Cable operators have an opportunity to overcome impediments to immediate expansion of video on demand (VOD) programming capacity through use of new technology that leverages existing network resources to produce significant gains at minimum costs.

This new approach to maximizing VOD bandwidth efficiency is based on transrating technology that’s designed specifically for on-demand applications. It effectively expands VOD channel capacity for both standard and high definition (SD and HD) content by as much as 50 percent without requiring additional quadrature amplitude modulation (QAM) modulators and without incurring the delays and disruptions associated with techniques such as switched digital video (SDV) or node splitting. In fact, VOD transrating can serve as a complimentary linchpin in network migration that adds significant efficiency to these and any other expansion strategies operators might pursue.

This "drop-in" approach to VOD capacity expansion entails placement of the transrating platform between VOD servers and edge QAM modulators with connectivity into network management systems (NMSs) and session resource managers (SRMs). To be effective, this solution must process content streams in real time and be fully compatible with all varieties of VOD servers, QAM modulators, encryption platforms and management systems commonly used in cable systems, and it must be equipped to ensure failsafe redundancy.


Real-time VOD transrating uses several techniques to augment existing capacity efficiency, including rate shaping, statistical multiplexing and other processes. While rate shaping and stat-muxing have long been staples in cable broadcast transmissions, real-time transrating of VOD content has taken longer to develop because of the processing challenges posed by constantly changing content streams. VOD transrating solves this problem through dynamic assignment of each on-demand flow to continually fill available channel spaces across multiple QAM streams.

The rate-shaping process supports conversion of constant bit rate (CBR) streams to variable bit rate (VBR) while stat-muxing allocates bits to match individual stream complexity and statistically averages the complexity across several channels to ensure that all available capacity is utilized to maximum advantage. The platform also applies session-aware processing intelligence to ensure that trick play functionalities are maintained on all VOD streams.

Of course, VOD transrating must integrate with existing VOD architectures, most of which are based on pre-configured routing or port mapping. In this mode of operation, the 16-bit user datagram protocol (UDP) port targeted at a specific set-top is dynamically assigned to each stream with each customer order. Simply by reading the UDP header, the edge QAM modulator and VOD transrater always know where the ordered program is going. This contrasts with the session-based routing architecture, which, absent the pre-configured UDP assignments used in pre-configured routing, requires the SRM to instruct the transrater to pick up and map a specific stream to a specific QAM channel. (See Figures 1a and 1b.)

ArrayArrayMarket drivers

It’s clear that cable operators need a fast-track, low-cost approach to accommodating what promises to be accelerating demand for more VOD channel space, not only because the volume of on-demand content is expanding, but also because ever more linear HDTV channels and increasing allocations of bandwidth to DOCSIS broadband service are competing for available capacity.

Competitive pressures from satellite companies boasting 100-plus HDTV channel counts have been the greatest force behind operators’ efforts to free up space for more linear HD. But as telcos expand their TV footprints, they, too, are adding to the competitive pressures, on both the linear and on-demand sides of the ledger. Once telcos have enough capacity over their switched access networks to deliver at least three simultaneous HDTV channels to the home, they are not burdened by throughput constraints in the race for bragging rights to high volumes of HD linear and on-demand content.

Cable operators recognize they must be in a strong position to meet these challenges if they are to retain and build their customer bases. The penetration of personal video recorders (PVRs) and Web-based video content into the viewing habits of the mass market and the increasing popularity of online movie rental services from Netflix, and others signal that time-shifted availability of TV programming together with support for ever more VOD movie consumption are essential to cable’s ability to deliver what consumers want. HD capacity requirements for all this on-demand content on top of the linear HD programming requirements add to the urgency of the bandwidth conundrum.

Weighing options

There are many compelling options for operators to use in their efforts to draw more capacity from their HFC networks for on-demand content, linear HD programming and broadband service. SDV has proved to be especially attractive to many cable operators, who have discovered that niche programming offered in this mode consumes half or less the bandwidth that would be consumed in linear broadcast mode.

However, because SDV only serves to free up significant amounts of bandwidth associated with niche programming, it affects a relatively small percentage of the RF downstream spectrum. Thus, the capacity benefits with respect to meeting the need for increasing volumes of on-demand channels are limited. Moreover, in service areas where SDV does not yet exist, the time and costs associated with implementing the technology may work against immediate needs for more on-demand capacity.

Another important bandwidth reclamation strategy involves deployment of digital terminal adapters (DTAs), which convert a digitally delivered basic channel package into analog for distribution to analog TV sets in non-digital households. The very low-cost DTA is viewed by some cable operators as a cost-effective way to reclaim 200-300 MHz or more of analog bandwidth.

However, not all cable operators are convinced that DTAs deliver sufficient benefits to justify the costs when compared with other capacity-expansion strategies. Many believe the combination of SDV and node splitting, possibly in combination with fiber-deep extensions to further divide serving areas down to as few as 125 households, offers a better migration path to significant expansion of linear HD and dedicated on-demand channel capacity.

But again, whichever paths are taken, the costs and time required to execute these changes on metro-area scales represent constraints on operators’ responsiveness to changing market conditions. Once subscribers are lost to telco competitors with seemingly unlimited capacity to meet demand for linear HD, time-shifted content and on-demand movies, it will be very hard to win those subscribers back.

Of course, the cable industry could double its available digital TV programming capacity with no further (or minimal) reductions in node service areas by moving to MPEG-4/H.264 compression over all digital channels. And it could free up significant portions of capacity by moving incrementally in this direction, for example, by converting high-end tiers to MPEG-4/H.264 and introducing hybrid MPEG-2/MPEG-4/H.264 set-tops to premium customers. But the time delays and costs associated with such strategies offer even greater encumbrances to satisfying immediate needs than the other options discussed earlier.

Adding to the time pressures is the fact that many cable operators are contemplating introduction of network-based PVR services in the year ahead. This trend, together with expanding availability of other time-shift services such as Time Warner Cable’s Start Over promises to significantly expand near-term on-demand capacity requirements.

VOD transrating offers operators a way to immediately address all these needs over existing resources in advance of execution on any of these other strategies. Operators can turn up the VOD transrating platform in live operating environments with no disruptions to ongoing service, and they can do this incrementally across service areas where the threat of bandwidth exhaust is greater than in other areas.


The viability of a real-time VOD transrating architecture rests on extremely high-density processing power, which is essential to achieving low-cost processing on a per-stream basis. The optimal VOD transrating platform can transrate up to 1,000 video streams simultaneously from a high-density device with only milliseconds of latency, in contrast to traditional stat-muxing systems that may delay transmissions by as much as 2 to 3 seconds.

State-of-the-art processing density enables the session-aware functionalities associated with dynamic routing to multiple QAM channels in mixed SD/HD environments. Session awareness is also essential to maintaining "keepalive" status when end users activate trick play functions. The VOD transrater, for instance, must be able to read different VOD servers’ modes of communicating the pause command in order to recognize a session while a paused stream is still alive.

Redundancy is also essential to VOD transrating and should be offered at two levels to address varying budgets and needs. In hot standby chassis redundancy mode, primary and backup VOD transraters configured with identical virtual IP addresses are in constant "heartbeat" monitoring communications that keep tabs on the health of the devices and ensure synchronization between them as configuration changes are made to the primary device. In the event of failure, the virtual IP address under management of the Gigabit Ethernet (GigE) switch refers to the physical Internet protocol (IP) address of the backup unit, and the backup unit inherits the media access control (MAC) address of the primary device to establish the link with the VOD server, thereby ensuring a minimum amount of downtime. (See Figure 2.)


In the second, SRM-driven route-around mode, the SRM intelligence is configured to recognize a VOD transrater failure and then proceeds to route the signals "around" the transrater to establish a direct connection from the VOD server to the edge QAM modulators. Whereas the additional streams made possible by transrating are lost unless a cold spare VOD transrater is configured and brought online, this represents a low-cost redundancy architecture for two-thirds of the VOD streams.

Operators who want to prepare for VOD transrating should keep in mind certain "dos" and "don’ts" that are essential to maximizing the benefits of the technology. They should avoid pre-encryption of VOD content and implement encryption downstream or possibly integrated into the VOD transrater. From a capex planning standpoint, VOD transrating allows much closer matching of revenue to investment by avoiding over-provisioning of QAM modulators and fiber backbone capacity. And, because VOD transrating can be used to deliver video quality that surpasses today’s VOD streams while still serving to reduce overall bandwidth consumption, operators should encode optimally at higher CBR bit rates than they otherwise would. (See Figure 3.)


Operators have a wide range of architectural options to tap as they seek to accommodate escalating bandwidth requirements for on-demand content amid competing strains on capacity imposed by linear HD and broadband services. But while SDV, node splitting, DTA deployments and MPEG-4/H.264 all deliver important benefits, the major efficiency gains provided by VOD transrating can be realized immediately at relatively lower costs on an as-needed basis across the metro service footprint.

The processing breakthroughs underlying VOD transrating also serve to help future-proof cable operators against competitive encroachments.

By implementing VOD transrating, operators not only address immediate needs for more on-demand capacity, but they compound the efficiency benefits to be gained as other approaches to capacity expansion are brought on line.

Nabil Kanaan is director of product marketing for RGB Networks.

The Daily


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