User Manual

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Understanding Industrial WLAN – IEEE 802.11
Components of the Expanded 802.11 Wireless System
Expanded 802.11 wireless systems consist of the following components, some of which are optional.
Access Points
Moxa supplies 802.11a/b/g/n (802.11n will be implemented in the future, the technology works by using
multiple antennas to target one or more sources to increase transmission power and throughput) wireless
AP/Bridge/Client devices to extend the wireless range.
IEEE 802.11a is a modified version of the IEEE 802.11 standard and was approved in 1999. IEEE 802.11a
adopts the same standards as IEEE 802.11 and operates in the 5 GHz band. It uses 52 Orthogonal
Frequency Division Multiplexing (OFDM) waves and has a maximum capacity of 54 Mbps. This has already
satisfied the standard requirement of network communication which needs around 20 Mbps of bandwidth.
It is also possible to drop the communication speed to 48, 36, 24, 18, 12, 9 or even 6 Mbps. IEEE 802.11a
has 12 parallel channels, among them 8 of which are used for indoor communications and 4 for point-to-
point communications. IEEE 802.11b is not inter-operable with IEEE 802.11a unless the communication
devices support both standards. IEEE 802.11a has the advantage of less interference than IEEE 802.11b
as IEEE 802.11b’s 2.4 GHz band is widely used. However, the high frequency also has some downsides.
IEEE 802.11a has a much narrower coverage, so it needs more access points. This also means that signals
can not be transmitted as far as IEEE 802.11b because it is much easier for signals to be absorbed by
surrounding objects.
Parameter Tuning
Wireless devices have traditionally been limited in range due to the inherent design of the 802.11 standard.
802.11 protocol uses acknowledge for each received frame. If an acknowledgement is not received, the
frame is re-transmitted. By default the maximum distance between transmitter and receiver is 1 mile (1.6
km). On longer distances the delay will force retransmissions so Moxa has tuned our Wireless product to
support long-range deployments using wireless 802.11.
Moxa Wireless Products are now enhanced with the ability to automatically adjust parameters such as slot
time, ACK time-out, and CTS time-out to fine tune the wireless device for optimal performance and achieve
a longer range.
Environmental Conditions:
Two factors are considered as below:
• 2.4GHz interference: There are literally hundreds of other sources of interference that aggregate into
a formidable obstacle to enabling long range use in occupied areas: microwave ovens, baby monitors,
wireless cameras, remote car starters, wireless phones, and Bluetooth products.
• Landscape interface: Obstacles are among the biggest problems when setting up a long-range wireless
application. Trees and forests degrade the microwave signal, and rolling hills make it difficult to establish
line-of-sight propagation. In a city, buildings will impact integrity, speed and connectivity. Steel frames
partly reflect radio signals, and concrete or plaster walls absorb microwave signals significantly, but sheet
metal in walls or roofs may efficiently reflect wireless signals, causing an almost total loss of signal.
Power Amplifier
Moxa supplies RF devices with 63/200/800 mW and boosters to extend your wireless range.
For example, if you have a 18 dBm (63 mW) device and replace Moxa’s 200 mW RF device, you can
increase 18 dBm (63 mW) to 23 dBm (200 mW); if you replace Moxa’s 800 mW RF device, you can increase
18 dBm (63 mW) to 29 dBm (800 mW). Based on our experience, you can increase the range by using
Moxa’s RF devices or boosters.