Density was a hot topic in the world of edge QAM (quadrature amplitude modulation) modulators at Cable-Tec Expo three months ago, and it remains today so for several reasons.

In an article scheduled for publication in the February issue of Communications Technology, Comcast engineering execs Jorge Salinger and John Leddy point to successfully deployed video on demand (VOD, especially of HD content), switched digital video (SDV) and DOCSIS 3.0 as driving the delivery of increased numbers of unicast QAM channels.

At the same time, they note that MSOs for several years have been reducing the size of their service groups.

“These two trends results in a continuous increase in the number of QAM channels per service group,” Salinger and Leddy write. “Moreover, the expectation from the current service projections is that such growth will continue and even expand, especially as MSOs reduce the number of analog channels available in the network.”

“As a result,” they continue, “the cable industry needs ever denser Edge QAM (modulators) to reduce both the cost from the equipment itself and the resulting environmental requirements in headends and hubs.”

The pace of density

How best to leverage such density is a key main point of their article, which is an overview of the Comcast-designed converged multiservice access platform (CMAP), part of the next generation network architecture (NGAA) that these authors first discussed publicly at SCTE’s Conference on Emerging Technologies in April 2009.

The precise nature of individual responses to Comcast’s initiative remains to be seen. However, based on conversations with various vendors over the past three months, it is clear that the question of ultra dense QAM modulators has been as much about when MSOs would be ready to take advantage of them as it has about whether vendors had the capability to manufacture them. (For a related article from November 2009, click here.)

One vendor that builds edge QAM modulators and supplies QAM modules as components for others, Vecima Networks, for instance, first unveiled its HyperQAM, which offers up to 128 QAM channels in a 2RU platform, in June 2007. And it has not been the only early mover.

“Given our core technology, nothing is preventing us from moving to 100 channels per RF port except the deployment plans of the MSOs,” Tony Pierson, vice president business development, LiquidXStream, which currently offers 36 QAM channels per RF port.

Currently, VOD, SDV and DOCSIS are separated by device and “combined to the service group,” said Charles Cheevers, vice president, product management, CTO Europe, ARRIS. No single service will require 32 QAM channels in the next two years.

“The key thing to leverage a 32+ channel capable QAM output is service combining and this is dominated by the back office capability to configure and manage the combining of multiple services to one QAM group before modulation. When an operator decides to do this is the key factor in the deployment of more dense and upgradeable QAM devices,” Cheevers said.

Cheevers described a three-phase evolution: Less than 24 channels per F-port and service group; combining two F-ports to less than 64 channels per service group; and then a single F-port QAM capable of outputting greater than 64 channels.

“The two future applications that could drive us to 100 QAMs per service group are 3D TV…or IPTV,” Pierson said.

Not just density

While density will prove to be important, so too will frequency agility, said Yoav Derazon, Harmonic senior manager, cable solution and strategy. “(This) is the ability to spread the QAMs cross the spectrum, rather than have them in a contiguous block.”

Operational costs and manageability concerns also have emerged.

“It takes substantial time and effort to be able to operate a large number of devices, account for their configuration, upgrade them and support different software features,” Derazon said. Harmonic’s HectoQAM, slated for release in mid-2010, will provide four-to-four-and-a-half times the density the company offers today.

Different applications use different numbers of QAM channels. “How can you mix and match QAM sizes to optimally meet bandwidth requirements?” John Holobinko, BigBand vice president, marketing, asked.

Bigband’s forthcoming BEQ 6200 8:1 QAM, which will offer 96 QAMs in a 1.5 rack unit, can be configured from a software standpoint alongside the BEQ 6000 4:1 QAM. “Instead of looking at two products, (it’s like) looking at a single product with two different densities,” Holobinko said.

The next step is a control plane that can dynamically allocate QAM capacity. “(This) can enable you to better utilize network bandwidth by, in the future, not tying that QAM or bandwidth on the network to a particular application,” Holobinko said, noting that BigBand currently offers some of this capability and expects enhancements throughout 2010.

Costs and consumption

As the initial purchase price of QAM devices is doing down, the total cost of ownership (TCO) is growing in importance. “The percentage of overall cost due to operation is (becoming) greater by default,” Holobinko said.

Reliability should be factored into the equation. “The cost of replacement and the disruption of the operation to have to…put new ones in and configure them. Those are real costs,” Holobinko said, suggesting that vendors should be asked about the return rate of their QAM platforms, not just failure rate.

Power is another component of operational costs, ARRIS’ Cheevers said. His company’s universal edge QAM, the D5, draws 2.7 watts per QAM channel, and he expects a move to 1 watt “over time.”

“How much power an Edge QAM draws directly impacts the UPS battery backup, the generator capacity, and the air conditioning required. The air conditioning is actually a double hit. One needs extra AC to dissipate the heat put off by more power draw, and the AC itself must be powered,” LiquidXStream’s Pierson said. The LxS-3616 uses 2.5 watts per QAM.

Holobinko noted that comparisons should be apples-to-apples, as some vendors list power consumption of the module, while others report the power supply for the chassis.

System simplification also will save money and rack space, John Dahlquist, vice president of marketing, Aurora Networks, said. As narrowcast services have increased, the combining network has become more complex with the potential for more loss. “Traditionally an operator would need to introduce a third party pre-amp module to ensure the signal integrity of the narrowcast network.”

Aurora’s AT3535G QAM transmitter, which supports 80-plus QAM channels, has an integrated pre-amplifier.

As it happens, simplification of RF combining network remains is one of seven key functional goals of the Comcast CMAP, the others being:

  • Flexible use of QAM channels;
  • Individually configurable assignment of QAM channels;
  • Efficient implementation of edge QAM blocks;
  • Implementation of sophisticated encryption systems;
  • Transport-agnostic network architecture; and
  • Operational (including environmental) improvements.

For more on CMAP, see the forthcoming February issue of Communications Technology.

–Monta Hernon

The Daily

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