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Digging Deeper Into 'DSG’

This week marks the beginning of an additional use of this column space: “Translation Please 2.0,” a deeper dive into the technologies used in cable and competing broadband networks. It will run in biweekly rotation with the original Translation Please column, now in its eighth year. This is the first one.

This time, we’re going deeper on DSG. That’s short for “DOCSIS Set-Top Gateway.” As discussed in the March 10 edition, DSG is a headend thing, necessary to talk to set-tops that include an embedded cable modem.


DSG is high on my watch list, for one big reason: It represents the first major intersection (some might say collision) of the Internet-protocol world and the traditional, MPEG-based digital-video world. This includes the people — most cable technologists tend to self-identify as “video people,” or “IP people.”

The Video People generally grew up through analog video, into digital video, and through to on-demand and two-way services delivered over the MPEG transport passageways of contemporary systems.

The IP People came in through the data side of the house. They tend to think in terms of software stacks. They’re all about how to maneuver data through routers, over the public or private Internet. They know all kinds of funky protocols.

DSG is one of the major technological realities that will pull these two groups of people together. Why? Because it’s the first time that stuff needed by the set-top will, in part, be requested through the cable-modem termination system (CMTS) — itself uniquely an IP, data-side piece of equipment.

Most MSOs are well down the path of installing high-end, dual-tuner HDTV set-tops into homes. These boxes generally include an embedded cable modem.

Some operators have already turned up DSG as an alternate, “out-of-band” signaling path; some are just starting.

DSG is tightly linked to the Open Cable Applications Platform, plus it’s a better mousetrap. So it’s not a case of whether DSG gets turned up, just when.


The rest of this translation is for those of you on or interested in the technical side of activating DSG. Maybe you want to start sending command-and-control mechanisms, like guide data and software updates, to set-tops. Maybe you want to use it to shuttle VOD trick-mode requests (fast-forward, rewind) to the servers, or to perform security exchanges and conditional-access queries. (Are you who you say you are?)

Let’s assume you’ve already upgraded your video headend controllers to the version that also handles OCAP. (It’s a pretty safe assumption.) Next step is to upgrade the CMTS. Tactically, that means both with firmware and a new configuration file that supports DSG.

This is where it can get tricky. Recall that the out-of-band path on existing digital set-tops — pre-DSG — was designed a decade or so ago to push and pull information into organized clumps of set-tops, within a system.

Then came high-speed data and broadband IP services, which use entirely different equipment. They were designed for load more so than messaging. It follows that service mapping zones don’t always correlate between digital video and broadband. Meaning that the CMTS and the existing out-of-band paths likely don’t have the same coverage areas.


Remapping which set-tops go to which CMTS units isn’t hard, but it takes time, say the engineers up to their elbows in it. It requires learning about IP multicast and talking in terms of “DSG tunnels.” (As it was being explained to me, it occurred to me that people good at Sudoku puzzles would probably thrive at remapping.)

Watch for warehousing changes, too, if you pre-provision set-tops for service prior to fielding them. Again: Not bad, just different.

Then there’s the Emergency Alert System. Dig out the manuals. It’s likely that your EAS encoders will need an upgrade, too, to properly relay information over the new DSG path. Up until now, many alert systems operated over a contact closure on the extant out-of-band path. Now, they’ll need a DSG tunnel through the CMTS, to specific multicast groups.

Expect security changes, too. Your headend life will suddenly involve way more firewalls and protective mechanisms.


Lastly: Pay attention to signal levels at the set-top. Homes that subscribe to cable-modem and digital-video service, for instance, usually get signal from a directional coupler to the cable modem first. One side goes to the rest of the house, with more splitters feeding other set-tops; the other to the modem. How you configured levels when that happened, and whether one side got a cleaner drop, may impact performance — at least initially.

Depending on system size and topology, it’ll probably take about a month to get DSG up and running, experts say. It’s worth it to have a faster, less-expensive and more flexible way of sending mission-critical business-side stuff to the boxes.

Once it’s activated, who knows what else it will do to further blend the MPEG and IP worlds.

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