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If your path is that short, then laser can give you seamless gigabit throughput with no interference or security issues. If you stretch the distance for FSO, then you’ll want to pay special attention to your backup because users will invariably be switched onto it. Consider also, that users who regularly ride on a gigabit connection will not be let down lightly upon a few T1s.
Accordingly, FSO manufacturers are now packaging their links with a high frequency radio backup as a means of mitigating the reliability problem. This is a good concept, though still limited in path distance and with a cost that approaches more robust alternatives.
Spread spectrum was developed for the Department of Defense and aptly, its chief advantage is security. Since it’s been put to commercial use, the technology has come way down in cost and is widely available. High security and low component cost has made this a popular technology for cordless phones.
Spread spectrum radios use either frequency hopping” or “direct sequence” schemes to scramble signals and hunt for the best available channel in a frequency band. Spread systems use radio signals, which are not impaired by weather and therefore achieve reliability rates upwards of 99.999% (the proverbial “five-nines”).
I’d devote more time to the topic here, except that spread spectrum falls short of the bandwidth needed for transparent connections. Bandwidth tops out at 11 Mbps, but actual throughput - what really matters, is only 4.5 Mbps each way, net of product overhead. Also, where spread systems once claimed a unique security advantage, they're now eclipsed by higher speed microwave products that also feature encryption. Cost of a spread spectrum link ranges from $2,500 to $15,000.
Unlicensed microwave is the most prolific wireless solution, mass marketed by LAN vendors and some traditional microwave outfits. Unlicensed radios offer instant gratification where delivery is priority one. Installation may be completed in single day by untrained personnel using friendly user features like "installation wizards" for automated link configuration.
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Frequency bands are 2.4, 5.8 and 60 gigahertz ("GHz") for practical distances of 0-20 miles. Bandwidths at 2.4 and 5.8 reach 420-480 Mbps each way and the 60 gigahertz radio pushes a full gigabit.
High bandwidth, solid reliability, convenience and speedy deployment are all pluses of unlicensed radios. The downside is that they're inherently prone to interference, and that's why so many wireless ISPs have had to replace (“upgrade”) unlicensed backhaul connections. The 2.4 band is chronically congested and 5.8 GHz is heading for the same fate. It’s not a matter of if, but when.
Cost for 2.4 and 5.8 microwave starts at about $2,500 for 10-20 megabits and upwards of $80,000 for 480 megabits. The best value per meg of bandwidth is the 60 GHz radio. Installed cost is about $30,000 per link for a full gigabit. Unfortunately, while these radios are big on performance, they're short on distance. They'll only get you about half a mile, limiting their usefulness to cross street or metropolitan area network (MAN) connections.
The relative low cost of unlicensed makes it compelling for alternate or "mixed-media" backup to corporate lifelines buried in the ground or strung across poles. Naturally no one appreciates the expense until there's a big telco outage and you're the only outfit on the block still conducting business while others are in damage control.
Unlicensed radios make good investments for light to medium duty and especially for temporary jobs, such as to reach a building under construction or one that you don't plan to occupy for long. They should serve the purpose remarkably well and with a quick return on investment. In the worse case, no one's head should roll over a $6,000 purchase gone bad.
That being said, I wouldn't drop the big bucks on higher speed unlicensed radios. Dealers make bold pronouncements, but I haven't met one that will guarantee interference free performance for any length of time. Therefore, I regard unlicensed products as ticking interference bombs. Yours could go off in five years or five weeks.
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