Nokia has achieved a connection speed of 5Gbps—about 625MB/sec—over 70 metres of conventional twisted-pair copper telephone wire, and 8Gbps over 30 metres. The trial used a relatively new digital subscriber line (DSL) protocol called XG.fast (aka G.fast2).

XG.fast is the probable successor of G.fast, which was successfully trialled in a few countries over the past couple of years and will soon begin to commercially roll out. (In an unusual turn of events, the UK will probably be the first country with G.fast.)

Fundamentally, both G.fast and XG.fast are best described as "VDSL on steroids." Basically, while a VDSL2 signal frequency maxes out around 17MHz, G.fast starts at 106MHz (it can be doubled to 212MHz) and XG.fast uses between 350MHz and 500MHz. This means that there's a lot more bandwidth (the original meaning of the word), which in turn can be used for transferring data at higher speeds.

By way of example, VDSL2 can do around 100Mbps over that 17MHz channel; G.fast can do about 700Mbps at 106MHz; and XG.fast can go all the way up to 10Gbps at 500MHz with two bonded telephone lines.

The problem with higher frequencies, though, is that crosstalk interference—interference between different copper wires in the same bundle—becomes a major issue, causing the DSL signal to attenuate (i.e. weaken) very quickly. To combat the interference, G.fast and XG.fast both use vectoring to keep the signal going over reasonable distances.

Vectoring, which has also been used to great effect with VDSL, works in a similar way to noise-cancelling headphones: the DSLAM (the networking kit on the other end of your telephone line) constantly assesses the crosstalk on a given bundle of telephone lines, and then generates an anti-phase signal that cancels out most of the interference.

Even with vectoring, though, the max distance of G.fast and XG.fast is rather short: about 100 metres for 700Mbps G.fast, down to just 30 metres for 10Gbps XG.fast. This means that the DSLAM has to be brought much closer to the premises (usually under the pavement outside or up a telephone pole), which in turn means you need a far higher density of DSLAMs, and thus much higher infrastructure costs. But, so the argument goes, it's still much cheaper than running fibre all the way inside the premises.

Nokia's 5Gbps over 70 metres is quite impressive; the original plan for XG.fast was only 2Gbps over 70 metres, so Nokia has squeezed a fair bit more blood from the stone. 70 metres would let you connect a few dozen homes/buildings to the same DSLAM in a dense urban environment.

Whether you'll actually get a 5Gbps or 700Mbps connection or not is another question entirely, though. Like other forms of DSL, G.fast and XG.fast are adaptive—they can fall back to slower speeds over longer distances. BT, which has committed to rolling out G.fast to 10 million UK premises by 2020, will initially offer max speeds of around 300Mbps, with DSLAMs every few hundred metres. Clearly the plan is to match rather than supersede its main competitor, Virgin Media.

Presumably other twisted-pair telephone wire ISPs (such as AT&T in the US) will follow in BT's footsteps, starting with a sparse G.fast rollout, and then increasing the DSLAM density—and thus max connection speed—as necessity (read: competing with cable and fibre operators) dictates.

Long-term, of course, assuming humanity's thirst for bandwidth continues to increase, it probably would've made more sense for those ISPs to just bite the bullet and roll out fibre to the premises in the first place, rather than flogging the dead twisted-pair horse... but hey, such is life.

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