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There are distinct differences between these forms of wireless. First, there's bandwidth. Licensed and unlicensed systems carry the highest payloads. Spread spectrum loses considerable throughput to the overhead it takes to process their signal.
For instance, there are spread spectrum radios commonly thought to deliver 11 megabits of Ethernet data - or at least that's how their touted by vendors. In tests we found that the throughput in each direction was measured at significantly less than the 11 megabit claim (actually about half that rate in each direction). With licensed and unlicensed radios, 10 or 100 megabits is exactly that, and nothing less.
Another difference is in the wireless medium's ability to simultaneously process both voice and data. Licensed radios can support Ethernet and T1s across a single wireless channel. With spread spectrum and most unlicensed systems you'd have one link for voice and a separate one for data. This is more a matter of convenience and of limiting the number of antennas on your roof along with the number of components that need to be supported.
Then there's interference. Of the three technologies, licensed microwave produces the least residual noise and gives you the greatest protection ("isolation") from interference. The difference lies mainly in the discrimination of the receiver, and that relates to its ability to distinguish the sliver of a signal it wants to see and ignore all the rest. Licensed systems will have tighter receivers than the more promiscuous unlicensed radios and that promotes greater signal quality with less noise reception. Unlicensed systems simply do not incorporate the level of interference protection that the higher cost licensed radios do. That, and the lack of adequate standards create a law of the jungle environment where no unlicensed vendor will guarantee that after spending seven to fifteen thousand dollars, you can be sure of even a single year of clear, interference-free operation.
Licensed radios also must use higher performance antennas as mandated by the FCC. Those antennas have a more narrow beamwidth than what's used in spread spectrum and unlicensed radios. There can be dozens of licensed microwave links on the same tower or rooftop, but put a handful of spread or unlicensed systems in a small area and in all likelihood, you'll be out of luck.
Where spread spectrum beats all is in security. It was developed specifically to ensure the security of military transmissions. Read our answer on the security question for more on this subject. It explains how certain licensed radio transmissions offer all the security you could ask for and why others may need to be encrypted. For all practical purposes, a high frequency licensed microwave link will give you ample data security with no compromise to throughput and signal quality.
And finally there's product cost. At face value, licensed microwave is several times more costly than unlicensed and spread spectrum radios. But then there's a big difference between what you pay for a product up front, and what the actual cost of operating that product is throughout the year.
Several factors should be considered when you assess the proposed cost of a wireless installation, and that's; 1) your real bandwidth needs, 2) the upgrade potential (if growth is anticipated), and; 3) the prospect of eventual downtime and attendant cost of your backup plan (e.g., spares, on-site support, leased line backup, etc.).
Just be aware that the dollar amount on the sales quote is just part of the story. Often a lower entry price leads to a deceptively high overall cost. Today you can purchase a marvellous printer for under a hundred dollars, but then you spend half that every other month just to keep it in ink cartridges. In that vein, a day of downtime can cost you a fortune, eliminating in a heartbeat, any up front savings there appeared to be.
Nowhere is the maxim, "you get what you pay for", more applicable than to the wireless business. If you've received a hundred fifty thousand dollar quote for fiber, don't expect that it can be replaced, apples for apples, with a three thousand dollar wireless gadget that sits on your desk. Instead you might have to spend forty thousand in wireless to get the type of bandwidth and level of reliability you'd expect from the higher priced fiber option. On the other hand, if you only need a few T1s and the data you're sending isn't vital or time sensitive, then unlicensed wireless is perfectly suitable.
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