Cable operators have historically taken a measured approach to improving their broadband networks—adding capacity incrementally through spectrum and modulation improvements. It is an approach that has served them incredibly well over the years, especially when it comes to managing capital expenditures. With fiber competition and pressure on cable operator executives to deliver a fiber strategy increasing, it should come as no surprise the path to fiber for the some of the largest cable operators will incorporate a phased approach.
The industry was already well-aware of Comcast’s approach to steady and continuous upgrades from DOCSIS 3.1 mid-split to full-duplex DOCSIS 4.0 and, ultimately, fiber based on a foundation of a virtual cable modem termination system (vCMTS), Remote PHY,and remote optical line terminals (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 fiber providers’ symmetric and near-symmetric residential offerings in the short-term, while also taking evolutionary steps towards DOCSIS 4.0 and/or FTTH.
While these DOCSIS-based architectures will serve as the backbone for the majority of their residential networks, it is clear that these operators have designs on delivering more FTTH services than previously expected. By relying on a combination of Remote PHY and vCMTS to re-architect existing DOCSIS networks and move to distributed access architectures (DAA), the MSOs can then 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. Traffic originating from those RPD and R-OLT modules can be transported back to the headend via Ethernet transport, with all subscriber endpoints—be they DOCSIS modems or PON optical network terminals (ONTs) managed through the same vCMTS and/or virtual broadband network gateway (vBNG) platforms.
Another option for MSOs with this architecture is the mixed use of optical nodes for DOCSIS-based residential services and fiber-based business services using the remote OLT. There are many instances of optical nodes covering services groups that include both residential and business customers. Historically, MSOs have only been able to offer business-class DOCSIS services to these customers, with an upcharge for higher service level agreements (SLAs), static IP addresses and other features. MSOs have done extremely well over the last decade of stealing away small and medium business customers from telcos who had more inflexible pricing plans or relied on T1 or business-class DSL lines. But recently, telcos and other fiber ISPs have pushed hard to get these business customers back by pitching the 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 truly become the crossover point for cable’s DOCSIS, PON, and even wireless networks and ambitions. From simple transport and aggregation elements, nodes are now the point at which layers 1-3 converge and where access and aggregation layers of the network meet. Further work is being done today to extend the utility of these platforms beyond just supporting DAA and remote OLTs.
CableLabs’ Coherent PON initiative aims to add coherent optical modules into nodes to be able to aggregate high-speed DOCSIS 4.0 and 10-25 Gbps PON traffic onto 100 Gbps aggregation links back to headends or regional data centers. In addition, these coherent connections could eventually deliver up coherent optics, while typically more expensive than single wavelength options, helps to solve a couple of cable operators’ major challenges: Long fiber spans between headend and node locations, as well as a limited number of fiber strands.
Over the long-term, as DOCSIS and HFC ultimately give way to full fiber networks, the combination of C-PON and remote OLT modules in optical nodes should serve as a strong platform for the delivery of fiber to the home and business. Existing amplifier stations can house splitters, AWGs and other passive elements typical of PON deployments while reducing overall power consumption across the network and increasing overall reliability thanks to the reduction of active network elements.
Other cable operators are bypassing this evolutionary progression and overbuilding with fiber today. Many of these either don’t pass millions of homes or are located in countries where the cost to change the outside plant--due to labor, permitting, or both—is simply too high for a step-by-step progression. Nevertheless, 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.