G.fast's 106MHz spectrum compares to 17MHz used for VDSL2, the current leading DSL standard. But the wider spectrum comes at a cost: greater signal attenuation and crosstalk.
Signal attenuation is a result of the phone wire's limited spectrum. Greater attenuation at higher frequencies cuts the distance over which data can be sent: VDSL2 operates over 1500m, while G.fast will typically be up to 250m.
Crosstalk describes the signal leakage between copper pairs in a cable that can hold tens or hundreds of wire pairs. The leakage means each pair not only carries the sent signal, but also noise; the sum of all the leakages from neighbouring DSL pairs.
Crosstalk increases with frequency and early G.fast trials by Alcatel-Lucent showed crosstalk was so severe at higher frequencies that the interference matched the strength of the received signal. "It's one reason why no one has developed G.fast technology until now," said Dudi Baum, CEO of Israeli start up Sckipio.
Vectoring, which tackles crosstalk by using noise cancellation, is already used with VDSL2. "Vectoring is considered a key aspect of G.fast, even more than for VDSL2," said Paul Spruyt, DSL strategist for fixed networks, Alcatel-Lucent.
While G.fast is viewed as an extension to VDSL2, it has clear differences. VDSL2 uses frequency division duplexing where 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, where the full spectrum is used alternatively to send or receive data.
If a cable carries both VDSL2 and G.fast services, G.fast will use frequencies of more than 17MHz to avoid overlapping with VDSL2; crosstalk between the two cannot be cancelled because of the different duplexing schemes.
Operators want G.fast to deliver aggregate data rates ranging from 150Mbit/s over 250m to 1Gbit/s over cable lengths of less than 100m (see fig 2). This compares to VDSL2's aggregate data rate of 70Mbit/s (50Mbit/s downstream, 20Mbit/s upstream) over 400m, a rate that doubles to 140Mbit/s with vectoring.
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 – generating anti-noise to null each line's crosstalk.
The crosstalk coupling between the pairs is estimated using modulated 'sync' symbols sent between data transmissions. A user's DSL modem expects to see the modulated sync symbol but, in reality, receives the symbol and crosstalk from modulated sync symbols transmitted on neighbouring lines.
The modem measures the crosstalk and sends the value back to the DSLAM. This correlates the received error values on the 'victim' line with the pilot sequences transmitted on all other 'disturber' lines and measures the crosstalk coupling for every disturber-victim pair. Anti-noise is generated and injected into the victim line on top of the signal to cancel the crosstalk; a process repeated for each line.
G.fast vectoring is, however, more complicated than VDSL2's. Besides the greater crosstalk, G.fast has a power saving mode in which the line is deactivated if no data is sent. The vectoring algorithm needs to stop generating anti-noise as soon as a line is deactivated and to respond rapidly once it restarts. VDSL2 modem lines also get deactivated, but far less frequently.
"The number of computations is proportional to the square of the number of lines," said Spruyt. For G.fast, the number of lines is 4 to 24, or even 48, since the distribution point unit is closer to homes (see fig 3). For VDSL2, 200 or even 400 pairs are possible.
Both DSL technologies use discrete multitones, but G.fast uses half the number – 2048 – of VDSL2's, with each tone 12 times broader in frequency. G.fast's symbol rate, related to the tone spacing, is 12 times faster, requiring the calculations to be performed a dozen times more quickly.
Since crosstalk cancellation is required for each tone and there are half the number of tones, G.fast's calculation rate is six times that of VDSL2 for the same number of lines. Thus, while G.fast vectoring is more complicated, the overall computation load and power consumption are lower, due to fewer DSL lines.
In contrast, G.fast's analogue front end requires much faster A/D and D/A converters due to the 106MHz bandwidth, upping the power consumption. "We should expect the first generation of G.fast to consume more power than VDSL2 silicon," said Spruyt.
The main functional blocks for G.fast and VDSL2 include the baseband DSP, vectoring, the analogue front end and the line driver. The degree to which they are integrated – 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," said Arun Hiremath, director of marketing for Ikanos Communications. For example, the G.fast modem could be implemented as a single chip – the baseband, home gateway and even the line driver, due to the short transmission lengths, he says.
Ikanos has yet to disclose G.fast silicon, but has announced its Neos development platform, allowing its customers to test and trial the technology. Hiremath says its G.fast silicon will be based on the Neos architecture, with samples expected later this year.
Neither has Sckipio revealed its G.fast chipsets, but it is expecting first samples. "We will provide more information in a few months," said Baum.
The start up has ported its RTL design onto a Cadence Palladium system and has DSL models – bundles of twisted copper pairs measured at greater than 200MHz – to test its design's performance. "We use those models to see the expected performance running our protocol over those wires," said Baum.
Alcatel-Lucent has its own vectoring know-how for VDSL2 and G.fast. "Having our own vectoring technology means we have our own vectoring processing," said Stefaan Vanhastel, marketing director for fixed networks, Alcatel-Lucent.
Alcatel-Lucent has conducted G.fast trials with A1, the home subsidiary of Telekom Austria. Over 100m, G.fast only achieved 60Mbit/s, due to crosstalk.
"Activating G.fast vectoring saw it rise to 500Mbit/s, almost a factor of 10," said Vanhastel.
Much work is still to be done before G.fast will be deployed, says Alcatel-Lucent. The ITU is only likely approve the G.fast PHY specification later this year, while there are interoperability, test, functionality and performance specifications still to be written by the Broadband Forum.
Sckipio is more upbeat about timescales, believing operators will start deployments in 2015. Operators have to respond to broadband competition from cable players and operators deploying fibre, says Baum. Sckipio has multiple field trials of its G.fast silicon planned this year.
"Both companies might be right," said Hiremath. "For G.fast, you need fibre closer to your house to get a gigabit and that is not available with most carriers." G.fast will start with small scale deployments, he says, but the carriers will wait a little more for things to mature.
