Fred Morris, Vice President, Comtech EF Data

By | October 12, 2009 | Broadcasting, Telecom

Comtech EF Data Vice President Fred Morris has seen global networks evolve and standards come and go over his career. As his company does business in the DVB-S2 era, Morris is faced with several challenges ranging from meeting the demands of an increasingly diverse end-user base to making new products as efficient as possible.

However, Morris has taken on another unexpected challenge – clearing up the misconceptions over adaptive coding and modulation (ACM), a new network architecture component, which aims to reduce latency and streamline traffic over an affected link. With a wide variety of interpretation over the reality of ACM, Morris defines the commonalities of ACM and explains the science behind the new technology.

Via Satellite: What is the basic function of ACM and what role does the technology play in the network architecture?

Morris: For a given set of operating points or signal-to-noise there is an associated given forward-error correction and modulation. As a rain fade hits the link, it eats away at the signal-to-noise. ACM basically downshifts on the modulation and forward-error correction to maintain the link in a broadcast stream based on the feedback from each individual location. But, it does this at the penalty of using throughput.

    There are issues with ACM that are in the network design that have to be taken into account. Because if you consider the network, which is an IP-based computer network designed with a given amount of throughput in mind all the way through the network of routers talking to other routers over a satellite link, the bandwidth starts decreasing. ACM isn’t a sole solution to every problem. If vendors don’t do anything except for just reduce the bandwidth then there is the byproduct of increased latency, because the networks will try to send all of their traffic at once and create congestion. In situations where you’ve taken a megabit link and dropped it down to half a megabit or 56 kilobit, you still have that congestion from an IP sense.

    We have addressed this problem with our 625 modem. We put backward pressure on the link and we have pause frames that we send out from the modem when the ACM kicks in and starts. The pause frames tell the devices on the network to hold up their traffic and stop sending signals at the rate that they normally send. It works well. Several companies that have tested this technology have accepted it and implemented it.

Via Satellite: Does ACM have a standard definition?

Morris: ACM technology is not a standard. It is a concept. ACM works with Ethernet and IP. It does not work with serial. Since ACM is not a set standard, it’s more of a company-to-company interpretation and different companies and vendors have taken different approaches. DVB-S2 had envisioned ACM, as well as what had existed before, as concepts. DVB stopped at the concept, which is consistent, and basically allowed the industry come up with their own version of it. They didn’t settle on it.

Via Satellite: Does individual interpretation lead to misconceptions of ACM technology?

Morris: Yes. The misconceptions that come out of the technology are really side effects of the marketing hype that people have applied to ACM. There are a lot of claims that focus on ACM providing greater throughput and increased availability, which are true, but, I have a problem with some of the numbers attached to these claims that I see out there. ACM does not provide 192 percent greater throughput and 200 percent increases in availability. That’s crazy.

Via Satellite: What is the most realistic picture of the advances that come with ACM?

Morris: It gives you greater availability of a resource – the link between your two locations, or a broadcast to multiple locations – where rain fades would previously result in a complete loss of a link and now merely reduces availability for a brief period of time. It is a great improvement and that is what needs to be talked about. The rest of these claims are just noise that confuses quite a bit of the satellite equipment buyers.

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