802.11 Co-existence: 802.15.4 (Zigbee, 6LowPAN, Wireless Hart, MiWi, ISA 100.11a)
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. 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 the other frequencies the protocol operates in are not available globally. There are two types of communication used for 802.15.4, beaconless, and beacon.
Similar to 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.
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. Additionally to this recommendation, 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.