matthEW peach JOHN WILLIAMSON
I n October 2013, CableLabs, the not-for-profit research and development consortium for the cable industry, announced the availability of its DOCSIS 3.1 PHY (physical layer) and MULPI (MAC and Upper Layer Protocol) specifications. Inter alia, the DOCSIS 3.1 specification can support the following: • Higher speeds, defining support for up to 10 Gbits/s downstream and up to 1 Gbits/s upstream network capabilities; • Improved Quality of Experience, by utilising so-called Active Queue Management to significantly reduce network delay as data traffic grows in the home network; and a significant increase in network capacity, with the ability to transmit up to 50 percent more data over the same spectrum, on existing HFC networks. More recently, in a 2015 Infonetics/IHS survey of the cable operators worldwide that collectively control 87% of the world’s cable CapEx, it was revealed that about one third of their residential subscribers will be passed by DOCSIS 3.1-enabled headends by April 2017. In December 2015, Comcast – the American multinational mass media company and largest broadcasting and cable company in the world by revenue – reported that it had installed what it believed to be the world’s first DOCSIS 3.1 modem on a customer-facing network. DOCSIS 3.1 speeding things up
Beating OBI A number of approaches have been developed to mitigate or eradicate the undesirable effects of OBI. One is to limit the number of simultaneous upstream transmissions via the use of only a single upstream channel. However, this has the downside of not exploiting the full potential of RFoG. A second approach is to ensure that the network does not schedule transmissions from different R-ONUs at the same
combiner. This involves replacing passive splitters/ combiners with low-powered active combiners that eliminate OBI at the point where it occurs. With the OBI problem addressed, by whichever means, ARRIS is predicting great things for RFoG. “We needed to solve OBI before RfoG technology could be feasible for mass market deployments,” concluded Ciocirlan. “Now that we’ve solved it we see great potential for the technology.”
A third approach is to arrange the network and its elements such that separate R-ONUs which are sharing the same fibre splitter are wavelength- separated. This means either physically configuring the R-ONUs to transmit at slightly different wavelengths, or tuning the lasers in the R-ONUs to perform in that way from their installation. A fourth approach, and one embodied by ARRIS in its AgileMax Hybrid PON (HPON) product line, is to tackle the problem in the optical splitter/
time. Critics of this approach concede that it works well when only one or two upstream channels are considered, but contend that scheduling becomes inefficient much above three channels, particularly when DOCSIS 3.1 is part of the mix. DOCSIS 3.1 aside, there is also the suggestion by some in the industry that the scheduler method is best suited to low bitrate applications in the 20 Mbits/s to 30 Mbits/s range, at a time when more and more cable operators are increasingly looking to provide 50 Mbits/s to 100 Mbits/s.
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