PBS Moves Forward on New Pull Transmission Method


PBS's implementation of a transmission method whereby stations pull content from a video-server farm at PBS headquarters is nearing the end of the prototype phase.

Earlier this year, PBS began the pilot program called the Next Generation Interconnection System prototype (NGISp). Nine stations are participating in the project, which involves sending content via satellite using Internet Protocol. The system allows the stations to pull content from video servers at PBS almost as easily as PBS pushes content.

The system is based on Omneon video servers both at PBS headquarters in Alexandria, Va., and at the stations; all have 880GB of storage (PBS is working on deploying 2TB versions). The servers on the project have four standard-definition ports and two ASI ports for compressed HD delivery.

The system has two modes of operation. For scheduled series like Nova or Nature, content is automatically sent to the station's servers, requiring no manipulation of the media. In the other mode, a station pulls content out of the PBS library. In that case, the system determines when it can most easily deliver the content, making it easier for the station to prepare to receive it..

Jerry Butler, PBS senior director, interconnection projects, says the stations have been transferring significant numbers of files for several months. File-checking processes confirm arrival and integrity of the content.

The system originally used QPSK modulation (the equivalent of 4QAM) to optimize the content for delivery via satellite. According to Jim Kutzner, PBS senior director, enterprise networking, the network is now using 16QAM for non-real-time content delivery. In non-real-time delivery. the program is broken down into packets, which are sent out to the station and reassembled there. Any missing packets can be resent immediately, ensuring proper delivery.

Use of 16QAM for non-real-time content (QPSK is still first choice for real-time content) allows more content to be sent across a smaller pipe, Kutzner says. "When we fully ramp up, we'll need half as much bandwidth and half as many transponders. And with transponders costing more than $1 million per year, you're talking real money over a 10-year time frame." PBS's current transponder contracts expire in 2006.

PBS has been spending more time on fine-tuning working with 16QAM than on the broader issue of file transfer. Fade characteristics, Butler explains, are more severe with 16QAM, requiring PBS to be more cognizant of how a file is packaged and what kind of error correction is used.

The program runs through December, after which PBS will have a better sense of how the system changes the workflow. "We have the before picture," Reilly says. "Now we're waiting to get the after picture."