Cisco Aironet Antennas and Accessories
This antenna reference guide explains issues and concerns about antennas used with a Cisco Aironet wireless LAN system or wireless bridge system. It details deployment and design, limitations and capabilities, and basic theories of antennas. This document also contains information about the Cisco antennas andaccessories, as well as installation scenarios, regulatory information, and technical specifications and diagrams of the available antennas.
Overview of Antennas
Each Cisco Aironet radio product is designed to perform in a variety of environments. Implementing the antenna system can greatly improve coverage and performance. To optimize the overall performance of a Cisco wireless LAN, itis important to understand how to maximize radio coverage with the appropriate antenna selection and placement. An antenna system comprises numerous components, including the antenna, mounting hardware, connectors, antenna cabling, and in some cases, a lightning arrestor. For a consultation, please contact a Cisco Aironet partner at:http://tools.cisco.com/WWChannels/LOCATR/jsp/partner_locator.jsp. Cisco partners can provide onsite engineering assistance for complex requirements.
In the mid-1980s, the U.S. Federal Communications Commission (FCC) modified Part 15 of the radio spectrum regulation, which governs unlicensed devices. The modification authorized wireless network products to operate in the industrial, scientific, and medical (ISM) bands using spreadspectrum modulation. This type of modulation had formerly been classified and permitted only in military products. The ISM frequencies are in three different bands, located at 900 MHz, 2.4 GHz, and 5 GHz. This document covers both the 2.4- and 5-GHz bands. The ISM bands typically allow users to operate wireless products without requiring specific licenses, but this will vary in some countries. In theUnited States, there is no requirement for FCC licenses. The products themselves must meet certain requirements to be certified for sale, such as operation under 1-watt transmitter output power (in the United States) and maximum antenna gain or effective isotropic radiated power (EIRP) ratings. The Cisco Aironet product lines utilize both the 2.4- and 5-GHz bands. In the United States, three bandsare defined as unlicensed and known as the ISM bands. The ISM bands are as follows:
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900 MHz (902-928 MHz) 2.4 GHz (2.4-2.4835 GHz) - IEEE 802.11b 5 GHz (5.15-5.35 and 5.725-5.825 GHz) - IEEE 802.11a, HIPERLAN/1 and HIPERLAN/2. This band is also known as the UNII band, and has three subbands: UNII1 (5.150-5.250 GHz), UNII2 (5.250-5.350 GHz), and UNII3 (5.725-5.825 GHz).
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Each set of bands has different characteristics. The lower frequencies exhibit better range, but with limited bandwidth and hence lower data rates. The higher frequencies have less range and are subject to greater attenuation from solid objects.
802.11 Modulation Techniques
The IEEE 802.11standard makes provisions for the use of several different modulation techniques to encode the transmitted data onto the RF signal. These modulation techniques are used to enhance the probability of the receiver correctly receiving the data and thus reducing the need for retransmissions. The techniques vary in their complexities and robustness to RF signal propogation impairments. Direct-SequenceSpread Spectrum The direct-sequence spread spectrum (DSSS) approach involves encoding redundant information into the RF signal. Every data bit is expanded to a string of chips called a chipping sequence or Barker sequence. The chipping rate, as mandated by the U.S. FCC, is 10 chips at the 1- and 2-Mbps rates and 8 chips at the 11-Mbps rate. So, at 11 Mbps, 8 bits are transmitted for every one bit...