The Tale of DOCSIS and cable operators
May 2007

When anyone that uses the Internet on a regular basis is presented with an opportunity to upgrade their access speeds they will usually jump at the opportunity without a second thought. There used to be a similar analogy with personal computers with operating systems and processor speeds, but this is a less common trend these days as the benefits to be gained are often ephemeral as we have recently seen with Microsoft's Vista. (Picture: SWINOG)

However, the advertising headline for many ISPs still focuses on "XX Mbit/s for as little as YY Pounds/month". Personally, in recent years, I have not seen too many benefits in increasing my Internet access speed because I see little improvement when browsing normal WWW sites as their performance are not now bottlenecked by my access connection but rather the performance of servers. My motivation to get more bandwidth into my home is the need to have sufficient bandwidth - both upstream and downstream - to support my family's need to use multiple video and audio services at the same time. Yes, we are as dysfunctional as everyone else with computers in nearly every room of the house and everyone wanting to do their own video or interactive thing.

I recently posted an overview of my experience of Joost, the new 'global' television channel recently launched by Skype founders, Niklas Zennstrom and Janus Friis - Joost’s beta - first impressions and it's interesting to note that as a peer-to-peer system it does require significant chunks of your access bandwidth as discussed in Joost: analysis of a bandwidth hog.

The author's analysis shows that it "pulls around 700 kbps off the internet and onto your screen" and "sends a lot of that data on to other users - about 220 kbps upstream". If Joost is a window on the future of the IPTV on the Internet, then its should be of concern to the ISP and carrier communities and it should also be of concern to each of us that uses it. 220kbits/s is a good chunk of of the 250kbit/s upstream capability of ADSL-based broadband connections. If the upstream channel is clogged, response time on all services being accessed will be affected. Even more so if several individuals are are access Joost of a single broadband connection.

It's these issues that make me want to upgrade my bandwidth and think about the technology that I could use to access the Internet. In this space there has been an on-going battle for many years between twisted copper pair ADSL or VDSL used by incumbent carriers and cable technology used by competitive cable companies such as Virgin Media to deliver Internet to your home.

Cable TV networks (CATV) have come a long way since the 60s when they were based on simple analogue video distribution over coaxial cable. These days they are capable of delivering multiple services and are highly interactive allowing in-band user control of content unlike satellite delivery that requires a PSTN based back-channel. The technical standard that enables these services is developed by CableLabs and is called Data Over Cable Service Interface Specification (DOCSIS). This defines the interface requirements for cable modems involved in high-speed data distribution over cable television system networks.

The graph below shows the split between ADSL and Cable based broadband subscribers: (Source: Virgin Media) with Cable trailing ADSL to a degree. The link provided provides an excellent overview of the UK broadband market in 2006 so I won't comment further here.

A DOCSIS based broadband cable system is able to deliver a mixture of MPEG-based video content mixed with IP enabling the provision of a converged service as required in 21st century homes. Cable systems operate in a parallel universe, well not quite, but they do run a parallel spectrum enclosed within their cable network isolated from the open spectrum used by terrestrial broadcasters. This means that they are able to change standards when required without the need to consider other spectrum users as happens with broadcast services.

The diagram below shows how the spectrum is split between upstream and downstream data flows (Picture: SWINOG) and various standards specify the data modulation (QAM) and bit-rate standards. As is usual in these matters, there are differences between the USA and European standards due to differing frequency allocations and standards - NTSC in the USA and PAL in Europe. Data is usually limited to between 760 and 860MHz.

The DOCSIS standard has been developed by CableLabs and the ITU with input from a multiplicity of companies. The customer premises equipment is called a Cable Modem and the Central Office (Head End) equipment is called the a cable modem termination system (CMTS).

Since 1997there have been various releases (Source: CableLabs) of the DOCSIS standard with the most recent being version 3.0 being released in 2006.

DOCSIS 1.0 (Mar. 1997) (High Speed Internet Access) Downstream: 42.88 Mbit/s and Upstream: 10.24 Mbit/s

  • Modem price has declined from $300 in 1998 to <$30 in 2004

DOCSIS 1.1 (Apr. 1999) (Voice, Gaming, Streaming)

  • Interoperable and backwards-compatible with DOCSIS 1.0

  • “Quality of Service”

  • Service Security: CM authentication and secure software download

  • Operations tools for managing bandwidth service tiers

DOCSIS 2.0 (Dec. 2001) (Capacity for Symmetric Services)  Downstream: 42.88 Mbit/s and Upstream:30.72 Mbit/s

  • Interoperable and backwards compatible with DOCSIS 1.0 / 1.1

  • More upstream capacity for symmetrical service support

  • Improved robustness against interference (A-TDMA and S-CDMA)

DOCSIS 3.0 (Aug. '06) Downstream: 160 Mbit/s and Upstream: 120 Mbit/s

  • Wideband services provided by expanding used bandwidth through the use of channel bonding e.g. instead of a single data channel being delivered over a single channel, they are multiplexed over a number of channels. ( A previous post talked about bonding in the ADSL world Sharedband: not enough bandwidth? )

  • Support of IPv6


With the release of the DOCSIS 3.0 standard it looks like cable companies around the world are now set to be able to upgrade the bandwidth they will be able to offer to their customers in coming years. However, this will be an expensive upgrade for them to undertake with the need to upgrade head end equipment first and then followed by field cable modem upgrades over time. I would hazard a guess that it will be at least five years before the average cable user will be able to see the benefits.

I also wonder about what price will need to be paid for the benefit of gaining higher bandwidth through channel bonding when there is limited spectrum available for data services on the cable system. A limit in subscriber number scalability?

I was also interested to read about the possible adoption of IPv6 in DOCSIS 3.0. It was clear to me many years ago that IPv6 would 'never' (never say never!) on the Internet because of the scale of the task. It's best chance would be in closed systems such as satellite access services and IPTV systems. Maybe, cable systems are an another option. I will catch up on IPv6 in a future post.

Back to home