A Comprehensive Scope of Work: Fiber optic cable plant design questions

Published On
Apr 15, 2021

A city manager called the Fiber Optics Association for advice regarding a fiber optic network his organization was planning. They were developing the scope of work and a request for proposal on a fiber optic network that involved about a 15-mile-long (25 km) cable run with a dozen locations where drop cables would be needed. The backbone cable was planned to be 192 fibers and each drop was only a four-fiber cable. He had several technical questions.

He wanted to know if it was necessary to plan for 194 splices at each of the 12 locations—190 fibers plus the four fibers for the drop cables. (With a four-fiber drop cable, you can drop two fibers and two continue on a ring network. Then you splice two fibers of the drop cable to two incoming fibers and the other two to the same two fibers continuing down the cable, which totals 194 splices.)

I asked if he had ever heard of the term “midspan access,” a technique that would allow only making four splices at each location. He had not, and that made me wonder. I bet quite a few managers and supervisors, and even some installers, are not aware of this time- and cost-saving process.

Midspan access refers to the technique of opening up the jacket of a fiber optic cable, separating just the fibers to be connected to a second cable (usually called the drop cable) and allowing all other fibers to pass through undisturbed. The only splices that need to be made are on the fibers being connected to the drop cable.

The basic process is simple, so I’ll use a loose tube cable as an example.

The first step is to remove enough length of the jacket from the cable to separate the tubes of fibers. You need to open a fairly long section of the cable and have enough length of buffer tubes to coil the pass-through tubes and secure them in a splice closure underneath the splice tray. The actual length will be determined by the cable design and size.

The cable’s center strength member must be cut off to fit the cable in the splice closure, but sufficient length needs to be left for securing the cable to the closure.

The tube with fibers to splice is opened with a special tool that shaves off part of the tube to allow accessing the fibers. Then the proper length of the tube is cut off, which leaves bare fibers. These fibers are carefully placed in a splice tray.

The drop cable is then prepared for splicing and the fibers placed in the same splice tray. The drop fibers are separated, prepared and spliced to the backbone fibers. All other fibers are secured in the tray undisturbed. The splice tray is ready for closure.

Midspan access is really that simple, widely used and obviously a big time- and cost-saver. If another drop is needed at the same point in the future, the closure can be opened, and more fibers spliced to the fibers in the backbone cable.

This is a hard process to describe in words or visualize, so I’ve created an online guide to midspan access. If you are looking for more information, see “How to Calculate Fiber Optic Power and Loss Budgets."

15-mile-long cable

The caller had another question about his cable plant design. He wanted to know, could he get a length of cable 15 miles long, or would he need to splice together several reels of cable?

I had to discuss that question with cable manufacturers, because it’s a matter of cable type, size and weight on the reel. For his 192-fiber backbone cable, the longest length of cable made would be about 3 miles (5 km), so he would need a minimum of 5 segments where he would have to splice together all 192 fibers.

Since the network had 12 drops, it made sense to make the splices between lengths of cable coincide with the drops to share manholes or handholes and not install any extras. That led to a further discussion on how to plan that, given the need for extra lengths of cable for service loops and the midspan access points. That was going to keep him busy for a while.

Questions like this arise all the time when designing a fiber optic network. Many design issues are not independent; simply designing around where the cable goes and where drops or junctions are to be located is not the end of the design. The design is also dependent on the type of installation (aerial or underground) and the types of components chosen. Networks may require special installation processes such as midspan access.

That’s why it’s important to develop a comprehensive scope of work. Then pass it around to others, including potential contractors, to get their feedback.

About the Author

Jim Hayes

Fiber Optics Columnist and Contributing Editor

Jim Hayes is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com.

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