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How Switched Digital Video Works

Switched Digital Video Architecture
An example of a switched digital video system's 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.