There are lots of different ways
that electronic devices can connect to one another. For example:
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
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.
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
- 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
while printers tend to use
- 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
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 is a standard developed by a group of electronics manufacturers
that allows any sort of electronic equipment -- from computers and
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
- 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
(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
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
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.
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
spectrum. Since every Bluetooth transmitter uses spread-spectrum
transmitting automatically, itís 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
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
Letís take a look at how the Bluetooth frequency hopping and personal-area
network keep systems from becoming confused. Letís say youíve got a typical
modern living room with the typical modern stuff inside. Thereís an
entertainment system with a stereo, a DVD player, a
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
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 itís 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
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, itís
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 networkís
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
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
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.
Here are some specification details from
the Bluetooth Web site
- 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
- 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
- ACL slaves can only transmit when requested by the master.