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DOCSIS – Change is Coming and it is Good

Post 29 of 178

DOCSIS - Change is Coming and it is Good

Everyone is talking about it. Everyone in cable will be impacted by it. You may not realize it but DOCSIS 3.1 will change the cable industry — whether cable operator, vendor or consumer. Even if you don’t plan to upgrade to DOCSIS 3.1 you will be impacted. There are two fundamental technologies that DOCSIS 3.1 has me excited about. First, DOCSIS 3.1 enables gigabit speeds, which allows us to extend the life of coax cable without the expense of pushing fiber closer to the home. Second, DOCSIS 3.1 enables test capabilities like we have never imagined possible before in cable modems (CM) and CMTSs. Conceivably we can automate the detection and location of RF impairments in the coaxial network with unprecedented accuracy — even better than the amazing capabilities of today’s proactive network maintenance (PNM) tools and far superior to most of todays conventional test equipment. I will briefly touch on the speed aspect of DOCSIS 3.1, more importantly what I consider the trickle down effect, but my main topic of this article is the test capabilities built in to DOCSIS 3.1.

Speed Trickles Down

Other articles will cover DOCSIS 3.1 speed, so it is sufficient to not get into details. What may be more relevant is that as large cable operators upgrade their DOCSIS 3.0 CMTSs to DOCSIS 3.1 we can anticipate surplus equipment. This is great news for Tier 2 and Tier 3 cable operators who are struggling to meet subscriber’s needs on older DOCSIS 2.0 and small DOCSIS 3.0 systems. Over the next couple of years we should see a rapid decline in DOCSIS 3.0 CMTS prices. This means smaller operators can afford upgrades in regions that have generally been underserved. Subscribers in those areas will see some much needed relief. All ships rise with DOCSIS 3.1.


Testing in a DOCSIS 3.1 environment is the real game changing opportunity for cable operators. As an industry we have learned a great deal about how to utilize the power of CM and CMTSs to do troubleshooting with DOCSIS 2.0 and 3.0 devices. This includes upstream and downstream spectrum analysis, detecting and finding impairments in coax cable, identifying intermittent connections that are likely leaking ingress into the upstream plant and more. At the same time we realized there was a lot we could not do because CM and CMTSs were not designed to be test equipment. This changes in DOCSIS 3.1. As the DOCSIS 3.1 specification was being drafted we included as many testing features as we could think of that we could not currently do with pre- DOCSIS 3.1 devices. In my previous article I mentioned laser clipping detection and spectrum analysis. This was just the tip of the iceberg. The way I believe it has been best described is that DOCSIS 3.1 CMTSs and CM is close to having an $80k vector signal analyzer in every subscriber’s house and headend. This is a pretty bold statement. However, the reason a cable operator would use such an expensive piece of test equipment is to capture and identify transient RF events, like burst noise. DOCSIS 3.1 equipment can do this. Let’s not stop at just a vector signal analyzer. In the first sentence of section 9.2 of the DOCSIS 3.1 PHY specification it reads “...the CMTS and CM contain test points which include essential functions of a spectrum analyzer, vector signal analyzer (VSA), and network analyzer...”. I really love that statement. It sets the ambitious tone the authors had for the entirety of section 9, called “Proactive Network Maintenance” (PNM). You see this was the first DOCSIS specification to ever have PNM written into the specification. The authors really understand the challenges of cable operators and cable plants, and therefore created a goal of integrating very powerful test tools into the CMTS and CMs. Having a test & measurement background, this makes DOCSIS 3.1 equally as powerful as its speed capacity.

Spectrum Analyzer

Let’s expand upon that sentence of section 9.2 in more detail to understand the power that we should expect from DOCSIS 3.1 CMTSs and modems. Today we already have upstream spectrum analysis performed by the CMTS and downstream spectrum analysis by DOCSIS 3.0 CM. In DOCSIS 3.1 we add noise power ratio notch (NPR) testing at the CM and triggered spectrum analysis at the CMTS. You’ve likely heard of NPR testing for characterizing return path lasers. Having this ability in the downstream is really cool in a DOCSIS 3.1 world because we will be able to see impairments in an OFDM DOCSIS 3.1 channel such as LTE interference, intermodulation products or any type of ingress that could be causing our 4096- QAM channels to be dropping to a lower modulation, all without having to turn off the entire DOCSIS channel. With triggered upstream spectrum analysis at the CMTS we will have much more flexibility to identify and capture upstream impairments. When we couple this with new CM that also capture upstream RF noise from the home we can correlate the source of ingress noise (in the home) and know exactly which homes to send techs in order to eliminate ingress noise. Gone are the days of pulling pads in amps at 3AM.

Vector Signal Analyzer (VSA)

The vector signal analyzer (VSA) is the second piece of test equipment mentioned in the sentence. If you have ever used a VSA you know that it is incredibly powerful for RF measurements and also incredibly difficult to setup and use. The latter makes it a challenge in cable networks in addition to its $80k price tag. What’s really amazing is that the bulk of the VSA features end up in the CM. These include: • Upstream pre-eq coefficients (also have pre-eq in the CMTS) • Downstream channel estimates • Downstream constellation display • Modulation Error Ratio (MER) per sub-carrier Pre-equalization coefficients are nothing new. Most of you are likely familiar with these by now from using PNM tools and have seen 8-tap and 24- tap equalizers. A DOCSIS 2.0 and 3.0 modem has these taps spaced over the 3.2 MHz or 6.4 MHz upstream. For a 6.4 MHz upstream that is one post-main tap per 400 kHz. With DOCSIS 3.1 things are a bit different. When in OFDMA mode each cable modem upstream subcarrier will only have a single complex pre-equalizer value vs. the 24 complex values of a DOCSIS 2.0 or DOCSIS 3.0 modem. However this is actually better because each sub-carrier will have a spacing of 25 or 50 kHz. Instead of one complex tap per 400 kHz we will have one complex tap 50 kHz minimum. Therefore our tap resolution will go up by at least eight times. This means impairment location accuracy will improve dramatically. Similarly, downstream channel estimates allow for the CM to provide estimates of the downstream performance back to the CMTS. Downstream constellation display and modulation error ratio (MER) are fairly self explanatory, but consider that we are getting these from a CM and not an expensive test meter. In a recent project I worked on we determined that a DOCSIS 3.1 CM with a battery pack and an iPhone could do everything and more than competing hand-held test equipment. This included tasks such as sweeping the network (assuming you only swept the 192 MHz DOCSIS band). With respect to sweep, however, there are some techniques in the upstream that are being finalized which allow complete RF characterization, so it is likely upstream sweep is complete. I am optimistic that we will be able to accomplish the same in the downstream once DOCSIS 3.1 equipment is more readily available.

Network Analyzer

The network analyzer functionality in the both the CMTS and CM is something that I believe will be heavily used once DOCSIS 3.1 systems are deployed. The concept is simple. Downstream and upstream channel estimates are sweet features and can be best envisioned as a sweep system performed by the CMTS and the CM. This is accomplished because the CMTS knows every symbol it transmits, which is stored in memory. The CM then receives data from the CMTS and reports this set of channels (i.e. channel estimate) back to the CMTS upon request. A comparison of the transmitted data by the CMTS and received data by the CM is performed which results in a channel response. An example channel response is shown in figure 1. This is similar to sweeping the cable plant because it shows the frequency response of the cable plant, however it is much more powerful because it is a complex frequency response which shows both magnitude and phase impairments along with any impaired symbols. This process also works in the upstream, which is very powerful when using OFDMA in the upstream. In this case sweeping the return would not be possible. In a DOCSIS 3.1 environment one could sweep the return from any subscriber’s home 24x7 without ever leaving the headend. This is powerful technology and I’m really excited about it as you should be too.


The impact for cable operators is extraordinary. CAPEX and OPEX for cable operators will be shifted around the change in the organizational maintenance structure. The benefits also extend to the subscriber. As an industry we will be positioned to provide better quality of service and experience to our subscribers by advanced proactive network maintenance (PNM) tools than ever before. PNM is very powerful, effective, and desired by cable operators today without question. I am even more excited to see PNM in a DOCSIS 3.1 network!  Change is coming and it is good.


By Brady Volpe

Mr. Volpe has over 25 years of communications industry experience. He is focused on the cable and telecom industry with deep technical and business skills. Mr. Volpe is currently the president and chief technologist of the Volpe Firm and holds an MSEE with honors.

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