Cisco Aironet AIR-ANT3351 Reference Manual page 13

Figure 8 Point-to-Multipoint Wireless Bridge Solution

• Ampliﬁers—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 US. If it has been certiﬁed, then it must be marketed and sold as a complete

system including transmitter, antenna, and coax. It also must be installed exactly this way.

If using a system that includes an ampliﬁer, remember that the above rules concerning power are still in effect. If the ampliﬁer

is 1/2 watt (27dBm) this means in a multipoint system the maximum antenna gain is only 9dBi, and in a point-to-point system,

it is only 15dBi.

ETSI

The European Telecommunication Standardization Institute 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 US.

• Antenna Gain and Power Output—The ETSI regulations specify maximum EIRP as 20dBm. 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 +20dBm. 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 are typically not permitted in any ETSI system.

Frequencies and Channels Sets

IEEE 802.11b Direct Sequence Channels

Fourteen channels are deﬁned in the IEEE 802.11b Direct Sequence (DS) channel set. Each DS 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 DS system (11 usable in the US), only three non-overlapping (and hence,

non-interfering) 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 multi-access points environment such as an ofﬁce or campus.

Access points are usually deployed in "cellular" fashion within an enterprise where adjacent access points are allocated

non-overlapping 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.

Cisco Systems, Inc.

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