Information Technology Reference
In-Depth Information
The third mode is the
Extended Service Set (ESS)
. As shown in Figure 4-15, ESS is a set of BSS
where APs have connectivity in the WLAN, which provides a distribution system for roaming
capabilities.
Wireless ESS Mode
Figure 4-15
Wired LAN
Access point
Access point
AP
AP
WLAN clients
WLAN clients
Frequencies Used by WLANs
The IEEE 802.11b standard uses the 2.4 GHz band of the Industrial, Scientific, and Medical
(ISM) frequencies. The Federal Communication Commission (FCC) authorizes ISM
frequencies for unlicensed use in the United States. The three ISM frequency bands are as
follows:
•
902 to 928 MHz
•
2.4000 to 2.5000 GHz
•
5.725 to 5.875 GHz
IEEE 802.11, 802.11b, and 802.11g standards all use the 2.4 ISM band.
The IEEE 802.11a standard uses the 5 GHz bands of the Unlicensed National Information
Infrastructure (UNII) frequencies. The three UNII bands are as follows:
•
5.15 to 5.25 GHz (lower band)
•
5.25 to 5.35 GHz (middle band)
•
5.75 to 5.85 GHz (upper band)
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