Harald Bluetooth was king of Denmark in the late
900s. He managed to unite Denmark and part of
Norway into a single kingdom then introduced Christianity
into Denmark. He left a large monument, the Jelling
rune stone, in memory of his parents. He was killed
in 986 during a battle with his son, Svend Forkbeard.
Choosing this name for the standard indicates
how important companies from the Nordic region
(nations including Denmark, Sweden, Norway and
Finland) are to the communications industry, even
if it says little about the way the technology
works.
Why Bluetooth Works
Bluetooth takes small-area networking to the next
level by removing the need for user intervention
and keeping transmission power extremely low to
save battery power. Picture this: You're on your
Bluetooth-enabled cell phone, standing outside
the door to your house. You tell the person on
the other end of the line to call you back in
five minutes so you can get in the house and put
your stuff away. As soon as you walk in the house,
the map you received on your cell phone from your
car's Bluetooth-enabled GPS system is automatically
sent to your Bluetooth-enabled computer, because
your cell phone picked up a Bluetooth signal from
your PC and automatically sent the data you designated
for transfer. Five minutes later, when your friend
calls you back, your Bluetooth-enabled home phone
rings instead of your cell phone. The person called
the same number, but your home phone picked up
the Bluetooth signal from your cell phone and
automatically re-routed the call because it realized
you were home. And each transmission signal to
and from your cell phone consumes just 1 milliwatt
of power, so your cell phone charge is virtually
unaffected by all of this activity.
Bluetooth is essentially a networking standard
that works at two levels:
• It provides agreement at the physical
level -- Bluetooth is a radio-frequency standard.
• It provides agreement at the protocol
level, where products have to agree on when bits
are sent, how many will be sent at a time, and
how the parties in a conversation can be sure
that the message received is the same as the message
sent.
The big draws of Bluetooth are that it is wireless,
inexpensive and automatic. There are other ways
to get around using wires, including infrared
communication. Infrared (IR) refers to light waves
of a lower frequency than human eyes can receive
and interpret. Infrared is used in most television
remote control systems. Infrared communications
are fairly reliable and don't cost very much to
build into a device, but there are a couple of
drawbacks. First, infrared is a "line of
sight" technology. For example, you have
to point the remote control at the television
or DVD player to make things happen. The second
drawback is that infrared is almost always a "one
to one" technology. You can send data between
your desktop computer and your laptop computer,
but not your laptop computer and your PDA at the
same time. (See How Remote Controls Works to learn
more about infrared communication.)
These two qualities of infrared are actually advantageous
in some regards. Because infrared transmitters
and receivers have to be lined up with each other,
interference between devices is uncommon. The
one-to-one nature of infrared communications is
useful in that you can make sure a message goes
only to the intended recipient, even in a room
full of infrared receivers.
Bluetooth is intended to get around the problems
that come with infrared systems. The older Bluetooth
1.0 standard has a maximum transfer speed of 1
megabit per second (Mbps), while Bluetooth 2.0
can manage up to 3 Mbps. Bluetooth 2.0 is backward
compatible with 1.0 devices.
Flexible Transmission
Most of the time, a network or communications
method either works in one direction at a time,
called half-duplex communication, or in both directions
simultaneously, called full-duplex communication.
A speakerphone that lets you either listen or
talk, but not both, is an example of half-duplex
communication, while a regular telephone handset
is a full-duplex device. Because Bluetooth is
designed to work in a number of different circumstances,
it can be either half-duplex or full-duplex.
The cordless telephone is an example of a use
that will call for a full-duplex (two-way) link,
and Bluetooth can send data at more than 64 kilobits
per second (Kbps) in a full-duplex link -- a rate
high enough to support several voice conversations.
If a particular use calls for a half-duplex link
-- connecting to a computer printer, for example
-- Bluetooth can transmit up to 721 Kbps in one
direction, with 57.6 Kbps in the other. If the
use calls for the same speed in both directions,
Bluetooth can establish a link with 432.6-Kbps
capacity in each direction.
