Gotcha! Ups and Downs of Satellite NetworkingBy David F. Carr | Posted 2004-07-01 Print
The pros and cons of very small aperture terminal satellite networking.
Very small aperture terminal satellite networking has long been a favorite of retailers seeking rapid geographic expansion, from Dollar General to drugstore chain Walgreens. VSAT is sometimes the only choice for organizations that need to provide network services to remote locations where terrestrial telecommunications aren't available. More often, however, network managers must evaluate the trade-offs between VSAT and terrestrial services such as frame relay.
Problem: Even at the speed of light, data bounced off a satellite faces a transmission delay of 250 milliseconds, four times that of a frame relay connection.
Resolution: Using small aperture technology means compensating for delay, or "latency."
Latency tends to bog down interactive applications where the user sits waiting for a response from a remote server to appear on screen. Particularly slow to load are Web applications in which a browser makes multiple requests for images or other page elements to display on a single page. For years, drugstore operator Walgreens custom-designed key applications to minimize network round trips. Walgreens makes Web application performance acceptable by using technology from FineGround Networks that improves performance fivefold by caching content in memory and optimizing page display, says Phil Herrera, director of telecommunications information technology.
Herrera says the effort pays off because VSAT saves the drug chain "hundreds of millions of dollars over any other technology we could have used."
Problem: It's easier for headquarters to send large files to remote users than vice versa.
Resolution: Make sure remote sites don't have to trasmit large amounts of data regularly. VSAT connections are asymmetric, meaning they broadcast data more effectively than they receive it. So only a headquarters operation should transmit video to remote locations. Field sites should only have to send text and simple graphics.
According to a Gartner Inc. analysis, distance learning requires a headquarters to send 100 times as much data to the field as users do in reverse. For Web and client-server applications, the ratio is more like 20 to 1.
Problem: Severe weather, particularly heavy rainstorms, can block connections.
Resolution: Network design should include a backup alternative, such as dial-up connections. Higher-power satellite dishes may be able to penetrate all but the most severe storms. Overall, VSAT connectivity uptime averages about 99.6%, compared with 99.8% for frame relay service, according to Gartner. But in Florida and other volatile weather areas, this is a serious problem.
Problem: Startup costs can be high.
Resolution: Look at long-term costs, traffic requirements and alternatives. The cost of VSAT equipment is coming down, but remains about $2,000 a location. Payback comes with time, with a typical monthly service cost of about $150 per location versus $300 for frame relay, according to Gartner.
Alternatives may be costly, too. Wal-Mart announced years ago it was switching from VSAT to frame relay in search of higher performance, yet still has thousands of stores operating on VSAT connections, according to Simon Bull, a satellite communications analyst with Comsys. "I think they hadn't realized how expensive it would be to move to frame relay," Bull says.
But the economics change. With lots of traffic, frame relay can be cheaper for applications that don't require large amounts of outbound data, which VSAT handles better.
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