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Bluetooth & 802.11b - Part 2

    Although aimed at different markets and roles, there has long been speculation about the effects 802.11b ( also called Wi-Fi, IEEE 802.11, Wireless LAN, ... ) and Bluetooth have on each other, not just technical interference, but also whether they are in competition with each other commercially. Often these are simplistic press reports attempting to compare the two and seeking to decide a winner. This article however explains the similarities and differences between Bluetooth & 802.11b on a broad level and tries to show that there is no serious competition between the two. In fact both have their own advantages and disadvantages, and both technologies can complement each other.

Note: in this article 802.11 refers to 802.11b unless specified.

Bluetooth & 802.11b Co-existence Factors

    It would be a bit embarrassing to argue that we need both Bluetooth and 802.11b, and then discover that they mightn't work with each other. This perception might arise when we consider that both transmit and receive in the 2.4GHz ISM range. This fact lead many in the popular press to assume that the technologies are at odds with each other. The reality is much more complex. Bluetooth & 802.11b DO cause interference with each other, increasing as they approach one another. However it is the nature of radio links to experience interference. At the same time the two devices are causing interference , more interference is also being caused by environment factors, stray radiation, multi-path delays etc. Both technologies are designed to handle interference.

    Bluetooth uses it innovative hopping method to hop over 79 frequencies that are 1MHz wide. thus , eventually, Bluetooth devices occupy 79MHz, but at any specific moment only 1 MHz is occupied. Each of these hops goes to a slot that is 625us long, (packets can last 1,3 or 5 slots , but the hop frequency remains the same for each packet).The hopping sequences allow multiple bluetooth piconets (networks) to co-exist simultaneously.

    The 802.11b standard defines 11 possible channels that may be used. Channel hopping also occurs, but 600 times slower than Bluetooth. Each channel is defied by its 'centre frequency'. the centre frequencies are at distances of 5MHz from each other. since the high bandwidth (20dB) could give a signal as wide as 16MHz, multiple co-located networks channels have to be spaced out from another . Thus one 802.11b network could operate at any channel, but 2 co-located networks would have to have enough spacing, say channel 2 and 10, giving a minimum of 24 MHz in between them. Similarly 3 co-located networks would have to choice something like channels 1, 6 & 11, to ensure enough spacing. More than 5 co-located channels is not recommended. The duration of each Wi-Fi packet varies based on the amount of data in the packet.

     By their nature, as both device occasionally hop to the same frequency, packets will be lost and throughput reduced. however this interference is not as bad as once feared. Several papers, such as [1] examines the mutual interference a bluetooth and 802.11 enabled device have on each other. This report found that in extreme conditions, where a Bluetooth or 802.11 interfere is positioned right beside to a receiver of the opposite technology (example 1 above), throughput is significantly reduced. However when the interferer is positioned further away, such as 10m in the Bluetooth interferer case (example 2 above), throughput is only minimally reduced compared to normal. One thing to note, as shown in case studies, is that Bluetooth will cause more interference with 802.11, than the other way around due to Bluetooth's much faster hop rate. The reasoning behind this is that while an 802.11 device is transmitting on a particular frequency, a Bluetooth device might hop to this frequency several times before 802.11 device hops to the next frequency. Although both technologies drop packets, 802.11's packets are bigger so more information has to be re-transmitted[4]. In general though, the study[1] shows that both technologies hold up very well when standard and every day scenarios are studied, which therefore puts the whole co-existence story to rest.

    Or does it?  In certain kinds of application any kind of interference is undesirable. Also many companies are now attempting to develop co-located bluetooth and 802.11 chip sets, and thus these extreme situations where 802.11 and Bluetooth are transmitting and receiving right beside each other must be handled as well.

Enabling Bluetooth & 802.11 Co-existence: Both on-chip and off

    The basic principle behind enabling Bluetooth & 802.11 Co-existence is to ensure a Bluetooth and 802.11 device located near each other never use the same frequency at the same time. Section 5 of [1] gives a variety of different methods which can be used by both 802.11 & Bluetooth. For 802.11 it proposes using dynamic channel section to ensure a channel picked which will interfere as little as possible with bluetooth piconets. A different method of ensuring a reduction of interference on 802.11 transmission is to fragment 802.11 packets to ensure that any interference is minimised. This however also reduces throughput by the increasing overhead of packet headers. Finally transmit power control could be used to ensure that the transmitted power in the band need be no higher than it has to, ensuring that other users of the band need not be needlessly interfered with [1],[6].

    For Bluetooth , interference reduction is not so easy. In [1] , TI propose 'Intelligent Frequency Hopping', a non-spec method of never hopping to certain frequencies. This would require changes in FCC regulations. Otherwise, Bluetooth could assemble a collection of known 'good' frequencies to hop through (hop frequencies where little or no packets are dropped), and subsequently avoid 'bad' hop frequencies (hop frequencies where an unacceptable percentage of packets are dropped. It is presumed this is the method TI and some other companies are currently announcing in their latest Bluetooth offerings[12]. However the fact that these methods stray from the bluetooth spec must be kept in mind. A non-'Intelligent Frequency Hopping' master in a piconet would hop as normal, meaning any advantage a 'Intelligent Frequency Hopping'-enabled slave possessed would be negated. Finally like 802.11, a golden rule of transmit power could be used, to ensure that other users of a congested band need not be needlessly interfered with [1], [7].

    The above methods would handle most general cases. However in the situation where Bluetooth and 802.11 chipset would be located on the same device, or even the same chipset, even more methods may have to be used. A simple signalling scheme with a co-ordination unit could be used by the 802.11 and Bluetooth device to reserve transmit and receive slots[1]. Although details are not announced, it is possible that a scheme like this may be what a number of companies are already using such as the TrueRadio chip from Mobilian. Additionally a more complex method of adaptive reservation could be used to maximise throughput by resolving conflicting reservations to ensure both devices avoid long traffic delays and to take into account the fact that one device may be busier than the other. This would ensure that a  Bluetooth/802.11 chipset would be able to function almost seamlessly side by side and thus maximise the potential of the wireless device using the joint chipset.

Conclusion

    Since their inception, Bluetooth and 802.11 have always aimed to provide their respective devices seamless radio connections to other devices, both wired and wireless. This initial similarity then drove some to assume that both technologies were in competition with each other, and led to  arguments, comparisons, judgements regarding one or the other. As has been shown above, and is being borne out now, both technologies each have there own unique role to play in setting up a wireless environment, and any market niches that they overlap in is but a small part of each device's respective market area. A more concrete and immediate debate is whether Bluetooth and 802.11 can physically live with each other. Alas, there is no end word on this yet, but as shown in this and other more practical and authoritative papers, the interference in everyday terms is not as great as once believed and even when this interference exists, moves are afoot (with already proven results) to reduce this interference to negligible effects.

    The future of both technologies remain bright, Bluetooth's low cost and low-power features means it will continue on course to become the technology of choice for small devices to replace cables, enable cheap short-range communication and form the backbone of future wireless PANs. 802.11 's greater throughput and functionality means it will continue to go from strength to strength providing the wireless LAN environment in the home and office. Although newer technologies and specs like ZigBee and 802.11g may strive to enter the same area, it seems that Bluetooth and 802.11b will come to dominate their respective wireless markets, not as adversities, but allies to each other, where the combination of both technologies gives greater functionality and potential than either could achieve on their own. 

  Part 1 of this article     Back to Cathal's Corner     More Bluetooth Articles