The Future of Coaxial Networks

My blog devotes a lot of time looking at fiber deployment, but since the majority of people in the US get broadband from cable companies using hybrid fiber/coaxial (HFC) technology, today’s blog looks at the next generation of changes planned for HFC.

DOCSIS 4.0. The current generation of HFC technology is DOCSIS 3.1 This technology uses 1.2 GHz of spectrum over coaxial cable. DOCSIS 3.1 has several competitive drawbacks compared to fiber. First, while the technology can deliver gigabit download speeds to customers, the dirty secret of the industry is that gigabit speeds can only be given to a limited number of customers. With current node sizes, cable companies can’t support very many large data users without sacrificing the performance of everybody in a node. This is why you don’t see cable companies pricing gigabit broadband at competitive prices or pushing it very hard.

The other big drawback is that upload speeds on DOCSIS 3.1 are set by specification to be no more than one-eighth of the total bandwidth on the system. Most cable companies don’t even allocate that much to upload speeds.

The primary upgrade with DOCSIS 4.0 will be to increase system bandwidth to 3 GHz. That supplies enough additional bandwidth to provide symmetrical gigabit service or else offer products that are faster than 1 Gbps download. It would also allow a cable company to support a lot more gigabit customers.

The big drawback to the upgrade is that many older coaxial cables won’t be able to handle that much bandwidth and will have to be replaced. Further, upgrading to 3 GHz is going to mean replacing or upgrading power taps, repeaters, and other field hardware in the coaxial network. CableLabs is talking about finalizing the DOCSIS 4.0 specification by the end of 2020. None of the big cable companies have said if and when they might embrace this upgrade. It seems likely that many of the bigger cable companies are in no hurry to make this upgrade.

Low Latency DOCSIS (LLD). Another drawback of HFC networks is that they don’t have the super-low latency needed to support applications like intense gaming or high-quality video chat. The solution is a new encoding scheme being called low latency DOCSIS (LLD).

The LLD solution doesn’t change the overall latency of the cable network but instead prioritizes low-latency applications. The result is to increase the latency for other applications like web-browsing and video streaming.

This can be done because most of the latency on an HFC network comes from the encoding schemes used to layer broadband on top of cable TV signals. The encoding schemes on coaxial cable networks are far more complex than fiber encoding. There are characteristics of copper wires that cause natural interference within a transmission path. A coaxial encoding scheme must account for attenuation (loss of signal over distance), noise (the interference that appears from external sources since copper acts as a natural antenna), and jitter (the fact that interference is not linear and comes and goes in bursts). Most of the latency on a coaxial network comes from the encoding schemes that deal with these conflicting characteristics. The LLD solution bypasses traditional encoding for the handful of applications that need low latency.

Virtual CMTS. One of the more recent improvements in coaxial technology was distributed access architecture (DAA). This technology allows for disaggregating the CMTS (the router used to provide customer broadband) from core routing functions, meaning that the CMTS no longer has to sit at the core of the network. The easiest analogy to understand DAA is to consider modern DSLAM routers. Telephone companies can install a DSLAM at the core of the network, but they can instead put the DSLAM at the entrance to a subdivision to get it closer to customers. DAA allowed cable companies to make this same change.

With virtual CMTS a cable network takes DAA a step further. In a virtual CMTS environment, the cable company might perform some of the CMTS functions in remote data centers in the cloud. There will still be a piece of electronics where the CMTS used to sit, but many of the computing functions can be done remotely.

A cloud-based CMTS offers some advantages to the cable operator:

  • Allows for customizing portions of a network. The data functions provided to a business district can be different from what is supplied to a nearby residential neighborhood. Customization can even be carried down to the customer level for large business customers.
  • Allows for the use of cheap off-the-shelf hardware, similar to what’s been done in the data centers used by the big data complies like Google and Facebook. CMTS hardware has always been expensive because it’s been made by only a few vendors.
  • Improves operations by saving on local resources like local power, floor/rack space, and cooling by moving heavy computing functions to data centers.

Summary. There is a lot of discussion within the cable industry asking how far cable companies want to push HFC technology. Every CEO of the major cable companies has said that their eventual future is fiber, and the above changes, which each bring HFC closer to fiber performance, are still not as good as fiber. Some Wall Street analysts have predicted that cable companies won’t embrace bandwidth upgrades for a while since they already have the marketing advantage of being able to claim gigabit speeds. The question is if the cable companies are willing to make the expensive investment to functionally come closer to fiber performance or if they are happy to just claim to be equivalent to fiber performance.