Cisco Aironet 1520 Series Manual do Utilizador Página 13

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© 2011 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information. Page 13 of 73
Amplifiers
In the FCC rules, Section 15.204-Part C states: “External radio frequency power amplifiers shall not be marketed
as separate products.” Part D states: “Only the antenna with which an intentional radiator (transmitter) is originally
authorized may be used with the intentional radiator." This means that unless the amplifier manufacturer submits
the amplifier for testing with the radio and antenna, it cannot be sold in the United States. If it has been certified, it
must be marketed and sold as a complete system, including transmitter, antenna, and coaxial cable. It also must
be installed exactly this way.
If you are using a system that includes an amplifier, remember that these rules concerning power are still in effect.
If the amplifier is one-half (.5) watt (27 dBm), this means in a multipoint system, the maximum antenna gain is only
9 dBi, and in a point-to-point system it is only 15 dBi.
ETSI
The European Telecommunication Standardization Institute (ETSI) has developed standards that have been
adopted by many European countries as well as many others. Under the ETSI regulations, the power output and
EIRP regulations are much different than in the United States.
Antenna Gain and Power Output
The ETSI regulations specify maximum EIRP as 20 dBm. Since this includes antenna gain, this limits
the antennas that can be used with a transmitter. To use a larger antenna, the transmitter power must be reduced
so that the overall gain of the transmitter, plus the antenna gain, less any losses in coax, is equal to or less than
+20 dBm. This drastically reduces the overall distance an outdoor link can operate.
Amplifiers
Since the ETSI regulation has such a low EIRP, the use of amplifiers is typically not permitted in any ETSI system.
Frequencies and Channel Sets
IEEE 802.11b/g Direct Sequence Channels
Fourteen channels are defined in the IEEE 802.11b/g direct-sequence channel set. Each direct-sequence channel
as transmitted is 22 MHz wide; however, the channel center separation is only 5 MHz. This leads to channel
overlap such that signals from neighboring channels can interfere with each other. In a 14-channel direct-
sequence system (11 usable in the United States), only three nonoverlapping (and hence, noninterfering)
channels, 25 MHz apart, are possible (for example, channels 1, 6, and 11).
This channel spacing governs the use and allocation of channels in a multiple-access-point environment such as
an office or campus. Access points are usually deployed in “cellular” fashion within an enterprise, where adjacent
access points are allocated nonoverlapping channels. Alternatively, access points can be collocated using
channels 1, 6, and 11 to deliver 33 Mbps bandwidth to a single area (but only 11 Mbps to a single client).
The channel allocation scheme is illustrated in Figure 9, and the available channels in the different regulatory
domains are defined in Table 4.
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