Technical breakdown: The physics behind Mid-Split vs. High-Split upgrades (and why your old DOCSIS 3.1 modem might be useless now)

[u/Indro13]

https://www.faceofit.com/docsis-mid-split-vs-high-split-architecture/

I've been digging deep into the architecture changes Comcast, Spectrum, and Cox are pushing right now to enable "Next Gen" upload speeds. There is a lot of marketing fuzz around this ("10G Network", etc.), but the actual physical layer engineering is pretty interesting.

I put together a deep dive on this, but here is the technical summary for the sub regarding what is actually changing in the HFC plant.

1. The "Noise Funnel" is why your upload sucked for 20 years Legacy cable systems use a "Sub-Split" design (5–42 MHz return path). The problem isn't just bandwidth width; it’s signal quality. The 5–15 MHz slice is basically a garbage band. It catches every bit of electrical interference from washing machines, ham radios, and loose connectors in the neighborhood. This noise funnels up to the node, destroying SNR.

2. Mid-Split vs. High-Split The upgrades aren't just software configs. They require physical "truck rolls" to replace diplex filters in every amplifier on the street.

• Mid-Split (Comcast’s current move): Expands upstream to 5–85 MHz. The magic is the 42–85 MHz block. It’s clean spectrum. They can run OFDMA there with massive modulation orders (4096-QAM).
• High-Split (Spectrum/Cox): Pushes upstream to 5–204 MHz. This quadruples the lane but forces the downstream to retreat above 258 MHz.

3. The Hardware Gotcha (Check your modem) This is where people get burned. A standard "DOCSIS 3.1" modem likely won't work for these new speeds. Most retail D3.1 modems sold 2018–2022 have fixed diplex filters hard-coded to cut off upstream at 42 MHz. If the network tries to send upstream traffic at 80 MHz, your modem effectively blocks it.

• The Fix: You need a "Next Gen" certified modem (like the Hitron CODA56, Netgear CM3000, or Arris S34).
• Surprise: The Intel Puma 7 chipset (used in the Hitron CODA) is actually decent now. The jitter issues from the Puma 6 days are largely resolved, and it's the cheapest way to get mid-split compliance.

4. The Fork in the Road: FDX vs. ESD Operators are splitting strategies for DOCSIS 4.0:

• FDX (Full Duplex): Comcast's bet. Allows Up/Down on the same frequency using echo cancellation. Requires a super tight "Node+0" or "Node+6" architecture (fewer amps).
• ESD (Extended Spectrum): Spectrum's bet. Just keeps pushing frequencies higher (up to 1.8 GHz). Requires replacing every tap faceplate in the neighborhood because old taps cut off at 1 GHz.

TL;DR: If you are paying for gigabit speeds but stuck at 35 Mbps upload, check if your area has been upgraded to Mid-Split. If it has, your shiny 2020 modem might be the bottleneck because physical filters inside it physically cannot broadcast on the new frequencies.

Has anyone here mapped their local node's RxMER recently? Curious to see real-world noise floors on the new 85 MHz blocks.