Cisco Aironet 1520 Series Manual de usuario descargar pdf (Pagina 12)

At 2.4 GHz, the maximum transmitter power is also 1 watt. Using this maximum power, the maximum antenna gain

is 6 dBi. However, the regulations also define the maximum values in regards to the following two different system

scenarios.

Point-to-Point and Point-to-Multipoint Systems

In point-to-multipoint systems, the FCC has limited the maximum EIRP to 36 dBm. EIRP = TX power + antenna

gain. For every dB that the transmitter power is reduced, the antenna may be increased by 1 dB. Thus, 29 dBm

TX, +7 dB antenna = 36 dBm EIRP; 28 dBm TX +8 dB antenna = 36 dBm EIRP.

The Cisco Aironet 2.4-GHz bridge transmitter power is 20 dBm, which is 10 dBm lower than maximum. This then

allows the use of antennas up to 10 dB over the initial 6 dBi limit, or 16 dBi.

In point-to-point systems for 2.4-GHz systems using directional antennas, the rules have changed. Because a

high-gain antenna has a narrow beamwidth, the likelihood is great that it will cause interference to other area

users. Under the rule change, for every dB the transmitter is reduced below 30 dBm, the antenna may be

increased from the initial 6 dBi, by 3 dB. Thus, a 29-dB transmitter means 9-dBi antenna; a 28-dB transmitter

means 12-dBi antenna. Because we are operating at 20 dBm, which is 10 dB below the 30 dBm level, we can

increase the antenna gain by 30 dB. Note that Cisco has never tested, and therefore has not certified, any

antenna with gain greater than 21 dBi.

The main issue that comes up here is: What differentiates a point-to-point from a multipoint system.

In Figure 7, point A communicates to a single point (point B), and point B communicates to a single point A;

therefore, it is simple to see that both locations see this as a point-to-point installation.

In Figure 8, point A communicates to more than one (or multiple) points; therefore, point A is operating in a

multipoint configuration, and the largest antenna permitted is 16 dBi. Point B or point C can each communicate to

only one point (point A); therefore, point B and point C actually operate in a single-point or point-to-point operation,

and a larger antenna may be used.

Figure 7. Point-to-Point Wireless Bridge Solution

Figure 8. Point-to-Multipoint Wireless Bridge Solution

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Cisco Aironet 1520 Series Manual de usuario Pagina 12