WLAN Interference Raises Doubts about ZigBee, IEEE 802.15.4 Products
April 6, 2007 // Published as a news service by IHS
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Electronics & Telecom Standards Collections IEEE Standards News Electronics & Telecom StandardsThe findings, which were published in a white paper, showed substantial and frequently debilitating levels of signal interference between devices based on so-called "15.4" and typical residential WLAN technologies such as IEEE 802.11 (Wi-Fi) and all its variants.
The measurements demonstrated that control products using IEEE 802.15.4 technology are seriously compromised and often inoperable even within the most basic residential WLAN, according to the white paper.
Triggered in part by earlier reports of interference problems, comprehensive simulations of WLAN interference against ZigBee and other IEEE 802.15.4 based devices were conducted by employees of IEEE 802.15.4 and ZigBee chip vendors last year and published as part of the IEEE 802.15.4-2006 version of the specification.
Despite clear evidence of harmful interference levels, these findings went largely unnoticed, overshadowed by the marketing efforts of the ZigBee community, according to the paper.
By contrast, WLAN interference tests in Europe were conducted by an independent original equipment manufacturer (OEM) under conditions that closely resemble typical European and U.S. residential and commercial environments. None of the measurements and simulations quoted in the new report came from a ZigBee competitor.
The results confirmed existing independent industry observations, which had previously concluded that common WLAN environments based on IEEE 802.11 unavoidably cause significant and even debilitating interference with IEEE 802.15.4 devices, according to the paper.
Previous test results shown by the ZigBee community summarized IEEE 802.15.4 as an operable technology within a residential or light commercial environment where WLAN traffic is also present. However, those tests were conducted under overly unrealistic conditions, according to the paper.
For example, some tests measured the ZigBee link over a short distance of only 2-3 feet, while placing the interfering WLAN some 20 to 50 feet away; hardly characteristic of a typical home setup. Other tests operated the WLAN only at unrealistically low network loads not taking into account typical home activities such as web surfing, Voice over Internet Protocol (VoIP) and e-mail, nor to mention video and audio streaming.
Moreover, the earlier tests did not take other important real-world considerations into account such as the dramatic degradation of command response time that IEEE 802.15.4 devices would display in a typical WLAN environment and the effects of battery life shortened from multiple years down to weeks, said the paper.
In the study, four market-leading IEEE 802.15.4 products were tested within a typical single-story residence with carpeting and a combination of brick and sheetrock walls. Other available IEEE 802.15.4 devices were preemptively rejected as test subjects because even without WLAN interference, their radio frequency (RF) performance was judged to be far below the expected performance quality of such devices.
The WLAN interferer was operated at varying distances to the victim receiver so as to reflect typical installation scenarios. These ranged from having both devices in the same audio-video rack all the way to installations of WLAN anywhere in the house even approaching the maximum range a WLAN can achieve under ideal circumstances.
Even under these non-taxing conditions, the observed data was conclusive, according to the paper:
- If both WLAN and IEEE 802.15.4 are resident in the same audio-video rack or present in devices that are only 2-3 feet apart from each other, the interference from WLAN blocks IEEE 802.15.4 signals, independent of which channel is used by the IEEE 802.15.4 device.
- If the IEEE 802.15.4 device is operated in a channel that is "between" or "besides" the non-overlapping WLAN channels, the level of WLAN interference causes packet error loss rates in the same room making IEEE 802.15.4 devices unusable and also degrading the response time and reducing battery life for all other IEEE 802.15.4 devices in the home.
- Even if the use of RF channels by WLAN and IEEE 802.15.4 is ideally coordinated in one home, WLAN devices used by a nearby neighbor or public Wi-Fi network can easily create harmful interference.
In summary, the claim of the ZigBee community of the existence of "safe" IEEE 802.15.4 and ZigBee channels that are immune to WLAN interference is contradicted by the test results, according to the paper. Further, the test results also showed that ZigBee's attempts to implement an automatic change of the IEEE 802.15.4 channel (a solution planned for release in future products several years away) may be futile for many applications that would be critical to the success of ZigBee.
Ironically, the consistent test conclusions were reached under conservative conditions that were designed to show a minimum level of WLAN interference and deliberately underestimated the level of interference one would experience from a typical residential WLAN, according to the paper.
For example, the tests used only a single WLAN device as the "interferer." Actual interference in home environments would almost certainly be higher, considering that many, if not most households have multiple WLAN devices and often more than one WLAN. Even these conditions don't take into account a neighbor's WLAN or situations where many wireless devices would be operated at the same time as they might be in a typical home.
Applications based on ZigBee or IEEE 802.15.4 that are designed to operate over multiple years using battery power face an even higher risk, according to the paper. Any installation of a nearby WLAN such as at a neighbor's house or apartment, a city network or other public service could degrade the operation of ZigBee at a later stage.
The fact that this high risk of interference from WLAN is well known - from both simulations and tested measurements - could possibly expose corresponding ZigBee vendors to the risk of litigation and tort.
With the emergence of new high-definition video and audio streaming products based on IEEE 802.11n and other WLAN technologies, the amount of interference in a typical home WLAN will increase exponentially in the near-term future, according to the paper. It is clear that IEEE 802.15.4-based control technologies attempting to coexist within this framework might face even greater challenges regarding reliable or even achievable operation.
Technologies that operate in frequency bands that are impervious to Wi-Fi interference are emerging as another choice for control applications in wireless homes. Due to a sub-1-GHz band design, the Z-Wave Alliance said its technology has advantages that loom definitively in the wireless control space.
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The white paper can be found at http://www.z-wavealliance.org/modules/iaCM-DocMan/?docId=53&mode=DE.
Source: Z-Wave Alliance.