VDSL2: now available at 500 Mbps

Just when you thought DSL couldn't possibly get any faster, someone goes and demonstrates VDSL2 running at half a gigabit over copper.

Ericsson did just that in March this year, announcing it had pulled off a live demo of "vectorized" VDSL2 running at 500 Mbps downstream over a loop length of half a kilometer. The secret lies in utilizing a combination of line bonding - in this case, using six copper lines at the same time - and vectoring, a crosstalk cancellation technique that improves VDSL2 performance by reducing noise originating from the other copper pairs in the same cable bundle that would otherwise interfere with each other.

Reducing that interference means faster VDSL2 throughput speeds at longer distances, says Ericsson broadband product marketing chief Don McCullough.

"Reducing the interference also improves power management considerably, so using vectoring also helps reduce power consumption," McCullough says.

Vectorized VDSL2 is essentially the art of combining existing (albeit fledgling) technologies to boost line performance. Line bonding is based on the ITU-T's G.998.2 (or G.bond) recommendation specs for Ethernet bonding for multipair DSL. Line bonding also features in the IEEE's Ethernet in the First Mile (EFM) standard (802.3ah), although EFM is a Layer 2 spec that runs on fiber as well as copper.

Vectoring technology, meanwhile, isn't the only crosstalk cancellation technology being bandied about for VDSL2. Dynamic Spectrum Management (which is based on the same basic principles as software-defined radio) is a similar solution being pushed by specialist players like ASSIA (Adaptive Spectrum and Signal Alignment), as well as big names like ECI Telecom and Alcatel-Lucent. The ITU-T is working on a standard for vectoring, and Ericsson expects the G.vector spec to be completed by the end of this year.

However, as impressive as 500-megabits over copper may sound, it's not yet clear whether vectorized VDSL2 will give fiber a run for its money.

Six lines of copper

The sexiness of FTTH is due in part to the fact that xDSL access speeds are already under pressure as broadband usage skyrockets. The growth of heavy apps like HD video and P2P are driving the need to beef up access speeds past the 50-Mbps range in the next few years. However, DSL providers still want to maximize use of existing copper as much as possible, especially since fiber CPE is still more expensive compared to xDSL.

Consequently, line bonding and crosstalk cancellation are drawing plenty of interest. It's also why Nokia Siemens Networks has dropped its work on GPON in favor of what it says is a more cost-effective FTTN/FTTC strategy that shortens copper loops enough to run VDSL2 at a potential 100-Mbps downstream.

If nothing else, VDSL's star is on the rise, according to Dell'Oro, which says that while ADSL port shipments fell almost 20% last year, VDSL port shipments grew 40%. That trend is expected to accelerate this year, driven by VDSL upgrades in markets like the US, Japan and Taiwan.

However, vectorized VDSL2 is a highly specialized version of VDSL, as it requires six copper lines to be available. McCullough says there are plenty of scenarios where six-line vectorized VDSL2 can be deployed, such as residential access for MDUs where fiber's reach is limited to the curb or the basement. But vectorized VDSL2 will more likely be for enterprises or mobile backhaul, he says.

"The demand will come from areas where copper infrastructure are already available and can be reused," McCullough says. "But of course in greenfield areas fiber deployment will be used instead."