A Programmer’s Guide to Switched Video

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Last week I witnessed a very funny young waitress walk up to the booth next to me, intent on clearing the plates. (It was at my favorite Denver diner, Annie’s Café, 8th and Colorado.) “You gonna eat that?” she said to the two men sitting there. They nodded no.

“No?” she said, eyebrows arching: “Can I have it?”

At that moment, over a chicken burrito and a glass of iced tea, it hit me: What a perfect way to add levity into the mind-numbing topic that is switched video! Imagine she was pointing at bandwidth. (Whatever bandwidth looks like. I see it as having stripes.) Then substitute the word “use” for the word “eat.”

OK, maybe not. But if my e-mail basket is any indication, people from the program-network side of life are going into this week’s Cable & Telecommunications Association for Marketing Summit laden with questions about this new video-switching thing.

As it turns out, that same topic was top-of-mind for the industry’s technical ranks, too, at their big conference last month — the annual Cable Tec-Expo, hosted by the Society of Cable Telecommunications Engineers.

In presentations and on the SCTE Expo show floor, operators and their switching suppliers were full of early detail about how things are going. The one that struck me was word from BigBand Networks that nearly 7 million homes are currently getting their cable digital video through switches. (“And if they know about it, you did something wrong,” said one presenter.)

That’s a surprisingly big number. The first trials happened three years ago this month; “official” launches began last spring. If nothing else, it shows how swiftly this switch — to the switch — is moving.


The queries from the programming community go like this: Switching is a way to make room for us to launch our new, fledgling channels, right? How do operators decide what channels to put on the switch? Then what happens — what’s the step-by-step of putting in a video switch?

Let’s take them one at a time. Switching is a way to make room for more digital stuff, yes. What it makes room for, though, is pretty much up to the person who owns the shelf.

As for how operators pick what programs go in the switch: In general, “lightly viewed” programming is loaded up first. It’s not to remove channels, the reasoning goes, because viewers should be able to watch TV as they wish.

But transmitting rarely watched channels isn’t ideal, either, because what nobody’s watching is using up space that could be used for something else.

Time of day matters, too. If a channel contains programming that’s heavily viewed during the day, but not so viewed at night, or vice versa, that channel becomes a switching candidate at the moment lots of people tune off.

Step one in switching video is the digital simulcast — taking the entire analog tier, and replicating it digitally. Step two: Learning utilization statistics, over time. Utilization statistics are a set of data, collected from a storage server, which shows what channels are selected, at what part of day, and in what serving groups.

Then, switch loading begins. If an operator sets aside switching bandwidth equivalent to eight digital channels, and each of those channels can hold 10 standard definition video streams, then, the starting point is to tuck 80 channels into the switch. As more streams are moved, the bandwidth savings happen: 160 channels is a 2:1 loading, or, a 50% bandwidth savings.


The trick is to load up the switch as much as possible, short of “blocking” — or what telephone engineers refer to as “the Mother’s Day Factor.” Translation: Telephone traffic is typically highest on that day. Thus, telephone networks are designed to hold up to peak loading on Mother’s Day.

As far as timing goes, the vendor community says it takes about 90 days, from the time equipment orders are placed to the time the first hubs are activated for switched video. (For safety’s sake, double it.)

And know that behind the scenes, it’s the video switch that decides who’s gonna eat that bandwidth. And how. And where.

Stumped by gibberish? Visit Leslie Ellis atwww.translation-please.com.