Accurately Estimating D3.1 Channel Capacity (2017)

By Karthik Sundaresan, CableLabs

The DOCSIS 3.1 specification fundamentally changes the nature of information delivery across the cable plant, and how the cable plant will be maintained and managed. The modulation order and FEC can be optimized based on actual plant conditions at individual devices. Devices which receive clean signals will utilize very efficient high-order-modulation across each of the subcarriers, devices with a degraded signal will use more robust modulation, all on the same channel. To manage this optimization the CMTS uses the concept of downstream OFDM profiles and upstream OFDMA profiles. D3.1 allows defining multiple profiles, each tuned to account for plant conditions experienced by a set of CMs.

Estimating downstream and upstream channel capacity was relatively straightforward for SC-QAM channels. But in D3.1, since the modulation orders of each subcarrier could be different and different across profiles, estimating the DOCSIS channel capacity is no longer simple. With multiple modulation profiles in use simultaneously, the capacity of the channel as seen by each CM may change instantaneously. The aggregate channel capacity calculation from a CMTS point of view with the different CMs using different profiles becomes more complicated and varies with which CMs and profiles are in use. How does one account for the NCP, FEC, PHY, MAC layer overhead and other variable factors in determining the effective channel throughput? How does profile definition, number of CMs, and heavy vs light traffic users on the channel affect throughput? The channel capacity affects how many subscribers can be assigned to use the same set of channels. It also affects traffic engineering and when an operator would need to split the node to increase available capacity. What is the reliable method for an operator to get a handle on the network capacity? This paper will present a framework to calculate the D3.1 downstream and upstream channel capacity accurately and answer the above questions and considerations.

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