As noted in my colleague John Downey’s blog earlier this week, unprecedented demand continues on local broadband networks. Cable operators are working to stay ahead of the curve. John’s blog includes tips for optimizing cable modem termination system (CMTS) performance. This blog includes my checklist for the outside plant.

Proper operation of the optical and coax portions of the network is critical for the best performance. Impairments can degrade DOCSIS® 3.0 (and earlier) single-carrier quadrature amplitude modulation (SC-QAM) and DOCSIS 3.1 orthogonal frequency division multiplexing (OFDM) signals, reducing their effective throughput. Forward error correction (FEC) helps, especially the much more powerful FEC in DOCSIS 3.1, but good old Cable 101 is still an important part of ensuring reliable operation. As well, taking advantage of the available spectrum to expand DOCSIS signal carriage can add to the ability to increase throughput.

Bit Errors, Dropped Packets

  • SC-QAM: Identify signals with pre-FEC and post-FEC bit errors. Fix the causes of post-FEC errors first, since they mean packet loss. Then find and fix the causes of pre-FEC bit errors, before they become post-FEC bit errors.
  • OFDM: Identify and fix the causes of uncorrectable codeword errors. Because of the way DOCSIS 3.1’s error correction works, it’s okay to ignore correctable codeword errors.

More Speed: Legacy SC-QAM Signals

  • If any of your downstream legacy digital signals are still configured for 64-QAM, moving to 256-QAM will increase the per-channel throughput.
  • Doubling upstream channel bandwidth – for instance, changing from 3.2 MHz to 6.4 MHz – and switching to a higher modulation order can provide improved throughput.
  • Does your network have vacant spectrum available? If so, consider adding more DOCSIS channels, which can in turn increase overall capacity.

More Speed: DOCSIS 3.1 OFDM Signals

  • If you can do so, increase the RF bandwidth of your DOCSIS 3.1 downstream OFDM channel (remember, the maximum supported channel bandwidth is 192 MHz). That will bump up the throughput.
  • Can the network support higher modulation orders than what is being used now? You might be surprised at how well DOCSIS 3.1’s powerful FEC deals with plant impairments. Higher modulation orders equal more bits through the pipe.
  • For more reliable operation, ensure that the OFDM signal’s physical layer link channel (PLC) is on a frequency that is not susceptible to ingress or other interference. The PLC’s frequency is user-configurable.

Physical Plant Changes

  • Move ahead with those node splits where the needs are greatest and look at upstream segmentation.
  • Have you been contemplating deploying a distributed access architecture (DAA), such as Remote PHY? One advantage: The digital fiber links used in DAA can provide an improvement in downstream and upstream receive modulation error ratio (RxMER), which supports higher modulation orders.

General Network Performance

  • Optimize headend/hub RF signal levels and outside plant active device alignment.
  • Optimize downstream and upstream analog optical fiber link performance. You may be able to achieve improved RxMER through the optical links, which is good for higher modulation orders.
  • Place more short-term emphasis on signal leakage and ingress management, which can improve signal quality on ingress-impacted channels.


  • Some cable operators have had good luck asking subscribers to check the tightness of F connectors on customer premises equipment (CPE), and tighten if loose. This can reduce truck rolls and has been found to improve or resolve some upstream noise and ingress issues.
  • While not directly related to capacity, check outside plant standby power supplies. Ensure that batteries are present and in good condition, standby operation works correctly, and batteries have the adequate capacity (replace old or defective batteries as needed). At the least, this will help reduce nuisance power outages in the plant.
  • If CPE has functional backup batteries, remotely check their condition and backup function (if possible).
  • Consider sharing CableLabs’® guidelines to improve Wi-Fi performance with your subscribers.

For more information visit our Cable Access website. Also, be sure to register for the upcoming Cisco Knowledge Network webinar scheduled for May 7, “How Is Your Broadband Holding Up?” to hear from my colleagues John Chapman, Cisco Fellow and CTO, Cable Access Business Unit, and John Downey, Senior CMTS Technical Leader, Cable Access Business Unit, where they’ll share valuable information you can use today to quickly adapt your cable network.


Ron Hranac

Technical Marketing Engineer, Cable Access

Service Provider