Hybrid fiber coax upgrades intended to increase upstream bandwidth can take a number of forms. Shrinking the serving areas; switching to fiber-to-home and re-architecting the network for different frequency plans are the typical choices.
For operators who want to delay the shift to FTTH, moving from the standard HFC low-split design, and substituting a mid-split or high-split frequency plan, are the two architectural choices other than shrinking the fiber node serving areas or moving to an entirely-new FTTH network.
As always, incrementalism is favored. Comcast appears to prefer the mid-split option, while Charter seems to be leaning towards a more-radical high-split approach. In terms of capital investment, the mid-split choice might be a shorter-window bridge to FTTH, while high-split might allow a longer window before FTTH is required.
More symmetrical bandwidth is a large part of the thinking.
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. That is an option for DOCSIS 4.0 networks, and is a break from the frequency division HFC has used.
Full duplex networks would allow the upstream and downstream traffic to use the same spectrum at the same time. That would require an HFC upgrade to a node-plus-zero amplifiers” design that is similar to fiber to the curb. The drop to the user location still uses coaxial cable, but without any radio frequency amplifiers.
The whole point of all these interventions is to supply more upstream or return bandwidth than HFC presently provides.
Cable operators are a practical bunch, and will prefer gradualism when possible. So one might hypothesize that either mid- or high-split designs will be preferred.
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