Fiber Length Options

In Fiber Manager there are two categories of length: cable length (also known as sheath length) and glass length. Inside a cable, the fibers twist around a central core, and this twist adds length to the individual fibers. In other words, if you were to take out a fiber and lay it flat, it would actually be longer than the cable it came from.

Both cable and glass lengths are stored at the fiber cable feature class level. In short:

• Cable Length: the physical length of the cable

• Glass Length: the physical length of the cable multiplied by the twist factor, or in other words, the length the light travels inside the cable

Cable Length

There are four ways to calculate the cable length. No method is necessarily better than another, it simply depends on what information you have from the field.

• GIS Length + Slack Loop Length

• This method takes the length of the cable as drawn in the GIS and adds any length stored in slack loops, risers, or other point features that represent additional cable. For example, if the cable drawn on the map is 3,000 feet and there are 2 slack loops each containing an additional 75 feet, the cable length would be 3,000 + 75 + 75 = 3,150 feet.

• Footstamps

• This method depends on data entry in the Start and End Cable Length Mark fields. Fiber cables are typically stamped with measure marks on the protective, outer sheath. As the crew is installing cable, they note the measure at the beginning and the end of the installation. This calculation method simply takes the difference between the two numbers. For example, if the Start is 3,000 and the End is 6,500, the cable length would be 6,500 – 3,000 = 3,500 feet.

TIP: The application does not care whether the End is greater than the Start or vice versa. The final difference is the absolute value, never a negative number.
• Documents/Field Notes

• This is similar to Footstamps, but it is a single data entry. In other words, the field crew simply wrote down the length of cable installed.

• OTDR Length

• This is the same concept as Documents/Field Notes, but in this scenario, the crew performed an OTDR Trace and wrote down the length result from the device. Where this calculation differs from Documents/Field Notes is in the calculated glass length, which is covered below.

The cable length calculation method is set at the feature class level. The chosen method can vary among cables, but it is recommended to set the most common method as the default in the fiber cable favorites.

Glass Length

As stated above, the glass length is a calculated value depending on the cable length and twist factor.

The twist factor is typically provided by the cable manufacturer, and it is entered in the Twist Factor field on the fiber optic cable feature class.

The Glass Length depends on the Cable Length method used for the particular cable:

• GIS Length + Slack Loops, Footstamps, and Documents/Field Notes

• All three of the these methods use the same final calculation: cable length x twist factor. They simply arrive at the cable length part of the equation differently. For example, if the cable length is 3,500 and the twist factor is 1.04, the glass length would be 3,500 x 1.04 = 3,640 feet.

• OTDR Length

• This length comes from an OTDR trace, which is a trace of light length in the cable. In other words, the OTDR length already factored in the twist. Thus, in this method, the glass length = the cable length entered.

Splitting an Existing Cable

When you split an existing cable (with a new splice case, for example), the application uses the GIS Length + Slack Loop method by default to arrive at the lengths of the cable before and after the split. After the split, you can then edit the two sides in the Attribute Editor to input crew-noted lengths, if necessary.