User Manual

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2009 Industrial Wireless Guidebook
Understanding Industrial WLAN – IEEE 802.11
2
DSSS (Direct Sequence Spread Spectrum)
DSSS divides a stream of information to be transmitted into small pieces, each of which is allocated to
a frequency channel across the spectrum. DSSS generates a redundant bit pattern for each bit to be
transmitted. This bit pattern is called a chip (or chipping code). Even if one or more bits in the chip are
damaged during transmission, statistical techniques embedded in the radio can recover the original data
without the need for retransmission. Direct sequence spread spectrum is also known as direct sequence
code division multiple access (DS-CDMA). This modulation technique is officially accepted and used by the
IEEE 802.11b and IEEE 802.11g standards.
Signal Level
2400 2412 2417 2422 2427 2432 2437 2442 2447 2452 2457 2462 2467 2472 2477
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Frequency
(MHz)
Channel 2 Channel 6 Channel 10
OFDM (Orthogonal Frequency Division Multiplexing)
OFDM is a modulation scheme that divides a single digital signal across 1,000 or more signal carriers
simultaneously. The signals are sent at right angles (orthogonal) to each other so they do not interfere with
each other. OFDM has the ability to overcome multi-path effects by using multiple carriers to transmit the
same signal. OFDM is commonly used in IEEE 802.11a and 802.11g standards. Non/near line-of-sight
associations can be achieved using the OFDM technique.
The following table summarizes the modulation techniques:
Modulation Technique DHSS FHSS OFDM
Narrowband
Interference
Less resistance
(22 MHz wide contiguous
bands)
More resistance
(79 MHz wide contiguous
bands)
Much less
(multicarrier
modulation)
Interference
susceptibility
Medium High Low
Collocation Less More
Uses several parallel
sub-carriers
Compatibility 802.11b (WiFi Alliance) None 802.11a, 802.11g
Implementation Cost Comparatively Less Comparatively more High
Throughput 5 – 6 Mbps 2 Mbps for 802.11 25 Mbps