As we start 2022, much of the world is in a very different place vs. where we were a year ago. Most kids are back in school, and many of us are beginning to return to the office. Despite these baby steps toward normalcy, it is clear that broadband upstream demand and usage models have forever changed. Cable providers weathered the initial storm well and are now positioning themselves through innovation and industry-wide collaboration to remain ahead of the bandwidth demand curve.
Cable operators are not totally reinventing the wheel to stay ahead - the same three primary options for increasing upstream capacity that cable operators have relied on for decades still remain in play:
- Mode Nodes – Segmenting or splitting nodes to shrink service group sizes
- More Hz – Increasing spectrum available in which to widen and/or add carriers
- More Bits/Hz – Increasing modulations to gain higher spectral efficiency.
But how they go about leveraging these options is certainly changing (Table 1).
More Nodes (Via R-PHY/R-MACPHY)
Distributed access architectures (DAA) like Remote PHY (R-PHY) and Remote MACPHY (R-MACPHY) are rapidly replacing standard node splits to appease hub rack space and power/cooling constraints. In the early days of DAA operational concerns existed about how to maintain and troubleshoot the plant once RF was no longer present in hubs/headends for ingress, leakage, and sweep tools. These problems are now behind us as the functions previously handled by rack-mounted gear have been virtualized, enabling reuse of existing workflows and field meters. As with any new technology rollout, new challenges continue to emerge as deployments progress.
10+ Gbps Optical Ethernet in the Access Network – While the old point-to-point analog optical links for legacy nodes were finicky to set up and maintain, overall they were simple compared to the complex optical networks feeding some DAA nodes (Figure 1). Technicians in the field today often must be versed in DWDM technologies and be equipped with the proper tools to confirm that they have not just the right light levels but also at the right wavelength. Verifying network continuity through a vast array of switches and muxes/demuxes also creates challenges. Technicians must troubleshoot SFP issues, be aware of how PTP timing can impact services, and not forget the fundamental importance of fiber inspection and cleaning. No longer can cable operators rely on a small subset of fiber experts; now everyone must have this baseline knowledge and toolset.
RF Video Verification – Early adopters ran into unanticipated issues with linear video in early deployments. RF video is created for the first time at the Remote-PHY Device (RPD), so traditional test points in headends or RF combining networks are no longer available. Beyond just typical RF issues, opportunities exist for missing programs or packet identifiers (PIDs), stalled PIDs, out-of-band (OOB) carrier issues, logical/physical channel plan mismatches, and more. Each service group within each RPD/Remote-MACPHY Device (RMD) can have its own unique configuration, which makes being able to confirm that everything is correct for each service group a daunting task. Field meter-based solutions have since been developed to check everything above and more in about 5 minutes, minimizing this challenge and simplifying handoffs between field technicians and video engineering when necessary (Figure 2).
Flexible MAC Architecture (FMA) – The idea of managing the MAC layer from anywhere in the network has been alluring since the early days of DAA, and while some vendors have been offering R-MACPHY solutions for multiple years the true promise of ultimate flexibility and complete interoperability are just now heading toward reality. The underlying technology enabling FMA deployments is complex, but from a Tech Ops standpoint very little changes for monitoring and troubleshooting FMA networks. All of the systems developed to enable continuity of physical layer testing and troubleshooting from legacy to R-PHY networks work the same for R-MACPHY networks. The same generally goes for PNM and QoE monitoring; the transition will generally be transparent for maintenance techs in the field.
Many More - These are just a few of the test challenges created by DAA. Table 2 below is a summary of other test considerations associated with DAA deployment and maintenance.
More Hz
While node splits are typically the first option considered for increasing upstream bandwidth available to each service group, they are not always the most economical or practical option. Sometimes the best answer is to instead move from a sub-split (42 or 65 MHz) upstream to a high-split (204 MHz) architecture. While a typical node split will double the capacity/service group, a high split can yield a 5X increase or more vs. 42-MHz upstreams – eliminating or at least deferring the need for future node splits. It’s important to note that the two aren’t mutually exclusive; they can be deployed together for maximum impact.
High-split transitions aren’t simple from an outside plant standpoint; every active and some passives must be visited before the full cutover can occur. Operators with 1-GHz or even 860-MHz networks can generally tolerate the downstream spectrum loss via analog reclamation and video compression techniques, but 750-MHz and below plants typically need downstream expansions to compensate. Benefits of high-split transitions are limited to 204-MHz-capable CPE, but fortunately many recently deployed DOCSIS 3.1 CPE have switchable diplexers in place already. In addition to these fundamental challenges, other test-related challenges have emerged.
Signal Leakage/CLI – Governmental regulations in some countries require aeronautical band (108-136 MHz) signal leakage monitoring, a task made simple by widely deployed downstream leakage monitoring systems. High-split networks break this paradigm as the aero band moves to the upstream band (Figure 3). Leakage systems designed to detect injected downstream signal “tags” or OFDM carriers leaking out from the downstream no longer have anything to detect. After evaluation of several alternative techniques, the industry has converged on detecting OUDP (OFDMA Upstream Data Profile) test bursts from CPE upstream transmissions for detection in high-split plants. OUDP burst functionality is already covered in current DOCSIS 3.1 specs for CPE/CCAP, and detectors from leading vendors are software-upgradable to provide OUDP burst detection. Whether it be for government-mandated testing or general plant hardening, good solutions exist for this “More Hz” challenge.
FM Ingress – FM ingress has been problematic for cable operators due to the always-on nature of FM signals. Anywhere there is a shielding weakness in a cable network, there are likely multiple FM signals ready to flood in and disrupt services. As with the aero band, the FM band moves into the upstream with high-split architectures (Figure 4). Now the funnel effect applies, creating a cumulative effect on the combined upstream signal and making localization more difficult. The wider OFDMA carriers specified for use above 85 MHz make ingress visibility even more challenging. Many operators are reporting success with using heatmap spectral analysis in headend systems and field meters to restore FM ingress visibility in the upstream for troubleshooting.
More Bits/Hz
The first two options have involved some pretty heavy lifts from a field operations standpoint – lots of truck rolls, plant impact, and project management challenges to deal with. Squeezing more bits out of each hertz of spectrum that you already have sounds much simpler – just turn on OFDMA and reap the benefits of a more-efficient pipe – right? This is partially true – there are a ton of DOCSIS 3.1-ready CPE deployed in many parts of the world and updating/configuring/licensing CCAPs to be capable is less intrusive vs. tinkering with the outside plant. But this doesn’t mean that there aren’t challenges to be overcome.
Ingress detection and troubleshooting – OFDMA, featuring Low Density Parity Check (LDPC) error correction, was designed to be more resilient against ingress, although mixed reviews have come in from early adopters regarding performance in bursty noise environments. Modulations can be adjusted on a per-subcarrier basis within profiles, but this is complex to manage without mature artificial intelligence/machine learning (AI/ML) solutions to help. Even as the industry learns to optimize configurations to fully realize OFDMA benefits, upstream ingress will remain the largest consumer of opex and largest creator of network-related trouble tickets. Again, heatmap-based spectral analysis has proven successful to help see both bursty and always-on ingress under these carriers (Figure 5).
Reverse Sweep – Return sweep in legacy SC-QAM networks generally involves placing sweep pulses in vacant spectrum and in the guard bands between the SC-QAM upstream carriers to minimize service disruption potential. With OFDMA carriers as wide as 96 MHz, new approaches must be considered for reverse sweep. How sweep is accomplished tends to vary by use case.
- Critical Outage Troubleshooting – During critical outages, traditional sweep is the go-to tool as it works even when DOCSIS services are down and provides instant feedback to any repair actions taken. Any minor service impact created by sweeping through OFDMA carriers is of minimal concern compared to restoring services during these times.
- Amplifier Balance/Alignment – Traditional sweep is generally used here also, including a new version that uses sweep pulses specifically designed to minimize any service impact for OFDMA carriers. Even absent the OFDMA-optimized sweep protocols, service impact is normally brief and minimal for most OFDMA carrier configurations.
- General Plant Maintenance – Sweepless reverse sweep is sometimes used for this use case as it requires no hardware beyond a field meter, instantly updates to any changes in channel lineups, and provides a higher resolution view of in-band response. The only downsides to return sweepless sweep are that it requires active DOCSIS services to work (not good during outages), is much less responsive (challenging to tweak amp alignment), and only covers occupied spectrum.
Conclusion
While the upstream demand spike that occurred in early 2020 has since backed off a bit, we are still left with a step change in upstream capacity and service quality requirements. The shock to the system caused by the pandemic and its disruption of steady growth models challenged the standard methods and processes that operators had relied on for decades, especially in the upstream. But as with past disruptive events and technologies, the industry showed innovation and dedication to overcome these challenges using the three proven pillars of upstream expansion. The future looks bright for HFC to continue as the broadband service delivery architecture of choice for many years to come.
Jim Walsh is solutions marketing manager at VIAVI Solutions.