Getting to Know the DOCSIS Gateway

Last time, we examined the virtues of the embedded cable modem as the nimblest of available signal paths to move two-way information.

The cable modem, it turns out, isn't just about broadband Internet services.

That development, informally called "eDOCSIS," for "embedded DOCSIS" (Data Over Cable Service Interface Specification), is one part of the puzzle. Another part — especially relative to an embedded modem within set-tops, or within digital TVs with embedded set-tops (yes, that's a double embedding, set-top and cable modem) — is the "other end" of the network.

What happens when the embedded modem sends something for handling? Where does it go?

Those questions represent the bulk of a 35-page CableLabs specification known as "DSG," for "DOCSIS Set-Top Gateway."

Marketers, hold your fire. This is an acronym that almost certainly will not come up in everyday conversations with retailers or customers, any more than "CMTS" does when you're talking broadband Internet. It's a back-end detail term. (Whew.)

Very very new

Also know that this is all new stuff — the DSG spec was first released by Cable Television Laboratories Inc. a year or so ago. Equipment development is next. Lots more needs to be done.

There are two ways to envision the teamwork between the embedded modem and the DSG. One is by signal path. Say you have a cable-ready HDTV, developed for two-way cable communications via an embedded modem. (This could also be a digital video recorder, or a game console, or a portable video player, or anything else imaginable as a cable-connected consumer device.)

As far as timing goes, it's important to note that this discussion occurs at the output of the second part of the cable/consumer-electronics "plug-and-play" agreement.

The first part of that agreement is being fleshed out now. We're thinking ahead here.

Say this HDTV set is under the control of a customer who just initiated a VOD session. Because this column is being written as the writer looks out at four-foot snowdrifts, let's be really indulgent and say this customer just pressed the button to watch Fargo.

In a simplified map, her request is routed to the embedded cable modem, which passes it upstream, over the cable plant, to the headend.

There, it goes where traffic from cable modems always goes: to the CMTS, or cable-modem termination system.

The CMTS looks at the incoming packets. "Aha," it says. "You're a session-set-up for a set-top. Over there, please." It passes the VOD initialization request to the DSG, which talks back and forth with the set-top controller, which talks with the VOD controller to set up the session.

Not annoying

The confirmation — and session details — move back to the embedded cable modem and HDTV, in reverse order. Back at the couch, everything seems the same. Maybe Fargo
arrives a little faster, but faster in a way that isn't obvious, like when you notice that something isn't annoying.

This pattern repeats for everything that needs to occur, or is planned to take place, between cable-connected devices and the servers and routers that handle the flow of information. Masterminding the flow is the DSG.

In a diagram, the DSG attaches to the CMTS on one side, and other background controllers (such as existing set-top controllers) on the other side.

It's partly a router, partly a proxy server and mostly an intermediary.

Its primary role is to manage the business policies that describe how different devices are allowed to connect with embedded cable modems, and the circumstances under which that linkup may take place.

That brings us to the second way of envisioning the DSG: It's sort of like the rooms of a house. A new house, specifically.

Changing rooms

Say you want to move, because you want more room. Maybe you pick a place with three additional rooms.

Because the whole moving thing is exciting and new, you tend to spend a lot of time thinking about those three new rooms. Maybe one is to be a home theater, which requires lots of daydreaming and research. Maybe another is a workshop, and the third a home office.

Meanwhile, the dining room furniture still needs to go into the dining room, the beds still need to go to the bedrooms and the couch to the living room.

In a DSG sense, the conditional access still needs a room, as does guide data, billing information, code revisions, and session management.

We tend to spend a lot of time thinking about the new stuff, whether it's rooms in a new house, or cool new two-way services in future CE devices. However, the bulk of the initial and ongoing work necessarily must continue to support the ongoing services of digital cable, pay-per-view, VOD and guide updates.

The focus has to start with where to put the existing stuff, so that it works better. That's the point of the DSG, and the embedded cable modem: To make today's digital businesses work better, while leaving plenty of extra rooms for the new stuff.