The global telecommunications infrastructure is navigating a complex evolutionary turn. For nearly twenty-five years the Hybrid Fiber-Coaxial networks serving most North American homes operated on a foundational asymmetry. Designed for broadcast video and downstream data these networks allocated the majority of spectrum to the home. This left a narrow sliver for the return path.

This decision created the familiar “1000/35” speed tiers. Massive gigabit download capabilities paired with double-digit upload speeds. The post-2020 landscape of high-definition video conferencing and real-time applications rendered this asymmetry untenable.

This report analyzes the industry response: the migration to “Mid-Split” and “High-Split” DOCSIS architectures. These technologies represent a fundamental re-engineering of the cable plant physical layer. We explore RF spectrum allocation physics, Orthogonal Frequency-Division Multiple Access introduction, strategy divergence between operators like Comcast and Charter, and the role of next-generation Consumer Premises Equipment.

The Physics of HFC Spectrum Allocation

To understand the engineering magnitude of Mid-Split and High-Split upgrades we must deconstruct the legacy architecture governing cable systems since DOCSIS 1.0.

The Noise Funnel Mechanics

The HFC return path (upstream) functions as an inverted tree. Noise generated at every home in a service group (typically 60 to 500 homes) combines as it travels upstream toward the node. This accumulation is known as the “funnel effect.”

The lowest frequencies (5 MHz to 15 MHz) are the most vulnerable. This band is susceptible to impulse noise from household motors, shortwave radio interference, and electrical grid feedback. In a legacy Sub-Split system, nearly 30% of the available upstream spectrum is often unusable due to this high noise floor.

Mid-Split opens the 42 MHz to 85 MHz band. This spectrum resides above the “garbage band” of low-frequency interference. Consequently, operators can run higher modulation orders (4096-QAM) in this new upper block, yielding significantly higher spectral efficiency per Hertz than is possible in the legacy 5-42 MHz block.

The Diplex Filter Barrier

Transitioning frequencies is not merely a software configuration change. The barrier is physical. Inside every amplifier in the network—often every 1,500 to 2,000 feet—resides a component called a “diplex filter.” This passive device mechanically separates the upstream and downstream frequencies.

To activate Mid-Split (85 MHz) or High-Split (204 MHz), field technicians must physically access every amplifier housing in the cascade to replace these modules. This “truck roll” requirement explains the neighborhood-by-neighborhood deployment pace seen with Comcast and Charter.

The Mid-Split Architecture (5–85 MHz)

Mid-Split represents the primary expansion of the upstream in the DOCSIS 3.1 era. By moving the diplex filter boundary operators expand upstream allocation from 5–42 MHz to 5–85 MHz. This expansion creates a clean slate of spectrum from 42 MHz to 85 MHz. This upper band allows for high-efficiency OFDMA channels running at high modulation orders.

The High-Split Architecture (5–204 MHz)

High-Split pushes the upstream boundary to 204 MHz. This quadruples available upstream spectrum compared to legacy systems. It theoretically supports symmetrical speeds of 1 Gbps or greater. The engineering cost is substantial. Pushing the upstream to 204 MHz forces the downstream to retreat to approximately 258 MHz.

Modulation Technologies: The Role of OFDMA

Expansion of raw Hz is the first step. Efficiency determines throughput. This is the domain of Orthogonal Frequency-Division Multiple Access (OFDMA).

Legacy DOCSIS 3.0 systems utilized Single Carrier QAM (SC-QAM). While robust, SC-QAM is spectrally inefficient. OFDMA divides the upstream spectrum into thousands of tiny subcarriers. This provides ingress noise cancellation and variable bit loading.

OFDMA Subcarrier Dynamics

In DOCSIS 3.1 upstream, the channel is composed of subcarriers spaced at either 25 kHz or 50 kHz. This granularity allows the Cable Modem Termination System (CMTS) to perform “profile management.”

Unlike SC-QAM, where the entire channel fails if noise spikes at one frequency, OFDMA can simply turn off specific subcarriers that are experiencing interference while keeping the rest of the channel active. This resilience is vital for maintaining stability in the 42-85 MHz mid-split band.

Interactive Modem Database (2026)

The shift to Mid/High-Split has bifurcated the consumer market. A “DOCSIS 3.1” label no longer guarantees access to the fastest plan speeds. Use the tool below to find the right hardware.

Chipset Architecture: The Puma Paradox

For years, the enthusiast recommendation was “Avoid Intel Puma.” The Puma 6 chipset (latency/jitter bug) caused severe reputation damage. However, the current landscape has shifted.

• Hitron CODA56 (Puma 7): This modem uses the MaxLinear (formerly Intel) Puma 7. It is widely certified for mid-split upload speeds. Testing confirms the Puma 7 does not suffer the severe TCP/UDP jitter of its predecessor. It is a viable, cost-effective option.
• Broadcom BCM3390 (Netgear/Arris): The gold standard. Broadcom chipsets utilize a different packet processing architecture that historically offers slightly lower latency under heavy load. For competitive gaming or L4S implementation, Broadcom remains the preferred silicon.

Network Infrastructure Transformation

The upgrade to Mid-Split or High-Split involves physical labor at every active point in the network. Technicians must visit every amplifier location. Actives must be upgraded with new modules containing updated diplex filters.

Physical Plant: The Hardline Reality

The “hardline” coaxial cable (aluminum shielded, 0.500 to 0.875 inches thick) hanging on utility poles is robust. However, the “Taps” (passive devices that split signals to individual homes) are often a bottleneck. Many legacy tap faceplates were designed with a hard cutoff at 1 GHz.

To enable 1.2 GHz (Mid-Split) or 1.8 GHz (DOCSIS 4.0), operators often must unscrew and replace the faceplate of every tap in the system. If a tap is not upgraded, high-frequency signals simply hit a wall, preventing the new spectrum from reaching the home.

Distributed Access Architecture (DAA)

Operators are moving away from centralized monolithic units toward Distributed Access Architecture.

• Remote-PHY: Moves physical layer generation to the fiber node. The link between headend and node becomes digital Ethernet, eliminating analog laser noise (SNR improvement).
• vCMTS: Comcast has deployed virtualized CMTS solutions allowing rapid scaling and easier implementation of features like Low Latency DOCSIS.

Future Path: FDX vs. ESD

As operators look beyond Mid-Split toward DOCSIS 4.0, a strategic divergence has emerged regarding how to achieve multi-gigabit symmetry.

Node+0 Economics

Originally, Full Duplex DOCSIS (FDX) required a “Node+0” architecture, meaning the fiber node connects directly to the passive coax network with zero amplifiers in between. This eliminates echo, allowing simultaneous upstream and downstream transmission on the same frequency.

However, stripping all amplifiers from a network is prohibitively expensive (requiring massive fiber construction). Consequently, Comcast developed “FDX Amplifier” technology, allowing FDX to work in a “Node+2” or “Node+6” environment, saving billions in construction costs while still utilizing the FDX band.

Full Duplex DOCSIS (FDX)

Favored by Comcast, FDX allows upstream and downstream traffic to occupy the same frequency spectrum simultaneously. This is achieved via advanced echo cancellation.

• Advantage: Does not require extending total spectrum beyond 1.2 GHz initially, saving tap upgrades in some areas.

Extended Spectrum DOCSIS (ESD)

Favored by Charter and Cox, ESD maintains the traditional frequency separation but pushes the ceiling higher—up to 1.8 GHz.

• Requirement: Upgrading all taps and passives to support 1.8 GHz.
• Advantage: Simpler operational model (no echo cancellation complexity), closer to traditional RF management.

The Latency Frontier: LLD and L4S

The most significant improvement for real-time applications is latency reduction. The “Next Gen” upgrade includes support for Low Latency DOCSIS.

LLD implements the IETF standard L4S (Low Latency, Low Loss, Scalable Throughput). This allows applications to mark packets as latency-sensitive. The modem maintains two separate queues. A “Classic” queue handles bulk traffic while a separate “LLD” queue handles marked traffic. This queue is kept short ensuring immediate transmission for video calls or gaming packets.

Operator Strategies

Throughput Reality: Overhead Calculations

Users upgrading to 2 Gbps downstream tiers often face a bottleneck at the modem port. Due to TCP/IP overhead, a Gigabit Ethernet port saturates at roughly 940 Mbps.

Final Thoughts

The transition to Mid-Split and High-Split architectures marks the end of the “download-only” mindset. Through physics and advanced mathematics cable operators have engineered a mid-life kicker for HFC networks.

For the consumer hardware matters. Specific “Next-Gen” certification is required to unlock network potential. Latency is improving alongside bandwidth quantity. Understanding whether your provider is deploying Mid-Split or High-Split is essential for selecting equipment.