How Switched Digital Video Works

In a traditional video delivery system, the cable companycombines all channels into one data stream,even if no one is watching most of them.

Cable service providers are facing a troublesome obstacle. Consumer demand for bandwidth is approaching -- and in some cases exceeding -- network capacity. Many cable companies offer a wide range of services, including cable television programming, digital video and audio channels, high definition (HD) television channels, video on demand (VOD), cable modem Internet services and digital phone service. While coaxial and fiber-optic systems have a relatively large bandwidth capacity, service can suffer if too many customers are using it at the same time. Cable companies are considering several options to meet customer needs, including switched digital video (SDV).

To understand how SDV works, we'll first need a quick lesson on how cable companies traditionally provide service to customers. In older cable systems, the cable company broadcasts every channel in a single stream of programming to every section of its network all the time. When you use your set-top box (STB) to tune to a particular channel, the STB searches the stream for the channel's frequency and carries the signal to your television.


Because the cable company sends every channel through the entire network, there's not much spare bandwidth available for Internet service or digital video channels. High definition channels take up more bandwidth than normal digital video, so spare bandwidth decreases as cable companies add HD channels to their lineups. Because HDTVs have become more affordable and more households have acquired them, demand for HD programming has increased. Cable companies have to find ways to meet customer expectations, particularly in light of the fact that HD television providers already offer dozens of channels in HD format.

Switched digital video uses a different delivery system. Instead of combining all channels into one programming stream throughout the network, the cable company selects only the most popular channels for a network-wide stream. For less popular programming, the company responds to individual customer demands as the customer tunes in to that channel. In other words, the service provider sends only the channels customers are actually trying to watch. Because the system only sends customer-demanded channels, there's a lot of spare bandwidth left over for other services.

According to IneoQuest, a company that offers SDV solutions to cable companies, around 80 percent of all viewers in a given area are watching the same top 20 channels [source: Switched Digital Video Solutions]. Providing only the requested channel feeds would free up enough bandwidth for the cable company to:

  • Increase the number of available channels in any given region, including HD video feeds
  • Customize channel selection for different regions, including niche programming that might appeal to one segment of the company's customer base but not another
  • Create more VOD content, giving users even more choices of what to watch
  • Experiment with interactive television formats
  • Target specific ads to individual markets, or even individual customers
  • Allow cable modem customers to download content at a much higher rate

In this article, we'll examine the architecture and equipment used in an SDV system, and we'll learn about the potential advantages and disadvantages of SDV. We'll also look at some companies and markets that are already using SDV.

In the next section, we'll learn how the SDV system connects the cable provider to the customer.


Switched Digital Video Architecture

An example of a switched digital video system's architecture

There is no standardized way to set up an SDV system, but every system can be categorized into four sections: the headend, the transportsystem, the accesssystem and the customer network.

The headend section of SDV architecture is where the video and Internet feed sources enter the system. It includes the equipment directly connected to the cable company:


  • MPEG encoders, which convert the raw digital or analog signal into an MPEG format
  • A bulk encryptor, which scrambles the signal in such a way that only the appropriate set-top box (STB) can unscramble it
  • Internet servers, which allow customers to access the Internet using cable modems
  • Applications servers, including a Session and Resource Manager (SRM), which determine how much system bandwidth each application can access, and another server that monitors and manages other system component (SDV systems can be very complex and require powerful machines dedicated to keep them properly configured.)

Some elements of the headend only flow one way into the system, such as the cable company's video feed. Others, like the applications servers, communicate back and forth with the network to ensure that everything is running smoothly.

Once the headend converts the video feed into MPEG format and encrypts it, it sends the signal on to the transport system of the SDV architecture. The transport system's purpose is to route the video and Internet feeds from the cable company to the access system. This section consists of nodes, points where cable connections intersect and branch off, and routers. Nodes and routers redistribute the signal to other nodes and routers so that the original feed covers the cable company's entire customer base. The transport system's path connects the headend to the access system.

The access system is where the actual digital switching takes place. The core of the access system is the SDV server. It's the SDV server's job to keep track of customers' channel change requests. SDV servers are dedicated computers that run software designed to interpret each channel request. The server sends commands to an Edge Resource Manager (ERM) and several Edge QAM devices to meet demands.

The customer network includes a set-top box that receives and decrypts signals from the access system and a cable modem if the customer subscribes to cable Internet service. Each company uses a different interface between the customer network and the access system.

In the next section, we'll learn about some of the advantages and disadvantages of SDV service.


Switched Digital Video Advantages and Disadvantages

Switched digital video systems' most obvious advantage is that they address one of the biggest problems cable companies have -- running out of bandwidth. As more people sign up for HDTV services, high speed cable modems and VOD, cable companies have to find a way to deliver. An SDV system helps cable companies implement a solution without replacing miles of cable, since the system can work on top of an existing infrastructure.

Increased bandwidth can also lead to more services. A cable company could choose to increase the number of channels it offers, allowing it to compete more effectively with satellite television companies. Additional channels could include more HD feeds or programming catered to a particular region.


Cable companies provide Internet access in much the same way they provide television feeds. Companies feed Internet downstream through 6-megahertz (MHz) channels in the cable bandwidth. Upstream feeds, the information sent back through your modem to the Internet, goes into a 2 MHz channel. When customer demand gets heavy, the cable company creates a new 6 MHz channel to relieve stress on the system. With SDV, cable companies have more bandwidth available to convert into Internet channels during periods of high customer demand.

A switched digital video system sends only the channels requested by viewers, saving the cable provider bandwidth. Click play to learn more.­

SDV systems provide some options that are more important to the cable company than the consumer. Using SDV systems, cable companies can get more information about what their customers watch -- most SDV systems preserve individual customer privacy, but they still allow cable companies to see which channels are in high demand in various regions. Some SDV companies offer systems that would allow a cable company to target specific regions, or even specific households, with local advertising based on viewing habits. That can be a powerful bargaining chip when the cable provider meets with potential advertisers, though consumers might not be as thrilled about it.

Increased bandwidth could also let cable companies offer more interactive options. Let's say you've decided to order a boxing match pay-per-view special. The cable company could process your request and send you a message that gives you the option to order pizza from a local restaurant. You use your remote control to navigate a menu, choose your pizza and toppings, and confirm your order. Half an hour later, you're watching the pay per view and enjoying a hot pizza pie.

The biggest disadvantage to SDV systems is that there are many components in the system. This means that when problems arise, cable companies might find it difficult to figure out the source of the issue. Problems could start in any section of an SDV system's architecture, and different problems require different solutions. Problems originating from the customer's STB will only affect that one household, but a problem with an SDV server or SRM has the potential to affect thousands of customers and might require teams of technicians to get it back on track. In many cases, cable companies will have to rely on products from multiple vendors to create one SDV system. Some products may have compatibility issues with others, making it challenging for cable companies to deliver reliable service. Even so, SDV systems are an attractive alternative to replacing fiber-optic and coaxial cables with higher capacity lines.

In the next section, we'll take a look at some markets that have SDV systems in place.


Switched Digital Video Market

Companies that design and produce SDV system components include big names like Scientific Atlanta and Motorola. Some companies build all the components an SDV system needs, but others specialize in specific hardware like encryptors or routers. The variety of products allows cable companies to shop around to find the system that best suits customer needs and demands.

Because the technology is relatively new and there's potential that problems in an SDV system could be difficult to diagnose and correct, implementation has been a gradual process. Cable companies like Comcast Corporation, Time Warner, Cox Communications Inc. and Cablevision Systems Corporation are experimenting with SDV equipment by implementing it in select markets [source: Cable Digital News].


These markets include Cablevision's New York metro region, Time Warner Cable's markets in Austin, Texas, and Columbia, S.C., and Comcast's markets in Denver, Colo. and New Jersey, to name a few. These pilot markets include millions of cable customers, but it's still a relatively small percentage of the whole consumer base. There haven't been any major issues with the pilot programs so far. Several cable companies plan to implement SDV systems in more markets beginning in 2008.

Assuming the pilot programs work well, we may see cable companies embracing SDV technology over the next few years. It will also take time for companies to develop strategies to provide customer support and solve problems when they arise in the network. This could include hiring consultants to develop new customer service approaches and training staff members so that they know how to solve technical issues as they arise.

For customers, changes might be subtle at first. Some SDV systems can work with existing set-top box technology, which means customers won't have to upgrade their equipment at home. For them, changes could seem to happen overnight. They might discover they have a faster Internet connection or more channel options. Other systems require new customer equipment, which will require existing customers to switch out their STBs.

Every time you change the channel in an SDV system, your set-top box engages in a complex digital conversation with the SDV network. Press play to learn more.

To learn more about switched digital video and related topics, follow the links on the next page.


Lots More Information

Related HowStuffWorks Articles

More Great Links

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  • "Quadrature Amplitude Modulation." University of Delaware Department of Physics and Astronomy.
  • Boudreau, Yves and Dion, Gino. "Understanding Switched Digital Video." IneoQuest Webinar. August 15, 2007.
  • Breznick, Alan. "MSOs Switch Digital Gears." Cable Digital News. January 26, 2007.
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  • Koch, Chris and Hartkemeyer, Ron. "Switched digital video: Hollywood's happy ending." Lightwave Online.
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  • McCarthy, Shira. "Switched digital video steals the show." Telephony Online. July 19, 1996.
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  • Spangler, Todd. "Comcast Picks Motorola for Switched Digital Video." Multichannel News. October 25, 2007.