Proactive Network Maintenance: Monitoring DOCSIS 3.1

The evolution of DOCSIS – especially the DOCSIS 3.1 release –  has it made possible to expand the capacity of the network dynamically by introducing new possibilities for cable operators. Managing these new opportunities will require standard monitoring tools as well as new predictive tools that allow them to make appropriate decisions in a timely manner to improve operational cash flow. So let’s talk about monitoring DOCSIS 3.1.

Although there are many aspects of DOCSIS 3.1 worth analyzing, in this post I will share a summary of the measurement possibilities cable modems and CMTS must support according to section 9 of the DOCSIS 3.1 Physical Layer Specification:

The following table shows all measurement possibilities grouped by the test point function.

DOCSIS 3.1 Physical Layer Specification

Monitoring DOCSIS 3.1

Here’s an explanation for each item of the table according to the specification mentioned above:

Downstream PNM Requirements:

  • Downstream Symbol Capture: It is being measured in the Cable Modem and the CMTS. It provides partial functionality of a network analyzer which represents how a symbol is transmitted by the CMTS and how it is received by the cable modem. Using this, the exact quality of the transmitted and received DOCSIS signal can be determined and compared.
  • Downstream Wideband Spectrum Analysis: It allows a downstream wideband spectrum analyzer function in the DOCSIS 3.1 CM similar to the capability provided in DOCSIS 3.0. It gives the CM the opportunity to be the end-of-line probe, which allows checking the real spectrum received by each subscriber. A quick look at this can determine service problems, as there are direct translation of known issues to spectrum figures and shapes.
  • Downstream Noise Power Ratio (NPR) This Measures the noise, interference and intermodulation products underlying a portion of the OFDM (Orthogonal frequency-division multiplexing) signal. This allows determining the amount of available dynamic range. Previously NPR testing was only available in the lab.
  • Downstream Channel Estimate Coefficients: The purpose is for the CM to report its estimate of the downstream channel response. Each coefficient represents the gain of each tap in the adaptive equalizer.
  • Downstream Constellation Display: Provides received QAM constellation points for display. Each dot in the constellation represents a symbol.
  • Downstream Receive Modulation Error Ratio (RxMER) Per Subcarrier: This item provides measurements of the receive modulation error ratio (RxMER) for each subcarrier. The higher the MER, the cleaner the received signal.
  • Downstream FEC Statistics: Monitor downstream link quality via FEC and related statistics.
  • Downstream Histogram: Measurement of nonlinear effects in the channel such as amplifier compression and laser clipping.
  • Downstream Received Power: Measures the average received downstream power in a set of non-overlapping 6 MHz bands for any DOCSIS 3.0 and 3.1 signals in the receive channel set (RCS) of the CM including the DOCSIS 3.1 PLC.

Upstream PNM Requirements:

  • Upstream Capture for Active and Quiet Probe: It measures plant response and views the underlying noise floor, by capturing at least one OFDM symbol during a scheduled active or quiet probe. An active probe provides the partial functionality of a network analyzer since the input is known and the output is captured.
  • Upstream Triggered Spectrum Analysis: Provides a wideband spectrum analyzer function in the CMTS which can be triggered to examine desired upstream transmissions as well as underlying noise/interference during a quiet period.
  • Upstream Impulse Noise Statistics: Gather statistics of burst/impulse noise occurring in a selected narrow band as defined in [DOCSIS CCAP-OSSIv3.1].
  • Upstream Equalizer Coefficients: This item provides access to CM upstream pre-equalizer coefficients, and CMTS upstream adaptive equalizer coefficients, which taken together describe the linear response of the upstream cable plant for a given CM.
  • Upstream FEC Statistics: Provide for monitoring upstream link quality via FEC and related statistics.
  • Upstream Histogram: Measurement of nonlinear effects in the channel such as amplifier compression and laser clipping.
  • Upstream Channel Power:  Estimate the total received power in a specified OFDMA channel at the F connector input of the CMTS line card, or other agreed measurement point, for a given user.
  • Upstream Receive Modulation Error Ratio (RxMER) Per Subcarrier: This item provides measurements of the upstream receive modulation error ratio (RxMER) for each subcarrier.The higher the MER, the cleaner the received signal

Sources & further reading:

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