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RF Bands
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Wireless communications can use various RF bands Some of these are licensed (you have to pay a governmental agency to use them) and some are unlicensed For example, radio broadcasts, such as AM and FM, are licensed through your country s government Other bands, such as the 80211 bands, are unlicensed, so you don t have to buy a license from the government to use these frequencies However, unlicensed bands are still regulated by governments, which might define restrictions in their usage WLANs use three unlicensed bands:
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900 MHz 24 GHz 5 GHz
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Used by older cordless phones
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Used by newer cordless phones, WLANs, Bluetooth, microwaves, and other devices Used by the newest models of cordless phones and WLAN devices
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5: Wireless
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A hertz (Hz) is a unit of frequency that measures the change in a state or cycle in a wave (sound or radio) or alternating current (electricity) during 1 second A megahertz (MHz) is 1 million cycles per second and a gigahertz (GHz) is 1 billion cycles per second The word hertz is from Heinrich Hertz, a German physicist who first discovered that you could send and receive waves through the air The 900 MHz and 24 GHz frequencies are commonly referred to as the Industrial, Scientific, and Medical (ISM) bands and the 5 GHz frequency the Unlicensed National Information Infrastructure (UNII) band
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The advantage of using unlicensed bands for WLANs is that you, as a company, don t have to pay a license fee to the government to use these frequencies However, their main
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drawback is that many types of devices use them, such as cordless phones, microwave ovens, and Bluetooth devices, which can create interference for WLAN devices using the same RF band
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Wireless is becoming very popular in today s LANs, since little cabling is required Four basic standards are currently in use: 80211a, 80211b, 80211g, and 80211n, as shown in Table 5-1
Be familiar with the contents of Table 5-1, especially the data rates: 80211b 11 Mbps; 80211a 54
Mbps; and 80211g 54 Mbps Remember that 80211b and 80211g support DSSS and 80211a and 80211g support OFDM
The speeds listed in Table 5-1 are optimal speeds based on the specifications the actual speeds that you might achieve in a real network vary according to the number of devices you have, the distance that they are from an access point, and any physical obstructions or interference that might exist
WLAN Standards
TABLE 5-1
WLAN Standards
80211a
Data Rate Throughput Frequency Compatibility Range (meters) Number of Channels Transmission 54 Mbps 23 Mbps 5 GHz None 35 120 3 OFDM
80211b
11 Mbps 43 Mbps 24 GHz With 80211g and the original 80211 38 140 Up to 23 DSSS
80211g
54 Mbps 19 Mbps 24 GHz With 80211b 38 140 3 DSSS/OFDM
80211n
248 Mbps (with 2 2 antennas) 74 Mbps 24 and/or 5 GHz 80211a, b, and g 70 250 14 MIMO
Direct Sequence Spread Spectrum (DSSS) uses one channel to send data across all frequencies within that channel Complementary Code Keying (CCK) is a method for encoding transmissions for higher data rates, such as 55 and 11 Mbps, but it still allows backward compatibility with the original 80211 standard, which supports only 1 and 2 Mbps speeds 80211b and 80211g support this transmission method OFDM (Orthogonal Frequency Division Multiplexing) increases data rates by using a spread spectrum: modulation 80211a and 80211g support this transmission method 80211n uses MIMO (Multiple Input Multiple Output) transmission, which uses DSSS and/or OFDM by spreading its signal across 14 overlapping channels at 5 MHz intervals Use of 80211n requires multiple antennas Of the four IEEE 80211 standards, 80211b has been deployed the most, with 80211g being the most common one sold today 80211n is very new and not commonly found in company networks, but is slowing making its way into SOHO networks; its main drawback is that 80211n hasn t been ratified as a standard yet it s still in a draft state One advantage that 80211b and 80211g devices have over 80211a is that 80211b and 80211g can interoperate, which makes migrating from an all-80211b network to an 80211g network an easy and painless process Note that 80211g devices are compatible with 80211b devices (but not vice versa) and 80211a devices are not compatible with the 80211b and 80211g standards 80211n is backward compatible with all three standards; however, when running it in compatibility mode, you will not reach its maximum data rate or throughput 80211n is currently in a draft state and should be ratified as a standard in 2008 or 2009; therefore, you might experience compatibility issues between different vendors if using 80211n equipment
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