A Technical Guide to DOCSIS 3.1 and Beyond – Part 1: Increasing Capacity

It has been 10 years since the DOCSIS 3.0 specification was released, and in those 10 years bandwidth demand has continued to grow unabated. Butter’s Law states that optical transmission speeds are doubling every nine months. But how does that compare to the latest specification of DOCSIS?

From 1997 to 2006 we saw downstream capacity grow from 38Mbps of DOCSIS 1.x to as much as 1Gbps of DOCSIS 3.0 with 32 bonded downstreams; with the latest 3.1 specification we are at 10Gbps. Let’s look at what DOCSIS has provided with the latest revision of the standard, and consider whether the capabilities it affords are sufficient motivators to upgrade today.

At the most basic level, DOCSIS 3.1 has boosted bandwidth capacities from 1Gbps downstream to 10Gbps and allows 100Mbps upstream to 1Gbps. The standard is backwards compatible with the previous generation and even coexists with networks running DOCSIS 3.0. Although this is a significant improvement over the previous iteration, the bandwidth growth rate is roughly half of the prediction from Butter’s Law.

However, using a DOCSIS 3.1 modulation of 4096-QAM we end up just north of 64.32Mbps while the previous capped modulation of 256-QAM is only able to achieve a theoretical rate of 42.88Mbps. This has a theoretical advantage of 50 percent increase of data capacity using the same 6Mhz channel. In reality, the two numbers end up being 54Mbps and 38.81Mbps respectively (due to DOCSIS overhead and spectral efficiencies).

How is this all achieved? Spectral efficiencies of higher order modulations. Using 256-QAM, the highest modulation available in DOCSIS 3.0, as a benchmark we can compare at a basic level what the spectral efficiencies are using this formula:

bandwidth = rate* x log2(modulation)
(*where 256-QAM rate as defined by the ITU-T standard J.83 (12/07) at 5.361 Msym/s.)

So what does this mean for service providers?

There are loads of technical advantages that we will explore in part two of this guide, but what really gets the end-users’ attention is additional bandwidth capacity. As we can see from the evidence above, enabling a 50 percent increase of data capacity is no minor feat for a service provider. The advantages gained from this include faster, more reliable service speed and increasing capacity to add additional users.

While many operators still employ DOCSIS 2 equipment, now is the time to extend the life of your copper plant with DOCSIS 3.1. Check out my next post for a list of the technical advantages enabled by DOCSIS 3.1.