How IPTV Differs From Cable TV

Every January, usually right after the Consumer Electronics Show, cable's technologists pack their bags and head to the Society of Cable Telecommunications Engineers' annual Conference on Emerging Technologies.

Attending ET means sitting in a darkened room for two days, listening to technical people deliver technical papers. Translations, context, and interstitials come from VIP moderators — Tony Werner, chief technology officer of Liberty Global, and Chris Bowick, cheif technology officer of Cox Communications Inc., each anchored a half-day session at the Tampa, Fla., gathering last month.

Rising up from the dense thicket of terminology this year was an ever-tantalizing subject: How telco-styled “IPTV,” or Internet Protocol Television, is unique from cable-delivered video.

The goods came from Nimrod Ben-Natan, a vice president in Harmonic Inc.'s Convergent Systems Division. Although Harmonic sold a pile of gear to Verizon Communications Inc. last year, through Tellabs Operations Inc., Ben-Natan's remarks focused more on video delivered over telco DSL (digital subscriber line) networks, like what AT&T Inc. is putting together.

Note: DSL is rooted in IP. Thus, IPTV is synonymous with sending TV over DSL.

And with no further ado, here's his take on the Two Big Things that make video over DSL different than “traditional” digital cable networks. And you can bet your bippy, as my mother would say, that these two things will be heavily marketed by IPTV purveyors like AT&T.


No. 1 is that much-touted fast channel change. How fast is fast? Less than 200 milliseconds, technically. Visually, 200 milliseconds looks as fast as channel changes used to look, back when the F-connector plugged into the back of the TV set.

If you've been following the IPTV action, you've heard this one before. Lots. So far, what's said about how it works is a plausible-enough explanation: The tuners aren't nested inside the box. They're up in the network somewhere. (Make a left at the pedestal, go a couple miles.)

Here's more on how it works, technically: Say that's Consumer Jane there on the couch. She gets digital video from her local telephone company, which sends it to her over a DSL connection.

Remote in hand, Jane thumb-surfs via the channel-up arrow. The set-top (or media center) that came with the service issues what's known as a “join” request. It wants to dip into a pre-cached set of video frames.

The request zings up the phone wire, to that buffer. Maybe it's in an “edge aggregator,” or maybe it's in the “D-SLAM,” or Digital Subscriber Line Access Multiplexer. (The latter is telco-speak for the thing that sits between a bunch of incoming DSL lines and a fast Internet backbone.)

Either way, the tuners aren't inside anything at Jane's house.

The bits that make up the video frame in the buffer zing back into the box.

The boxes used by cable and satellite operators use work differently. On-board tuners work by literally jumping frequencies each time somebody invokes a zap with the remote control. Then they need to demodulate the incoming signal, stabilize it and deal with any error-correction activities. If the processing chip isn't beefy enough, that to-do list can bog down. Symptom: Slower zapper action.


Let's switch to Consumer Bob — on game day. He's watching his favorite team. On the same screen, in smaller video boxes, are three other games. Or, maybe he's watching three other camera angles.

A heavy offering of picture-in-picture screens is the second offering unique to video over DSL, Ben-Natan said at the conference.

Technically, here's what happens: Each smaller video box gets filled with a lower resolution, 200 Kilobit-per-second stream of video. Using advanced compression, ordinary (not high-def) streams earmarked for DSL video weigh about 1 Megabit per second. Those smaller, picture-in-picture boxes can get skinnier streams because … well, because they're smaller.

Serving video multitaskers with cable or satellite boxes is generally limited by the number of tuners in those boxes. That places picture-in-picture near the top of the benefit list for IPTV's “tunerless” architecture.

Telco video isn't without challenges, though, Ben-Natan said — notably bandwidth. Supporting a home with two HD sets, each eating up 6 Mbps, removes more than half of the available bandwidth on a 20 Mbps connection — the current deployable max for advanced DSL gear.

In closing: The good thing about technical conferences is the possibility of seeing that devil lurking in all of those details.

The bad thing about technical conferences is waiting for the right details, depending on what devil you're after.

If your devil is “IPTV,” this year's ET Conference delivered.