Splits refer to the separation between Return/Upstream and Forward/Downstream spectrum allocation in DOCSIS. Mid and High refer to moving that separation higher, allocating more bandwidth to the Return channels for more upload capacity (speed).
Figure 1. Examples of mid/high-split and extended spectrum DOCSIS. They offer significant improvements to upstream and downstream bandwidth and performance.
"Split" is a reference to the separation or spectrum allocation between Return (Upstream) and Forward (Downstream) signals in DOCSIS coaxial cable plants (HFC, R-PHY, DAA).
- In the original spectral allocation splits, the Return (US) signals were limited to around 42 MHz (USA) or 65 MHz (Europe).
- Mid-Split allocates a bit more spectral bandwidth to the Return signals, to around 85 MHz, allowing faster and more reliable upstream data rates.
- High-Split moves the Return spectrum allocation even higher, to around 204 MHz, giving more bandwidth to the upstream (US) for more speed. This is normally combined with 1.2 GHz Extended Spectrum DOCSIS 3.1 (ESD) for the Downstream (DS), to offer much higher (more competitive) upstream and downstream data rates.
- Ultra High-Split moves the spectral allocation split even higher, to around 684 MHz, giving a significant amount of bandwidth to the upstream for much more upload speed. The idea is to combine it with 1.8 GHz Extended Spectrum DOCSIS 4.0 (ESD) for the downstream, to offer much higher (more competitive) upstream and downstream data rates and certain symmetrical high-speed services.
- ESD refers to extending the usable coax spectrum to 1.2 GHz in the short term and up to 1.8 GHz (and maybe 3 GHz) in the long term, to allow existing HFC plants to evolve along FTTx optical distribution networks (ODN) and stay competitive during the transition to fiber optics.
Of course, the implementation of new bandwidth allocations requires updating, qualifying, validating and troubleshooting all new passive and active elements in the coax plants being updated, to make sure they comply with the new bandwidth requirements. For modern hybrid R-PHY/DAA/PON networks, this can be verified by (frequency) sweeping the coax segments with a new generation of portable meters (test sets) supporting the new extended frequency ranges. Portability is key because the fiber feeders are now digital and the remaining coaxial segments are at the edge of the HFC network. Centralized sweep generators are no longer viable for this application.
The latest (2024) DOCSIS 4.0 Cable Modems can now achieve downstream speeds of 9 Gbit/s and upstream speeds greater than 2 Gbit/s, using ultra-high split (684 MHz). This 2024 interop testing was performed against various DOCSIS 4.0 Remote Phy Devices and DOCSIS 3.1 CMTSs. (Source: CableLabs D4.0 Interop).
Future Spectrum Allocations? At the 2024 SCTE TechExpo trade show, we saw credible development announcements from Broadcom, Charter and Comcast for supporting future 3 GHz HFC networks, which could be considered DOCSIS ‘5.0’. The goal is to support 25 Gbit/s downstream speeds. (Press release: 25G DOCSIS)
For more details, refer to the Extended Spectrum DOCSIS (ESD) & High Split Article and visit the DOCSIS 3.1/4.0 resources page.
Related Test Solutions
- SWP-BOX - Portable Sweep Generator (supports extended spectrum)
- CX380C - Advanced Plant Maintenance Meter (handheld sweep receiver)
- CX310 - DOCSIS 3.1 Installation Tool