In a recent trial the equipment was able to send a 5 Gbps signal over copper for 100 meters and 8 Gbps for 30 meters. This is much faster than the G.fast top speed in trials of about 700 Mbps. In a real life situation using older copper the speeds will not be nearly this fast. G.fast in real life trials has gotten about half of the speeds seen in labs, and it would be impressive if that can also be achieved for XG.fast.
The technology works by utilizing higher-band frequencies on the copper. Traditional VDSL uses frequencies up to about 17 MHz. G.fast uses frequencies between 106 MHz and 212 MHz. XG.fast climbs the spectrum even further and adds on spectrum between 350 MHz and 500 MHz.
There are a lot of issues involved in using all of this frequency on a small-gauge copper. The main problem is crosstalk interference – when adjoining copper wires interfere with each other, and this degrades the signal and drastically cuts down on the distance the signal can be transmitted.
Nokia mitigates the crosstalk using vectoring, the same as is done with VDSL and other DSL technologies. Vectoring generates an –out of-phase signal that can cancel out some of the interference. But there is so much interference at thise frequencies that vectoring can only keep the signal coherent for the short distances seen the trial.
To date there has not been a lot of interest in G.fast. Adtran, the other competitor in the G.fast space claims to have now conducted ninety field trials of the technology worldwide. That’s an extraordinarily low number for a technology that can add speed to existing copper. But it looks like most phone companies are not interested in the technology, and they have some good reasons.
The short distances make G.fast and its new successor impractically expensive in the copper plant. In order to use the technology the telco would have to mount an XG.Fast transmitter at the pole outside each home, or in dense neighborhoods to perhaps serve a few homes. But if the telco wants to take advantage of the faster speeds that XG.Fast can get into the home they also would need to string fiber to feed the XG.Fast transmitters.
XG.Fast is largely a fiber-to-the-curb technology and the cost of the building fiber up and down streets is the big hurdle to using the technology. Any company willing to spend the money to build that much fiber probably isn’t willing to trust copper for the last 100 feet.
There is one application where XG.fast makes good economic sense. It can be extremely costly to rewire older apartment buildings with fiber. But every apartment building has existing telephone wiring and XG.fast can be used to move data from a telephone closet to the apartment units. This sounds to be far less costly than trying to snake fiber through older buildings. Since a lot of companies have avoided older apartment buildings this might offer a relatively inexpensive way to bring broadband.
You can’t fault Nokia for continuing to pursue the technology. There is a huge amount of copper still hanging on poles and the world keeps shouting for more broadband. But I get nervous about recommending any technology that isn’t widely accepted. I can picture a telco deploying this technology and then seeing support dropped for the product line.
But I can’t see this ever being much more than a niche technology. Telcos in the US seem to be looking for reasons to tear down copper and don’t seem willing to take one more shot at a copper technology. There might be a good business case for using the technology to extend broadband inside older buildings. But US telcos seem completely uninterested in using this in older copper networks.