At a technical session at Cable-Tec Expo last month, two operators discussed their experiences with implementing switched digital video (SDV) systems. Paul Conway, regional vice president of engineering at Time Warner Cable, talked about getting SDV equipment physically in place in New York City, while John Civiletto, executive director of platform architecture with Cox, talked about an SDV field trial.

Cox Communications conducted the field trial to validate that cable networks can support a nearly infinite number of channels while mitigating pressure on bandwidth utilization. Civiletto’s presentation was based on a technical paper co-authored with Ludovic Milin, director of product management SDV with BigBand Networks, called "Switched Infinity: Supporting an Infinite HD Lineup with SDV."

The field trial encompassed 23 SDV QAMs deployed on four service groups in a selected Cox market.

"We looked for demographic diversity in our service groups," said Civiletto. "Consumption behaviors need to be taken into account. We also picked relatively large service groups for a good representation."

SDV provided enough bandwidth to support the equivalent of 43 broadcast QAMs and a lineup of 268 standard definition (SD) and 75 high definition (HD) channels without sacrificing video quality across a total of 34 QAMs. Service group size was significantly larger than the current SDV standards at between 700 and 1,100 tuners per service group.

According to the paper, "With all other parameters being equal, the number of tuners per service group is the variable with the most effect on the efficiency and bandwidth gains of the SDV system. Going from 700 to 1100 tuners per service group required up to four more QAMs (23 percent more spectrum) to support the same SDV lineup."

"Service group size materially impacts the performance of the system," said Civiletto. "Today, we target around 750 tuners with an eye to reduce to 500 over time. As you drive down service group sizes, you are in effect creating bandwidth. Look for that optimal sweet spot."

Interestingly, the study found that only two HD channels appeared in the Top 25 most popular broadcast channels, despite HD tuner penetration being about 50 percent.

Those channels viewed less than 50 percent of the time are good prospects to move to SDV for additional bandwidth savings. "We move channels back and forth maybe once a quarter," said Civiletto. "It’s definitely an ongoing process."

SDV in New York

In the TWC case study, because of New York City’s density, it was challenging to find rack space for QAMs, said Conway. Time Warner requested that the vendors do the wiring for the racks off site and have them shipped in. Even then, tight quarters became an issue because some of the racks were too big to load onto elevators at the hub sites.

A technical paper, "Successfully Deploying Switched Digital Video Systems," was associated with Conway’s presentation. The paper, co-authored by Conway, Jim Kuhns, senior deployment project manager with BigBand Networks and Michael Melendez, lead project engineer with BigBand Networks, stated, "Physical equipment placement in the racks should be done with sufficient space to allow for future QAM expansion."

Conway noted he strictly limited access to the implementation sites and recommends the practice. With open access, things happen – such as power cords getting kicked – that can cause major delays.

For the New York SDV project, Conway used Microsoft Project as his project-management software, and he highly recommends it. He said paying attention to such small details as consistent color-coding for cables was important: "We didn’t want stuff that would confuse our hub techs later on."

According to the technical paper, "Naming and numbering conventions should be established for the hubs and service groups before the deployment begins. It is recommended that the conventions used for the SDV QAM service groups match the existing service groups as this will assist in future troubleshooting."

Finally, as far as lessons learned, Conway said it was imperative to conduct service group testing, but it took a lot longer than expected. "It needs to be done; it’s the only way to make sure it’s going to work when you turn the switch. But it is time-consuming," he said.

-Linda Hardesty

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