Cable operators and mobile operators share one business commonality: capacity improvements hinge on the availability of spectrum and the degree of frequency reuse (smaller cells or serving area sizes).
Both mobile and cable operators can effectively boost capacity by using different modulation techniques as well. But cable operators face a bigger problem, architecturally. “At some point” in the future a shift to fiber to home designs seems inevitable.
But there are many ways to upgrade the hybrid fiber coax network before then, with varying degrees of capital investment and complexity, as well as capacity improvements. So each upgrade path embeds assumptions about what the market will require in terms of both upstream and downstream capacity , and for how long.
DOCSIS 4.0 is going to force decisions about which path to take to support symmetrical multi-gigabit-per-second speeds of as much as 10Gbps downstream and up to 6 Gbps upstream.
Hybrid fiber coax networks still use frequency division, separating upstream and downstream traffic by frequency. So when a cable operator contemplates adopting mid-split or high-split designs, there are implications for active and passive network elements, especially for the more-radical high-split design.
At this point, executives also will ask themselves whether, if radical changes are required, whether it would not be better to simply switch to fiber-to-home.
Our notions of mid-split and high-split frequency plans have shifted a bit over the years, as total bandwidth has grown beyond 450 MHz up to 1.2 GHz. A designation of “mid-split” made more sense in an era where total bandwidth was capped at about 450 MHz or 550 MHz. In those days, 108 MHz to 116 MHz of return bandwidth was perhaps 42 percent of the usable bandwidth.
Hence the “mid-split” designation.
Likewise for high-split designations, where as much as 186 MHz was designated for the return path, the return bandwidth represented as much as 67 percent of usable bandwidth on a 450-MHz coaxial cable system.
Definitions remain, though with some new standardization of return bandwidths. “Mid-split” now features 85 MHz of return bandwidth, while “high-split” offers 204 MHz of upstream bandwidth.
“Ultra-high-split” designs also are being investigated, where the upstream spectrum’s upper frequency limit can be 300 MHz, 396 MHz, 492 MHz, or 684 MHz, says Ron Hranac, consulting engineer.
What remains true is that the ability to wring more performance out of hybrid fiber coax plant has proven more robust than many expected a decade ago.
Also being considered are full duplex designs that swap time division for frequency division multiplexing.
Each technology upgrade path has business implications, especially the cost to upgrade HFC in some way without shifting to FTTH. The other assumption is the competitive environment and how long each alternative upgrade can support the expected business model.
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