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 article, 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.
Photo courtesy Bluetooth SIG Bluetooth wireless PC
card
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.
Other Wireless Connections 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.
The Bluetooth Solution 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.
Photo courtesy Bluetooth SIG Bluetooth-enabled Palm Pilot
PDA
Bluetooth Frequency 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: Low Power 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.
Photo courtesy Bluetooth SIG Bluetooth-enabled cell
phone
With many different Bluetooth devices in a room, you might
think they'd interfere with one another, but it's unlikely. On
the next page, we'll see why.
Avoiding Interference: Hopping It is
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.
Example: Networks 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.
Example: Half/Full Duplex 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.
Logo courtesy Bluetooth
SIG
For more information on Bluetooth and related topics, check
out the links on the next page.
