Broadband Access Technology Comparison
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Broadband Access Technology Comparison The goal of this document is to provide a handy, convenient and accurate comparison of the available broadband technologies including DOCSIS 3.0 (and 3.1), vectored VDSL technologies and G.fast variants, and G.hn which is used as an access technology (G.Now and Korea Telecom GiGa Wire™) and the most recently announced MoCA Access™. There is much information but no singular repository or document which fairly and accurately represents all the broadband technologies, thus MoCA contracted with Peter White and his team at Rethink Research to produce such a document. The firm gathered and analyzed the data which already existed in their database, from discussions and demonstrations at tradeshows and conferences, publicly available documents and by lengthy interviews with companies such as Nokia, MaxLinear, Arris, Sckipio and Adtran. Rethink Research also spoke with a few key operators and Professor John Cioffi. Throughout the information gathering process, an attempt was made to better understand attenuation, QoS layers, latency, security, real world performance, and whether the networks can operate with or without a home gateway. Additional attention was given to speeds the protocol can support upstream as well as downstream. This typically meant establishing whether the protocol is TDD or FDD based, and if the FDD spectrum or the TDD timings, are fixed, programmable and/or dynamic. This paper also captures other relevant details such as whether vectoring is required or built in, if system is point-to-point (PtP) or point to multipoint (PtMP), how many clients it can support in the real world, and where in the spectrum the usable bandwidth resides and took that into consideration when addressing actual throughput. G.fast, VDSL and their variants VDSL has been around a long time. There are significant variants which use 8.5 Mhz, 17.7 MHz and 35.33 MHz and a second version of VDSL (VDSL2) which supports longer distances and use vectoring, speeds up to 360 Mbps have been achieved when the DSLAM connection is under 100 meters from the home falling all the way back to 50 Mbps as far as 1 kilometer from the home. 1 VDSL2 mostly adhered to the 17.7 MHz version (at a top rate of 130 Mbps) and the 35 MHz version (VPlus or 35b). These are often used as operators migrate to G.fast. In some cases telcos replace VDSL with a combination of 35b alongside G.fast, each in different parts of the network. The selection is made based on loops lengths, especially if no greater benefit than 35b’s 300 to 360 Mbps is possible. Unlike coaxial cabling, unshielded twisted pair is not protected from interference. These pairs tend to have been installed for VDSL in large bundles so the likelihood of crosstalk interference is high. Vectoring was invented to record and cancel such crosstalk. The calculations for this reside either in the same device in a separate component within or close to the DSLAM, or a nearby processor which can take on the crosstalk workload for multiple DSLAMs. Typically this resides in a low volume ASIC or FPGA, which could adversely affect cost. The closer G.fast and VDSL components are located to the home, the fewer cables there are in a single binder creating crosstalk. This actually makes managing crosstalk easier. Sckipio has a distributed crosstalk cancelling system across multiple DPs (distribution points) which cancels crosstalk on up to 24 lines each. They talk to other G.fast DPs to share crosstalk information on other pairs in the same binder. G.fast itself comes in two flavors, using baseband frequencies up to 106 MHz and the more recently introduced systems reaching 212 MHz. When these sit alongside 17a or 35b VDSL devices, the crosstalk has to be coordinated between the two types of systems (as one is FDD and the other TDD), but also part of the spectrum (0 to 17.7 MHz or 0 to 35 MHz) can have no G.fast signals resident at all as it will impede performance. G.fast is organized by time, not frequency. In principle this means a system could dynamically allocate more time in one direction when needed and later change to accommodate more traffic in the other direction depending upon load. All broadband lines have had to accommodate changes to user habits, where there are more apps which upload data and which are increasingly creating large file sizes. An example might be UHD video from onboard cameras. It is these applications and greater need of bandwidth which is changing the asymmetric nature of broadband to one which needs to be dynamically reversible. But this is not easily done despite G.fast chips adopting TDD. Crosstalk from the client (far end cross talk or FEXT) and crosstalk from the DP (near end cross talk or NEXT) are also affected by attenuation. To cancel them, a complex calculation related to their declining power at various points on the line is needed. 2 A recent development standardized under the ITU, is called Collective Dynamic Time Assignment (cDTA). It allows the DP to adjust up/down ratios in response to traffic demands automatically, squeezing more out of the theoretical two-way capacity of a line. Sckipio insists this actually allows all of the capacity in either direction when needed, which it claims enlarges the area over which G.fast can be effective. It remains true that if crosstalk cancellation was not needed these complexities would not be required. Fundamentally if twisted pair (TP) is used, crosstalk cancellation (vectoring) remains necessary. DOCSIS A recent change to DOCSIS has been the move from using 6 MHz channels – the equivalent of a traditional analog TV signal – as the core building block for data. These had to be multiplexed in numbers as high as 32 separate channels to provide sufficient bandwidth in DOCSIS 3.0. In DOCSIS 3.1, the system relies on basic OFDM using sub-carriers between 20 KHz and 50 KHz. These can be multiplexed together with up to 192 MHz of spectrum (32 channels) for one user at a time. DOCSIS 3.0 tops out at 1.2 Gbps real world downstream. Upstream capacity is capped at 200 Mbps. DOCSIS 3.1 has a theoretical upper limit of 10 Gbps downstream and two (2) Gbps uplink. Real world performance is closer to 7.3 Gbps downstream and 1.4 Gbps upstream as maximum upstream capacity, when using 4 separate OFDM clusters of 3840 sub- carriers using up 768 MHz. Real world services will be configured way below that for some years although the standard has room for maneuver, but the price of CPE and backhaul are the limiting factors. Comcast currently offers a DOCSIS 3.1 service with one Gbps download and 35 Mbps upload. In DOCSIS 3.0 environments, typically only 300 Mbps home gateways are supported in the U.S. Anything requiring faster speeds, such as enterprise, services tend to use fiber all the way to the building. DOCSIS 3.0 spectrum begins at 111MHz and can go all the way up to 1002 MHz. DOCSIS 3.1 can use up to 1.2GHz of spectrum if the plant allows it and theoretically all the way up to 1.8 MHz. Usually the lion's share of any spectrum (about 80 percent) is allocated to downstream traffic. It would be more effective of course, if both upstream and downstream could use the same big block of spectrum. If DOCSIS 3.1 is ever configured to use all available spectrum on its plant, it will mean that any MoCA connections, especially MoCA Access, will have trouble coexisting with it, using just 500 MHz of the highest frequency spectrum (which has the highest attenuation) available to it. Increasingly, DOCSIS will be unable to coexist with other coax technologies. Currently, G.hn cannot coexist with DOCSIS without the use of a special RF front end. 3 Similarly it makes little sense to look in detail at the latest DOCSIS Full Duplex technology, which is still in laboratory trials and by no- means deliverable with any certainty regarding price or timeframe. Full Duplex relies on receivers which treat all communication in the reverse direction as if it were interference, so that both directions can have the full capacity of a DOCSIS channel. This is the cable dream, to push 10 Gbps in each direction through multiple bonded channels, at some future point. G.hn Korea Telecom offers a version of G.hn branded as GiGAWire™ which is marketed in South Korea and worldwide. The chips were formally branded as G.Now by Marvell, but the product line has since been acquired by MaxLinear. It was initially available at 500 Mbps performance but is now capable of 1 Gbps. KT claims to have installed 100,000s of MDU lines with this technology. The Wave 2 G.hn chips which emerged during 2016 use a full 200 MHz of spectrum to deliver 2.0 Gbps PHY rate, of which about 1.5 Gbps is usable. G.hn uses up to 4096 OFDM subcarriers in coax and the spectrum can be shifted up into RF frequencies with an RF front end to avoid baseband and other RF segments. When operating over twisted pair however, G.hn does not have full crosstalk cancellation. Consequently, speeds are somewhat lower and KT does not use it at any faster speed than 1.2 Gbps across both directions (1 Gbps down and 200 Mbps up). G.hn is TDD so any performance numbers for it are an aggregate of up and down speeds.