Switched Digital Video (SDV) is rapidly gaining recognition as the best way to squeeze additional bandwidth out of a cable system. Notably, Time Warner Cable has proven SDV can work and provide value. In fact, Mike La Joie, Time Warner Cable's chief technology officer, said in the pre-opening session of the 2005 SCTE Cable-Tec Expo: "We have two systems in full production, and it's going quite well. The result [of switched digital] is a 60 percent efficiency gain, or 10 channels into 6."
Figure 1: Today’s typical HFC downstream bandwidth usage.
A switched digital video system can offer an unlimited channel line-up by transmitting video channels to subscribers only when they request them. In addition, SDV answers a lot of questions about getting to all on-demand; if you can switch all digital channels in real time, all on-demand is just an exercise in scaling the content server farm.
Scaling-up SDV implementations to support millions of subscribers is a big challenge and will require industrial-strength software control systems with a provision for hardware redundancy and hitless upgrades.
Meanwhile, satellite and telco operators are leapfrogging cable in their adoption of MPEG-4 AVC technology, which has twice the bandwidth efficiency of MPEG-2. The problem for cable operators is that the prospect of replacing millions of existing MPEG-2 set-top boxes (STBs) is a multi-billion-dollar proposition.
This article proposes solutions to these growing pains and paints a clear vision toward an advanced switched digital video systems approach.Allocation of bandwidth
A brief review of how bandwidth allocation of cable services developed will help to explain the difficulty in adding high definition (HD) services to an already congested cable plant. Analog cable channels were the first big "spectrum grab," taking roughly 100 6 MHz channels and consuming 70 percent of total spectrum. 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.
Constrained by many factors, such as FCC regulations, municipal franchise agreements, broadcaster and programmer pressures, advertiser agreements, and subscriber preferences, the cable industry has been unable, as yet, to reclaim significant analog bandwidth. Although analog PPV and some analog premium channels have been reclaimed, the rest are difficult to recover without buying a lot of digital STBs.
Today, approximately 80 to 95 percent of the available cable plant bandwidth is spoken for, with most systems at the upper end of that number. With the exception of the premium channels, the MPEG-2 standard definition (SD) programs are predominantly simulcast analog channels. The MPEG-2 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.Current SDV technology
The scarcity of spectrum combined with the demand for new programming (especially high definition programming) has provided momentum for the move to SDV. While current SDV implementations do a good job of reclaiming bandwidthi,ii they can be best viewed as the first step toward development of more sophisticated solutions.
Figure 2: Today’s SDV system.
Current SDV systemsiii consist of the following functional elements (Figure 2):
- 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/resource manager (RM), 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 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.
The sequence of operation is:
1: A software client in the set-top box sends channel change requests to the SDV server.
2: The SDV server/RM supports the channel switching, by allocating the requested channel to an edge QAM, and telling the set-top client where it can be found (its frequency and MPEG program number).
3: 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.
4: While the channel is active, the SDV server/RM periodically generates a "fast channel map" of active channels and carousels it out to the clients.
5: When the SDV server/RM determines that no subscribers are tuned to a channel, it tears the channel down, freeing the bandwidth for use by other channels.Channel change confusion 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.
Let's take ABC as an example. Today, the channel guide shows that the SD version is on channel 2 and the HD version is on channel 701. Notice that there is no correlation in the numbering scheme to relate the two as the same programming, just different video resolutions.
- The analog cable tuner will tune to the analog cable frequency (54 to 60 MHz) and display the analog channel 2 display version of ABC.
- The SD-capable set-top box when tuning to display channel 2 looks at its physical channel map and tunes to the RF program number shown there (let's say RF channel 91 or 624-630 MHz, MPEG program number 1) and displays the SD digital version of channel 2, ABC.
- The owner of the HD set-top has to be aware that the HD version of ABC is located on a different display channel (701) and physical channel. If the customer does not know to choose this display channel, and chooses channel 2, his set-top will tune to the SD version of ABC, and he will probably ask why his HD service, and wonderful HD television, still looks just like it did before he spent all that extra money.
Figure 3: The device-aware SDV system.
Managing different video resolutions and compression formats by offering each as a separate display channel, and allowing or, indeed, forcing, the subscriber to choose one format over another will continue to cause subscriber confusion.
The solution is to add set-top awareness to the SDV system, or more generally, device-awareness.
To add device-awareness to the SDV system, a number of new functions need to be implemented (see Figure 3):
- A device manager 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 connects a subscriber to the devices that are used to deliver service to that subscriber, and the services to which he subscribes. 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
- dynamically creates the 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.
An immediately noticeable difference is that the subscriber no longer has to choose the display channel, but just tunes to channel 2 for ABC. This reduces the confusion when switching out set-tops upon joining a new tier (HD or MPEG-4, for example). The subscriber only needs to know that he is interested in watching ABC. The SDV system takes care of getting him the best available format for ABC.
As MPEG-4 set-tops are deployed, a device-aware SDV system allows the operator to benefit from bandwidth savings immediately, and without the need for simulcast.
For example, if a subscriber with an MPEG-4 set-top is first in the service group to request a particular channel, an MPEG-4 version of the program can be supplied with significant bandwidth savings. Conversely, if there is already a subscriber watching the program in MPEG-2, the best efficiency is to have an MPEG-4-capable set-top join the MPEG-2 channel, since all MPEG-4 set-tops are also able to decode an MPEG-2 channel for backward compatibility.
Another scenario is to supply MPEG-4 set-tops to all subscribers who choose an extended tier, ensuring that programming in that tier will always be delivered in MPEG-4.
Figure 4: The overall architecture. —Click Here to Enlarge 
Finally, a hybrid model is possible where STBs are gradually swapped out to customers based on their programming preferences and location in the system.
In each case, 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.
As cable systems become larger through regional clustering, an intelligent layer is needed to ease configuration, as shown in Figure 4.
The following are functional elements of the intelligent 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.
The advanced SDV system that has been described will: Support a managed transition to MPEG-4; provide a simplified program paradigm; and support scaling of SDV implementations to millions of subscribers.
1: i Ron Hranac, "Hit the Switch: MSO and NCTA Update," CT Pipeline, April 18, 2006.
2: ii Nishith Sinha and Ran Oz, "The Statistics of Switched Broadcast," SCTE Conference on Emerging Technologies, January 2005.
3: iii Luis Rovira, Lorenzo Bombelli, and Paul Brooks, "Switched Digital Video," Communications Technology, February 2006.