How Cable Will Conquer the Unknowns of DOCSIS 4.0
By Mike O’Dell
As we begin to deploy the initial implementations of Docsis 4.0, I am reminded of a phrase from the opening of the iconic series, Star Trek: “…to boldly go where no man has gone before.” With so much promise, but also a portion of uncertainty, how do we boldly go where we haven’t been before?
The old saying, “we don’t know what we don’t know” looms large, but we have learned an awful lot over the years about the successful implementation and management of new technology. Not so long ago, digitally modulated video channels were new and exciting, the earliest DOCSIS implementations presented challenges, and expansion of the return path into mid-split (85 MHz) was a matter of no small concern. Yet, in every instance we have managed to adjust our construction, activation, and maintenance routines to adapt to each new thing. After all, as Mr Spock taught us, “change is the essential process of all existence.” Now that DOCSIS 4.0 is deploying at scale we are learning about the essential changes, the unknown is becoming known, and I predict DOCSIS 4.0 will, as oft stated by Mr. Spock, “Live Long and Prosper.”
Let’s start with what we do know:
- RF is still RF
- Coaxial cable is still coaxial cable
- Attenuation as a function of frequency over coaxial cable is well-understood
- Gain has its advantages and risks
- Reliability is paramount
- Customers must be always connected
Let’s examine each of these in turn. Radio frequencies are comfortably familiar to us. Whether digitally modulated, or analog, we have been reliably transporting these signals over coaxial cable for decades. Regardless if there is a diplex filter involved to separate the upstream and downstream paths, RF over coaxial cable behaves predictably, even in the presence of impairments.
I am a big fan of coaxial cable. When properly installed and maintained, it is an incredibly reliable delivery medium with excellent behavioral performance. Because we have been delivering RF over coax for so long, the impacts of coax impairments to the transported RF signals are well understood. Further, our industry has developed many excellent intelligence tools to identify, isolate, and localize these impairments. This results in the effective prioritization of impairments and allows operators to dispatch and manage their workforces efficiently and effectively.
One of the fundamental behaviors of coax is attenuation. Attenuation is a decrease in the power of a signal or signals. Attenuation in coax is also a function of frequency, and is not linear across the spectrum of interest. Let’s assume we want to use bandwidths out to 1,794 MHz, or roughly 1.8 GHz. At these higher frequencies, there will be significant increases in the amount of natural loss, or attenuation, as the frequency increases to the band edge. Note that DOCSIS 4.0 FDX does not extend the bandwidth to 1.8 GHz. Instead, FDX leverages new technology to use the existing spectrum bi-directionally, and thus more efficiently, avoiding these additional frequency dependent losses and implications. For systems that do increase the frequency, such as DOCSIS 4.0 FDD, it requires a decision by the operator in their design philosophies. They can reduce the distance between the amplifiers, requiring additional construction and power consumption. Or the operator can deploy amplifiers with higher gain, while maintaining the same amplifier spacings. Higher gain amplifiers can consume more power but will retain the same number of amplifiers inside the service group area. Note that new devices leveraging these higher frequencies and subject to this additional loss, also are expected to be able to receive these frequencies at lower levels, albeit with implications to RxMER. Equipment in the network and home such as passives, taps and splitters may need to be changed due to their attenuation out to 1.8 GHz. Fortunately, the cable industry now has a unified system-on chip technology supporting either FDD or FDX; or even both at the same time.
About that gain: It makes sense to maintain amplifier spacings and node service area boundaries, through installing higher gain, higher bandwidth actives. The amplifier manufacturing community has done an exceptional job of developing platforms that can deliver high quality, and high gain. The proper set-up of wideband systems, with high gain requires precision of RF management through the active devices. Proper equalization, cable simulation, and attenuation management at specific points will be critical. Upskilling our workforce to understand the operating constraints, and capability of the new network components will be essential. The consequence of improperly aligned active devices can be significant in terms of signal quality, service delivery, and the customer perception of reliability. Smart amplifiers are being developed for DOCSIS 4.0, with the trend towards virtualization and automation happening across the industry software; smart meters are emerging, simplifying this process for technicians and automating it for even greater reliability. For more on downstream amplifier performance, read Diana Linton’s article in the Spring 2022 edition of Broadband Library. In that same edition, Brady Volpe wrote an excellent article that can provide some insight on return path management including OFDMA channels.
Last but certainly not least, is reliability. Connectivity is an integral part of everyday life. Whether wireless, or wireline, our society’s desire to constantly be connected to information, news and entertainment has transformed the way we think about the entirety of the connected customer experience. It is table stakes for any operator serving the needs of their current and potential customer base. The customer’s perception of the reliability of the network is as important as any metric telemetry that might be collected from the network itself. Having said that, there are many best practices that have been learned and improved upon from early DAA and other deployments that will continue to ring true in the next phases of DOCSIS 4.0 implementations.
First, note that DAA itself adds powerful new telemetry and visibility by distributing intelligence into the digital nodes and smart amplifiers streaming telemetry infrastructure built on top of the available metrics from RPDs and CMs brings yet another level of reliability benefits. Then, consider power supplies: Having a structured, scalable, and consistent power supply health management program can ensure the consistent performance of your outside plant powering network. This aspect of the outside plant is often overlooked as a pivotal part of the service delivery mechanism. No power, no packets. Non-invasive troubleshooting is another pillar of effective and efficient outside plant management models. We know that all outside plant networks are living, dynamic, ever-changing organisms, that can and will be subject to impairments over time. There are many implementations of machine learning informed artificial intelligence engines, and some that have become very powerful in cable systems. The profile management applications, or PMA, bring the full value of DOCSIS 3.1 multiple modulation profiles. PMA allows periodic adaption of QA
M profiles to maximize their efficiency and ultimately the network capacity available while assuring the best customer experience, based on plant metrics measured multiple times daily. Another AI/ML application in use can diagnose and ultimately remediate troublesome RF signatures. These AI models identify evidence of impairment through near-real-time telemetry, take steps to mitigate the effects of the impairment to the customer, and restore the network to its optimal performance states, with little to no manual intervention.
From the lens of the customer, these are invisible, and do not detract from their perception of reliability. There are instances, however, when manual intervention from the technical workforce is required or when the impairments are too severe for software driven mitigation efforts. It is these times, when the technician’s impact to the network is most keenly felt by the customer base and can impact their perception of the reliability of the network. Network maintenance practices that are built around the use of test points, impedance matched test probes, and other troubleshooting best practices that minimize or eliminate unnecessary customer downtime will be valued not only by the customers, but by the operators themselves. Note that while such issues requiring manual intervention will occur, new visibility through telemetry and diagnosis capability leads to improved MTTR for the customer even for these cases. This is because deep telemetry today enables the dispatch of the right technician with the right tools and information to a pinpointed location in the plant where repairs must be made.
In summary, we are off and flying at warp speed with DOCSIS 4.0 deployments at Comcast, and I boldly predict that the lessons we have learned from the past will serve us well in identifying, and resolving any obstacles that do emerge, as well as paying our learnings forward to the professionals who will build and deploy whatever comes next.
Mike O’Dell
Mike is a Distinguished Engineer at Comcast Corporate Headquarters. Mike has nearly 30 years of cable television experience, specializing in outside plant. Mike has been involved in SCTE for nearly 20 years, as a chapter leader with the Penn-Ohio Chapter, past Chair of the Cable-Tec Games Subcommittee, and participates in standards on the Network Operations Subcommittee. Mike is a graduate of SCTE’s Georgia Tech Engineering Leadership Development Program. Mike is committed to training and is active in training content creation through the Penn-Ohio Chapter’s social media presence and YouTube training channel.
—
Images: Shutterstock
