Switched Digital Video (SDV) - issues and challenges

The SDV is becoming more and more attractive among telco and cable operators for number of benefits it brings to telcos. In this article some of the common and major issues of SDV with possible solutions is described.

Prior to SDV system

When satellite DTH started to offer digital video, the cable industry response was a mixed analog and MPEG-2 digital plant with provisions for:

• 70 to 90 analog 6 MHz channels
• 8 to 10 MPEG-2 simulcast standard definition (SD) 6 MHz QAM channels
• 8 to 10 high definition (HD) 6 MHz QAM channels
• 4 to 6 video-on-demand (VOD) 6 MHz QAM channels
• 1 to 2 high-speed data (HSD) and voice-over-IP (VoIP) channels

SDV Benefits:

• Optimizes bandwidth; SDV uses 50 to 75 percent less bandwidth as compared to traditional broadcast model. Without SDV, with the exception of the premium channels, the MPEG-2 standard definition (SD) programs are predominantly simulcast analog channels. The MPEG-2 High Defination (HD) content consists of between 15 to 30 HD programs and is also, in large part, duplicating the SD content. Worse, MPEG-2 HD content is a bandwidth hog, where only two to three programs can be put in one 6 MHz QAM channel, so there is limited space to add more HD content.
• Provides ability to monitor channel programming effectively
• Provides ability to transmit the best format to the subscribes
• Delivers the requested content to subscribers in real-time
• Provides efficeint mechanism for migrating from MPEG2 to MPEG4
• Facilitates the creation of new services
• Boosts total number of channels available
• Enables market-specfic niche, including high definition programming
• Increases the level of concurrent video on demand (VOD) sessions being served to various nodes
• Provides methods for advanced targeted ad-insertion solutions
• Frees up bandwidth for data delivery

SDV Issues and challenges:

Some of the common issues cable and telco operators face when deploying SDV are as outlined below:

• RF access network
• Long-haul transport issues
• Signaling between many hetrogence components that must operate in unison
• Scalability of SDV implementation is one of the major challenges of telco operators which will require industrial-strength software control systems with a provision for hardware redundancy and hitless upgrades to support millions of subscribers
• Adopting MPEG4 technology is another major challenge for the cable operators, which has twice badwidth effieciency as MPEG2. Replacing millions of existing MPEG2 set-top-boxes with MPEG4 is a multi-billions dollars proposition.

SDV common implementation requirements

Among the most common SDV implementation requirements are the following:

• Converting video streams from multi-program transport streams (MPTSs) to single program transport streams (SPTSs)
• Converting from variable bit rate (VBR) to constant bit rate (CBR)
• Increasing exponentially the number of multicast flows
• Increasing the required network computing resources of routers, switches, edge quadrature amplitude modulation (QAM) modulators and other components
• Reducing the relative throughput of those devices while increasing the chances of queuing overflows or underflows
• Increasing network communication overhead for both the video delivery (data plane) and control plane networks (control and signaling)
• Associating and switching video flows to the home to enable seamless channel changes

Current SDV technology

Current SDV systems consist of the following functional elements:

• A staging processor, which breaks a multiple program transport stream (MPTS) into single program transport streams (SPTS), and "clamps" the video bit rate of the streams (that is, converts any variable bit rate streams into constant bit rate streams). It also assigns Multicast IP/UDP port addresses to each SPTS.
• The SDV server and Session/Resource manager, which processes set-top box channel change requests, sets up and tears down active channels on the QAMs, and generates a list of the current service group (collection of users) active channels for inclusion in the mini-carousel that is continuously sent to the STB.
• A bulk encryptor, which provides payload scrambling to support conditional access.
• A content routing network that transports the multicast streams to the QAMs.
• An edge QAM that modulates the MPTS and places the active channels on the HFC plant.
• An SDV software client in the set-top box.

Also see "SDV Architecture" for more information on current SDV architecture.

Issue with current SDV technology

Because the SDV system is not aware that there are multiple types of set-top boxes, and only HD set-tops can decode HD channels, the cable operator must rely on an often-times confused subscriber to make the right channel choice. That's because the SDV system transmits to the subscriber the same program in two different format, one in HD and another one in SD, available in two different channels. There is no correlation in the numbering scheme to relate the two as the same programming, just different video resolutions. This creates confusion for the viewer. The owner of HD set-top-box must know which channel is showing HD version of the program, otherwise, he might be watching SD digital version of the program which of course has lower quality of HD but paying for more expensive HD set-top-box.

A possible solution for this confusion problem is to add a set-top awareness capability to SDV system. By adding this capability, the SDV selects the format that's desirable for the subscribers set-top-box and displays on one channel. Implementing set-top awareness capability requires a number of new functions as described below:

• A device manager that maintains information on the capabilities of set-tops, edge QAMs, service groups, transcoders, and stream processors, as well as the source of different formats of the content for delivery to each service group.
• A subscriber manager that connects a subscriber to the devices that are used to deliver services to that subscriber. It also maintains a heuristic analysis of subscriber viewing patterns.
• An SDV manager provides centralized management and control of the SDV system. It registers all of the SDV servers/RM, edge QAMs, and service groups, maintains a central database for the events associated with the channel change requests for each subscriber, and tracks set-up and tear-down of active channels in each service group. Additionally, it provides the interface with the global session resource manager (GSRM) for request and allocation of shared QAM resources. Finally, it provides control, configuration management, and reporting functions of the entire SDV system.
• A bank of shared transcoders for dynamically creating different formats for replicated versions of the same program.
• A monitoring/reporting function is critically important for optimizing and troubleshooting the network, and ensures that the operator knows when a service is switched to a service group and that the content was actually transmitted over the HFC.

A hybrid model to managed transition to MPEG-4 set-tops

A hybrid model provides ability to cable operators to gradually swap out the STBs to customers based on their programming preferences and location in the system. With hybrid model, the SDV system automatically makes the most bandwidth efficient format decision for each program being transmitted within each service group based on which subscribers are watching the program and their STB capabilities.

An intelligent management layer

An intelligent management layer can help cable operators to configure the SDV system easier as cable system become more complex through regional clustering. The following outlines the elements of an intelligent management layer:

• A workflow engine manages and tracks the processes needed to implement new services, devices and even software applications. Additionally, it manages and tracks the creation of the different video resolutions and compression formats for all content to support installed STBs.
• An element manager monitors and assists in the configuration management of hardware and software within the network, and provides reports to the operator on impairments, loading and outages.
• A device (and subscriber) manager maintains a database of each device in the network and in use by subscribers, including hardware and software versions, compatibilities and incompatibilities of the same, and the video resolutions and compression formats supported by each device.
• A global session resource manager (GSRM) arbitrates bandwidth usage between services based on loading balancing, value of the services, and subscriber experience.
• A service manager maintains a database for how each subscriber device receives and supports service information.
• A code download manager manages the distribution of code objects, easing the operator involvement in upgrading code to the set-tops and other network devices.
  

About me:

bruce atlasi is a professional computer engineer, skilled in telecomm and datacomm technologies and architecture. He has diverse working experience with many telecomm start-ups and fortune 100 companies, including Cisco Systems, IBM, and Siemens. He regularly blogs on About Telecomm site.

References:
"Raising the bar on SDV", by Chip Paryzek and Michael Adams.
"Gaining Visibility into the Complexities of SDV", by Gino Dion