Wireless+Networking+in+the+Developing+World_Part5

Wireless+Networking+in+the+Developing+World_Part5 - Path...

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Unformatted text preview: Path Loss = 40 + 20log(5000) = 113 dB Subtracting the path loss from the total gain 40 dB - 113 dB = -73 dB Since -73 dB is greater than the minimum receive sensitivity of the client ra- dio (-82 dBm), the signal level is just enough for the client radio to be able to hear the access point. There is only 9 dB of margin (82 dB - 73 dB) which will likely work f ne in fair weather, but may not be enough to protect against ex- treme weather conditions. Next we calculate the link from the client back to the access point: 15 dBm (TX Power Radio 2) + 14 dBi (Antenna Gain Radio 2)- 2 dB (Cable Losses Radio 2) + 10 dBi (Antenna Gain Radio 1)- 2 dB (Cable Losses Radio 1) ¡¡¡¡¡¡ 35 dB = Total Gain Obviously, the path loss is the same on the return trip. So our received sig- nal level on the access point side is: 35 dB - 113 dB = -78 dB Since the receive sensitivity of the AP is -89dBm, this leaves us 11dB of fade margin (89dB - 78dB). Overall, this link will probably work but could use a bit more gain. By using a 24dBi dish on the client side rather than a 14dBi sec- torial antenna, you will get an additional 10dBi of gain on both directions of the link (remember, antenna gain is reciprocal). A more expensive option would be to use higher power radios on both ends of the link, but note that adding an ampli f er or higher powered card to one end generally does not help the overall quality of the link. Online tools can be used to calculate the link budget. For example, the Green Bay Professional Packet Radio ¡ s Wireless Network Link Analysis ( http://my.athenet.net/~multiplx/cgi-bin/wireless.main.cgi ) is an excellent tool. The Super Edition generates a PDF f le containing the Fresnel zone and radio path graphs. The calculation scripts can even be downloaded from the website and installed locally. The Terabeam website also has excellent calculators available online ( http://www.terabeam.com/support/calculations/index.php ). Chapter 3: Network Design 71 Tables for calculating link budget To calculate the link budget, simply approximate your link distance, then f ll in the following tables: Free Space Path Loss at 2.4 GHz Distance (m) Loss (dB) 100 500 1,000 3,000 5,000 10,000 80 94 100 110 113 120 For more path loss distances, see Appendix C . Antenna Gain: Radio 1 Antenna + Radio 2 Antenna = Total Antenna Gain Losses: Radio 1 + Cable Loss (dB) Radio 2 + Cable Loss (dB) Free Space Path Loss (dB) = Total Loss (dB) Link Budget for Radio 1 ¡ Radio 2: Radio 1 TX Power + Antenna Gain- Total Loss = Signal > Radio 2 Sensitivity 72 Chapter 3: Network Design Link Budget for Radio 2 ¡ Radio 1: Radio 2 TX Power + Antenna Gain- Total Loss = Signal > Radio 1 Sensitivity If the received signal is greater than the minimum received signal strength in both directions of the link, as well as any noise received along the path, then the link is possible....
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Wireless+Networking+in+the+Developing+World_Part5 - Path...

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