G.fast adds to the broadband options of the service providers

Feature: G.fast

Source: Alcatel-Lucent

Competition is commonly what motivates service providers to upgrade their access networks. And operators are being given every incentive to respond. Cable operators are offering faster broadband rates and then there are initiatives such as Google Fiber.

Internet giant Google is planning 1 Gigabit fibre rollouts in up to 34 US cities covering 9 metro areas. The initiative prompted AT&T to issue its own list of 21 cities it is considering to offer a 1 Gigabit fibre-to-the-home (FTTH) service.

But delivering fibre all the way to the home is costly, and then there is the engineering time required to connect the home gateway to the network. Hence the operator interest in the emerging G.fast standard, the latest digital subscriber line (DSL) development that promises Gigabit rates using the telephone wire.

"G.fast eliminates the need to run fibre for the last 250 meters [to the home]," says Dudi Baum, CEO of Sckipio, an Israeli start-up developing G.fast chipsets. "Providing 1 Gigabit over a copper pair is cheaper and faster to deploy, compared to running fibre all the way."

For G.fast, you need the fibre closer to your house to get the Gigabit and that is not available today with most carriers

Until recently, operators faced a choice of whether to deploy FTTH or use fibre-to-the-node (FTTN) and VDSL to boost broadband rates. Now, such boundaries are disappearing, says Stefaan Vanhastel, marketing director for fixed networks, Alcatel-Lucent. Operators are more pragmatic in their deployments and are choosing the most suitable technology for a given deployment based on what is most cost effective and fastest to deploy.

"It is very much no longer black and white," agrees Julie Kunstler, principal analyst, components at market research firm, Ovum. "The same service providers will be supporting multiple access networks."

The advent of G.fast will enhance the operators' choice, boosting data rates while using existing copper to bridge the gap between the fibre and the home. But the technology is still some way off and views differs as to whether deployments will begin in 2015 or 2016.

"For G.fast, you need the fibre closer to your house to get the Gigabit and that is not available today with most carriers," says Arun Hiremath, director, marketing at DSL chip company, Ikanos Communications. It will likely start with some small scale deployments, he says, "but the carriers will wait a little more for things to mature".

G.fast

G.fast enables Gigabit rates over telephone wire by expanding the usable spectrum to 106MHz. This compares to the 17MHz spectrum used by VDSL2, the current most advanced deployed DSL standard. But adopting the wider spectrum exacerbates two local-loop characteristics that dictate DSL performance: signal attenuation and crosstalk.

Operating at higher frequencies induces signal attenuation, shortening the copper reach over which data can be sent. VDSL2 is deployed over 1,500m links typically, G.fast distances will more likely be 200m or less.

Dudi BaumCrosstalk refers to signal leakage between copper pairs in a cable bundle. A cable can be made up of tens or hundreds of copper twisted pairs. The leakages causes each twisted pair not only to carry the signal sent but also noise, the sum of the leakage components from neighbouring DSL pairs.

Crosstalk becomes more prominent the higher the frequency. "One reason why no one has developed G.fast technology until now is the challenge of handling crosstalk at the much higher frequencies," says Baum. Indeed, from G.fast field trials, observed crosstalk is so severe that from certain frequencies upwards, the interference is as strong as the received signal, says Paul Spruyt, DSL strategist for fixed networks at Alcatel-Lucent.

Vectoring

Vectoring is a technique use to tackle crosstalk and restore a line's data capacity. Vectoring uses digital signal processing to implement noise cancellation, and is already used for VDSL2. "Vectoring is considered a key aspect of G.fast, even more than for VDSL2," says Spruyt.

G.fast can be seen as a logical evolution of VDSL2 but there are also differences. Besides the wider 106MHz spectrum, G.fast has a different duplexing scheme. VDSL2 uses frequency-division duplexing (FDD) where the data transmission is continuous - upstream (from the home) and downstream - but on different frequency bands or tones. In contrast, G.fast uses time-division duplexing (TDD) where all the spectrum is used to either send data (upstream) or receive data.

If a cable carries both services to homes/ businesses, G.fast is started from the 17-106MHz band to avoid overlapping with VDSL2, since crosstalk cannot be cancelled between the two because of their differing duplexing schemes.

Paul Spruyt

Both DSL schemes use discrete multi-tone, where each tone carries data bits. But G.fast uses half the number of tones - 2,048 - with each tone 12 times the bandwidth of the tones used for VDSL2.

Operators can also configure the upstream and downstream ratio more easily using TDD. An 80 percent downstream/ 20 percent upstream is common to the home whereas businesses have symmetric data flows.

Only transmitting or only receiving also simplifies the G.fast analogue front-end circuitry since it is less susceptible to signal echo, whereas such an echo is an issue with VDSL2 due to the simultaneous sending and receiving of data.

Operators want G.fast to deliver 150 Megabit-per-second (Mbps) aggregate data rates over 250m, 200Mbps over 200m, 500Mbps over 100m and up to 1 Gigabit-per-second over shorter spans. This compares to VDSL2's 70Mbps (50Mbps downstream, 20Mbps upstream) over 400m. With vectoring, VDSL2 performance is doubled: 100Mbps downstream and 40Mbps for the same span.

Vectoring works by measuring the crosstalk coupling on each line before the DSLAM - the platform at the cabinet, or the fibre distribution point unit for G.fast - generates anti-noise to null each line's crosstalk.

The crosstalk coupling between the pairs is estimated using special modulated ‘sync’ symbols that are sent between data transmissions. A user's DSL modem expects to see the modulated sync symbol, but in reality receives the symbol distorted with crosstalk from modulated sync symbols transmitted on the neighbouring lines.

The modem measures the error – the crosstalk – and sends it to the DSLAM. The DSLAM correlates the received error values on the ‘victim’ line with the pilot sequences transmitted on all the other ‘disturber’ lines. This way, the DSLAM measures the crosstalk coupling for every disturber–victim pair. Anti-noise is then generated using a vectoring chip in the DSLAM, and injected into the victim line on top of the transmitted signal to cancel the crosstalk picked up, a process repeated for each line.

Such an approach is known as pre-coding: in the downstream direction anti-noise signals are generated and injected in the DSLAM before the signal is transmitted on the line. For the upstream, post-coding is used: the DSLAM generates and adds the anti-noise after reception of the signal distorted with crosstalk. In this case, the DSL modem sends modulated sync symbols and the DSLAM measures the error signal and performs the correlations and anti-noise calculations.

G.fast vectoring is more complex than vectoring for VDSL2.

Besides the strength of the crosstalk at higher frequencies, G.fast uses a power-saving mode that deactivates the line when no data is being sent. The vectoring algorithm must stop generating anti-noise each time the line is deactivated, while quickly generate anti-noise when transmission restarts. A VDSL2 modem line can also be deactivated but this is much less commonplace.  

"The number of computations you need to do is proportional to the square of the number of lines," says Spruyt. For G.fast, the lines used are far less - 4 to 24 and even 48 in certain cases -  because the G.fast mini-DSLAM is much closer to the home. For VDSL2, the number of lines can be 200 or 400.

However, the symbol rate of G.fast is related to the tone spacing and hence is 12 times faster than VDSL2. That requires faster calculation, but since G.fast has half the number of tones of VDSL2, and crosstalk cancellation is performed for each tone, the overall G.fast processing for G.fast is six times greater.

G.fast vectoring may thus be more complex but the overall computation - and power consumption - of the vectoring processor is lower than VDSL2 due to the fewer DSL lines.

We should expect the first generation of G.fast to consume more power than VDSL2 silicon

Chip developments

The G.fast analogue silicon requires much faster analogue-to-digital and digital-to-analogue converters due to the broader spectrum used, while the G.fast line drivers use a lower transmit power due to the shorter reach requirements. "We should expect the first generation of G.fast to consume more power than VDSL2 silicon," says Spruyt.

Stefaan Vanhastel

The main functional blocks for G.fast and VDSL2 include the baseband digital signal processor, vectoring, the analogue front end, and the line driver. The degree to which they are integrated in silicon - whether one chip or four if the home-gateway functions are included - depends on where they are used.

"The chipsets will be designed differently for the different segments where they are used," says Hiremath. For example, the G.fast modem could be implemented as a single chip that includes the baseband, home gateway, and even the line driver due to the short lengths involved, he says.

Moreover, while the G.fast standard does not require backward compatibility with VDSL2, there is nothing stopping chipmakers from supporting both. The same was true with VDSL2 yet the resulting chipsets also supported ADSL2.

Ikanos has yet to unveil its G.fast silicon but it has announced its Neos development platform for customers to test and trial the technology. Hiremath says its G.fast design is based on the Neos architecture and that it expects first samples later this year. 

Start-up Sckipio has also to detail its G.fast silicon design but says it will provide more information in the coming months. G.fast has system requirements that are difficult to meet, says Baum: "The challenge is not to show the technology working but to meet the standard's boundary requirements with a small, efficient design that provides 1 Gigabit." By boundary conditions Baum is referring to performance requirements that the modem needs to achieve, such as certain speeds and distances with a given packet loss, for example.

Sckipio already has first samples of its silicon. The company ported the RTL design of its silicon onto a Cadence Palladium system - a box with hundreds of FPGAs that allows the complete hardware design to be built. The company also has DSL models - bundles of twisted copper pairs measured at greater than 200MHz - to test the design's performance. "We use those models to see the expected performance running our protocol over those wires," says Baum.

Alcatel-Lucent has developed its own vectoring know-how for VDSL2 and has now added  G.fast. "Having our own vectoring technology means that we have our own vectoring processing," says Alcatel-Lucent's Vanhastel.

Alcatel-Lucent has conducted G.fast trials with A1 Telecom Austria. "The good news is that we have been able to show that with vectoring, you can get really close to single-user capacity; the same capacity you have if there is only a single user active on the line," says  Vanhastel. In the trial using over 100m of cable, G.fast achieved 60Mbps due to crosstalk. "Activating G.fast vectoring it rose to 500Mbps - almost a factor of 10," he says. 

Much work remains before G.fast is deployed in the network, says Alcatel-Lucent. The International Telecommunication Union's G.9701 G.fast physical layer document is 300 pages long and while consent has been achieved, approving the standard is expected to take the rest of the year. Interoperability, test, functionality and performance specifications are still to be written by the Broadband Forum and then there are regulatory issues to be overcome: G.fast's 106MHz spectrum overlaps with FM radio, for example.   

Sckipio is more upbeat about timescales, believing operators will start deployments in 2015 due to competition including the cable operators. The start-up says it has multiple field trials of its G.fast silicon this year.

Meanwhile, extending the spectrum to 212MHz is the next logical step in the development of G.fast. "Bonding is another concept that could be applied," says Spruyt.

There is life in the plain old telephone service yet.

This is an extended version of an article that first appeared in New Electronics, click here.