User Manual

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Understanding Industrial WLAN – IEEE 802.11
ISM and Licensed Band
The FCC (Federal Communications Commission) regulates the usable frequency bands and the maximum
allowable power in these frequency bands for the United States. WLAN devices are allowed to use the ISM
(Industrial/Scientific/Medical) band by the FCC. The ISM band consists of 3 different sub-bands: 902 MHz, 2.4
GHz and 5.8 GHz. The FCC has also further defined the UNII (Unlicensed National Information Infrastructure)
band for WLAN usage. The following diagram shows the spectrum overview of the ISM and UNII bands.
Advantages and Disadvantages of Using Unlicensed Bands
ISM and UNII are both un-licensed bands which means anyone can transmit in these bands without a
license from the FCC. It is the opening of these un-licensed bands that has allowed the WLAN business to
grow in small businesses and homes. The freedom of these license-free bands also means a great number
of un-licensed users may share the bandwidth with you.
Our discussion only includes the 2.4 GHz ISM band and 5 GHz UNII band because these 2 frequency bands
are the most commonly used in WLAN applications.
802.11g Data Rate
(Mbps)
Transmission
Type
Modulation
Scheme
54 OFDM 64 QAM
48 OFDM 64 QAM
36 OFDM 16 QAM
24 OFDM 16 QAM
18 OFDM QPSK1
12 OFDM *
a
QPSK
11 DSSS CCK2
9 OFDM BPSK3
6 OFDM BPSK
5.5 DSSS *
b
CCK
2 DSSS QPSK
1 DSSS *
c
BPSK
*
a
QPSK: Quadrature Phase Shift Keying
*
b
CCK: Complementary Code Keying
*
c
BPSK: Bi-phase Shift Keying
Figure: ISM and UMI Bands
Lastly, let’s use the 802.11g standard as an example for how the transmission type and modulation scheme
corresponds to each data rate: