DSL: Will phantom channels become real deployments?

Alcatel-Lucent’s Bell Labs has announced it has achieved a data rate of 300 megabits-per-second (Mbps) over 400m using digital subscriber line (DSL) technology. 

Alcatel-Lucent is promoting its DSL Phantom Mode technology as a complement to fibre-to-the-x (FTTx) technology. Operators can use the technology to continue to extend services offerings to existing DSL subscribers as they roll out FTTx over the next decade or more.

But one analyst believes the technology could take years to commercialise and questions whether the announcement is not sending a wrong message to the industry by providing an alternative to fibre.

“The investment required to upgrade DSL is quite small”

Stefaan Vanhastel, Alcatel-Lucent

What has been achieved?

The 300Mbps data rate is achieved using two copper wire pairs between the access equipment and a DSL modem although three DSL ports are required at each end. The rate drops to 100Mbps when the reach is extended to 1km. In comparison very high speed Digital Subscriber Line 2’s (VDSL2) data rate over a single line ranges from 20 to 40Mbps over 1km.

None of the three techniques that Alcatel-Lucent uses – bonding, vectoring and the phantom mode that creates an extra virtual channel alongside the two bonded pairs - is new. What the company claims is that it is the first to combine all three for DSL.

In March Ericsson announced it had achieved 500Mbps over 500m but it used six bonded pairs and vectoring only.

Why is the Phantom Mode important?

The significance of the announcement, according to Alcatel-Lucent, is that operators can continue to offer existing DSL customers new bandwidth-intensive services as they roll out FTTx.

“Rolling out FTTx will take a significant amount of time,” says Stefaan Vanhastel, director of product marketing, wireline networks at Alcatel-Lucent. “Operators are looking to reuse their copper infrastructure in the short-to-medium term - the next 5 to 10 years.”

An operator must have a central office or cabinet equipment 1km or less from the user’s residence as well as having two wire pairs per building or residence. “In many countries two pairs are available,” says Vanhastel. 

However, one analyst questions the development and promotion of such copper-enhancing technology.

“I think Alcatel is being disingenuous when they say "fiber will take long to implement, this is an intermediary solution’,” says the analyst, who asked not to be named. “They know full well that customers would see this as a way to hold back on deploying fibre.

“Ultimately to me this is schizophrenia at work. Alcatel-Lucent wants to be all things to all service providers and may be sending the wrong message to the market that they need not invest to sustain the bandwidth demand growth, which is suicidal both for service providers and for Alcatel-Lucent in the long run.” 

Alcatel-Lucent does believe operators will invest in DSL alongside FTTx.

“The investment required to upgrade DSL is quite small,” says Vanhastel. “Even with two ports it is a bargain; you get the investment back in one or two months.” 

Even operators more advanced in their FTTx deployments will want to offer new higher bandwidth services such as high-definition TV to all their customers.

“What are you going to do? Offer your services to just 50% of your customers?” says Vanhastel “They [the remaining customers] will go elsewhere.”

Method used

The Bell Labs research arm of Alcatel-Lucent has used three techniques to enhance DSL’s speed and reach performance.

• Bonding: The combination of copper line pairs to boost the number of channels – in this case two are bonded - and hence the data rate between access equipment and the DSL modem.

• Vectoring: Noise cancellation techniques using digital signal processing to improve the overall signal-to-noise performance. “It involves measuring the noise on all the lines and generating anti-phase – the inverse signal – such that the two cancel out,” says Vanhastel.

• Phantom mode: The phantom mode technology uses two physical wires to create a third virtual one. The technology was first proposed in the 1880s as a way to add an extra virtual telephone line.

Two physical pairs and the third phantom one. Source: Alcatel-Lucent

Using the phantom mode, only two wire pairs are needed to connect the end equipment. The information on the third “virtual” line is shared over the two physical channels. Using analogue electronics, the data on the third channel is processed and recovered.  “We add and subtract through the use of a bunch of transformers,” says Vanhastel.  Where the circuitry is placed, whether in the DSLAM access equipment or elsewhere, is to be decided.

To create the virtual wire, a modem supporting three-pair bonding is required. In addition the chipset in the DSL modem must have sufficient processing performance to execute vectoring on three channels. That's because adding the phantom mode degrades the performance of all the channels due to crosstalk. The crosstalk is removed between the channels using vectoring.

What next?

The technology needs to be brought to market. “At the earliest it will be 2012,” says Vanhastel.

But the analyst points out that the technology is lab tested: “Between test labs and implementation, count a significant number of years.”

The concept could even be extended using more wire pairs. The relationship is (N-1) phantom channels for N wire pairs i.e. 1 virtual channel with two pairs, 2 with 3 pairs etc.

Alcatel-Lucent says it has already completed two VDSL2 bonding trials in Asia Pacific, while three operators are undertaking VDSL2 vectoring tests in their labs and will move to testing in the field using a single line this year.

“Bonding is here today, vectoring will be 2011 and the phantom mode will be after that,” says Vanhastel.