AT&T gets asked a lot about whether its fiber-to-the-node, or FTTN, architecture brings enough bandwidth to the home to meet future demands for, say, multiple high-definition television streams. Lately the company has been answering those questions by promising to boost bandwidth by pair-bonding VDSL2, starting as early as the second half of this year.

AT&T expects to be able to “not quite double, but significantly increase” the bandwidth of its FTTN networks by enlisting a second copper pair to each house, AT&T's Chief Financial Officer Rick Lindner said last month. (Depending on proximity, AT&T claims to be delivering 30 Mb/s to 60 Mb/s per home already.)

How much more bandwidth can pair-bonding yield? It depends on a few factors, said Piyush Sevalia, vice president of marketing for VDSL2 chip vendor Ikanos Communications. If the pairs in question are in separate binders, they might offer roughly twice the bandwidth when bonded, but in the same binder, they pose the risk of crosstalk, limiting their performance, he said. Bonding pairs in close proximity might only boost bandwidth 25% to 30%, he said.

Earlier this month, Conexant, which supplies VDSL chips for AT&T's FTTN deployment, introduced two chipsets for VDSL2 bonding: one for central office gear and one for customer premises gear (CPE). Equipment might hit the market in the second quarter, Conexant said. But although pair-bonding must be supported at the chip level, it must be performed at the line card level to avoid confusion over which pairs lead to which houses, Sevalia said. “Say you have a forty-eight-port line card. It starts getting interesting from an operational point of view. You have to find out which two lines are getting into the consumer's home.”

Alcatel-Lucent, which supplies the VDSL2 access gear for AT&T's FTTN network, hasn't yet shipped access gear that can bond VDSL2 because CPE vendors haven't done so, a spokesman said. “We will have VDSL2 bonding-ready equipment going into production soon, and we will add the bonding software to the equipment once the CPE for VDSL2 bonding is available.”

AT&T declined to speak in detail about bonded VDSL2, but a spokesman said it entails assigning two DSLAM ports to each customer and deploying a standard set-top along with a special residential gateway outside the house that can terminate two pairs. In-home wiring won't change, and neither will installation times, the spokesman said. However, any bridged taps on the second pair would have to be removed, Sevalia said, and those taps could be inside the house or 50 meters away from it. Plus, there's another problem.

“In most neighborhoods, there's not enough [copper] for every home to have two pairs,” said Tom Starr, vice president of the DSL Forum.

It's unknown what fraction of homes in AT&T's FTTN footprint have the requisite second pair. In particular, the Pacific Bell and Southwestern Bell regions are “very tight on extra pairs,” posing a “potential problem,” said Teresa Mastrangelo, broadbandtrends.com principal analyst. Instead, AT&T might do more VDSL2 bonding in BellSouth territory, where copper is more abundant. BellSouth, which started deploying three pairs to every newly built home in the 1990s, already has deployed bonded G.shdsl for mid-band Ethernet services and has shown an interest in bonded VDSL2, Mastrangelo said.

Another question facing AT&T's plans is how it will handle discrepancies in copper plant. If only one part of a given city — or only one house in a given neighborhood — has twin pairs, U-verse speeds and services could vary greatly from house to house. How AT&T would address that in its marketing efforts is one more unknown.

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