Fiber Cable Length and Glass Length

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. If you were to take out a fiber strand and lay it flat, the strand would be longer than the cable it came from.

Both cable and glass lengths are stored at the fiber cable feature class level. The cable length represents the physical length of the cable. The glass length, the distance light travels inside the cable, is calculated by multiplying the cable length by the twist factor.

Cable Length

There are four ways to calculate the cable length. The method you use depends on what information you have from the field. The chosen method may vary among cables; it is recommended to set the most common method as the default when building a fiber favorite.

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. Example: If a cable drawn on the map is 3,000 feet long and there are 2 slack loops where each loop contains an additional 75 feet, the full 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. Example: If the mark at the beginning is 3,000 and the end is 6,500, the total 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. If the measured length recorded from the field crew is 3,150 feet, this value is recorded in your cable length field.
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. This calculation is different from Documents/Field Notes because the value includes the calculated glass length. The same Field Note or OTDR Length field is used for entering this value.

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 this method, the glass length = the cable length entered.

Splitting an Existing Cable

When you split an existing cable (with a new transition point, 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 attributes for the two sides to input crew-noted lengths, if necessary.