Do you know your FDD from your FDX?
Data Over Cable Service Interface Specifications, much better known as DOCSIS is the technology that is used to communicate and transfer data across RF or coaxial cable. This technology has been around since 1997 and continues to evolve today.
The goal for the DOCSIS protocol was to bring high-speed internet into homes and businesses across the planet. Again, this was 1997…if anyone had internet, it was through a phone dial-up system and buffering took so long, they needed to create a word for it. DOCSIS was originally established using pre-existing cable TV systems. It was a very clever way to leverage an existing infrastructure for such a massive undertaking.
How Does DOCSIS Work?
Up until 2013 (DOCSIS 1.0 - 3.0) the modulation used for downstream DOCSIS was (and is) SC-QAM; specifically, 64-QAM and 256-QAM. As SC-QAM is a single carrier technology, it means that there is only one carrier with a specific bandwidth. To transmit data, it is modulated with the single carrier, and it sends the information.
Upon the release of 3.1, DOCSIS began leveraging OFDM (Orthogonal Frequency Division Multiplexing) and OFDMA (orthogonal frequency-division multiple access). OFDM transmits each piece of data across multiple narrowband subchannel frequencies as opposed to only one wideband channel. What happens with OFDM is that instead of only 1 carrier for specific bandwidth, you have 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. OFDMA, as multiple access implies, takes a channel and subdivides it, which can be distributed to many devices more efficiently. By employing higher and more efficient modulation orders as 1k-QAM to 4k-QAM instead of the 256-QAM, cable providers can pack in more bits per hertz.
This switch came with many benefits including:
• Easy adaptation to severe channel conditions.
• Efficient use of the spectrum by sharing the spectrum between several users with the multiple access technology.
• Reduction of interferences.
• Flexible modulation schemes.
Along with these benefits, DOCSIS 3.1 brought a massive change to modem and chipset manufacturers. They needed to deal with what was essentially a brand-new technology and it didn’t happen overnight. MSOs (Multiple System Operators) required complete hardware redesigns and was a major cost for the deployment of new CMTS (Cable Modem Termination System) passive and active devices.
In some cases, the infrastructure itself needed to be redesigned. The 85MHz upstream band (which is referred to as mid-split) was significantly less problematic as existing DOCSIS equipment used the same bandwidth but the 204MHz band (high-split) required new hardware including the customer premises equipment. Also, upgrading the network to raise the upper limit of the downstream spectrum was quite challenging as it required replacing passive and active devices.
Moving into the Future of DOCSIS
DOCSIS will always remain relevant because the demand drives the technology. So, while some will continue to pursue the introduction of multi-gigabit services, there are other benefits of broadband connectivity including reliable network and low latency that drives the DOCSIS industry to extend HFC investments and the path to 10G.
DOCSIS 3.1 has a set limit for upstream and the market is showing that it is not high enough. With the normalization of constant streaming and video calls, the usage is growing by 50% every year. At this rate the maximum capacity for upstream will be surpassed very soon. Additionally, the demand is for not only for symmetric multi-gigabit speed over HFC networks, but it is for low latency, constant jitter and reliability. This was the catalyst to move into DOCSIS 4.0 and 10G, which is the next step. 10G offers higher speed, more reliability, better security and lower latency.
As a result, MSOs must boost the speed and capacity of upstream by increasing its bandwidth and this is being solved with DOCSIS 4.0. The next revision of DOCSIS has a broader frequency range for upstream at 684MHz which has the potential to deliver speeds of 10Gbps for downstream and up to 7Gbps upstream.
The Flavors of DOCSIS 4.0
DOCSIS 4.0 comes in 2 different flavors to meet the demands of its users. These are FDD (Frequency Division Duplex) and FDX (Full Duplex DOCSIS). MSOs have the flexibility to pick the technology they will be moving forward with, whether that is FDD or FDX, but cannot choose to do both, as it is too expensive. It is undecided which is preferred but CableLabs has developed 4.0 specifications for both technologies.
FDD mode is essentially an extension of DOCSIS 3.1, leveraging the same technology. FDX is a new technology that hasn’t been used before.
FDD is also known as Extended Spectrum DOCSIS (ESD). This implies that it extends the upstream-only spectrum and the downstream-only spectrum independent of one another. With FDD, 5-85MHz is used for legacy (as before) while the upstream spectrum can be extended as high as 684MHz. This leaves 108-684MHz for upstream-only while the downstream-only spectrum is extended to 1.8GHz. MSOs can offer more bandwidth and higher data rates for both upstream and downstream, and it is symmetrical.
FDX technology improves capacity by enabling network devices to simultaneously transmit both downstream and upstream channels in the same 108-684MHz spectrum. Additionally, the downstream-only spectrum further extends up to 1.2GHz. The increased upstream capacity with FDX is within the shared spectrum and it is more flexible and efficient. Different users can use the 108-684 band for different purposes and there is no issue. With people using both upstream and downstream in the same channel, it should cause interference. It is the CM (cable modem) that needs to stop that interference. This is not an issue for the 3.1 architecture and, of course, FDD.
Comcast has announced that they are releasing an FDX modem in 2023. On the other hand, Charter Communications and other MSOs are leaning towards FDD. Charter demonstrated speeds of about 8.5 Gbit/s downstream and 6 Gbit/s upstream in its trials. As for silicon availability, there aren’t a lot of options as few chip makers have focused on DOCSIS 4.0. Based on news and trials from MSOs there is silicon for remote PHY FDX and FDD devices and both Broadcom and Maxlinear are reported to be involved in developing silicon for DOCSIS 4.0. That said, everything is changing so quickly that this may not be the case tomorrow.
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