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DOCSIS Comes In for Out-of-Band Signaling

For years, digital set-top makers have been throwing cable modems into their more advanced box products. Now there is a move afoot to harness that for more than just Internet data duties tied to future interactive-TV applications.

Fueled by a set of specifications from Cable Television Laboratories Inc., there is a growing drive toward switching the set-top box's out-of-band signaling — the basic communications conduit between the box and the digital headend controller — onto the Data Over Cable Service Interface Specification channel.

Such a move would cut box prices by eliminating the out-of band receiver, and potentially simplify the headend by combining key video and data-control systems.

Out-of-band signaling provides the link between the box and the cable operator's headend controller system. It relays authentication, encryption and authorization signals from the headend to the box, and ferries back subscriber requests, including pay-per-view and video-on-demand selections.

"If you make the transition to all-digital, one of the ways to simplify the overall architecture is to move to a common out-of-band signaling channel. The logical choice for that, of course, is DOCSIS," said Ralph Brown, vice president and chief software architect at CableLabs. "It's the most broadly accepted, two-way communication channel on cable today."

In making the DOCSIS channel the out-of-band signaling conduit, the operators can take advantage of the less-expensive DOCSIS cable-modem termination system and modem gear.

In tapping the open-standard DOCSIS provisioning capabilities, it will also make it easier for other manufacturers to enter the cable box market, Brown noted.

"It's a step along the way of opening up these traditionally proprietary systems to make them a more open system architecture," Brown said.

CableLabs has come up with a related set of specifications to govern how this DOCSIS signaling would work. Key among these is the DOCSIS Set-Top Gateway, or DSG, specification, which spells out how out-of-band signaling can be moved onto the DOCSIS channel.

But while the boxes might be able, so far the idea has not made it from the drawing board to the test bench.

"How conformant they are to the DSG and OpenCable spec is not clear, because we haven't had OpenCable advanced host set-top boxes in here for CableLabs to do any kind of certification testing against," Brown noted.

As with other box suppliers, Motorola Inc. is working with MSOs to develop DOCSIS out-of-band signaling for its network systems and its higher end DCT-5100 and DCT-6200 product lines, according to Ray Bontempi, director of system engineering for Motorola Inc.'s Broadband Communications Sector.

But there are still crucial decisions on how to put the DOCSIS signaling scheme into the plant, including how to subdivide data and video traffic.

"There are different ways of dividing it up that are easier and harder to do to pull apart in the box," Bontempi said. "Being that it is sort of a clean sheet of paper here, in our discussions everyone seems to be open to doing it the right way and the way that makes the most sense."

In the headend, moving to a DSG signaling scheme would require some modifications, including network routing, he added.

1700 with modem?

MSOs would still need to maintain the systems that control their deployed boxes without modems. But if the idea really catches on, modems could be added to newer versions of lower-end boxes, such as Motorola's workhorse DCT-2000 and the newer DCT-1700, Bontempi said.

Cable operators "are asking us for this functionality both on the headend side and in the set-top," he added. "They are definitely saying they want this in their networks and in their set-tops."

While it is possible to get such a system running the lab, "we're just starting to talk about implementation and timelines, and it's kind of hard to read into the future to say how quickly we would see it in the field," Bontempi said. "It's not like tomorrow we are going to go out and put modems in out 1700 product line.

"We will follow how the industry goes, and to the extent that they want more set-tops with modems, we will produce more."