 DEC Alpha servers, 4 DEC 4000-90 hosts, 20 DEC 4000 workstations and 11 Siemens programmable logic controllers. The optical network is composed of multiple runs exceeding 2,000 feet and more than 1,000 termination points. Approximately 15,000 feet of tube cable was installed, through which 132,000 feet of 62.5/125 µm multimode fiber were blown using nitrogen at 100 psi. The time required to blow in the longest run in the plant (approximately 2,200 feet) was about 15 minutes.
Plant specifications were written to accommodate expansion. Tube cables containing 2, 4, 7 and 19 cells were installed, with parallel runs specified for redundancy.
Modifying the Network The real payoff or justification for an ABF system is its ability to accommodate changes to the network. All guesswork elements are removed except selecting tube bundles with the required number of cells. However, this point is moot, since fiber can be blown out of tube cells as quickly and easily as it can be blown in, making way for new fiber. Moreover, fiber that is removed can be used elsewhere in the network.
The 3-step process for making modifications at the West Point plant is (1) tap into the existing tube cable segments at the closest tube distribution unit, (2) couple the appropriate tube cells together to either direct or redirect the fiber bundle to the new location and (3) blow in the new fiber bundle.
Modifications are accomplished quickly. New fiber can be installed, or damaged fiber replaced, in a half day or less.
Economics Network complexity is a key factor in deciding whether to adopt a conventional or air blown fiber system. In instances where straight-run installations are required, economics may favor a conventional system. As routes become more complex (typical of our requirements) economics will favor the ABF technique. The most significant factor is measurable savings realized when the costs of future changes are figured in. These costs were key in our decision to adopt this technology.
About the Author: Philip M. Daniels is the Supervising Engineer for the Water Pollution Control Division of Department of Natural Resources, Metropolitan King County, Seattle, Washington.
Table 1: Ten Steps to Installing an Air Blown Fiber System
- Establish current point-to-point fiber requirements based on existing network topologies.
- Determine fiber bundle counts and routes based on current point-to-point requirements (from step 1).
- Determine tube cable cell counts required for each cable route by multiplying the current fiber bundle requirements (from step 2) by a pre-determined growth factor (typically 3 to 5).
- Determine tube cable types needed based on installation environment and tube cable cell count requirement (from step 3).
- Install tube distribution hardware at tube interconnection locations and fiber termination hardware at fiber termination locations.
- Install tube cables.
- Connect requisite cells in tube distribution hardware to establish point-to-point tube routes (from step 1).
- Perform end-to-end cell pressure test and obstruction test.
- Blow fiber bundles into the tube routes (from termination point to termination point).
- Terminate and test the fiber.
Tube cables contain up to 19 individual coded tube cells, and are constructed of a durable material that can be routed through underground conduit or in plenum, riser or general purpose indoor and outside plant applications. During installation, no fiber is in the tube bundles.
Point-to-point optical fiber paths are defined by using push-fit tube couplers to interconnect individual tube cells in a "junction box" (tube distribution unit).
Once a given route is established, fiber bundles containing up to 18 fibers (single mode or 50/125 µm or 62.5/125 µm multi-mode) are installed by blowing them through the cells using compressed air or nitrogen. Fiber bundles can be installed at rates to 150 feet per minute. Standard blowing distances are 3,300 feet for 2- to 6-fiber bundles and 1,650 feet for 12- to 18-fiber bundles. Distances can be doubled by running two sets of blowing equipment in tandem.
Optical fiber bundles used in the ABF system are about 1/40th the size (and substantially lighter in weight) than those used for conventional systems. This is because there is no need for strength elements to resist the strains and stresses encountered when pulling fiber through conduit and innerduct.
In addition to its comparatively small size and light weight, the fiber bundle has a jacket designed to resist friction with tube cell walls and to provide an aerodynamic characteristic similar to dimples on golf balls. These dimples catch the current of air in the tube, propelling and supporting the fiber as it "floats" to its destination.
Splicing and connectorization - major cost elements - are not required at tube distribution units because the installation is point-to-point as defined by the tube cell path. A splice-free installation improves overall system reliability and provides optimum fiber performance.