Split Plans & DOCSIS Drivers
Why High-Split (MSO / Spectrum-class plant)
Legacy North American HFC used a sub-split: upstream about 5–42 MHz and downstream from about 54 MHz. That was enough when traffic was mostly download. Today homes upload continuously (video calls, cloud backup, gaming). Operators re-split the plant so more spectrum is available upstream.
Industry classes (CableLabs band-split guidance): mid-split moves upstream to about 5–85 MHz with downstream starting near 108 MHz; high-split moves upstream toward 5–204 MHz with downstream starting near 258 MHz. Exact edges are always on the node package — markets and projects differ.
DOCSIS 3.1 OFDM/OFDMA and DOCSIS 4.0 multi-gig goals need that wider upstream. Published industry capacity examples often cite roughly ~450 Mbit/s shared upstream on a mid-split service group versus on the order of ~1.5 Gbit/s class capacity on high-split (shared, not per home). Field plant quality decides whether those numbers survive in production.
For hardline techs supporting MSO high-split upgrades (including Spectrum-class projects), the job is matched hardware, clean return path, correct diplexers, and documented test results — not guessing the split from the amp casting.
Frequency split comparison (typical North American classes)
- Sub-split (legacy)
- US 5–42 MHz · DS from ~54 MHz — still on unupgraded plant
- Mid-split
- US 5–85 MHz · DS from ~108 MHz — common interim upgrade step
- High-split
- US 5–204 MHz · DS from ~258 MHz — large upstream expansion on HFC
- Diplex / guard
- Transition band between US and DS — filter roll-off, not free spectrum
- Extended spectrum
- Downstream toward 1.2–1.8 GHz where plant and passives support it (ESD / 1.8 GHz actives)
- Field marker
- Job package lists split class per node — never assume from amp model alone
HFC path — where split changes take effect
Forward (downstream): services travel toward the customer. Click each stage.
QAM + DOCSIS origin; reverse terminates at CMTS/CCAP
Upload capacity is bounded by upstream MHz and modem count sharing that spectrum. High-split addresses the upload bottleneck — field return-path quality determines whether lab speeds survive in production.
Reading Spectrum Charts
Job packages show upstream upper edge (42, 85, or 204 MHz), downstream lower edge (54, 108, or 258 MHz), and max operating frequency. Sweep/OneCheck limits reference these charts — not universal constants.
Module balanced for mid-split shows 'tilt errors' on high-split sweep template because band edges moved. Always load correct limit file for deployed split.
High-split class example allocation
- Upstream data band
- 5–204 MHz — OFDMA and legacy SC-QAM upstream
- Diplex transition
- ~204–258 MHz — filter roll-off; not customer-usable
- Downstream data band
- ~258–1218 MHz — OFDM + SC-QAM downstream
- Legacy overlap risk
- 42 MHz diplexers misroute energy above legacy edge
- Meter implication
- Ingress scan and reverse sweep must cover full deployed upstream width