As the cable industry prepares for its annual trade show season, there’s arguably no single technology that continues to generate more buzz, opportunity, excitement – and confusion – than switched digital video.

By now, the benefits of switching video are well-understood. Switched video and its known aliases – switched broadcast, switched broadcast video, switched digital broadcast – can allow cable operators to significantly expand channel lineups, support additional HDTV and "long tail" programming, dramatically improve network efficiency, create new revenue opportunities and increase subscriber satisfaction, all at a fraction of the cost of most alternatives.

Switched digital video’s popularity has put it in the crosshairs of many of cable’s technology suppliers. Multiple suppliers can be good for operators. Agreement among operators and suppliers on the definition and adoption of open standards, however, could increase the likelihood of multi-vendor, switched video’s success. Fortunately, a strong foundation of open protocols already exists around switched video. If these protocols are supported by technology suppliers, multi-vendor switched video can be a relatively seamless exercise. The DOCSIS model The cable industry has been somewhat distinctive in its technology evolution. From the 1950s through the 1980s, the technology was dominated by end-to-end network architectures – from headend to set-top box (or TV set) – typically provided by a single vendor. In the United States, the advent of CableLabs, and standards to support interoperability, changed all that.

That was a watershed moment for the industry because it allowed (or forced, depending on a particular point of view) technology suppliers to interoperate with one another and gave cable operators the ability to choose viable network solutions from multiple suppliers for the first time. There is a valuable lesson for switched video development in the origins of DOCSIS. Open standards, and particularly the interoperability they enable, are a good thing.

The team at BigBand Networks began developing switched digital video technology for the cable industry in 1999, around the same time that the first DOCSIS products were being certified. In keeping with the spirit of openness, BigBand built early switched digital video technology on open-source, MPEG constructs that were available to anyone on the Internet. Using protocols called digital storage media command and control (DSM-CC) and program and system information protocol (PSIP) allowed our switched video components to communicate with each other and with the operator’s existing equipment.

The premise was that operators could implement and develop more easily on top of these open source constructs than on any closed alternatives, which ultimately would accelerate the technology’s adoption. The prospects for switched video were attractive. The industry was in the midst of spending $100 billion on network upgrades; switched digital video could achieve benefits similar to those of expanding capacity, at a fraction of the cost. Open architecture A major operator approached BigBand in 2004 with this offer: "Open your protocols to us, and we’ll develop a standard for switched digital video that the entire industry can use."

Because the technology was based on two open protocols – DSM-CC and PSIP – this became a fairly straightforward exercise. The DSM-CC construct, with adjustments specified by the operator, became the channel change protocol (CCP). This enables a seamless channel change in a switched services environment, a critical element to get right.

The PSIP construct, also with some operator modifications, became the mini-carousel protocol (MCP), which enables a range of important functions, including making it possible for set-top boxes to initialize into a switched digital video system. These adjustments having been made, the first switched digital video deployments followed thereafter in 2005.

Fast-forward to today. Many of the operators in North America are either deploying, or looking at how to deploy, switched digital video.  An ecosystem of protocols has emerged to ensure interoperability among vendors.

Figure 1 illustrates an existing architecture for open, switched digital video. Switched digital video is not a CableLabs specification, and thus a single spec is not defined to drive interoperability among all vendors. However, all of the key interaction among the elements of a switched digital video system is defined by readily available, open technologies. The switched architecture utilizes the ubiquitous Internet protocol (IP) for transport of video content from the headend to the switch and router. From the switch to the quadrature amplitude modulation (QAM) device, the Internet Group Management Protocol (IGMP) allows the switched video to be distributed from the single source at the headend via multiple edge routers to any number of QAM hubs.

What performs switched video’s technical "magic" or "smarts" is  a server that communicates with each set-top box. The session server collects information about what programs are being watched in real time and makes a decision to deliver programs based on that data. When the programs are not being viewed, the spare bandwidth can be used for other purposes, including adding additional programming options or channels.

Operators and suppliers have two protocol choices – remote procedure call (RPC) or real time streaming protocol (RTSP) – to enable communication between the edge QAM modulator and the switched session server. The standard channel-change protocol mentioned previously, CCP, in turn allows the server to communicate with the set-top box. The edge QAM modulator must also maintain communication with a global session or edge resource manager (GSRM or ERM), which is responsible for QAM resource sharing. A session setup protocol is used for this purpose. Open systems? Ideally, the purpose of open protocols should be to enable open systems. That is, if technology suppliers adopt the protocols, operators know that they can insert those suppliers’ equipment into their networks with the assurance that they will interoperate with other suppliers’ equipment supporting the same protocols.

The benefits of this approach are clear. Interoperability gives the supplier the option to choose a "best of breed" component in one case, or not overpay for another component simply because alternatives are limited. Interoperability also encourages innovation.

However, open protocols don’t automatically equate to open systems. If protocols aren’t fully adopted by suppliers in a way that allows for multi-vendor interoperability, then their value is diminished. Operators who are looking to deploy switched digital video may want to make certain that their technology suppliers not only "support" common switched protocols, but also have implemented them in such a way that the component they are supplying interoperates with the other components in their network. This is particularly true for QAM modulators, set-top boxes and conditional access systems.

For example, proprietary encryption schemes must be decoupled from common network elements, such as QAM modulators, to allow for interoperability. Alternative approaches to encryption, such as stand-alone, low-cost Gigabit Ethernet (GigE) appliances or bulk encryption devices, are available to achieve this result. DVB standards have already allowed this to happen economically in many international markets. This will be a key issue as QAM resource sharing across switched digital video and video on demand (VOD) services is implemented.

Other proprietary elements, such as VOD session resource managers, will need to be modified to accept higher level or independent bandwidth or throughput managers or appliances in the network. Clear promise Because switched digital video is not a CableLabs-defined specification, some confusion may exist about the status of the protocols that enable multi-vendor interoperability. As the preceding paragraphs indicate, considerable effort has gone into developing open protocols for switched video. These protocols effectively open the switched digital video opportunity to a range of suppliers.

The switched architecture promises a range of benefits beyond bandwidth savings, including advertising for individual recipients and an expanding library of programming. An industry-wide development effort could help the technology achieve its full potential. Doug Jones is BigBand Networks’ chief cable architect. Reach him at

The Daily


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