Industrial and Internet of Things (IoT) wireless protocols are becoming more prevalent in sites across the globe.  The amount of different wireless protocols sharing the ISM 2.4GHz band is rapidly increasing, making interference avoidance more important. Luckily, most of the protocols utilize the same physical and MAC technologies – 802.15.4 or Bluetooth.

7Signal Spectrum Analyser showing 2.4GHz band saturated

When left unmanaged, the potential exists to negatively impact both corporate and industrial wireless networks. Additionally, there are situations where interferance avoidance is not possible.

This post focuses on 802.15.4 technologies

802.15.4 is a protocol defined by the IEEE standards body.  To have a standard approved by the IEEE, the standards committee must provide a document outlining coexistence assurances for other IEEE protocols in the same band.  802.15.4 only consists of physical and data link layers. 

OSI model vs 802.15.4

It is the basis for many other protocols, such as zigbee, thread, 6LoWPAN, etc.  802.15.4 is often found in the ISM 2.4GHz band because this is the only globally defined band.  Other bands are defined per region.  There are two types of communication used for 802.15.4, beaconless and beacon.

Like 802.11, 802.15.4 uses CSMA/CA for collision avoidance.  When the device wants to transmit data on the medium, it will check the channel for energy, and if the channel is busy the device will enter a back-off algorithm.  There are 5 modes used for clear channel assessment:

Mode 1)           Energy Detect: If the client detects energy above a specified threshold, it will report the channel busy

Mode 2)           Carrier sense only:  The channel is reported busy only when direct-sequence spread spectrum signals are detected.

Mode 3)           Carrier Sense with Energy Detect: if the client detects direct-sequence spread spectrum signal above a defined threshold, the channel is marked busy

Mode 4)           Carrier sense detection with timer: A timer is set, in milliseconds, and the device monitors the medium whilst the timer is counting.  If a high-rate PHY signal is detected, the channel is marked as busy

Mode 5)           A combination of modes 3 and 1

802.15.4 consists of 16 channels in the ISM 2.4GHz band, 2 MHz wide with 1 MHz of spacing.  As seen below, only 4 channels avoid the usual 802.11 channels used in this band.  They are channels 15,20,25,26.

zigbee channels

Within the Coexistence Assurance documents for 802.15.4, it suggests a separation of between 12 and 15M for the transmitters of competing protocols.  These distances were found to produce an acceptable level of frame error rates for each system, improving interference avoidance. 

Additionally to this recommendation, if utilising Direct Sequence Spread Spectrum, only channels 15, 20, 25, 26 should be used.  Where required, additional channels may be used, however depending on the duty cycle of either competing protocols, performance may be degraded.

Want more information about 5G? Read more of our articles by visiting our 5G category. 802.15.4 is a standard developed by the IEEE.

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