ZyXEL Communications ZyAIR B-5000, User Manual

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ZyAIR B-5000 User’s Guide
Site Planning
A-6
A signal degrades as it moves through space. The longer the path, the more loss it experiences.
This free-space path loss is a factor in calculating the link viability. Free-space path loss is easily
calculated for miles or kilometers.
Availability represents the quality of a link. It is the ratio of the time that the link is available to
the total time. This serves as a guide to the service that you can expect, on average, over a period
of one year.
Availability
Your application determines what availability is required. A critical application where downtime adversely
affects business and revenue requires a high percentage of availability. Somewhat lower availability might
be acceptable by an application used to gather data, where occasional outages can be tolerated.
Availability is largely a function of fade margins and the amount of signal fading. Paths obstructed by trees
have larger fades than paths with no trees. Longer paths tend to have more fading than shorter paths. Larger
fade margins yield better link availability.
The International Telecommunications Union (ITU) publishes a reference for link planning, which is
available at
http://www.itu.ch/ .
ITU Recommendation G.826 contains definitions for "availability" and related terms used to describe link
quality. It also contains recommendations for link quality objectives.
ITU Recommendation P.530 contains information on how to plan for high reliability in clear, line-of-sight
links.
Availability is much more difficult to predict for non-line-of-sight links. It is best determined by field
measurements.
Unlicensed Frequencies (U-NII)
The FCC has identified the frequencies from 5.725 to 5.825 GHz as Unlicensed National Information
Infrastructure (U-NII). This band can be used by anyone without having to obtain a license. However, you
must use radio equipment that is "type approved" by the FCC for use within the specific band. If you are
installing a U-NII band link between two buildings, across a parking lot, or across town, you will find that
this type of system is much simpler to implement than licensed systems. By using very directional antennas
in the installation, you are not likely to experience interference.