Know How Applications Will Perform
Although the new generation of Ka-band systems is built to support higher bandwidth at lower cost, there is no way to avoide problems associated with latency, the half-second delay created when signals travel to and from the satellite, says DC Palter, president of California-based Apposite Technologies Inc., which develops satellite simulators and network test equipment. "Various acceleration technologies mitigate the impact of that delay on protocols and some applications, but for anything that depends on real-time communications, particularly voice and Internet gaming, the satellite delay will be noticeable," he says. "The only way to know how applications will perform under high latency conditions is to test them."
A team at the Center for Satellite & Hybrid Communication Networks, a NASA-funded organization within the University of Maryland's Institute for Systems Research, is performing modeling and simulation studies on power allocation in multiple spot beam satellite communication systems. One of the team's objective is to identify ways to optimize network performance and traffic aspects of the network. The team found that for some users in deep fade periods, full recovery is not possible within reasonable session times even if a forward error correction code capable of generating many encoding packets is used. It is better for the transport protocol to drop the connection to these users rather than wasting system resources with multiple connection attempts. "Our focus, in a follow-up work, was on how a reliable channel condition value can be obtained for a spot beam without asking or collecting periodic feedback from every active user, as this would place a heavy burden on the limited or scarce network resources in the return path, either a satellite return channel or a terrestrial dial-up return channel," says Gun Akkor, a staff scientist in Patton Electronics Co.'s engineering department,
Testing live satellite links in order to troubleshoot problems and optimize performance is time-consuming and expensive, whereas simulators offer a simple and efficient alternative way to test systems in the lab under a variety of conditions. For example, consider the possible combination of a Ka-band or Ku-band satellite link with a wireless Wi- Fi last-mile solution. Palter recently ran some performance tests on this type of deployment using Apposite's Linktropy 4500, a hardware-based simulator. A 10-megabyte file took approximately 100 seconds to download directly via satellite. Using a Wi-Fi connection plugged directly into the terrestrial Internet, the same file downloaded in about 90 seconds. "But here is the unfortunate surprise: Combining the packet loss of Wi-Fi with the latency of satellite caused the same file to take nearly 900 seconds to download," he says. "And using Windows file sharing instead of FTP caused the download to take nearly 1,200 seconds. The implication is that it is critical to test the whole end-to-end network. Testing individual sections may not show important interactions between the component links."
Not Just A Consumer Play
New Ka-band markets, like digital signage, are still works in progress and not yet on the fast track like applications such as direct broadcast satellite Ka-band deployments for HDTV local into local broadcasting or the Wildblue consumer broadband initiative. Newtec is preparing the delivery of a Ka-band network to Arqiva for multicast delivery of digital signage information which can also be used to distribute digital cinema, says d'Oreye.
"Satellite news gathering is where the advantages of Ka-band are expected to yield an immediate result, but questions about reliability are having an effect," says Osborne. "Today, there is some high-end demand for satellite news gathering applications in Ka-band," he says, adding that enterprise applications suffer from the perception that Ka-band is unreliable due to rain fade. "Although this technology has been proven reliable, this perception remains," he says.
Ka-band research continues. In addition to the work being done at the University of Maryland, Japan's Wideband Internetworking Engineering Test and Demonstration Satellite project is moving ahead. The spacecraft will be equipped with a Ka-band active phased array antenna, a multibeam antenna and a high-power multi-port amplifier. Although the program was put on hold in 2004, a launch in 2007 is a strong possibility, says an official at the Japanese Ministry of Internal Affairs and Communications.
Although we may be ready to declare that the first phase of the Ka-band revolution is officially over, it still is a bit too early to proclaim that profitable Ka-band operations are a reality.
Peter Brown Is Via Satellite's Senior Multimedia and Homeland Security editor.
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