Out of band, out of mind

Fri, 06/30/2006 - 8:00pm
Brian Santo, Senior Editor

The cable industry is on the verge of moving out-of-band (OOB) signaling from a proprietary channel to a DOCSIS channel. The primary means to achieve this is the DOCSIS Set-top Gateway, or DSG.

With DSG, cable is continuing to follow the way of all technology, the path toward specifications and standardization. It started three decades ago with agreements on connectors, hit a high note with DOCSIS, and continues with the gradual migration to IP transport and the imminent rollout of OpenCable Application Platform (OCAP) middleware—which DSG complements.

Motorola and Cisco Systems' recently acquired Scientific Atlanta operation each have their own proprietary approach to transmitting program guide information and other non-video program data to and from set-top boxes. Both use spectrum outside of the frequencies allocated for video, hence the term out-of-band (OOB).

Figure 1: A DSG-based network.

OOB signaling has its upstream drawbacks. One is a susceptibility to interference. Another is a bandwidth limit of 256 kbps—fine for applications such as pay-per-view, where set-tops had to be occasionally polled, but certain to be inadequate for interactive, real-time services that operators are preparing to offer within the year.

Bandwidth limits affect things as simple as changing channels. A VOD stream, meanwhile, requires real-time, two-way communication, difficult if not impossible with traditional OOB signaling, but easily provided by a DOCSIS channel.

DSG does not replace the proprietary Motorola and SA set-top signaling. Rather, it transmits those same control and command signals through a faster, two-way, and open DOCSIS channel. It is explicitly meant to be an end-around their duopoly control of the cable network, without completely alienating the two of them.

Nor is DSG an either/or proposition. Operators do not have to switch from the old way of doing things to DSG. Both can be supported in parallel, in order to support both legacy and new set-tops.

The impetus for DSG is that it gives operators more control of their own set-tops.

Opening up the DSG

One of the first envisioned advantages will pertain to electronic program guides (EPGs). The largest operators usually have different set-tops in different systems; they sometimes have a mix of STBs in a single market. That can force them to run different program guides on different boxes. With OCAP and DSG working in concert, cable operators can run EPGs with a common, branded look and feel across their entire operations.

But when it comes to control of the STB, operators are most concerned with conditional access (CA) issues. In most U.S. digital cable environments, Motorola and SA incorporate their own embedded CA technologies that are not only proprietary, but closed—meaning the operators do not have access to them, nor can they be changed out easily or inexpensively.

That means if an operator wants to run an application, it cannot always be sure it will be able to without first consulting the CA provider, which tends to also be the set-top manufacturer.

"Over DSG, we can set up a tunnel that the set-top is reasonably unaware of. The MSO can go in and configure a third-party app, get it up and running, and not have to go to the set-top vendor to get something approved," says Dan Hession, Cisco's director, worldwide cable marketing.

DSG not only affords operators greater flexibility with CA, it also lays the foundation for the downloadable conditional access system (DCAS) project, which will give operators the option not only to update their CA from Motorola and SA, but to also source them from another third-party vendor.

In fact, DSG lays the foundation for downloadable anything. In the near term, that will likely be electronic program guides, EPG updates, and then games and other interactive applications. With DOCSIS transport at multi-megabit-per-second transmission rates, anything that can be downloaded to a PC—including video—could conceivably be downloaded to a set-top box.

DSG took a little doing. The engineers specifying the standard had to satisfy the basic requirement of creating a messaging path, called a tunnel, on DOCSIS transport.

DSG also needed to establish within DOCSIS a one-way protocol for a number of reasons, including supporting CableCARDs, the current method of separating out the CA from the digital set-top.

The new DOCSIS tunnel had to be able to withstand back-channel interference, and if it did succumb to interference, it could not resort to the typical DOCSIS solution of an immediate reboot.

"With set-top boxes, if you lose your reverse channel, you still have to be able to watch TV, shop, get access to the interactive program guide," says CableLabs Chief Technology Officer Ralph Brown, a primary contributor to the DSG specs. "Rebooting is not good for TV."

DSG could be implemented on cable modem termination systems (CMTSs) or on dedicated equipment. But as a practical matter, since CMTSs already have DOCSIS control resident, DSG is likely to be deployed through that equipment. Under most circumstances, if an operator already has DOCSIS on its plant, deploying DSG will require the addition of a modest amount of software to the CMTS.

From an operational standpoint, DSG software performs a forwarding function. It identifies set-tops by MAC address and then ships IP packets from the headend controller to individual set-tops. If the application hasn't identified the set-top yet, DSG is able to dynamically assign MAC addresses.

The four major CMTS vendors—ARRIS, BigBand Networks, Cisco Systems and Motorola Inc.—all have DSG systems qualified by CableLabs.

The other key requirement is that set-tops are equipped with a DOCSIS modem, available in many new set-tops. "The challenge is getting cable modems into set-tops at the right price point," says Brown. "Set-tops have to have HD, DVR, OCAP—vendors have to integrate all of that."

DSG goes hand-in-hand with OCAP. OCAP was designed to enable interactive applications to run on any OCAP-enabled set-top box independent of the underlying operating system, in contrast to the current situation in which a developer would have to create a different version of its application for every set-top. The availability of a high-bandwidth, real-time, two-way DSG tunnel will be of great utility in interactive applications. OCAP is in fact built to look for DSG channels and enable applications to use them if available.

"With DOCSIS signaling, we expect to see more openness to do OCAP apps, even within the duopoly," says Steven Krapp, director of C4 CMTS product management at ARRIS. ARRIS expects the introduction of OCAP and DSG, and the applications they enable, to drive the volume of CM-equipped set-tops up, which would drive costs down, including those related to memory.

Rollouts of DSG are likely to coincide with rollouts of OCAP, which several major operators have committed to do before the end of this year.

Cablevision Systems has been running a version of DSG for years with its Sony-made STBs, Brown says, and now is doing it with some SA boxes. Most of the other major MSOs are testing or trialing DSG.

ARRIS claims it has several "serious" DSG deployments scheduled for this year with one of the top MSOs, upgrading systems in as many as 10 markets.

ARRIS expects it will be able to start with one downstream channel dedicated to 600 or 700 cable modems. The company expects up to 4,000 could be accommodated on a single DSG downstream channel.


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