As services like HDTV clamor for ever-scarcer bandwidth, much attention is being directed toward switched broadcast services.
Expand customer choice and enhance your network’s bandwidth utilization by sending selected programming only to nodes when and where subs actively request it. In today’s cable network, the status quo practice of filling the entire spectrum with live broadcast programming at all times has recently been supplemented by unicast video in applications like video-on-demand (VOD) and subscription VOD (SVOD). Stored video programming is directed from a server for consumption only by an individual subscriber. Switched broadcast—a method of sending selected programming only to nodes where and when subscribers are actively requesting it—represents the further evolution of the cable network by expanding the concept of traditional broadcast programming with a multicast programming overlay. This overlay is driven by subscriber requests, and can dynamically adjust network transmission capacity resource usage across nodes, service groups and hubs. This capability efficiently provides an expanded programming experience to subscribers that is customizable, personalizable and can potentially accommodate a limitless content selection. In a switched broadcast paradigm, cable spectrum is no longer looked at as a static collection of channels classified by classical EIA frequency plans. Rather, it’s a dynamic pool of digital transmission resources that, at any moment, could be serving any broadcast programming from any source. This could eventually be extended to also encompass narrowcast VOD content, interactive TV or any future digital service. Demands on networks While it seems like only yesterday when the major plant upgrades took place—many to 750 and even 860 MHz—operators are quickly finding out that they are again running out of bandwidth. Advances in encoding and rate-shaping technology have allowed as many as 14 digital programs to occupy the same frequency spectrum as a single analog channel. But even this expansion of capacity has been quickly filled. Many programmers, including HBO, Showtime, Discovery, MTV and ESPN have expanded their packages to offer vertically oriented variants. New programmers also have emerged to offer more narrowly focused content. The 500-channel packages of broadcast programming initially articulated by cable visionaries were first realized by direct broadcast satellite (DBS) operators. The continental footprint and abundant bandwidth resources available to DBS serves as a perennial motivator for cable to cost-effectively build and sustain networks that are capable of offering an equivalent or greater array of programming. The emergence of high-definition TV (HDTV) and its rapid deployment by North American operators serves as an additional business opportunity, but also as a further bandwidth burden on the HFC plant. Whereas 10 to 15 standard-definition TV (SDTV) signals can be carried in 6 MHz of spectrum, the bandwidth consumed by HDTV programming only allows two or three programs to be carried. The emergence of HD programming on cable networks almost can be regarded as a fait accompli, with commitments offered by the top 10 MSOs to the FCC to provide five simultaneous HDTV signals in the top 100 media markets. With all of the networks and some premium broadcasters offering 8 to 12 HD programs, operators may find themselves scrambling to find three to six extra quadrature amplitude modulation (QAM) channels. As the cost of HD sets continue their drop, and the popularity of crystal-clear HD programming explodes, consumers will demand ever-greater portions of HD, and vote with their remote controls. Video-on-Demand VOD has emerged as both a credible response to the DBS competition, and a promising service that can generate incremental subscriber revenues. VOD is widely regarded as the “silver bullet” for the cable industry because its capabilities cannot be practically duplicated by satellite. VOD is a unicast service, which means that a separate digital program stream is transmitted to each individual user. This is a far cry from the broadcast paradigm, where a program is ever-present and available to all users. To size the bandwidth requirements of VOD, operators typically make a mathematical assumption about how many digital subscribers will be using the VOD service simultaneously during the peak viewing period. Empirically, this number has typically been 5 to 10 percent of the number of digital households. For a headend with 100,000 digital subscribers, this equates to 5,000 to 10,000 streams, or 500 to 1,000 QAM channels for VOD alone. This number may seem daunting compared to the 20 or so QAMs required to carry the digital tier of broadcast programming. However, it is mitigated by the fact that the downstream bandwidth needed for VOD can be reused in each HFC node, or by a clustered group of nodes commonly referred to as a service group. The true amount of capacity available for any particular service, like VOD, is actually the product of the QAM count times the number of service groups in the headend system for the particular service. The reuse of bandwidth by node or service group, a road paved by VOD, provides the perfect vehicle for switched broadcast to provide an augmentation of transmission capacity to serve the programming demands of subscribers. Niche programming Switched broadcast offers the opportunity to reuse bandwidth that would otherwise be allocated systemwide to deliver niche and ethnic programming to all demographic groups. With switched broadcast, it is not necessary to know in advance where the viewership of these programming types is concentrated, nor to define hub or node boundaries by demographic. Enabling forces The HFC architecture is easy to sectionalize into discrete systems. These small systems are served by optical nodes (with a granularity as fine as 500 homes passed), and often are aggregated into service groups of 1,000 to 10,000 homes passed by virtue of RF splitting and combining. The aggregation ratios (service group sizes) can be different for each service, and usually are optimized based on subscription rates (traffic engineering) and performance (S/N+I) tradeoffs. For instance, traffic-based aggregation is commonly done for high-speed data services, where in high-usage cases an individual node may be provided with its own input port on the cable modem termination system (CMTS), while in other, lower traffic areas, several nodes may be aggregated. This architectural capability happened almost by accident. Because of the limited loss budgets of early linear analog optical links, one laser transmitter could only serve an average of three or four optical receivers over short links, as few as two or even a single receiver in links of more moderate length. This led most of the industry to build what are essentially “home-run” or star topology connections to the optical service area nodes. In conjunction with this, it also was found that the optimum number of RF repeater amplifiers in cascade is typically in the range of two to eight, depending on cost-based coaxial distribution design tradeoffs. These short cascades limited the service area sizes to 500 to 2,000 homes passed. The combination of home-run optics and small node sizes yielded enormous increases in bandwidth per subscriber. This capability only recently has been taken advantage of, first by high-speed data and more recently by VOD. Switched broadcast is the logical extension of efficient use of this accidental characteristic of HFC. Just as cellular telephony providers reuse common frequencies in their cells, switched broadcast implementations can reuse common pools of QAM channels and program bandwidth across service groups to provide a larger logical programming capacity than what is physically available with traditional broadcast. Two-way communication capabilities provided by segmentable HFC plants, combined with the sophistication of real-time interactive set-top designs provide the necessary environment for deployment of switched broadcast. We believe that existing set-tops and existing HFC outside plant architectures do not need to be physically modified for this service. Specialized programming, and the emergence of personal video recorders (PVRs) have modified traditional patterns of TV viewership. While networks like CNN and ESPN will always command a certain level of viewership, the spatial and temporal fragmentation of viewership complicates the task of determining the relative values of less-popular broadcast programming transmitted to subscribers 24 hours a day—perennially consuming scarce spectral resources. Switched broadcast components Three components are essential to the provision of switched broadcast: client software in the set-top, controller functionality at the headend and the switch itself. Other issues that must be considered are transport network enhancements to deliver content to the switch input, and a conditional access system to allow the operator to control access and monetize the service. Switched broadcast client (SBC): An SBC is a small software application in the set-top. This client functionality can just as easily be integrated into the tuning firmware of future set-tops. When a switched broadcast program is selected, this software component conveys the channel request via the upstream, along with information that identifies the set-top box. Switched broadcast manager (SBM): The SBM application runs on a machine located in the hub or headend. It uses the channel number received to identify the requested program, and consequently, the MPEG-aware switch (MAS) input port where the program is being received. Similarly, the SBM uses the set-top ID and associates the service group information to determine the downstream connection (MAS output port) where the subscriber can be reached. In many cases, a subscriber can be reached by more than one downstream QAM channel. This collection of one or more QAM channels, and the dozen or so programs that can be carried in each QAM channel, represent a pool of resources that the SBM has at its disposal to fulfill programming requests within each service group. When an available downstream QAM and the program resource are identified, the frequency and program information is returned via the downstream out-of-band channel to the set-top, which decodes and displays the program using the normal tuning mechanisms. While the specific frequency and program number for a switched broadcast program may vary in time, the channel number as seen by the subscriber always will remain the same. MPEG-aware switch: Completing the solution and providing real-time video switching capability is an MAS. To perform the switching role effectively, an MAS should exhibit the following: High input fan-in: An effective switched broadcast solution allows a substantial and theoretically unlimited amount of programming to be switched into a bandwidth-limited set of downstream transmission resources. To achieve this, an MAS should support an input bandwidth that greatly exceeds its per port output bandwidth. Fast switching time: The amount of time required to switch an input program to an output port should be as small as possible, because this switching time is a component of the overall channel change time. The switching ought to take place in such a way that the switching process does not syntactically corrupt the output stream. Supplemental media processing:In addition to the pure switching process that takes place, it is useful for an MAS to have supplemental media processing capabilities as well. Such processing functions include, but are not limited to remapping of packet IDs (PIDs) and reconstruction of program-specific information (PSI); processing of transport streams to correct program clock reference (PCR) jitter; remultiplexing and rate-shaping of output streams; and transcoding of elementary streams from other formats as necessary. Operation The actions that take place when tuning to a switched broadcast program differ from traditional broadcast tuning. In a traditional broadcast environment, all programming is sent to the set-top box, along with data streams that convey the PSI and system information (SI). Using these tables, a set-top can determine the associated EIA frequency and program number, and command the set-top tuner and MPEG decoder to receive and display the program. With switched broadcast, the same tuning methodology is used, with a modification. The tables that carry the frequency and program information for the broadcast channels now vary with time. Therefore, the set-top uses the out-of-band channel to acquire the now time-variant tuning information. When this is retrieved, the tuning process proceeds as normal. To the end user selecting a program via the electronic program guide or the remote control keypad, this is visually seamless. A switched broadcast channel change (Figure 1) starts with the SBC application on the set-top sending a channel request message upstream to the SBM. The SBM receives this, and reconciles the request against the available downstream resources. If a downstream resource is available, the SBM instructs the MAS to complete the downstream video connection (if it is not present already) and perform any needed stream processing. The downstream resource info (the QAM frequency and program number) is returned to the set-top, which tunes the program. Channel selection for subsequent viewers of the same program follows a similar , but simplified sequence. If the program is already being transmitted downstream, then the SBM just needs to return the tuning information to the set-top. Channel leave-off When a user is selecting a switched broadcast program, that user also may be leaving another switched broadcast program. Therefore, the channel request message also contains information about which program the set-top box is leaving. The SBM uses this to track the number of users watching a particular program. If the number reaches zero, the SBM can (but does not have to) reallocate this downstream resource for another program request. In addition to maintaining an accounting of programs being viewed, the SBM can interact with specific set-tops to confirm viewing information. The ability to recapture QAM program stream capacity is critical to aggressive deployment of switched broadcast. In the future, several different means to prioritize the stream dropping process can be implemented, such as program event boundary knowledge, time in stream timers, and set-top usage (button push) monitoring and timers. Other mechanisms, such as freeze-frame, may be used to encourage the subscriber to “push a button” to convey to the SBM the fact that the stream is being viewed. Such mechanisms would be used prior to dropping of a stream if the viewing status cannot be determined. If such mechanisms are not implemented, the QAM stream inventory can become clogged with streams that no one is using, such as when a subscriber turns off the TV set. Because switched broadcast is inherently a best-effort service, some type of message or barker would be provided to subscribers requesting a stream when none is immediately available. Stat collection While there are possible legal implications surrounding the gathering of individual subscriber data, it is advantageous and arguably necessary to the efficient use of transmission resources and subscriber satisfaction to collect viewership patterns. Fees in the future could be based on total subscriber viewing hours and/or through a method analogous to Web page “hits” where the number of subscribers tuning in is a variable separate from the hours viewed. MSOs may make programming purchasing decisions based on the actual popularity of the programming. This data also has interesting possibilities in the advertising business. Busy signals Given the fact that there will be more input programs available than downstream resources in a service group, this presents the possibility that all downstream resources will be utilized when a switched broadcast program is requested. This risk is mitigated by the fact that in a given broadcast channel lineup, the popularity of programming follows an inverse exponential distribution. In layman’s terms, a small handful of programs are watched often, and a large number of programs are not watched very much at all. It is these programs at the middle-to-far end of the curve that present themselves as ideal candidates for switched broadcast. The risk is further eased by the fact that switched broadcast is a multicast service. Users who watch the same program can share the same stream. This makes a switched broadcast implementation very stable in a “flash crowd” environment, where several users suddenly desire to watch the same program. Furthermore, an SBM implementation can provide a vehicle to prioritize certain programs (or subscribers, for that matter) over others. A set of business rules can be developed, and customized per location, to provide a better assurance that specific programming will be available. Additionally, rate-shaping techniques can be selectively applied to streams. Deployment topologies Switched broadcast can be deployed in a variety of configurations, and coexist with regular broadcast programs. It may be implemented across an entire headend, or deployed down to the individual service group level. This provides an opportunity to trial its behavior with a small set of programming and grow that programming set over time, and/or subdivide the subscriber group that accesses a common set of downstream resources. In the entry-level switched broadcast deployment, the dynamic adding and dropping of programs in a multiplex is combined with a rate-shaping engine to produce a solution that allows the seamless replacement of content while maximizing overall video quality. Large content model In larger deployments, the number of MASs distributed across hubs presents a more challenging business case to allow rate-shaping to be performed on such a wide scale. To optimize the economies of scale, a broadcast acquisition system can be located in the headend. The acquisition system performs a bit rate clamping function, where incoming broadcast programs are rate-shaped to a predetermined constant bit rate (CBR). The CBR rate is one that mimics the program parameters for VOD streams. Using a gigabit Ethernet transport structure, the clamped programs are multicast live to all hubs with switched broadcast MASs. Employing this model yields a number of advantages. The rate shaping function is performed once, and can be utilized by all hub MASs. Secondly, the new CBR nature of the resulting programs allows for a much simpler downstream resource allocation algorithm. Finally, because the new switched broadcast programs “look like” VOD streams, a higher-level bandwidth management function is possible, whereby downstream resources can be dynamically allocated between switched broadcast multicast requests, and VOD system narrowcast requests. This allows a larger pool of resources to be shared between the two apps, creating greater overall availability, and allowing each service to access a greater pool of resources during peak usage times. Tuning latency also is optimized because the addition of a stream to a QAM channel is reduced to a simple time division multiple access (TDMA) switching operation. It is not necessary to perform further bit rate adaptation for each program stream. As long as the stream is CBR and the rate is known, it is possible to develop allocation rules to optimize the “packing” of streams into each QAM channel. There is a specific benefit that is gained from reformatting broadcast programs into CBR streams that “look like” VOD streams. A higher-level resource manager could then manage switched broadcast and VOD downstream resources as a common pool, enabling efficiency gains for both services. Again, it is not necessary for each stream to have the same rate cap. Spectrum sharing In a VOD deployment, it is common to dedicate a certain number of QAMs to carry VOD streams. The number of QAMs is usually a percentage of the number of digital subscribers served. Whether all, most, some, or none of those VOD QAM resources are actually carrying VOD traffic, those QAM resources are “locked away.” Through the use of an intelligent downstream resource manager, the resources required by both VOD and switched broadcast can be shared, elevating the paradigm to the next level—switching across services and not just streams within a service. This allows VOD to have access to a larger pool of resources during its peak periods. It also allows for switched broadcast to access a larger pool of resources during its peak demand periods, which may be more random, enabling a more efficient statistical multiplexing effect due to the greater amount of multiplexed content. S.V.Vasudevan is chief architect at BigBand Networks. Paul Brooks is senior network architect at Time Warner Cable. The Switch to Switched Broadcast As new digital services like high-definition TV (HDTV) clamor for ever-scarcer bandwidth, and with capital-conscious operators still digesting over $70 billion spent in the last sweep of plant capacity upgrades, much attention is being directed toward switched broadcast. Switched broadcast is a method of sending selected programming only to nodes where and when subscribers are actively requesting it. Such services have the potential to promote revenue growth and reduce churn by expanding programming choices and enhancing bandwidth utilization. By shifting from a content-oriented delivery model to a subscriber-oriented one, MSOs can scale their delivery platform to track with the number of subscribers served. This is a move away from the less-stable model of compounding new programming expenses with additional capital investment to make room the new programming. Switched broadcast is becoming an operational reality, with initial trials by major MSOs commencing this year. Results will provide data for planning broader commercial deployments based on empirical subscriber usage patterns, validating the nature and magnitude of the technology’s benefits.