Phoenix Contact FL WLAN 24 AP 802-11, Руководство пользователя

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Functions and wireless technology
7190_en_02 PHOENIX CONTACT
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1.3.4 Modulation method
1.3.4.1 Direct Sequence Spread Spectrum (DSSS)
The DSSS method transmits information on a 22 MHz channel. The transmitter adds a
pseudostatic sequence of eleven characters (IEEE 802.11) or eight characters
(IEEE 802.11b) to each data bit. This means that the transmitted data flow contains
additional information, which enables the detection and correction of errors.
The relationship between the spread bandwidth and the transmission speed is called the
spreading ratio. If this ratio is 10, the conditions for transmission are ideal. In practice, this
ratio occurs if, for example, a 2 Mbps signal is spread over a bandwidth of 20 MHz. The
spreading gain, i.e., the ratio of the data flow clock rate to the code sequence clock rate, is
also an indication of the interference resistance for the transmitted signal.
Standard IEEE 802.11b (1999) only offered the DSSS method at a transmission speed of
11 Mbps. This was achieved through the use of the 22 MHz channel, but can fall as low as
1 Mbps in unfavorable environmental conditions.
Spreading the signal can reduce the signal level to such an extent that the transmission level
is less than the background noise. It is only by reversing the spread in the receiver that the
signal can be distinguished from the background noise again. The DSSS method therefore
offers increased immunity to interference and supports higher data throughput.
The standard recommendation is that only channels 1 | 6 | 11 are used, although in Europe
channels 1 | 7 | 13 or 2 | 7 | 12 or even 3 | 8 | 13 can be used in parallel without adverse effects.
1.3.4.2 Possible channel assignment with several access points
When the correct channels are selected, several access points can be operated in parallel.
If more than these three interference-free channels are required, channels 4 and 10 can
also be used in a full coverage wireless field, taking the local situation into account.
Figure 1-9 Non-overlapping channel selection
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