Tag Archives: CMTS

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.

New CableLabs Standard will Improve Cable Networks

coaxial cableCableLabs just announced a new set of specifications that is going to improve cable HFC networks and their ability to deliver data services. They announced a new distributed architecture that they are calling the Converged Cable Access Platform (CCAP).

This new platform separates functions that have always been performed at the headend, which is going to allow for a more robust data network. Today, the cable headend is the place where all video is inserted, all cable management is done, where the QAM modulation and RF Modulation is performed, and most importantly where the CMTS (cable modem termination system) function is done.

The distributed CCAP allows these functions to be separated and geographically distributed as needed throughout the cable network. The main benefit of this is that a cable operator will be able to push pure IP to the fiber nodes. Today, the data path between the headend and the neighborhood nodes needs to carry two separate paths – both a video feed and a DOCSIS data feed. By moving the CMTS and the QAM modulators to the fiber node the data path to the node becomes a single all-IP path that contains both IPTV and IP data. The new CCAP node can then convert everything to RF frequencies as needed at the node.

We’ve been expecting this change since for the last few years Chinese cable networks have implemented the distributed network functions. Probably one of the biggest long-term potentials for this change is that it sets the stage for a cable company to offer IPTV over DOCSIS frequencies, although there is more development work to be done in this area.

There are several immediate benefits to a cable system. First, this improves video strength since the TV signals are now originating at the neighborhood nodes rather than back at the headend. This will be most noted by customers who are currently at the outer fringes of a cable node. The change also will boost the overall amount of data delivered to a neighborhood node between 20–40%. It’s not likely this mean faster speeds, but instead will provide more bandwidth for busy times and make it less likely that customers lose speed during peak hours. Finally, it means that a cable company can get more life out of existing cable nodes and will be able to wait longer before having to ‘split’ nodes to provide faster data to customers.

Cable companies are not likely to rush to implement this everywhere. It would mean an upgrade at each node and most cable companies have a node for every 200–400 customers—that’s a lot of nodes. But one would think this will quickly become the standard for new nodes and that cable companies will implement it over time into the existing network.

This is the first step of what is being called the IP transition for cable companies. Most of my readers are probably aware that the telcos are working feverishly towards making a transition to all-IP. But cable companies are going to want to do that for a different reason. There is a huge amount of bandwidth capability on coaxial cable and if the entire cable network becomes IP from end-to-end then the huge data capacity in the cable network would be realized. Today cable companies use a broadcast system where they send all cable channels to every home and they then provide data services on whatever bandwidth is left. But in an all-IP system they would only send a customer the channels they are watching, meaning that most of the bandwidth on the system would be available for high-speed Internet services.

So think of this as the first step in a transition to an all-IP cable network. There are a number of additional steps needed to get there, but this pushes IP out to the neighborhood nodes and starts the transition.