Cable companies have always adopted a methodical approach to developing their broadband networks, gradually increasing capacity through the spectrum and modulation enhancements. It is an approach that has served them extremely well over the years, particularly when it comes to capital expenditure management. With increased fibre competition and pressure on cable operator executives to provide a fibre plan, it should come as no surprise that the path to fibre for some of the larger cable operators will be phased.
Comcast‘s approach to gradual and continual upgrades from DOCSIS 3.1 mid-split to full-duplex DOCSIS 4.0 and, eventually, fibre based on a foundation of a virtual cable modem termination system (vCMTS), Remote PHY, and remote optical line terminals was already well-known in the industry (OLTs). But it wasn’t until last week that Charter confirmed a similar roadmap using the same product portfolio—the major differences being Charter’s preference for high-split DOCSIS 3.1 and extended-spectrum DOCSIS 4.0. Both operators believe that the use of mid- and high-split architectures, which pushes their upstream bandwidth to anywhere from 400 Mbps to 1 Gbps, will be more than enough to counter fibre providers’ symmetric and near-symmetric residential offerings in the short-term, while also taking evolutionary steps towards DOCSIS 4.0 and/or FTTH.
While DOCSIS-based architectures will serve as the backbone for the majority of their residential networks, it is obvious that these operators want to provide more FTTH services than originally anticipated. MSOs can selectively deploy remote OLT modules alongside Remote PHY devices (RPDs) in existing node locations to peel off FTTH service groups of 32 to 64 homes by relying on a combination of Remote PHY and vCMTS to re-architect existing DOCSIS networks and move to distributed access architectures (DAA). Traffic from those RPD and R-OLT modules can be routed back to the headend through Ethernet, with all subscriber endpoints—whether DOCSIS modems or PON optical network terminals (ONTs)—managed by the same vCMTS and/or virtual broadband network gateway (vBNG) platforms.
Another option for MSOs adopting this architecture is to mix DOCSIS-based home services with fiber-based business services via the remote OLT. There are numerous examples of optical nodes serving service groups that comprise both residential and commercial users. Historically, MSOs could only provide these customers with business-class DOCSIS services, with an additional payment for higher service level agreements (SLAs), static IP addresses, and other features. Over the previous decade, MSOs have done an excellent job of taking small and medium-sized company clients from telcos who had more rigid pricing plans or relied on T1 or business-class DSL connections. However, telcos and other fibre ISPs have recently tried aggressively to reclaim these business clients by marketing higher reliability and technological advantage of fiber. Thus, the availability of remote OLTs—particularly those that can be added into existing node housings without significant upgrades—allows cable operators to offer a comparable fiber service to valuable business customers, all while evolving their access networks.
Optical nodes have genuinely evolved into the site of convergence for cable’s DOCSIS, PON, and even wireless networks and ambitions. Nodes have evolved from simple transport and aggregation elements to the point where layers 1-3 converge and the network’s access and aggregation layers meet. More work is being done now to expand the utility of these platforms beyond DAA and remote OLT support.
The Coherent PON program by CableLabs proposes to integrate coherent optical modules into nodes so that they may aggregate high-speed DOCSIS 4.0 and 10-25 Gbps PON traffic onto 100 Gbps aggregation lines back to headends or regional data centres. Furthermore, while coherent optics are often more expensive than single wavelength choices, they can help to alleviate a handful of cable operators’ major challenges: Long fiber spans between headend and node locations, as well as a limited number of fiber strands.
In the long run, as DOCSIS and HFC networks are phased out in favor of complete fibre networks, the combination of C-PON and remote OLT modules in optical nodes should provide a solid platform for fibre delivery to homes and businesses. Existing amplifier stations can house splitters, AWGs, and other passive parts common in PON deployments, decreasing total power consumption and boosting overall dependability by reducing active network elements.
Other cable companies are currently ignoring this natural cycle and overbuilding with fibre. Many of these either do not pass millions of homes or are located in nations where the expense of changing the exterior plant is simply too high for a step-by-step evolution due to labor, permitting or both. Regardless, a clear roadmap for the transition from DOCSIS to fiber now exists and is being operationalized at a growing number of MSOs around the world.