As the deadline approaches for broadcasters to transition to digital, cable operators will need to rethink the way they deliver services.
All forms of media are moving from analog to digital. While today’s children don’t recognize vinyl records and think cameras have always been digital, broadcast television remains the last bastion of analog. How will cable handle the transition to digital?
In the 1996 Telecommunications Act, the U.S Congress mandated that broadcast analog television transition to digital television (DTV). All broadcasters live under a set of rules requiring surrender of their analog broadcast licenses at midnight on 12/31/2006. While the Federal Communications Commission (FCC) may extend this deadline, it is likely that analog broadcasting will end in most markets before the end of this decade and probably sooner in larger markets where the transition to DTV is further along.
This presents a major challenge for operators. As analog stations go off the air, cable operators will want to remove analog signals—including the analog "expanded basic" channels—from the channel lineup and reallocate the spectrum to support new digital services.
During the transition from analog to digital, many homes won’t have digital receivers and few will have more than one. There will be an enormous installed base of analog TV devices for many years. To support them, cable operators will convert broadcast DTV signals to analog in the set-top box—as they do today with digital signals from satellite channels.
But the majority of analog TV devices currently are not connected to a set-top box. The cost of even a basic digital set-top is far too expensive when multiplied by hundreds of millions of analog TV receivers.
All-digital, all IP model
Outside North America, systems built from scratch today favor a plant model very different from ours. Some systems in Asia and Europe are taking an all-fiber, all-digital, all-IP approach.
Fastweb provides a good example of a modern plant design. As a new entrant, it is building an all-fiber, all-digital, all-IP network from scratch in major Italian cities. All services—including broadcast digital television, video-on-demand (VOD), high-speed Internet, and voice and video telephony—are carried over IP.
Fastweb is fully rolled out in Milan and is operational in five other cities during plant construction. In less than two years from service launch, Fastweb is now serving more than 175,000 subscribers. Italy has become a center for technical innovation for an all-digital, all-IP future.
All-digital plant design
With the end of analog on the horizon, it would be timely for operators to think about the appropriate plant design for an all-digital system. What elements of current plant design should be preserved for an all-digital future, and which should be reconsidered and possibly discarded?
Table 1 shows some of the key design parameters that operators might consider when planning tomorrow’s all digital cable plant.
The first question is whether to change the physical infrastructure from HFC to an all-fiber approach along the Fastweb lines. This might have the advantage of the best possible plant. But it would discard everything operators have learned over the past half-century and incur unacceptably high capital expenditures. The combination of fiber and coaxial cable has served operators well for many years, and there is reason to believe that HFC can be exploited to support new applications for many years to come.
New technologies promise to leverage the HFC infrastructure to extract additional bandwidth for new applications. Today these technologies are overlays on the existing plant architecture: the existing spectrum allocation and channelization are assumed to be sacrosanct.
A plant designed for an all-digital system would not have to preserve the 6 MHz frequency division multiplexing (FDM) channelization and the sub-split spectrum allocation. To allow higher efficiency in modulation and the assignment of services, the entire cable could be divided into several bands each carrying gigabits per second. It probably could not be used as a single wide-band channel, because it would need to carry existing services throughout a transition.
The split could be fixed at a much higher frequency. Ideally, technology would allow the split to be dynamically adjusted to match the actual degree of asymmetry in evolving user applications.
New compression techniques
New technologies are emerging for video compression. Moving Pictures Experts Group (MPEG)-2 is very inefficient by today’s standards. While some favor MPEG-4, many think that H.264 / MPEG-4 Part 10 will replace MPEG-2 for video compression of both standard and high definition video. It provides much higher compression than MPEG-2 and significantly better compression than MPEG-4.
This is especially critical for high-definition TV, which requires substantially more spectrum than standard-def. The cable plant’s ability to carry many channels of high-def is a major asset in competition with satellite; higher compression would permit carrying more channels in the same spectrum.
Some think that IP ought to replace MPEG-2 for some or all video transport. This would simplify digital-only set-top boxes and leverage the low cost of IP switching compared with MPEG switching in network equipment. By using IP formats, the industry could leverage the wealth of application development experience to enable innovative applications integrating video, data and interactivity.
Finally, operators must consider the timing of the transition. If an all-digital plant will be appropriate some time later this decade, should operators wait until the last analog TV station goes off the air to start the transition? Would it make sense to accelerate the change and reap the benefits earlier?
The operators’ views
We’ve discussed these ideas with several MSO senior engineers. We came away with several guiding principles. These include:
* MSOs can’t abandon their customers. They must find a cost-effective way to support the huge base of "cable-ready" TVs and VCRs through the all-digital transition.
* The investment community has a say. MSOs must leverage as much as possible of the huge capital investments they’ve made over the past decade in rebuilding plants and installing digital set-top boxes.
* The transition must be phased. A "flash cut" is unacceptable.
Richard Dowling, GCI’s senior vice president, corporate development, said the company wants to move to an all-digital solution as soon as possible. Alaska-based GCI currently provides video, Internet and telephone services over its 550 MHz systems. GCI wants to postpone re-building these systems until it’s clearer what plant technologies would be appropriate to support future digital services.
GCI plans to digitize all current analog channels at the headend, shut down analog service, and provide low-cost digital set-top boxes to all customers who want them. Original plans had called for GCI moving in this direction during 2003, but the current business slow-down might delay that until next year.
GCI wants to move away from a sub-split band plan and "provide more upstream bandwidth and clean upstream bandwidth" to meet increasingly symmetrical application requirements (see sidebar "Prepare for Symmetry"). Today’s FCC regulations make it difficult to remove the over-the-air analog channels. GCI would like the FCC to give the industry the opportunity to use the plant in a way that would be most beneficial to consumers. Ideally, GCI would like to "re-farm the spectrum" by installing technology to allocate bandwidth dynamically to match application symmetry ratios.
Another senior engineer in the industry believes that the huge inventory of legacy TVs and VCRs is a major obstacle confronting any all-digital approach. He thinks operators will keep carrying analog signals until the broadcasters go off the air, and have too much invested in the current infrastructure to consider moving to IP transport.
He suggested that operators start by creating a "digital basic package" with all channels of the present analog extended basic carried in the clear—digitizing at the headend those channels received only in analog. This would permit the creation of a digital-only set-top without the need to receive analog. It would also permit the creation of a low-cost "digital zapper" without conditional access or an electronic program guide (EPG). "If it had a low enough price, you could give it away to encourage migration to a basic digital tier," he said.
A third engineer agreed with many of these arguments but saw the need to resist a "build it and they will come" philosophy in next-generation networks. He said that the spectrum requirements for new applications should be carefully quantified as part of deciding what a new structure might be.
There are undoubtedly many other perspectives. Our goal is to stimulate this dialog, so that whatever course the industry takes, it benefits from lively debate, starting without any "sacrosanct" assumptions.
Cable operators are facing serious competition from satellite. Some systems have had net subscriber closses during the past year. Cable’s best opportunity to compete with satellite is to fully exploit the two-way and interactive capabilities of the cable system. But the system is constrained by long-held assumptions that inhibit cable from realizing its full capacity for emerging digital applications.
Working through CableLabs, the cable industry has achieved great success in developing specifications for new digital services. But each of these efforts has taken a long time to reach full market maturity—often much longer than participants believed when the projects were started.
If the industry started working together now to define, develop and test the appropriate technologies for an all-digital system, it could be fully proven before analog starts going off the air, and provide the industry with additional tools to compete with two-way satellite.
Prepare for Symmetry
When considering the transition to an all-digital future, cable engineers also need to examine the changing symmetry requirements of their networks. Currently the cable plant is highly asymmetric, with 15X or more bandwidth available downstream than upstream. While the typical downstream spectrum has expanded from 220 to 750 or 860 MHz, the upstream has expanded only from 30 to 42 MHz—constrained by the location of broadcast channel 2 at 54-60 MHz.
This asymmetry was not a problem in the early days of digital services. Digital television requires only downstream bandwidth. Early Internet services like Web browsing are highly asymmetric downstream. Both are good matches to the existing cable plant.
Many emerging applications are symmetric or nearly so. Some, such as PC backup, are asymmetric upstream. Studies by Terry Shaw of CableLabsindicate that the aggregate bandwidth requirements for data applications are tending toward symmetry.
Because upstream bandwidth is scarce, operators discourage the use of applications that consume it. They are increasingly acting to constrain bandwidth or to cut off high-consumption users.
While DOCSIS 2.0 will provide additional upstream capacity, it cannot overcome the inherent asymmetry in the plant. New applications such as video telephony and the sharing of consumer-created pictures and videos will add pressure on upstream bandwidth.
Ready for Digital?
Cable plant design has evolved over the past 50+ years, but its fundamental, broadcast-analog structure remains unchanged. Congress has mandated a transition to digital and high-definition TV. The coming end of broadcast analog TV, and the increasing use of the cable plant for digital applications—many symmetric and challenging the inherent asymmetry of the cable plant—suggest rethinking some long-held assumptions.
Although new entrants have the opportunity to start fresh and optimize their infrastructure for the coming all-Internet protocol (IP), all-digital world, cable operators would prefer to leverage their extensive physical plant assets, which have provided so much success in the TV and high-speed Internet markets.
While this transition imposes burdens on cable operators, it presents the opportunity to re-examine and consider changing some long-held assumptions so the cable infrastructure can provide the brightest competitive and financial future for the industry.
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