Cable operators are broadly deploying DOCSIS 3.1 technology around the world. Operators in North America and Europe have announced gigabit service offerings that use the technology across their footprint. In this blog, I focus on techniques by which operators can boost the capacity and robustness of their existing DOCSIS 3.1 network.
Introducing Toolsets for Optimizing the DOCSIS 3.1 Network
DOCSIS 3.1 specifications introduced orthogonal frequency division multiplexing (OFDM) technology to the access network. In DOCSIS 3.0 and prior technologies, a single-carrier downstream channel (6 MHz) would be configured with just the one modulation order (say, 256-QAM). The DOCSIS 3.1 technology divides a wider (e.g. 192 MHz) channel into a number of small subcarriers spaced at 25 kHz or 50 kHz apart, creating up to 7600 subcarriers. These subcarriers can be individually configured to operate at a different modulation order, anywhere from 256-QAM, to 1024-QAM, all the way to 4096-QAM.
A specific configuration of modulation orders for every subcarrier in the OFDM channel is known as a “profile.” A cable modem termination system (CMTS) can use multiple profiles on a channel; profiles differ in the modulation orders assigned to each subcarrier.
Different modems experience the network in different ways. Some modems may have a noisier environment, whereas others see a relatively clean plant. The figure shows an example of the interference seen by a modem in part of the OFDM channel.
Profiles allow the channel to operate at a higher modulation order in clean parts of the spectrum and then go down to more robust modulation orders when there is any interference present. The idea with multiple profiles is to have the CMTS use different profiles for such different groups of cable modems. This allows the operator to reach two goals:
- minimize transmission errors on the network and
- maximize network capacity.
The same concept applies to upstream OFDMA channels, where one can create upstream profiles for the channel, to increase reliability and throughput.
The Profile Management Application (PMA) Solution
If there are say 4 OFDM channels per port, 10 ports per line-card, and 10 line-cards on a CMTS, then there could be easily over a couple of million subcarriers requiring a specific configuration! This represents an immense configuration challenge which cannot be solved manually. This problem is made more difficult because profiles are not typically “one size fits all,” but rather they need to be tailored to the set of cable modems on the channel, the interference on the channel, and they also need to adjust to plant changes over time. The question then becomes: What is the most efficient method for automating the creation of multiple profiles to maximize robustness and capacity?
I am pleased to announce that CableLabs has met this tremendous challenge with the introduction of the Profile Management Application (PMA). The PMA creates a set of optimal profiles for each channel and assigns profiles to modems. And the PMA accomplishes this dynamically by proactively reading data collected from the network. The PMA can create optimized modulation profiles periodically, as well as backup profiles in case of errors. It can also intelligently decide when to roll out profile changes to the network. A single PMA instance can create and configure profiles for a number of CMTSs in a short period of time and help the operator understand the data capacity of each channel.
How It Works — PMA Architecture and Algorithms
CableLabs released a PMA Technical Report that describes the PMA architecture, use cases, and interface definitions. We also developed data models (YANG) and protocols to facilitate the design of a software application to configure and manage modulation profiles.
Determining the optimal set of modulation profiles to use on a DOCSIS 3.1 channel is a task for intelligent software, given the number of modems, the number of subcarriers and the differences in signal quality across the channel each modem experiences. This CableLabs paper on profile management algorithms describes methods to optimally compute the profiles. The paper also introduces a metric indicating the capacity gain a set of profiles obtains compared with running all traffic at 256-QAM. The key idea is to use clustering algorithms to group modems that have similar signal-to-noise signatures across the channel and then design a profile for each group. The PMA algorithm searches for the best set of profile definitions to maximize the overall capacity and at the same time keeps the individual profiles robust to observable interference patterns.
Some of the information that a PMA needs to compute the profiles includes the channel information from a CMTS, list of modems and network signal quality metrics (mainly, each modem’s per subcarrier RxMER data). Also using performance data (e.g., codeword error rates), a PMA can further refine the profiles.
How It Works — Deployment
A PMA field deployment would include the following:
- Data Collection: A centralized server is needed for modems and CMTSs to upload signal quality data. Operators can use the CableLabs DOCSIS Common Collection Framework (DCCF) to implement this.
- PMA: Using the data collected, this application creates optimized profiles per channel. It configures these profiles on the CMTS, assign CMs to profiles and can initiate performance tests.
- CMTS: Configures profiles for the individual channels, assigns the modulation profiles to CMs and ultimately sends/receives data using the profiles.
- CMs: Cable Modems use the profiles defined and assigned by the CMTS to receive/send data.
What’s Next? PMA Software Available
CableLabs has developed network-deployable software that operators can integrate and use within their DOCSIS 3.1 access network. This PMA solution can use the data generated by the network devices and create optimized OFDM/OFDMA profiles while considering different capabilities supported across CMTS platforms. The software is capable of calculating profiles for a channel with around 200 CMs in ~30 msecs using a single CPU process, so it is easily scalable across an operator’s network. Many operators are testing and planning PMA deployment using this solution framework. PMA is available via CableLabs C3. Contact us to try it out. (Watch for an upcoming video blog on this topic.)
Gains from PMA
A PMA will help increase the throughput per cable modem and maximize network capacity by optimizing the bit-loading of the subcarriers within a channel. Designing profiles around noisy areas in the plant make the system operation more robust. The bandwidth gains in running a well-designed set of profiles can be anywhere from 15% to 40% capacity increase on a channel, compared with running the whole channel at 256-QAM. This can translate to a solid 200 to 400 Mbps extra capacity on each OFDM channel! This enables an operator to match growing bandwidth demands and defer potential node-splits and new equipment costs.
With this profile management technology, operators can realize the full potential of their DOCSIS 3.1 network, by minimizing transmission errors and maximizing the data capacity of the OFDM/OFDMA channels.
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