How Bluetooth Operates

Bluetooth networking transmits data via low-power radio
waves. It communicates on a frequency of 2.45 gigahertz
(actually between 2.402 GHz and 2.480 GHz, to be exact). This
frequency band has been set aside by international agreement
for the use of industrial, scientific and medical devices
(ISM).

A number of devices that you may already use take advantage
of this same radio-frequency band. Baby monitors, garage-door
openers and the newest generation of cordless phones all
make use of frequencies in the ISM band. Making sure that
Bluetooth and these other devices don't interfere with one
another has been a crucial part of the design process.

One of the ways Bluetooth devices avoid interfering with
other systems is by sending out very weak signals of about
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 (32 feet), cutting
the chances of interference between your computer system and
your portable telephone or television. Even with the low
power, Bluetooth doesn't require line of sight between
communicating devices. 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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Bluetooth can connect up to eight devices simultaneously.
With all of those devices in the same 10-meter (32-foot)
radius, you might think they'd interfere with one another,
but it's unlikely. Bluetooth uses a technique called
spread-spectrum frequency hopping that makes it rare for more
than one device to be transmitting on the same frequency at
the same time. 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.

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. Let's check out an example of a Bluetooth-connected
system.

Bluetooth Piconets

Let’s say you have a typical modern living room with typical
modern stuff inside. There’s an entertainment system with
a stereo, a DVD player, a satellite TV receiver and
a television; there's also a cordless telephone and
a personal computer. Each of these systems uses Bluetooth,
and each forms its own piconet to talk between the main unit
and peripheral.

The cordless telephone has one Bluetooth transmitter in the
base and another in the handset. The manufacturer has
programmed each unit with an address that falls into a range
of addresses it has established for a particular type of
device. When the base is first turned on, it sends radio
signals asking for a response from any units with an address
in a particular range. Since the handset has an address in
the range, it responds, and a tiny network is formed. Now,
even if one of these devices should receive a signal from
another system, it will ignore it since it’s not from within
the network. The computer and entertainment system go through
similar routines, establishing networks among addresses in
ranges established by manufacturers. Once the networks are
established, the systems begin talking among themselves. Each
piconet hops randomly through the available frequencies, so
all of the piconets are completely separated from one another.

Now the living room has three separate networks established,
each one made up of devices that know the address of
transmitters it should listen to and the address of receivers
it should talk to. Since each network is changing the
frequency of its operation thousands of times a second, it’s
unlikely that any two networks will be on the same frequency
at the same time. If it turns out that they are, then the
resulting confusion will only cover a tiny fraction of
a second, and software designed to correct for such errors
weeds out the confusing information and gets on with the
network’s business.