Support for Bluetooth is solid, with more than 1,880 adopter/associate member companies signed up to the Bluetooth SIG (Special Interest Group). Although the technology has been the subject of tremendous hype (some reports would have us using Bluetooth in everything from the refrigerator to the toilet) and has the potential to explode on the market, Bluetooth was not designed for full WAN connectivity. At best, the technology can be used for ad hoc networking when needed, but it is not designed for more than limited use. While other standards, like 802.11b, are designed for wide-reaching applications, Bluetooth simply does not have the bandwidth to handle full network demands.
Bluetooth is a good, low-power data/voice transmission standard that will be a real boon for certain applications. Imagine a cell phone or pager that automatically transfers to vibrate mode when you enter a restaurant or theater. Or a laptop and GPS (global positioning system) that automatically feed a display in your car for directions. One of Bluetooth's design strengths is its low cost. The Bluetooth SIG says integrating this technology should add only $5 to the cost of a unit.
Specifications
Bluetooth operates in the 2.4-GHz ISM (industrial, scientific and medical) band and uses a fast frequency-hopping technology to minimize interference to and from non-Bluetooth sources. This frequency hopping occurs nominally at 1,600 hops per second. The system has 78 possible channels. Channel spacing is 1 MHz in every industrialized country but France. (In France, you are limited to 22 possible channels on which to hop. Negotiations have been ongoing with the French government to change its regulations.)
Bluetooth has three power classes for transmitting: 100 milliwatt, 2.5 milliwatt and 1 milliwatt. The range for 100 milliwatt is 100 meters, 2.5 milliwatt is 10 meters and 1 milliwatt is 10 centimeters. Remember that these ranges are variable and difficult to calculate. Materials, walls and interference from other 2.4-GHz sources all can change the range achieved. Digianswer (a division of Motorola), a DSP and Bluetooth developer, has proven that a 2.5-milliwatt PC Card can reach up to 100 meters in a completely unobstructed environment.
Throughput for Bluetooth is rated at 1 Mbps under optimal conditions. With error correction overhead and other environmental factors, however, the real throughput should be about 700 Kbps to 800 Kbps. This should be more than enough for the Bluetooth cable-replacement designation.
Several other factors can influence Bluetooth's speed. The presence of other 2.4-GHz devices, such as an 802.11b WAN, can degrade performance of a Bluetooth piconet. Bluetooth traffic is separated into two types: data and voice.
Voice traffic has a much lower tolerance for interference than does data traffic. Ericsson conducted a study of interference from 802.11b on Bluetooth and found that when 802.11b is operating at normal traffic conditions, the more sensitive voice operation of Bluetooth is not affected when the transmitter and receiver are less than two meters apart. At a range of 10 meters, the probability of noticeable interference increases to eight percent.
The data link gets more interference than the voice link but is less susceptible to it. At a 10-meter range, throughput loss of more than 10 percent can occur with a 24 percent probability. The Ericsson study reports that the total loss of throughput due to interference from 802.11 WANs cannot exceed 22 percent. The 801.11 networks take up 17 channels at 2.4 GHz, and Bluetooth has 79 available channels. As of press time, I was unable to get any data on Bluetooth's effect on 802.11's throughput. The IEEE's 802.15 working group is developing standards to let Bluetooth and 802.11b function better in shared space (see grouper.ieee.org/groups/802/15/).
Factor in Microwaves
The FHSS (frequency hopping spread spectrum) protocol that Bluetooth uses is sensitive to interference from microwave ovens. Basically, a frequency-hopping protocol like FHSS cannot avoid channels that have been obliterated by noise. It must hop through all 79 channels on a continuous cycle. Packet and frame loss will occur when it jumps into a noisy channel.
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