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, says 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 says. Analog fall-back One of the tools Hildebrand was referring to is Scientific-Atlanta’s receiver, which he says 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 says. “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 says 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 says. “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 some tunes to Fox News, for example, you put that in the channel map and begin broadcasting that channel map,” Hildebrand says. “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 says 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 says. “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 is associate editor for Communications Technology. Reach him at firstname.lastname@example.org.
C-Cor, OpenTV, et al.
Vendors are finding ways to fine-tune switched digital video (SDV) now that it has moved to real-world deployments. One recent example is the partnership between OpenTV and C-COR, who rolled out an SDV platform at the National Show that combines each company’s technology. The demo in Atlanta used OpenTV’s Core 2.0 Middleware solution as the client in Motorola’s DCT-2000/2500 set-top boxes while C-COR’s nAble global session resource manger (GSRM) provided the session and edge resource management. The result is an open architecture for SDV that can be deployed on legacy, two-way set-top boxes, including Scientific-Atlanta. Because the platform is an extension of the middleware in the set-top box, a program guide doesn’t need to be modified and integrated, which is the current scenario with SDV. “Because we’ve built this as a fundamental piece of the middleware, any program guide, in fact any third-party application, that is running on top of OpenTV’s middleware now has access to the switched digital channels,” says Steve Reynolds, CTO of the North American cable division for OpenTV. “The application, or the program guide, doesn’t even know that those channels have been moved into the switched space.” How it works The two companies began working on the project about 18 months ago after Time Warner Cable put out a specification for an end-to-end SDV system. While OpenTV focused on getting its Core middleware on Motorola’s set-top box, C-COR found that its nAble VOD software already supported SDV functions such as bandwidth management while also providing reports on what programs were being viewed. “We designed our global resource session manager to work with any of the Motorola or Scientific-Atlanta technologies,” says Cliff Aaby, C-COR’s VOD product architect, “At the (NCTA) booth, we demonstrated that we’re not only able to do switched digital, but that we can do it in an open environment.” C-COR’s GSRM interfaces with edge quadrature amplitude modulation (QAM) modulators, which is basically the same infrastructure a cable operator would use for VOD or network digital video recorder (DVR) deployments. The GSRM is the headend device that implements the SDV protocol. To deploy switched digital video, streams coming into a headend need to be clamped to make sure the bit rates are pre-established. Once the streams come in, they’re put on multicast—in most cases a Gigabit Ethernet (GigE)—at the headend. The switching is accomplished through a narrowcast edge device that allows multicasts coming into the headend to be targeted down to specific set-top boxes in service groups. In Atlanta, C-COR and OpenTV used Harmonic’s Narrowcast Services Gateway. “You get all of that headend infrastructure in, and then down on the client side, you have OpenTV’s middleware loaded in the set-top box,” Reynolds says. “That switched digital extension resides in a layer in the set-top software stack that is below the program guide.” Reynolds says the middleware stack maintains a service list, which includes a flag that establishes whether a channel is in the traditional broadcast or SDV space. If the particular channel requested isn’t being played down to the service group, the C-COR equipment at the headend will send messages to the narrowcast gateway instructing it to join the multicasts and begin transmitting that service down to the service group. Then the switched digital server sends a message down to the set-top box saying the service is now available and where to go to get it. Both Aaby and Reynolds think niche channels, such as foreign language or cooking channels, will be switched since the majority of viewers will already be watching shows on major stations such as CNN or ABC.