Tutorial: Wireless Ethernet transmission options


The most common approach to wireless Ethernet is radio frequency (RF) transmission in the spread spectrum bands. Globally, the 2.4 GHz and 5.8 GHz bands are available for license-free use in most countries. Wireless Ethernet transmission options include direct sequence and frequency hopping methods. The difference between these has implications for various applications, as explained by Jim Ralston of ProSoft Technology Inc .

Spread spectrum literally means spreading the RF energy across the entire (or a wide portion of the) available spectrum. This technique permits relatively high-speed communications while improving operation in noisy environments where multiple RF systems are present. The two major methods of spreading RF energy are direct sequence and frequency hopping. Both have advantages and disadvantages for industrial wireless communications.

Direct sequence uses a wide channel within the band to simultaneously modulate a highly encoded bit pattern. Direct sequence offers the fastest spread spectrum data rates, as the wide channel permits transmission of complex modulation schemes. Orthogonal frequency domain modulation (OFDM) is a complex modulation technique capable of fast data rates and is used extensively in the IEEE 802.11g standard, supporting RF data rates up to 54 Mbps.

Wi-Fi uses direct sequence
Direct sequence is the method used by all popular open Wi-Fi standards today including IEEE 802.11b, 802.11g (both transmitting in the 2.4 GHz band) and 802.11a (transmitting in the 5.8 GHz band). While the wide band modulation offers high speed, it also makes the RF system more prone to noise problems when multiple systems are operating in close proximity. For example, IEEE 802.11b has 13 available channels (only 11 channels in some countries), but only three channels don't overlap.

Due to overlapping channels and the popularity of Wi-Fi systems in plants, band overcrowding and RF saturation can lead to poor wireless performance.
Frequency hopping is a very popular technique for industrial systems because it has outstanding noise immunity techniques. Unlike direct sequence, frequency hopping uses many smaller channels in the spectrum and rapidly changes channels or "hops around" from channel to channel.

By incorporating error correction techniques, frequency hopping offers the best chance for successful data transmission, because the transmitter will send the packet over and over again using different channels until an acknowledgement is received.

Frequecy hopping reliability
The disadvantage of frequency hopping is that it is slower than direct sequence and has longer data latency. Most frequency hopping systems are limited to 1 Mbps or less RF data rate. But if the data rate is fast enough for the application, the reliability of frequency hopping is tough to beat, especially if more RF systems will be added in the future.

Frequency hopping modems are proprietary, meaning that each manufacturer uses its own technique and vendor X will usually not communicate with vendor Y. While this is potentially a disadvantage for commercial systems, it can be desirable for industrial systems for two reasons: security and isolation from the wireless IT system.

Because the frequency hopping technique is not based upon an open standard, the manufacturer can use unique authentication processes and sophisticated encryption techniques. While security has significantly improved in Wi-Fi systems with WPA and WPA2 standards, hackers will continue to look for holes. Many industrial Wi-Fi manufacturers now include an option to hide the access point by not transmitting its SSID beacon. This technique is effective at hiding the access point from potential hackers.

Frequency hopping also offers plant managers the ability to operate their own wireless network separate from the IT department. Because of the popularity of 802.11 technologies for wireless network access, warehouse barcode systems and video surveillance, proprietary frequency hopping systems may be the best choice for industrial systems.

—Jim Ralston, with more than 10 years' experience with industrial wireless systems,
is currently a wireless sales engineer for ProSoft Technology Inc. Reach him at jralston@psft.com .

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