Apprion discusses 802.11/802.15.4 coexistence

Larry Allhands, Apprion wireless advisor, discusses the coexistence of 802.11 and 802.15.4 networks in an industrial environment.

Interest in the application of wireless technologies in industrial settings has exploded over the last decade.

More specifically, technologies employing a range of wireless devices utilising the industrial, scientific and medical (ISM) bands that are unlicensed and open to a heterogeneous range of network technologies are growing at an unprecedented rate.

Of these growing technologies, this discussion will focus on the coexistence between two platforms that operate in the 2.4GHz ISM band; IEEE 802.15.4 Low-Rate Wireless Personal Area Network (LR-WPAN) and IEEE 802.11b Wireless Local Area Network (WLAN).

The 802.11 WLAN standard uses the 2.4GHz ISM band, operating in the US under Part 15 of the US Federal Communications Commission Rules and Regulations.

802.11 controls interference and susceptibility to interference by using spread-spectrum modulation.

802.11b and 802.11g use the direct-sequence spread-spectrum signaling (DSSS) and orthogonal-frequency division-multiplexing (OFDM) methods, respectively.

802.11a uses the 5GHz U-NII band, which, for much of the world, offers at least 19 non-overlapping channels rather than the three offered in the 2.4GHz ISM frequency band.

802.11 radios typically operate at relatively higher transmit powers, having an operational radio range of 100m.

The IEEE standard 802.15.4 offers the fundamental lower network layers of a type of LR-WPAN in the 2.4GHz ISM band focusing on low-cost, low-speed ubiquitous communication between devices.

The emphasis is on very low-cost communication of nearby devices with little to no underlying infrastructure, low data rate but very long battery life and very low complexity.

The basic framework for 802.15.4 radio systems operates at relatively lower powers, offering a 10m operational radio range with a data-transfer rate of 250kb/sec.

802.11 WLAN radios use a more powerful, longer-range radio than 802.15.4 LR-WPAN systems.

Since the 802.11 has radio range of 100m and 802.15.4 has radio range of 10m, the 802.11 has the potential of creating significant radio interference to an 802.15.4 system over a large area and from a long distance.

Therefore, large-scale 802.15.4-based sensor-network systems are potentially vulnerable to the interference from 802.11 networks.

There are many situations where 802.15.4 and 802.11 need to operate in the same location.

For example, a tank farm utilising 802.15.4 level sensors may reside next to a unit utilising 802.11 for mobile-operator applications.

In this scenario, the main concern regarding coexistence of 802.15.4 and 802.11 is the performance degradation of 802.15.4 caused by the interference of the adjacent 802.11 network.

For the purpose of this discussion, coexistence is defined as 'the ability of one system to perform a task in a given shared RF environment where other systems may or may not be using the same set of rules'.

There is reasonable concern regarding problems of crowding in the 2.4GHz ISM band, as the technologies using this spectrum become more ubiquitous.

However, the IEEE 802.15.4 standard has been designed from the ground up with coexistence in mind.

Coexistence is a top priority issue in system design for mission-critical applications using wireless systems.

For example, if an 802.15.4 sensor-network system is to be deployed in an industrial-production environment, a main design issue will be providing the coexistence of 802.15.4 and other wireless systems in the same frequency band.

If adjacent systems cause significant radio-channel interference in the 2.4GHz spectrum, the 802.15.4 sensor network system may not continue normal operational monitoring, potentially resulting in the loss of critical information such as level, vibration and temperature.

Many of the applications for 802.15.4 devices require a very low data rate.

Designers of RF systems for low-data-rate applications have traditionally exploited this fact by building transmitters with data rates as low as 9.6Kbps.

The designers of the IEEE 802.15.4 standard however, have chosen the relatively high data rate of 250Kbps.

The reasoning here is that one of the best ways to promote coexistence is to reduce channel occupancy and a radio with a higher data rate will occupy the channel far less and offer fewer opportunities for collision with other users than one with a low data rate.

IN DSSS, the term 'spread spectrum' refers to a modulation technique designed to promote coexistence and robustness in the face of RF interference.

The 802.15.4 signal is spread over a large signal bandwidth, allowing it to coexist with narrow-band signals, which generally appear to the spread-spectrum receiver as a slight reduction in the signal-to-noise ratio.

This technology is also utilised by the 802.11 WLAN standard and some mobile-phone platforms, effectively allowing sharing of a single channel by multiple users, resistance to intended or unintended jamming, reduced signal/background noise and determination of relative timing between transmitter and receiver.

In addition to DSSS, the 802.15.4 standard utilises a technique known as frequency-division multiple-access (FDMA), which divides the 2.4GHz ISM spectrum into sixteen distinct channels with two of these channels, 15 and 20, falling between the standard 802.11 channels 1, 6, and 11.

This technique allows experienced wireless engineers to design the seamless coexistence of IEEE 802.11 and IEEE 802.15.4 devices operating in adjacent channels.

The 802.15.4 standard makes use of Carrier Sense Multiple Access (CSMA) technology to listen to see if a channel is busy before transmitting.

CSMA has been used successfully in Ethernet networks for years and is very effective in regulating traffic in shared channels.

Devices under the 802.15.4 standard must acknowledge the receipt of a transmission within a specified time frame.

The transmitting device will wait to hear this acknowledgement and re-transmit the message if it does not receive an acknowledgement within the specified time frame.

Advancements in wireless technology have overcome many concerns regarding the implementation of wireless applications in process operations.

In particular, features of the IEEE wireless architecture and system design can effectively reduce or eliminate potential coexistence problems between 802.11 and 802.15.4 technologies.

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