ZyXEL Communications ZyAIR G-5100 User Manual page 240

ZyAIR G-5100 User's Guide
Calculating a Link Budget
A link budget is a rough calculation of all known elements of the link, to determine if the
signal will have the proper strength when it reaches the other end of the link.
To make this calculation, consider the following information.
• 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.
240 Appendix G Outdoor Site Planning