AT&T makes its call on home nets

By Rick Merritt
SOURCE: Embedded Systems Programming (San Francisco, CA)
03-05-2007

San Jose, Calif. -- AT&T is calling for interoperability standards in VDSL and developing a new class of low-loss splitters as two steps forward in home networking. While the telecom giant is betting on phone-line technology, it readily admits there is no silver bullet for home networks.

Those were some of the observations from a presentation by Vernon Reed, principal member of technical staff at AT&T Labs. Reed addressed the thorny problems of home networking and defended AT&T's choice of technology from the Home Phoneline Networking Association (HPNA) in a talk at the IPTV 2007 conference here last Tuesday.

All home network technologies have their pros and cons and none is a silver bullet, as they all face difficult conditions operating in the digital home, Reed said.

Addressing just one of those problems, AT&T Labs is submitting a request for a patent on a new low-loss splitter. Today's splitters can create signal loss of 20 to 35 dB, Reed said, but the new AT&T design has a signal loss of only 8 dB. The company expects to license the technology to manufacturers in Asia.

While AT&T is committed to HPNA as its primary home network delivery vehicle, it acknowledges that the technology has at least one major drawback. Unlike powerline plugs sold at the retail level today, Reed said, HPNA technology for AT&T's VDSL-based video and telephony services is not something users will be able to install themselves anytime soon.

The industry would have to set interoperability standards for VDSL, then develop a few generations of simpler systems based on those standards before AT&T could deliver an IPTV set-top that users could set up themselves, he said.

"It's doable, but it could take three to five years," Reed said. "We're still a long way from VDSL interoperability. The Ikanos, Broadcom and Infineon chip sets don't talk to each other."

Installing an external VDSL termination box outside a user's home for video and voice services can take five hours, Reed said. With better interoperability, the job could be cut to two hours, he said.

Reed said the ideal home network could be used over any medium-coax, twisted-pair, powerline or wireless network. Standards groups such as the Digital Living Network Alliance (DLNA) could do the industry a great service by benchmarking various home networks and defining a best physical layer, a media-access controller and remote management, although that would be difficult to accomplish, he said.

Choosing phone lines

AT&T chose HPNA for several reasons, Reed said. The technology has relatively high bandwidth, an efficient media-access controller and good quality-of-service features. More important, it can be used over coax or twisted-pair lines.

Some 37 of 41 cities targeted for AT&T's IPTV service had a significant number of apartment buildings that required home networking over coax, he said. By contrast, the technology from the Multimedia over Coax Alliance (MoCA) will not work over twisted-pair lines.

"That was the real killer," Reed said. "The ability to do stuff over coax and twisted-pair with HPNA has been incredibly handy."

On the other hand, HPNA isn't a complete solution for the digital home where more and more networked systems are emerging.

"I don't think you will ever see a Web camera with an HPNA phone jack, so you have to look at blending," Reed said.

AT&T has no plans to use powerline, which it considers unreliable. The technology faces difficulties with multiphase signals in U.S. homes that typically split two 220-V lines into two 110-V circuits, Reed said. Also, powerline is open to effects from a variety of devices on the line ranging from HDTVs to halogen lamps.

"You start stretching the dynamic range and open yourself up to noise" that can generate interference that impacts the power supplies and motherboards on the network, he said.

"There's not enough going for powerline to consider shifting from HPNA, even if they double their data rates," Reed added.

Wireless over any flavor is worse, he said. Even if a network works well, introducing a new wireless system or a neighbor's wireless net can cause problems.

"Wireless is subject to interference no matter what you do," he said. "If you want assured service, you need to be on wires."

Whatever the technology, all home networks face common problems. Reed said. Actual throughput is typically far less than advertised, and interference is always an issue.