A cordless
phone connects to its base unit with radio
waves, and it may have a headset that connects
to the phone with a wire.
In a stereo system, a CD
player and other audio devices connect to
the receiver, which connects to the speakers.
When you use computers, entertainment systems
or telephones, the various pieces and parts of
the systems make up a community of electronic
devices. These devices communicate with each other
using a variety of wires, cables, radio signals
and infrared light beams, and an even greater
variety of connectors, plugs and protocols.
The art of connecting things is becoming more
and more complex every day. We sometimes feel
as if we need a Ph.D. in electrical engineering
just to set up the electronics in our homes! In
this edition of HowStuffWorks,
we will look at a completely different way to
form the connections, called Bluetooth.
Bluetooth is wireless and automatic, and has a
number of interesting features that can simplify
our daily lives.
The Problems
When any two devices need to talk to each other,
they have to agree on a number of points before
the conversation can begin. The first point of
agreement is physical: Will they talk over wires,
or through some form of wireless signals? If they
use wires, how many are required -- one, two,
eight, 25? Once the physical attributes are decided,
several more questions arise:
Information can be sent 1 bit
at a time in a scheme called serial communications,
or in groups of bits (usually 8 or 16 at a time)
in a scheme called parallel communications.
A desktop
computer uses both serial and parallel communications
to talk to different devices: Modems,
mice and keyboards tend to talk through serial
links, while printers tend to use parallel
links.
All of the parties in an electronic discussion
need to know what the bits mean and whether
the message they receive is the same message
that was sent. In most cases, this means developing
a language of commands and responses known as
a protocol. Some types of products have
a standard protocol used by virtually all companies
so that the commands for one product will tend
to have the same effect on another. Modems fall
into this category. Other product types each
speak their own language, which means that commands
intended for one specific product will seem
gibberish if received by another. Printers are
like this, with multiple standards like PCL
and PostScript
Companies that manufacture computers, entertainment
systems and other electronic devices have realized
that the incredible array of cables and connectors
involved in their products makes it difficult
for even expert technicians to correctly set up
a complete system on the first try. Setting up
computers and home
entertainment systems becomes terrifically
complicated when the person buying the equipment
has to learn and remember all the details to connect
all the parts. In order to make home electronics
more user friendly, we need a better way for all
the electronic parts of our modern life to talk
to each other. That's where Bluetooth comes in.
Bluetooth Basics
Bluetooth is a standard developed by a group of
electronics manufacturers that allows any sort
of electronic equipment -- from computers and
cell
phones to keyboards and headphones -- to make
its own connections, without wires, cables or
any direct action from a user. Bluetooth is intended
to be a standard that works at two levels:
It provides agreement at the physical level
-- Bluetooth is a radio-frequency
standard.
It also provides agreement at the next level
up, 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 companies belonging to the Bluetooth Special
Interest Group, and there are more than 1,000 of
them, want to let Bluetooth's radio communications
take the place of wires for connecting peripherals,
telephones and computers.
There are already a couple of ways to get around
using wires. One is to carry information between
components via beams of light
in the infrared spectrum. Infrared refers
to light waves of a lower frequency than human
eyes can receive and interpret. Infrared is
used in most television remote control systems,
and with a standard called IrDA
(Infrared Data Association) it's used to connect
some computers with peripheral devices. For most
of these computer and entertainment purposes,
infrared is used in a digital mode -- the signal
is pulsed on and off very quickly to send data
from one point to another.
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.
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.
The second alternative to wires, cable synchronizing,
is a little more troublesome than infrared. If
you have a Palm Pilot, a Windows CE device or
a Pocket PC, you know about synchronizing data.
In synchronizing, you attach the PDA to your computer
(usually with a cable), press a button and make
sure that the data on the PDA and the data on
the computer match. It's a technique that makes
the PDA a valuable tool for many people, but synchronizing
the PDA with the computer and making sure you
have the correct cable or cradle to connect the
two can be a real hassle.
Bluetooth is intended to get around the
problems that come with both infrared and cable
synchronizing systems. The hardware vendors, which
include Siemens, Intel, Toshiba, Motorola and
Ericsson, have developed a specification for a
very small radio module to be built into computer,
telephone and entertainment equipment. From the
user's point of view, there are three important
features to Bluetooth:
It's wireless. When you travel, you
don't have to worry about keeping track of a
briefcase full of cables to attach all of your
components, and you can design your office without
wondering where all the wires will go.
It's inexpensive.
You don't have to think about it. Bluetooth
doesn't require you to do anything special to
make it work. The devices find one another and
strike up a conversation without any user input
at all.
Bluetooth communicates on a frequency of 2.45
gigahertz, which 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.
Why is it called Bluetooth?
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 Baltic region (nations including
Denmark, Sweden, Norway and Finland) are to
the communications industry, even if it says
little about the way the technology works.
Avoiding Interference
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.
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.
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.
An Example
Let’s take a look at how the Bluetooth frequency
hopping and personal-area network keep systems
from becoming confused. Let’s say you’ve got a
typical modern living room with the typical modern
stuff inside. There’s an entertainment system
with a stereo, a DVD player, a satellite
TV receiver and a television; there's a cordless
telephone and a personal computer. Each of these
systems uses Bluetooth, and each forms its own
piconet to talk between 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.
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 64,000 bits per second
in a full-duplex link -- a rate high enough to
support several human 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 kilobits per second (Kbps) in one direction,
with 57.6 Kbps in the other. If the use calls
for the same speed in both directions, a link
with 432.6-Kbps capacity in each direction can
be made.
The devices in a piconet share a common communication
data channel. The channel has a total capacity
of 1 megabit per second (Mbps). Headers and
handshaking information consume about 20 percent
of this capacity.
In the United States and Europe, the frequency
range is 2,400 to 2,483.5 MHz, with 79 1-MHz
radio frequency (RF) channels. In practice,
the range is 2,402 MHz to 2,480 MHz. In Japan,
the frequency range is 2,472 to 2,497 MHz with
23 1-MHz RF channels.
A data channel hops randomly 1,600 times per
second between the 79 (or 23) RF channels.
Each channel is divided into time slots 625
microseconds long.
A piconet has a master and up to seven slaves.
The master transmits in even time slots, slaves
in odd time slots.
Packets can be up to five time slots wide.
Data in a packet can be up to 2,745 bits in
length.
There are currently two types of data transfer
between devices: SCO (synchronous connection
oriented) and ACL (asynchronous connectionless).
In a piconet, there can be up to three SCO
links of 64,000 bits per second each. To avoid
timing and collision problems, the SCO links
use reserved slots set up by the master.
Masters can support up to three SCO links
with one, two or three slaves.
Slots not reserved for SCO links can be used
for ACL links.
One master and slave can have a single ACL
link.
ACL is either point-to-point (master to one
slave) or broadcast to all the slaves.
ACL slaves can only transmit when requested
by the master.
