June 26, 2008

SDV: Find the Right Mix

By Monta Monaco Hernon

Switched digital video (SDV) promises bandwidth savings for operators, but they must go beyond the how and why of the technology to strike a delicate balance.

"Trying to design SDV becomes a tradeoff between balancing supply and demand," said Phillip Gabler, Comcast distinguished engineer. "You want to reclaim as much bandwidth as possible from the greatest number of channels possible using the smallest number of QAMs."

"The larger your service groups, the greater the chance people will watch a greater variety of channels .... If you include more channels higher in popularity, the greater the chance your bandwidth is going to be consumed," he added.

Before launching SDV, Comcast measured viewership for approximately one month. Based on the data gathered, the company determined its "sweet spot" was 800 set-tops per service group and selected 80 long-tail channels. "We know we can fit them into a pretty small set of broadcast QAMs with little risk of blockage," Gabler said.

Non-interactive set-top boxes cannot switch digital video. Comcast has a non-responder rate that is 10 percent, but has experienced no significant increase in call volume because the channels currently being switched are "way out on the end of the curve."

"We have elected to be less proactive than we could be. Analysis says if we add more channels or allow service groups to get too big, we will have problems and will have to take the non-responder problem more seriously," Gabler said.

Comcast has continued to monitor the load on its systems. One or two channels have been dropped, but no "dramatic" changes have had to be made. The company currently employs four VOD QAMs and four SDV QAMs side by side. "Our goal is to merge (these) into a single pool and share across both applications," Gabler said.

A reduction in silos

Using network encryption rather than edge encryption is one way to enable sharing of QAMs and reduce the silo effect, said co-presenter Charles Hasek, principal engineer, Time Warner Cable. A company could then deploy clear QAMs that could be used for VOD or SDV.

"We can source edge QAMs from vendors who may not have ... the ability to support encryption in their product," Hasek said.

He acknowledged this would enable Time Warner to start "mixing" it up a bit and not be tied to Scientific-Atlanta's (now a Cisco company) PowerKey.

"We can support our embedded base, but still bring in other technologies," Hasek said.

- Monta Monaco Hernon

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