Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 122 RF Fresnel Zones Specifically, line-of-sight must be maintained within a signal’s ellipticalshaped Fresnel zone. If more than 40% of the lower radius of the Fresnel zone is obstructed, the signal will be negatively impacted from diffraction. Thus, it is imperative to maintain a minimum of 60% clearance in this radius. Calculating the radius of a Fresnel zone requires a complex formula, which is beyond the scope of this guide. Various factors must be accounted for, such as atmospheric refraction, the curvature of the earth, frequency, and the relative heights of the two antennas. A free Fresnel zone calculator is available online at: http://www.afar.net/fresnel-zone-calculator/ (Reference: CCNP BCMSN Official Exam Certification Guide 4 th Edition. David Hucaby. Pages 450-452) * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 123 802.11 Channels Recall that all amendments to the 802.11 standard operate in one of two frequency bands: • 2.4-GHz band (specifically, 2.4000 to 2.4835GHz) • 5.0-GHz band (specifically, 5.150 to 5.825GHz) The 2.4 GHz band supports a total of 14 channels, though the FCC limits this to 11 channels in the United States. The center frequency of each channel is separated by only 5 MHz. Channel # Center Frequency Channel # Center Frequency 1 2.412 MHz 8 2.447 MHz 2 2.417 MHz 9 2.452 MHz 3 2.422 MHz 10 2.457 MHz 4 2.427 MHz 11 2.462 MHz 5 2.432 MHz 12* 2.467 MHz 6 2.437 MHz 13* 2.472 MHz 7 2.442 MHz 14* 2.484 MHz * Restricted in US The 802.11 amendments that use the 2.4-GHz band (specifically, 802.11b and 802.11g) require a 22 MHz range to modulate the signal. Thus, with each channel’s center frequency separated by only 5 MHz, channel overlap will occur. In fact, the 2.4-GHz band supports only three non-overlapping channels. Specifically, these are channels 1, 6, and 11. Devices competing on the same or adjacent channels will interfere with each other, degrading performance and reliability. The less-often used 5-GHz band supports up to 12 non-overlapping channels (in the U.S.), and is further separated into three sub-bands (with four channels each). The lower and middle bands are dedicated for indoor use, and the higher band is dedicated for outdoor use. Remember that the 2.4-GHz band is unregulated, and the 5.0-GHz band is regulated. (Reference: http://en.wikipedia.org/wiki/List_of_WLAN_channels; http://www.cisco.com/en/US/docs/wireless/access_point/1200/vxworks/configuration/guide/bkscgaxa.html) * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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