AT&T said today it expects to begin pair-bonding advanced DSL lines in “late 2008,” pushing back the expected arrival of what the company says is an important part of its fiber-to-the-node (FTTN) initiative.

AT&T has long cited the promise of pair-bonding in response to criticisms regarding the bandwidth limitations of its FTTN architecture. Pair-bonding would “not quite double but significantly increase” the bandwidth AT&T delivers to customers’ homes, the company has said, by adding a second pair of copper lines to each house and combining their outputs at the customer premises.

A year ago, AT&T imagined using pair-bonding before 2007 ended. It later revised that outlook, predicting 2008 in general as the time frame. “Late 2008” is the most recent language the company has adopted, spoken by Chief Financial Officer Rick Lindner during the company’s quarterly earnings call today.

Promising the introduction of a “whole-home” digital video recorder service this year as well as a second stream of high-definition television (HDTV), Lindner added, “A third enhancement that will be important to us going forward--because it will enable additional HD streams where they’re required--will be pair-bonding, which we expect late in 2008.”

AT&T was not immediately available to elaborate on the subject, nor was the chief supplier of its FTTN network, Alcatel-Lucent. A year ago, Alcatel-Lucent said it was waiting for customer premises equipment vendors to introduce bonded VDSL2 gear before it could add the necessary software to its own access products.

If technological challenges were slowing the rollout of bonded VDSL2, it wouldn’t surprise Kermit Ross, a consultant with Millennium Marketing. “It sounds pretty simple,” Ross said. “You basically break the signal in half, send each half down the two [pairs of] wires, and put them back together in a single stream at the other end. But I suspect--especially when some packets, like video packets, need to arrive in order--it may be pretty difficult.”

“The practical problems may be pretty intractable too,” he added. “AT&T’s got to have a couple different kinds of residential gateway boxes--one for close work and one for far away work. Also, all the operating systems are oriented toward keeping track of one circuit--one pair of wires--at a time. All of a sudden, you have to keep track of two. It ain’t as easy as it sounds.”

In addition, others have pointed out that a second copper pair is not available in all areas. And the amount of bandwidth that pair-bonding adds is variable, depending on distance and the proximity of the bonded lines.