With sufficient switching capacity placed at the central office, the amount of content that can be delivered to a single household is infinite. The ability to offer such an amount of video programming in a telco network could be exploited as a competitive advantage. Switched digital video (SDV) is a cable technology that attempts to answer this challenge. It was designed as a cost-effective method to expand program availability, in a way different from the previously used methods of plant upgrades or video compression enhancements.
SDV is the first implementation of a broadcast video service in which only requested programs are sent to the group of subscribers (the subscriber group). SDV is the first broadcast video service that requires two-way communication with the subscriber for content selection. SDV is sent to a subscriber’s Set-Top-Box (STB) where it is decoded/uncompressed for playback on a TV.
With traditional digital broadcast a video program is carried to customer, whether it is being watched or not. With SDV, as with IPTV, programming terminates at the headend and hub does not explicitly send the video program unless requested. Instead, a receiver signals upstream to request programming, and a hub-based controller receives the request and enables the stream into the HFC network by means of a pool of allocated frequencies. See SDV Benefits for more.
In an SDV system, metadata that describes all broadcast programming is amended to indicate which programs are SDV programs. When an SDV program is selected, tuning software in the receiver sends an upstream signal. An SDV session manager receives the request and maps the program to a frequency within the allocated pool. This dynamic tuning information is returned to the receiver. If the program is already being viewed within the same subscriber group, then the task is as simple as reusing the session frequency information.
The following diagram depicts the SDV overall architecture.
Headend (HE)
The HE of SDV architecture is where the video and Internet feed sources enter the system. The HE is also where most of the backoffice systems, including billing systems, asset management systems, authorization systems, and so on, reside. HE includes the following equipment that are 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.)
- A Groomer which translates content into a maximum bandwidth, limiting the rate of video bursts.
The IP network
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. This section consists of nodes and routers where cable connections intersect and branch off. 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 hub (access system).
Hub
Hub is where the control of bandwidth-intensive applications, such as on Demand streaming, and HFC-connected components reside. The hub (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. The server sends commands to an Edge Resource Manager (ERM) and several Edge QAM devices to meet demands. An edge QAM modulates the multiple program transport stream (MPTS) and places the active channels on the HFC plant.
HFC network
HFC network is the fiber-coax access network for distribution to the subscriber. The HFC network is a shared medium, where groups of homes are connected on a common branch of coax cable. Groups of subscribers share access to the same downstream frequencies, and arbitrate for access to shared upstream frequencies.
Subscriber Site
Subscriber Site is a business or residential location where coax receivers, such as the Set-Top-Box (STB) and cable modems are placed. A set-top box 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. A STB also contains a SDV client that communicates with SDV server.
SDV sequence of operations:
- A software client in the set-top box sends channel change requests to the SDV server.
- The SDV server and Session/Resource Manager support the channel switching, by allocating the requested channel to an edge QAM, and telling the set-top client where it can be found, by providing the program's frequency and MPEG program number.
- The client then tunes the set-top to the channel. In addition, the client periodically sends a "heartbeat" message to indicate that it is still tuned to that service.
- While the channel is active, the SDV server in conjunction with Session/Resource Manager periodically generates a "fast channel map" of active channels and carousels it out to the clients.
- When the SDV server determines that no subscribers are tuned to a channel, it communicates with edge QAM and tears the channel down, freeing the bandwidth for use by other channels.
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