DOCSIS 4.0: Swimming Upstream

Improving the DOCSIS protocol based on user-demand.

coaxial cables for DOCSIS

Back in the day, content couldn’t be downloaded fast enough. There were movies, pictures, music…it was all take, take, take. These days, the clear divide between upload and download needs is closing fast. Over the past few years, mankind has been giving a lot back…to the network.

Upload speeds started growing with increased usage of sites like YouTube and Twitch, as well as home security cameras. Then suddenly, COVID-19 struck and multiple people within the same household are all video conferencing at all times in the day! Everyone was attending work and school from the comfort of their living room which resulted in a 63% surge in uploading, shifting the bandwidth priority. The DOCSIS protocol was required to evolve to satisfy the constant need for speed and intolerance for latency. Particularly now that Internet usage has become an absolute requirement for most employment, instead of simply Netflix and chill.

The Evolution of DOCSIS

DOCSIS, which stands for Data Over Cable Service Interface Specification, was created in 1997. It was a pretty ingenious solution for high-speed internet because it was designed to leverage the existing cable infrastructure. This clearly saved everyone involved millions of dollars and hours compared to starting from scratch.

At this time, you couldn’t assume people had internet access at home. Social media barely existed, and using email at work was just starting out. The concept of the cloud or video conferencing was still only accessible in sci-fi movies. It was all new and the traffic was nothing compared to what it is today. With that in mind, DOCSIS 1.0 could reach a bandwidth of 40 Mbps in the downstream channel and about 10 Mbps upstream. As Internet usage increased the protocol evolved with it, based on the demand. Less than 10 years later, DOCSIS 3.0 was released with the ability to connect 6 or 8 MHz channels downstream, with speeds of up to 340 Mb/s or 440 Mb/s for EuroDOCSIS. In the upstream direction, DOCSIS 3.0 reached about 120 Mbps.

DOCSIS 3.0 to DOCSIS 3.1

In 2013, DOCSIS technology took a hard left and introduced DOCSIS 3.1. As with previous iterations, the specification revision was designed to speed things up and lower latency. Considering the demand of the time and the newer competition in the field, bigger things needed to change in the next release.

Up until 3.0 the modulation used for downstream DOCSIS was (and is) single carrier quadrature amplitude Modulation or SC-QAM. 64-QAM and 256-QAM are identified in the specification. As SC-QAM is a single carrier technology, it means there is only one carrier, and that carrier has a specific bandwidth. To transmit data, the data is modulated with the single carrier and it transfers the information.

Transitioning to DOCSIS 3.1 was completely different because it leverages orthogonal frequency-division multiplexing OFDM and orthogonal frequency-division multiple access OFDMA. OFDM transmits each piece of data across multiple narrowband subchannel frequencies as opposed to one wideband channel. OFDMA, as multiple access implies, takes a channel and subdivides it, which can be distributed to many devices more efficiently. What happens with the OFDM is that instead of one carrier for specific bandwidth, there are multiple carriers, and they are orthogonal, so they cannot interfere with each other. Each carrier is then independently modulated, and you can efficiently use the bandwidth instead of using specific-bandwidth transmit only.

This was a major shift for the industry, and required the MSOs and chipset manufacturers to consider complete redesigns and validation requirements.

Cable Tomorrow: DOCSIS 4.0

Moving into the 2020s it was clear that the DOCSIS 3.1 upstream limit wasn’t going to cut it moving forward. This was the catalyst to move into DOCSIS 4.0, which will have a broader frequency range and wider channels. Potentially, it will deliver speeds of 10 gigs for upstream and 6 -7 g for downstream. 4.0 comes in two different flavors which are Frequency Division Duplex (FDD) and Full Duplex DOCSIS (FDX).

• FDD

FDD mode of operation ads on to how 3.1 was before but extending the upstream-only spectrum. So that means 5-85 MHz is used for legacy. 108 - 684 MHz can be used for upstream while also extending the downstream-only spectrum from 684 up to 1.8 GHz. MSOs can offer more bandwidth and higher data rates for both upstream and downstream, and it is symmetrical, but portions of the spectrum are reserved for upstream only or downstream only.

• FDX

FDX technology is full-duplex, which means it can transfer data in both directions simultaneously. Here it will again use 108-684 MHz spectrum but with the ability to simultaneously transmit both upstream and downstream. Similar to FDD, downstream, extends from 684 MHz up to 1.2 GHz, instead of 1.8. It has the ability to extend up to 1.8 but is unnecessary since 108-684 accommodates both.

DOCSIS 4.0 is well underway and will be accessible in the near future. Both FDD and FDX will be available, depending on the service provider and it is unclear today which is preferred.

For questions on new product validation for modems and chipsets, please contact Averna.