Last time, we looked at the many intersections between the technologies used for video on demand (VOD), and those used for switching video streams.
A brief review: Both work by setting up two-way “sessions” between a set-top and equipment higher up in the network — servers and streamers, in the case of VOD, and the switch, in the case of switched video.
Both also use the same transportation technique to move video streams to homes: quadrature amplitude modulation, or QAM. So does “regular” digital video, HDTV, simulcasting, broadband Internet, and voice services. That’s why operators are maneuvering to make those QAM devices capable of mixing and matching any digital traffic that moves through them.
In short, VOD and switching tend to hang out together, conversationally, because they have overlapping features, and they can potentially share resources.
This time, we’ll look at what’s different about VOD and switching, and at what operators are doing to get ready for the switch.
MANY 'CASTS’
Usually, if you agree to go more than two or three mental steps down the path of learning about switched video, you’ll run into the term “multicast.” It is one of the chief differences between VOD and switched video.
Here’s how multicast works, by practical example: Customer Bob, there on the couch, lands on a channel he wants to watch. Let’s say it’s Oxygen (Bob being an evolved man, and all). A session is set up between Bob’s set-top, and the switch. At this point, only Bob is watching Oxygen, although it’s flowing to everyone in Bob’s node (usually a grouping of about 500 homes) and in Bob’s service group (a cluster of four or so nodes).
Next door, Customer Jane also tunes to Oxygen. Her set-top says to the switch, “she wants Oxygen.” The switch says, “it’s already flowing, tap in here,” and tells the set-top where to tune so that Jane can “join” that stream.
As soon as Jane joined the Oxygen stream, Bob’s unicast became Bob and Jane’s multicast. Ditto for anyone else served by that node, inside that serving area group, watching Oxygen.
But wait, there’s more: The notion of the “switched unicast,” which will be detailed in a technical paper presented by Big Band Networks at this week’s National Show in Atlanta. (The session is Sunday afternoon, but if you miss it, there’s always the compilation of NCTA Tech Papers.)
Here’s how switched unicast works: Customer Ray, down the street from Bob and Jane, tunes a different channel. Let’s say Ray wants the Food Network. In a switched unicast environment, Ray’s set-top asks the switch for that channel, and the switch responds with a unicast session, just to Ray. In a way, it’s like a VOD stream, but without the trick modes (fast forward, rewind, pause).
If Bob flips over to Food, he doesn’t join Ray’s stream, even though it’s flowing into the same node.
The advantage, switched unicast proponents say, is everything that comes with one-to-one personalization with customers — the Ray-targeted ad, for instance. But it takes more bandwidth than multicast, and it costs more.
GEARING UP FOR SWITCHING
The week before last, our sister publication CED magazine, ran a Webcast about video switching, where technologists from Cox Communications Inc. and Time Warner Cable dispensed tips about how to get ready.
Among them: Allot plenty of get-ready time to rewire distribution hubs, which is where the switches often go. It’s the most time-consuming part of “going switched,” tech people say.
Also: Once and for all, get rid of any “non-responder” set-tops in the system. “Non-responders” are boxes that don’t or can’t communicate to the network, usually because they’re plugged into an outlet that gets turned off with the wall switch — or are otherwise “deaf” to the network.
Once the plant is ready, start slowly. Place “lightly viewed” networks on the switch first. Move only a few channels at a time, and determine what channels to move by using viewership analysis tools.
I’m not big into predictions, but switched video bears a strong resemblance to “digital simulcast,” in terms of unanticipated priorities. Like digital simulcast, switching is emerging fairly suddenly on operator to-do lists, with aggressive launch targets over the next few years.
The reasoning: You don’t leave the lights on when you’re not home. You don’t leave the water running in the sink when you’re not brushing your teeth. So why put channels down the pipe that aren’t being watched?
I’ve yet to find one cable operator complaining of excess capacity. Switching conserves their precious digital bandwidth.
Stumped by gibberish? Visit Leslie Ellis at www.translation-please.com.
