Wireless LAN – WLAN
BAT54-Rail/F..
Release
7.54
06/08
3.1
What is a Wireless LAN?
23
DSSS
For shielding against interferences by other transmitters, which have possi-
bly the same frequency band, the DSSS procedure (Direct Sequence Spread
Spectrum) is used for IEEE 802.11b in the 2,4 GHz frequency band. A trans-
mitter normally uses only a very narrow range of the available frequency
band for transmission. If exactly this range is used by another transmitter, in-
terferences in transmission would be the result. With the DSSS procedure
the transmitter uses a broader spread of the possible frequencies and be-
comes more insensitive to narrow-band disturbances then. This procedure is
also used in military range for increasing tap-proof security.
U IEEE 802.11g: 54 Mbps
The IEEE 802.11g standard works likewise with up to 54 Mbps data trans-
mission rate in the 2,4 GHz ISM-frequency band. Contrary to IEEE 802.11b,
the OFDM modulation is used for IEEE 802.11g, like already introduced for
IEEE 802.11a. IEEE 802.11g contains a special compatibility mode that en-
sures a downward compatibility to the popular IEEE 802.11b standard . How-
ever, in this compatibility mode you encounter reduced transmission speeds.
Due to the different frequency bands, IEEE 802.11g can not be compatible
to IEEE 802.11a. The transmission distances of IEEE 802.11g products are
comparable with those of IEEE 802.11b products.
Turbo mode
With the 802.11g standard in 'turbo mode' the transfer rate can be increased
to a maximum of 108 Mbps, by using two radio channels. But as a 2.4 GHz
band uses less channels than the 5 GHz band, the turbo mode limits in this
case the options of channels.
U Transfer rates
The indicated transfer rates are always to be interpreted as gross data rates,
i.e. the entire protocol overhead - as for example the complex protocols to se-
cure the radio transmission - is included in the indicated transfer rates. The
net data transfer rate can be thus lower than the indicated gross data rates,
typically over up to the half for all IEEE 802.11 standards mentioned above.
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