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@ Cable-Tec Expo: IP networks need QoS to combat dropped packets

Mon, 11/14/2011 - 4:20pm
Mike Robuck

Attendees of the SCTE's first Capacity Management Symposium got an early jump on this week's Cable-Tec Expo, as well as some planning tips for the migration to IP networks.

SCTE senior vice president of engineering and CTO Daniel Howard provided the level set for the symposium by citing Nielsen's Law, which says that the demand for broadband increases 50 percent every year. With that rate of increase, it's only a matter of time until cable operators transition over to IP networks.

Cisco's John Chapman, CTO of the cable access business unit and an Engineering Fellow, said attendees need to work on coming up with methods to manage the capacity on IP networks. One of the challenges for IP networks is dropped packets. Instead of changing a network, which Chapman said is an uphill battle, engineers need to adapt to it for video over IP.

"If you're going to drop packets, do it with style," Chapman said. "If you are going to build something, you drop with style. Quality of service is the art of dropping packets that don't fit, but at the same time making other packets fit better. Maybe when you go to drop the packets, you don't drop the video packets, you drop the data packets instead and let the video packets go through. That's quality of service."

Data services can overcome dropped packets via retransmission, while VoIP can use regeneration or concealment. Video also needs to survive packet drops, and it can use quality of service (QoS) three ways: Identify, or classify, a flow of packets that belong to an application; tag the packet; and apply a QoS policy in the network.

Chapman also spoke about average packet sizes, which he defined as a ratio of large and small packets. He urged symposium attendees to do their own studies on average packet sizes to get a better understanding of the normal distribution that can be used for traffic engineering. The cable industry needs to gather a lot of trend data in order to engineer video over IP networks, he said.

Chapman said there is no such thing as a real APS packet, so traffic engineering has to take into account a range of APS value. He spoke about the concept of a nominal, maximum and minimum APS value, but the minimum APS value needs to be used in traffic engineering calculations.

"The big point that I want to make today is traffic is asymmetrical," Chapman said. "There's really about two packet sizes on the Internet: big and small."

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