Originally featured on BroadcastEngineering.com
DOCSIS 3.1 comes top at Cable Congress
Cable operators are set to accelerate deployment of the latest DOCSIS 3.1 data over cable standard, in some cases to move quickly to an all-IP strategy, it emerged from the recent Cable Congress held in London.
This is the first new version of DOCSIS since 3.0 arrived in 2006 with channel bonding to boost headline data rates, and 3.1 achieves a similar trick by introducing OFDM (Orthogonal Frequency Division Multiplexing). Apart from bringing forward migration to all video over IP, welcomed by many operators as a route to reduce transmission costs and improved scalability at the network edge as well as easing deployment of cloud based delivery to multiscreen devices, DOCSIS 3.1 enables broadband speeds to keep pace with telcos without having to upgrade the infrastructure. This ability to extract more life from existing cable plant is seen as the key differentiator of DOCSIS 3.1 by many in the cable industry, rather than increasing bit rates since that can always be achieved by techniques such as node splitting that involve pushing fiber closer to the home. While telcos have to push fiber deeper to increase bit rates via VDSL2 technology, cable operators sense that they can compete successfully on price by avoiding having to do the same.
DOCSIS 3.1 does away with the 6MHz and 8MHz wide channel spacing used so far, replacing them with smaller 20KHz-to-50KHz-wide subcarriers that can then be bonded together inside a spectrum block up to 200MHz wide. OFDM is used to spread the symbols carrying data across this whole spectrum, with multiple carriers ensuring that overall high data rates are obtained.
Since there are multiple carriers for a given amount of information, each one is modulated more slowly than if there were just one carrier, in effect stretching each symbol out in time. As well as improving robustness this increases performance because it reduces the impact of the guard intervals, regions of spectrum carrying no information, needed to protect against interference. The guard interval take a fixed time to transmit and so because the symbol is spread out more in time, the proportion of that interval consumed by the guard interval is reduced.
OFDM combined with some other improvements enables DOCSIS 3.1 to boast headline download speeds up to 10Gb/s and 1 Gbps upstream, compared with up to 1.5Gb/s down and 240Mb/s up for some recent implementations of DOCSIS 3.0. Many operators have already deployed extra fiber to obtain higher bit rates, but are now hoping that DOCSIS 3.1 will avoid the need for any further upgrades for some time to come at a time when margins are being squeezed in many cases.
For example, UK cable company Virgin Media, which has 3.7 million pay TV subscribers and has just been acquired by the Liberty Global group for $23.3 billion, is evaluating use of DOCSIS 3.1 to deliver downloads in 10Gb/s bursts without overhauling its network.
Virgin Media currently runs on DOCSIS 3.0 at speeds of 1.5Gb/s for a trial service in London, but more generally 100Mb/s, over coaxial cable to street cabinets served by fiber following an earlier network upgrade.