Data-Over-Cable Service Interface Specifications (DOCSIS) version 3.1 offers a host of exciting possibilities and realities for cable operators. I have covered many of these topics in previous articles but the one thing that cannot be overlooked is the basics. It doesn’t matter how great these new technologies are if you do not pay attention to the foundation. It’s not any different than a house. If your foundation isn’t solid who cares if you have the best theater system in it. It can crumble under foot if the foundation is not any good.

As an industry we are fortunate to have new technologies such as DOCSIS 3.1 at our disposal to support the ever-increasing demand of subscriber traffic growth. However, many operators are still using single carrier quadrature amplitude modulation (SC-QAM) channels in the return path either due to budget constraints or just for simplicity. This means return path optimization is necessary in order to increase return speeds, keep modems online and improve subscriber quality of experience (QoE) as well as to ensure the DOCSIS 3.1 downstream is working optimally. This “back-to-basics” article covers some of the fundamentals DOCSIS experts should be putting into practice in order to ensure their upstreams are optimized.

Pre-equalization

At the top of my list is making sure that pre-equalization is enabled on every return port of your cable modem termination system (CMTS). We enable pre-equalization because it allows the CMTS and modems to overcome many upstream impairments simply by sending updates to modems so that they can predistort their radio frequency (RF) transmissions to adapt for the RF impairments. The CMTS sends correction signals to each cable modem during station maintenance in the form of a range response (RNG-RSP) message – [see section 6.4.6 of the DOCSIS 3.1 MUPLI Specification for details on station maintenance]. With pre-equalization enabled, a cable modem’s pre-equalizer can compensate for significant upstream group delay and micro-reflections in the return path.

What we believe: pre-equalization has been enabled on the CMTS and every cable modem. The reality: often times some upstream ports are overlooked and pre-eq is not enabled AND more frequently other configurations are not correctly enabled causing modems to have pre-eq disabled or improperly working.

Here are four must do items for ensuring pre-equalization is working in the upstream:

  • All modems must be DOCSIS 2.0 or higher (preferably DOCSIS 3.0 or higher for optimal load balancing and channel utilization)
  • Upstream must be using mixed mode TDMA/A-TDMA or preferably A-TDMA only If the upstream is configured for TDMA then cable modems will operate in DOCSIS 1.x mode, which has limited or no support for pre-equalization in modern modems
  • Config files sent to cable modems must have DOCSIS 1.1 QoS parameters enabled Legacy DOCSIS 1.0 QoS config files are sometimes not updated to reflect DOCSIS 1.1 QoS configurations leaving the advanced features in DOCSIS 2.0, 3.0 and 3.1 modems deactivated
  • Finally, ensure that pre-equalization is enabled on each upstream port of the CMTS—this can sometimes be overlooked, especially when activating new ports or adding new line cards

    Upstream Modulation Profiles

    The next recommended optimization that can improve upstream data throughput is a robust upstream modulation profile (mod-profile). What makes for a robust upstream mod-profile? It depends on the amount or return path ingress in the system, which in turn impacts the upstream modulation error ratio (MER). Upstream MER is often displayed as “SNR” on some CMTSs and monitoring systems, however this is really just MER and not an actual signal-to-noise ratio (SNR) measurement. It is important to understand this because MER is a much more accurate measurement of the digital information contained in an RF modulated signal once it has been demodulated back to baseband.

    It is generally recommended to stick to one of three upstream modulation profiles: quadrature phase shift keying (QPSK), 16-QAM or 64-QAM. While DOCSIS provides for other modulation levels, such as 32-QAM, the tradeoff between faster data rates and higher MER requirements in going from 16-QAM to 32-QAM is simply not justified. A more valuable approach would be to double the bandwidth of the upstream DOCSIS channel, resulting in doubling the data rate.

    Figure 1 provides a quick reference for each DOCSIS upstream modulation type (QPSK through 64-QAM) with its upstream bandwidth options (1.6 MHz to 6.4 MHz). Beside each bandwidth value are two columns, one for gross (or raw) throughput and one for net throughput. The gross throughput is given as this is the CMTS utilization while the net throughput is what is available to the subscribers.

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Optimizing the DOCSIS Upstream