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Juggling the VOD Numbers

The Second of a three-part Multichannel News Special report on topics that are sure to be front and center at the National Show in San Francisco April 3-5. This Week: High-definition TV.

Time Warner Cable has released a new bandwidth allocation algorithm for the commingling of high-definition and standard-definition content on video-on-demand systems for its various server, set-top box and software vendors.

The algorithm seeks to address a vexing capacity issue as cable operators add HD content to their VOD servers, by making maximum use of the bandwidth in each quadrature amplitude modulator.

Typically, Time Warner sets aside four QAMs, or four RF (radio frequency) channels, for each VOD service group. Each QAM has the capacity to transmit about 37.5 Mb of content at any one time. Since standard-definition VOD content is transmitted at 3.75 Mbps, a single QAM can handle 10 simultaneous SD VOD sessions, according to Glen Hardin, Time Warner director of digital video services.

But an HD VOD session accounts for four times the bandwidth, or 15 Mbps, per session. (Time Warner has rolled out HD VOD in a number of systems, carrying an average of eight Hollywood movies at one time.)

Currently, Time Warner assigns the first VOD session that comes in from any subscriber to the first QAM in that VOD service group. The second request goes to QAM 2, the third to QAM 3 and the fourth to QAM 4. The fifth VOD session initiated in that service group would go into QAM 1, thus filling 7.5 Mb of the 37.5 Mb capacity of that pipe, the sixth to QAM 2, and so on until all QAMs are filled. This is known as the “least loaded” algorithm method.

In a perfect world, the four QAMs never have to handle more than 40 VOD sessions. Anything beyond that would cause a busy signal, and the MSO looks at the traffic load each day to make sure that doesn’t happen. If usage gets close to capacity, the MSO will split service groups or add QAMs to handle the traffic loads.

But once HDTV is added to the system, the sequential nature of where to put new VOD sessions can become tricky, Hardin says. A QAM that is already filled with seven SD sessions, for instance, does not have enough bandwidth left to handle an HD session. It would, however, have enough capacity to handle three more SD sessions.

Time Warner’s new algorithm combines the best aspects of the “least loaded” and “most loaded” methods that have been available in the SD-only on-demand world. “We’re looking for the optimum implementation,” Hardin says.

The goal is to not strand any bandwidth, while preventing consumers from getting busy signals as HD VOD proliferates by placing HD sessions in QAMs that have at least 15 Mb of bandwidth available. Since subscribers might be constantly starting and stopping VOD sessions, mapping the QAM bandwidth is critical to maximum spectrum efficiency, Hardin says.

“HD VOD represents the first time cable has had a technical advantage that is unmatched in the industry,” Hardin says. “Satellite has no way in responding to it. We really need to exploit this advantage.”

In most Time Warner systems, it’s the server vendor’s software that calls the shots in terms of which QAMs get what content, Hardin says. Time Warner has four approved server vendors: Concurrent Computer Corp., SeaChange International Inc., nCube Corp. and Broadbus Technologies Inc. (The MSO’s set-top suppliers and N2 Broadband are also getting the new algorithm specifications.)

The digital controller from Scientific-Atlanta Inc. also has the capability to direct content into QAMs, Hardin says, an important feature as Time Warner looks at switched broadcast video. SBV would require the same QAM efficiency strategies from edge QAMs to the home to maximize bandwidth.

The server companies are now charged with augmenting or creating software that meet the new algorithms, he says. One vendor has completed its work, he says, and Hardin hopes to roll out the new algorithms in all Time Warner systems by year’s end.

Part of the math in the algorithm also makes some assumptions on the number of overall HD sessions, versus SD sessions, Time Warner will permit at any one time in any service group. Currently, the MSO wants to keep HD sessions to no more than two per service group. In other words, if two HD sessions are in progress, that service group can handle 32 SD sessions.

If no HD sessions are in progress, that service group could handle up to the traditional 40 SD sessions. But if more than two HD sessions are requested, the third request would see a busy signal, even if there is space available, he says, to protect the vast majority of subscribers who would be requesting SD feeds.

Today, the number of HD set-tops, combined with the number of HD movies available on VOD is still small, so contention isn’t seen as a serious issue. But as HD set-tops and programming increase, HD VOD contention could be an issue.

Time Warner plans to give systems reporting tools so when service groups start seeing more than two requests on a regular basis, adjustments can be made so subscribers don’t get busy signals. It’s a difficult balancing act, Hardin says, because the system doesn’t want to give substantial portions of the QAM VOD spectrum to HD sessions that take up four times the space but don’t necessarily provide four times the revenue. At the same time, subscribers with standard definition VOD requests shouldn’t be squeezed out by a boatload of HD VOD requests.

The MSO plans to adjust the strategy going forward, based on traffic patterns it sees in its individual VOD service groups.