10 Gigabit Plain Old Telephone Service

Bell Labs has sent unprecedented amounts of data down a telephone wire. The research arm of Alcatel-Lucent has achieved one-gigabit streams in both directions over 70m of wire, and 10-gigabit one-way over 30m using a bonded pair of telephone wires.

Keith RussellThe demonstrations show how gigabit-speed broadband could use telephone wire to bridge the gap between a local optical fibre point and a home. The optical fibre point may be located at the curbside, on a wall or in an apartment's basement.    

Service providers want to deliver gigabit services to compete with cable operators and developments like Google Fiber, the Web giant's one-gigabit broadband initiative in the US. Such technology will help the operators deploy gigabit broadband, saving them time and expense. 

"This kind of a technology is really going to be an enabler of fibre-to-the-home," says Keith Russell, senior marketing manager, fixed networks business at Alcatel-Lucent. "Service providers will have another tool, addressing those parts of the network where it is hard to drive fibre right to the home, whether it is a multi-dwelling unit or where they can't trench fibre those last few meters." 

Bell Labs delivers gigabits of data down the telephone wire by using more spectrum. VDSL2 uses 17MHz of spectrum while the first implementation of the emerging G.fast standard extends the frequency band to 106MHz. Alcatel-Lucent has gone beyond G.fast and uses even more spectrum: 350MHz for symmetrical 1 Gigabit, and up to 500MHz to demonstrate 10 Gigabit. Bell Labs calls its technology XG-FAST.

BT's chief executive, Gavin Patterson, has already described G.fast as a very exciting technology. "It allows us to get speeds of up to one-gigabit, and it builds on VDSL," said Patterson during BT's most recent quarterly results call. "It takes the fibre closer to the premise, so effectively you get a glass transmission closer to the premise but not always all the way in."

XG-FAST will take longer and will likely be commercially available only from 2018, says Teresa Mastrangelo, principal analyst at Broadbandtrends: "That timeline may still provide a quicker means to deploying gigabit services than having to deploy a full-blown fibre-to-the-home network." 

Source: Alcatel-Lucent Bell Labs

Using such a broad spectrum of the telephone wire, designed a century ago to carry voice signals several kilohertz wide, creates two challenges.

One is that signal attenuation grows with frequency. Hence the wider the spectrum, the shorter the copper loop length over which data can travel. VDSL2 has a loop-length of some 1,500 meters while XG-FAST achieves tens of meters.

The second issue is crosstalk, where the signal on a copper pair leaks into a neighbouring pair, generating electrical noise. The leakage can be so noisy at the higher frequencies that it can exceed the desired signal.

For the Bell Labs demonstration, crosstalk was only an issue in the 10-gigabit example that uses two wire pairs. However, for VDSL2 and for the emerging G.fast standard, crosstalk is a significant problem. Systems vendors have developed advanced digital signal processing techniques, known as vectoring, to reject such noise.  

Russell says that the G.fast standard's first phase - based on 106MHz of spectrum - will be ratified by year end. G.fast's second phase proposes doubling the spectrum to 212MHz. Alcatel-Lucent demonstrations using XG-FAST shows that digital subscriber line technology need not stop there.

"A lot of work is needed to take it [XG-FAST] into production," says Russell. First, there are engineering challenges, the broad spectrum used makes the analogue front-end chip design significantly more complex and expensive. Engineering effort will be needed before the cost of such a solution will match that of VDSL.   

XG-FAST would also need to be considered along with other proposals and the chosen outcome standardised before operators will embrace the technology in their networks. Meanwhile, operators will start testing G.fast from next year with products appearing mid-2015.

Another issue is the need for extensive copper characterisation in order to understand the state of the copper and whether it can even support this type of technology, says Mastrangelo.

"It will be very interesting to see what happens with G.fast given the operator interest in gigabit services," says Russell. "[G.fast] is a very strong option for operators wanting to offer such services quickly."   

BT estimates that the technology is two years away before it will play a role in the network.

* The article was further edited and added to on July 16th.