Avoiding
Interference
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One of the ways
Bluetooth devices avoid interfering with other systems is by sending out very
weak signals of 1 milliwatt. By comparison, the most
powerful cell phones can transmit a signal of 3 watts. The low power limits the
range of a Bluetooth device to about 10 meters, cutting the chances of
interference between your computer system and your portable telephone or
television. Even with the low power, the walls in your house won't stop a
Bluetooth signal, making the standard useful for controlling several devices in
different rooms.
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With many different Bluetooth
devices in a room, you might think they'd interfere with one another, but it's
unlikely that several devices will be on the same frequency at the same time,
because Bluetooth uses a technique called spread-spectrum frequency hopping. In
this technique, a device will use 79 individual, randomly chosen frequencies
within a designated range, changing from one to another on a regular basis. In
the case of Bluetooth, the transmitters change frequencies 1,600 times every
second, meaning that more devices can make full use of a limited slice of the
radio spectrum. Since every Bluetooth transmitter uses spread-spectrum
transmitting automatically, it?s unlikely that two transmitters will be on the
same frequency at the same time. This same technique minimizes the risk that
portable phones or baby monitors will disrupt Bluetooth devices, since any
interference on a particular frequency will last only a tiny fraction of a
second.
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When Bluetooth-capable devices come within range of one another, an electronic conversation takes place to determine whether they have data to share or whether one needs to control the other. The user doesn't have to press a button or give a command -- the electronic conversation happens automatically. Once the conversation has occurred, the devices -- whether they're part of a computer system or a stereo -- form a network. Bluetooth systems create a personal-area network (PAN), or piconet, that may fill a room or may encompass no more distance than that between the cell phone on a belt-clip and the headset on your head. Once a piconet is established, the members randomly hop frequencies in unison so they stay in touch with one another and avoid other piconets that may be operating in the same room.
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