Switched Digital Video Comes Home Thanks to the panel discussion by several of the top CTOs in the cable industry at the recent National Show, we know the deployment plans for switched digital video (SDV), or lack thereof, for some of the cable operators, but what are some of the key elements in getting the SDV house in order? The primary payoff of deploying SDV, or switched broadcast video, is that it frees up bandwidth for other services and technologies because it only sends the channel a customer is currently viewing instead of all of the digital channels. Viewers in the same node who are watching the same program can be grouped together. John Hildebrand, Cox‘s vice president of multimedia engineering, said there are several tools already available or in the offing, as well as a basic floor plan to put in place, that will help build toward SDV deployments. Cox, which plans on two switched digital deployments this year, trialed SDV two years ago in Tyler, TX, on a Motorola system with 2,000 participants. At the same time, Time Warner Cable ran its trial in Austin, TX, on a Scientific-Atlanta system, and both worked with BigBand Networks on the trials. "Our trial was small enough that we didn’t want to draw too many conclusions, but there are tools that allow us to accurately measure who is watching what and how many people are watching the same channel at the same time," Hildebrand said. Stocking the toolbox One of the tools Hildebrand was referring to is Scientific-Atlanta’s Receiver, which he said gives a good idea of what the viewership looks like before launching SDV. Cox plans on switching its digital simulcast channels in a number of its systems. To that end, Cox is also working with vendors to develop a technology called analog fall back. If subscribers are trying to access a digital simulcast channel and they hit a blocking situation but the analog equivalent is available, then the set-top boxes are instructed to tune in the analog equivalent. "The goal being, of course, that you don’t want any blocking," he said. "In simple terms, it’s (blocking) a video ‘busy signal.’ In a switched environment, it’s technically possible that if every subscriber tunes to a different channel there’s not enough bandwidth to carry the channel that someone wants at that point and time." Cox is working with its guide vendors-Scientific-Atlanta and Aptiv Digital-on client software for set-top boxes in the SDV environment that will enable analog fall back. SDV challenges Hildebrand said the first decision a cable operator has to make prior to entering the SDV realm is which channels are going to be switched and which will remain broadcast, while knowing that one-way CableCARD enabled TV sets can’t tune switched channels because they don’t have the switched client. "Secondly, you have to look at what the utilization will be because you want to make sure you don’t have any channel blocking," he said. "So how many QAMs can you dedicate to the number of channels you want to switch? "Third, and this is kind of an operational challenge, is looking at the return path. In a switched digital environment you have to have 24-by-7, two–way functionality. You also have to have a fully interactive network, and you need a very low nonresponder rate." For two-way capability, the set-top boxes need to have an out-of-band channel that is always available back to the headend. One way of keeping set-top boxes informed on what channels are currently being switched is through a channel map that is sent to them every few seconds. "With the multicast environment, as soon as someone tunes to Fox News, for example, you put that in the channel map and begin broadcasting that channel map," Hildebrand said. "For anyone else who decided they needed to go to Fox News, the first thing (for the set-top box) to do is look at the most recent channel map." Operating on the out-of-band channel, the set-top box doesn’t need to ask the headend where a channel is because the channel map has already said which particular QAM and MPEG pit has Fox News. DOCSIS Set-top Gateway (DSG) could be used to broadcast the "always current" channel map, but Hildebrand said it’s not required. Broadcast channel maps are broadcast every few seconds in today’s landscape so that when a set-top box reboots, it gets the current map. "In today’s world, it’s (channel mapping) fairly static, but in a multicast environment, it would be changing every few seconds," he said. "There are the technologies, like analog fall back and channel map, that are being used to ensure that customers always have access to video even in the switched environment." Mike Robuck

The Daily


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