The Tap Coefficients graph shows the strength of equalization corrections made to every tap on a modem. The x-axis shows time relative to the "main" tap. The y-axis shows the range of decibel values for all taps. There are usually 24 taps, represented by vertical bars on the graph.
- Main tap
- The time at which the modem transmits its energy. This tap should reflect the highest energy transmission level. The main tap is labeled "M" and generally occurs as the eighth tap in a set of 24.
- Pre-main taps
- Taps left of the main tap, which occur in time before the main tap. These are labeled with negative numbers.
- Post-main taps
- Taps right of the main tap, which occur in time after the main tap. These are labeled with positive numbers.
You can hover over any post-main tap to display more information about the tap.
- The position of the tap relative to the main tap (@ T+ [a number] where T is the main tap)
- Distance from the main tap based on a 3.2 MHz channel width
- Distance from the main tap based on a 6.4 MHz channel width
Each shaded line on a tap represents an issue, or a CMTS correction occurring at a specific frequency (y-axis) at a given time (x-axis). Modems that have similar corrections generate issues that are grouped together. Correction similarity is defined relative to a comparison threshold applied to the fiber node echo cavity. A higher threshold requires modems to be more similar before issues are grouped together, limiting the size of an issue group. A lower threshold will group more issues together, but a too-low threshold can collect too many issues in a group, making troubleshooting difficult.
You can filter the issues shown on the Tap Coefficients graph to show only those issues occurring at a specific frequency. When you display issues for only one frequency, the echo cavity comparison threshold defined for that frequency is shown by a red line over the post-main taps. Issues closest to or above this threshold generally indicate the most significant echo problems.
To filter issues shown according to frequency, click one of the frequency labels shown above the Tap Coefficients graph.