Back to Pathway Basics: Defining Fill, Capacity and Usability
Typically, when we look for references and resources, we search through recent memories and history because older technologies can become obsolete and reach end of life quickly. Unfortunately, this also means we sometimes mistakenly avoid sources of tribal knowledge or information that may be “old” in terms of when it was first established but is still accurate.
Sometimes, the industry likes to place its focus on what’s new in terms of technology: blinking lights, feeds and speeds. But just like any high-performance network relies on a strong infrastructure, a high-performance cabling infrastructure relies on a usable pathway system to support it. The pathway system may not have blinking lights or boast super-fast speeds, but it’s absolutely necessary for your cabling infrastructure.
If a pathway system isn’t usable, then issues will go far beyond not being able to get a cable from point A to point B: The performance of the cabling can be negatively impacted, which then impacts the performance of your network.
Pathway Basics: Fill, Capacity and UsabilityWhether you consider standards, best practices or manufacturer guidelines, we have access to vast numbers of resources. There are countless charts, graphs and tables that tell us what we can and can’t do—but sometimes it’s good to review the why behind these recommendations.
Charts, graphs and other resources are good starting points, but they’re just that—starting points. Site/project requirements and conditions must be considered and may sometimes take precedence over what the industry’s charts, graphs and tables may tell us.
In future blogs, we’ll discuss pathway design and installation. For now, however, let’s go over some basic language. Understanding these terms is key to getting pathways right. From the ICT perspective, pathways are designed to house, support, sometimes protect and, at times, act as a conveyance for cabling. These are common terms that help describe their capabilities.
Pathway FillWe often use or hear the term “fill”; more correctly, however, we should be saying “fill ratio,” which is the ratio of the cross-sectional area of cabling in the pathway to the cross-sectional area of the pathway itself.
“Fill” is often used to describe “capacity,” such as: “Is the pathway full?” In common English, “fill” and “full” are obviously related (“full” is an adjective that means “completely filled”); however, in ICT applications, we need one set of shapes to fit into another set of shapes—and these shapes that may or may not have similar geometry. (Example: Round cables fitting inside rectangle trays.) For that reason, “fill” doesn’t always equate to “full.”
If one square cable was placed inside a square tray, and the tray was there only for support—not for protection or conveyance—then we could achieve a fill ratio of close to 100%. A specious example, surely, but it’s an important one to make to demonstrate the need for a more realistic way to address the demands of pathway design, installation and use.
In practice, fill ratio is far less than 100%—it’s closer to 40%—and is impacted by multiple factors, including application type, future needs, ambient temperatures, etc. The percentage represents the usable area filled by cables, including the gaps between cables.
Pathway Capacity“Capacity” refers to the pathway’s ability to accommodate cables. Capacity is also a ratio but only from the minimum number (zero) to the maximum number of cables in a pathway. The maximum number can be a function of the current situation or future planning and depends on the type of cable installed in the pathway.
Pathway Usability“Usability” refers to the ability to install cables in a pathway so the cables function as expected. This applies to new cabling being installed, as well as any cabling that may be already in place, whether that pathway is used merely as a support/lay-in or as a conveyance.
We have lots more to say about pathways, so make sure you subscribe to our blog. In the next few months, we’ll talk about types of pathways, including unique pathway types, as well as fill-ratio examples, installation and what TIA and BICSI have to say about pathways.