SCTE Journal: Comcast's SDV Sweet Spot(s)


Philadelphia—Comcast distinguished engineer Phil Gabler provided some insight into the MSO’s technical trials to date with switched digital video.

Statistically, Gabler said, Comcast has found a sweet spot in its four test markets of sending SDV channels to service group sizes of about 800 set-tops. He gave his presentation, "Challenges and Design Considerations for Deploying Switched Digital Video," at a session here at SCTE’s Cable-Tec Expo.

Comcast is currently testing Motorola and Cisco switched video platforms, and hopes to start working soon with BigBand Networks. The MSO hasn’t said where those trials are happening, but two of them have been reported to be Denver and Cherry Hill, N.J.

In terms of channel selection, Comcast is "focusing on a selection of 80 long-tail channels," which are very rarely viewed. The MSO has allocated four QAMs for those 80 channels, an oversubscription ratio of 2:1.

Comcast is seeing channel-change times of about 1.5 second with SDV. "Our business requirement was to have no more than 20% difference with broadcast channel change," Gabler said.

The peak load on the SDV systems happens on Sunday nights. Comcast is putting some HD channels in the SDV system, but has seen more "blocking" events — in which all the QAMs allocated for switched video are used to capacity — with HD since high-def signals take up much more bandwidth than SD. 

Overall, Gabler said, Comcast is designing the SDV systems to have a blocking event very, very rarely.

Some set-tops, for various reasons, are unable to send channel-change requests up to the SDV system — such set-tops are referred to as "nonresponders."

Comcast has seen a nonresponder rate of 10% in one of its test markets, Gabler said, but "we saw no increase in call volume," which he attributed to the fact that the 80 channels in the SDV group are so rarely viewed.

Gabler also provided some hints into future projects. For example, he said, "We are starting to think about IPTV using switched video over DOCSIS."

In addition, Gabler said, Comcast is considering using SDV to deliver advanced advertising, but that’s a ways off. "We’re thinking more about video-on-demand advertising," he said.