Cable operators contemplating a migration to the narrowcast-focused architecture known as “CMAP” (Converged Multimedia Access Platform) have an opportunity to avoid what could be major outlays for physical network upgrades by deploying a new generation of optical components that serve both current and future needs at greatly reduced costs.

CMAP – conceived by Comcast and now under development industry wide – offers a much more efficient solution for expanding narrowcast channel counts and variations in the services carried over those channels compared to such previously devised migration paths as modular CMTS (cable modem termination system), DOCSIS bypass and stacking of universal edge QAMs (quadrature amplitude modulators) for multicast and VoD channels for each service area. In essence, CMAP combines CMTS and QAM functionalities at the headend so that all video, voice and data streams can be switched in IP mode for aggregation into narrowcast and broadcast QAM channels in whatever combinations are needed to meet market requirements at a given time.

But this approach to streamlining service migration has important implications for how optical transmission resources are allocated in the network. While engineers focus on the development of CMAP specifications and implementation strategies, it is essential that they begin taking into account the impact CMAP will have on their transport architectures. As they do, they quickly will find that the assumptions underlying current routine purchasing decisions must be altered significantly if they are to avoid unnecessary replacement costs later on.

Cable operators have greatly expanded their QAM channel counts per service area during the past two years, in some cases reaching a dozen or more. But this is just the beginning of a trend line some operators are projecting could lead to requirements for 30 or more QAMs per service area.

The Narrowcast QAM Challenge

Not only do operators anticipate sharp increases in narrowcast requirements for SDV (switched digital video), time-shifted TV programming and movies on demand, with higher proportions of HD channels counts consuming more bandwidth per dedicated stream and the spectre of 3D TV looming on the horizon, they also expect to be allocating more dedicated broadband channels for bonding for DOCSIS 3.0 services. And many are planning IPTV migration strategies that will utilize narrowcast bandwidth to deliver an increasing volume of premium content to hybrid set-top boxes.

Making matters more difficult, as operators continue to reduce the sizes of their service areas, the total headend QAM counts go up accordingly. While high-density processing technology rapidly is increasing the number of QAMs per chassis, the likelihood is that the rate of narrowcast channel expansion will outpace gains in density, resulting in a net expansion of space and power resources to accommodate all the QAMs. Moreover, the fact that QAMs are dedicated to specific services and are aggregated on a per-service-area basis makes it difficult to adjust how QAM resources are allocated as operators require more narrowcast capacity for DOCSIS channels with the relative proportions of one type of narrowcast service to another continually change.

The CMAP Transport Challenge

In today’s transport architectures, different QAMs for different types of narrowcast services – VoD, DOCSIS data and SDV – are aggregated at the headend onto wavelengths dedicated to specific nodes for delivery to the distribution hub. At that time, the narrowcast signals are handed off from each wavelength to individual fibers for distribution to each service area. Separately, broadcast QAM channels are combined with analog channels at the headend for transmission via high-power lasers over a separate fiber or wavelength to serve all the nodes connected to a given hub.

To take advantage of the great efficiency gains made possible by CMAP, operators must be able to shift a much larger share of the total downstream spectrum to narrowcast lasers than such lasers can carry today. Owing to their limited capability to perform well in high-dispersion fiber regions and to their relatively high second and third order distortion characteristics, older C-band lasers will not be able to propagate across the wider RF spectrum without introducing unacceptable increases in noise levels.

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