Connecting The Dots: Switching And VOD

Maybe it's happened to you lately. You're in a meeting. Engineers are present. They're talking about switched digital video. (Or maybe they call it “switched broadcast,” or “SDV,” or SDB.” They're interchangeable.)

You drift off to your checklist of basics: Switching is a bandwidth-conservation technique. Most people most of the time watch mostly the same stuff. That means dozens of channels are moving down the pipe, to few if any eyeballs.

The answer: Save room. Only send channels that viewers actually ask for.

From what you've heard, the math seems promising. In a digital video broadcast lineup of 160 channels, half aren't watched. A video switch makes 160 channels look like 80 channels — and that's a conservative estimate.

You tune back in to the conversation happening around you. You could swear you only mentally skipped out for a few sentences — yet your technical colleagues seem to have careened into a deep dive on the architectures used for video-on-demand delivery.


This week's translation seeks to connect the dots between the seemingly unrelated technology precincts of switched digital video and video on demand.

Commonality No. 1

The first commonality between switched video and VOD is perhaps obvious: Neither is broadcast to all subscribers.

When you pick something from the on- demand roster offered by your cable company, you're setting up a “session.” The session links your set-top with a storage server somewhere in the network, where the video is stored. When you demand it, a copy is sent to you.

Only you get what you selected. Not everyone on the system. (Tech people call this “unicast.” Translation: You-cast.)

Switched broadcast techniques also use “sessions,” to fetch the channel you pick, when thumb-surfing the remote. If you're the first in your neighborhood to ask for the channel, then your session initiates the transmission of the channel. If you're the second, you “join that stream,” as it goes by.

That's why technology people talk about “SRM,” for “session resource management,” when they're talking about either technique. They mean everything that's required, behind the scenes, to set-up that box-to-server link, maintain it, and tear it down. Amongst your tech-side peers, SRM is a chewy topic.

Commonality No. 2

The second commonality between switched video and VOD is the technology used to deliver those sessions to and from a set-top box. This is where the conversation usually veers into modulation talk, and especially “QAM,” for Quadrature Amplitude Modulation.

Sorry. I know how easy it is to tune out when QAM pops into a conversation. Here's a hopefully easy way to put it in perspective: QAMs are what move digital channels down the pipe. They are complex techniques for imprinting bits of data on the radio waves moving down the pipe.

One digital channel equals one QAM, equals one chunk of bandwidth capable of hauling 40 Megabits per second down the pipe.

QAMs enter conversations about switched video and VOD through the door marked “inventory management.” Say a cable operator earmarks four digital channels for its on-demand offerings. Engineers will say this as “I've got four QAMs for VOD.”

Right now, any operator considering a video switch is working out how many channels in their digital shelf space — how many QAMs — they should allocate to the switch. That raises the question: Should that switch also manage the four on-demand channels, and the QAMs that support them?

And those other digital channels, allocated to IP (Internet protocol) services, like voice-over-Internet protocol and broadband. They also use QAMs to move bits to homes. Should the switch manage those, too? And if so, are the switch people tuned in to the DOCSIS (Data Over Cable Service Interface Specification) 3.0 chapter, which carries similar implications?

Pull up, the engineers say. Right now, the installed base (1 million or so) of digital modulators (QAMs) is service-specific. VOD QAMs carry VOD. Broadcast QAMs carry the “regular” digital video channels. Data QAMs carry voice-over-IP, Web surfing, and everything moving over the cable modem.

In short, all bits stay in their respective “lanes.” Since switching video is a nascent technique, the time is now to decide whether those “single purpose” QAMs should be re-configured, to carry whatever traffic comes at them — data, video, or voice.

Feature Overlap

Because switched video is just emerging as a strategic priority, the vendor community is predictably a mixture of suppliers of other technologies. VOD suppliers make session resource managers. So do the makers of video switches. And that's not even factoring in the mass of voice switch vendors with video intentions. Partnerships are likely.

VOD and switching hang out together, conversationally, because they have overlapping features. More on that, and the differences between VOD and switching, next time.

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