Telekom Austria has demonstrated aggregate speeds above 1Gb/s over short copper access circuits using the emerging G.Fast technology being standardized by the ITU. Using equipment from Alcatel Lucent, the incumbent telco achieved 1.1Gb/s at distances of 70m, far more than is possible with currently deployed techniques combining VDSL2 with vectoring to eliminate the crosstalk interference that occurs between active wires carried in the same binder, when their signals interact.
“While this is only a trial, the impressive results show the enormous potential of G.Fast for service providers around the world,” said Federico Guillen, leader, business-line, fixed access, Alcatel-Lucent. “G.Fast is currently undergoing standardization and will be commercially available in the coming years.”
The limitation is that G.Fast only works effectively over very copper loops of standard 0.5mm copper cable deployed in typical Telco access infrastructures, up to 250m and ideally no more than 100m. This is because, like previous improvements in xDSL transmission, G.Fast trades distance for higher operating frequency, while taking advantage of advances in signal processing and computational power. The roadmap of xDSL is in tune with the strategies of many telcos and governments in building fiber out ever closer to the end user, while at the same increasing bit rates over the remaining increasingly short lengths of copper. This is possible not just because of improving technologies for mitigating interference, but also because there is less signal attenuation over short distances. This improves signal-to-noise ratios, and in turn, enables higher frequencies to be sustained.
The xDSL story began in the mid-1990s, when ADSL technology was introduced, supporting speeds of 4Mb/s at distances up to around 3Kms or correspondingly less farther away from the central office where the DSLAM equipment terminating the operator’s core fiber network is located. Then ADSL2 came along around 2004, bringing speeds up to 10Mb/s at around 1.5Kms depending on quality of the copper. VDSL and then VDSL2 were introduced about 2009, taking speeds up to 40Mb/s over copper lengths of around 1Km. The next step in 2010 was VDSL bonding, combining two wires in a single circuit, increasing bit rates to 60Mbp/s over about the same distance.
This was followed in 2010 with the first of a series of more significant technological breakthroughs, with vectoring to eliminate the crosstalk interference that had appeared to impose an ultimate constraint over performance. With VDSL2 vectoring the interference between all pairs in a binder is first calculated, and this information is then used to generate a noise cancellation signal on each pair. This removes virtually all crosstalk and so increases the ratio between signal and noise, which in turn enables the operating frequency to be raised for higher bit rates.
Vectoring along with higher frequencies was first deployed in 2011, bringing bit rates up to 100Mb/s, but only over distances up to about 600m. Next, Alcatel Lucent developed a further enhancement that works alongside bonding and vectoring, called Phantom Mode, creating a third virtual pair across two copper pairs by exploiting the differences in voltage between them, increasing bit rates to 200Mb/s. But, meanwhile, G.Fast development was coming along with potential for a further significant step forward by increasing the frequency range by six to 12 times. This involved shortening the maximum length further, and at the same time exploiting advances in silicon, given that at these even higher frequencies crosstalk interference was greater still and required more calculations per second to eliminate. One side effect of this is that the impact of vectoring is even greater at these speeds, especially over poorer-quality wires, as Telekom Austria has discovered in its trial.
Using prototype G.Fast technology from Alcatel-Lucent’s Bell Labs, Telekom Austria demonstrated aggregate speeds (combined upstream and downstream) of 1.1Gb/s over 70m and 800Mb/s over 100m lines. But this was over good-quality cable, and when tests were conducted on older, unshielded cables the speed dropped to 500Mb/s over just 60m. Then the telco tried the effect of adding a second G.Fast line, and performance collapsed to only 60Mb/s at 100m for these poorer lines because of the crosstalk interference, but that was without the help of vectoring. When vectoring was applied, the speed came back up to 500Mb/s at 100m.
The arrival of vectoring and particularly G.Fast is having a profound effect on the strategies of telcos, since they had assumed that they could only compete with cable operators with their higher- capacity coaxial cable access networks by taking fiber all the way to the home at prohibitive cost. This whole economic model is being overturned by G.Fast in particular and has certainly prompted Telekom Austria to change its strategy. In Austria many people in towns and cities live in Multi- Dwelling Units (MDU) and for new buildings the plan is still to deploy fiber inside new buildings because then cost is much lower. But the telco is now looking to delay FTTH deployments for some years to existing MDUs and terminate fiber in the basement, while continuing to use copper to reach individual apartments, avoiding costly structural changes inside the buildings.
Other telcos are looking at G.Fast to increase speeds to single buildings, with fiber terminated at the end of streets to avoid having to dig up roads for the last 100m. BT is evaluating G.Fast for this use case in the U.K., but over longer copper loop lengths up to 600m, so bit rates would not be as high as Telekom Austria is achieving, perhaps up to 250Mb/s.
Meanwhile Alcatel-Lucent has done some research on the economics of G.Fast and found that as a baseline, FTTH costs on average 15 times more per subscriber to deploy than the original ADSL offerings where the fiber is terminated at a central office. When fiber is taken to nodes in neighbourhoods, as in VDSL2, the cost saving is reduced to about three times compared with FTTH, coming down farther the closer the fiber termination point gets to the premise. In the case of FTTB (Fiber to the Basement) as deployed by Telekom Austria, the saving achieved over FTTH is about 30 percent, but Alcatel-Lucent points out that G.Fast has another crucial advantage, which is that it avoids disruption and reduces time to market, because no structural changes are required.
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