If you have the normal complement of electronic equipment in your
home, then you know all of the different ways that devices connect
to one another. For example:
The art of connecting things is
becoming more and more complex every day. Oftentimes we feel as if
we need a Ph.D. in electrical engineering just to set up the
electronics in our houses! 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 should 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 -- 1, 2, 8, 25 or more? Once the physical attributes
are decided, several more questions arise:
- Information can be sent one 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 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
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 some 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 new standard developed by a group of electronics
manufacturers that will allow 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 to 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 (a class of computer called Personal
Digital Assistant, or PDA for short), 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 won't have to worry about
keeping track of a briefcase full of cables to attach all your
components, and you can design your office without wondering where
all the wires will go.
- It's inexpensive. Manufacturers think that it will add about
$15 to the price of a product at first, and by mid-2001 it should
only add $5.
- 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 around the turn of the
last millennium. 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 will 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 new 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 new Bluetooth-capable devices come within range of one
another, an electronic conversation will take 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
cellphone on a belt-clip and the headset you're wearing. 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; 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 its
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 will weed out the confusing information and get on with the
network’s business.
Most of the time, a network or communications method either works
in one direction at a time, called asynchronous
communication, or in both directions simultaneously, called
synchronous communication. A speakerphone that lets you
either listen or talk, but not both, is an example of asynchronous
communication, while a regular telephone handset is a synchronous
device. Because Bluetooth is designed to work in a number of
different circumstances, it can be either synchronous or
asynchronous. The cordless telephone is an example of a use that
will call for a synchronous, or two-way, link, and Bluetooth can
send data at more than 64,000 bits per second in a synchronous link
-- a rate high enough to support several human voice conversations.
If a particular use calls for an asynchronous link -- connecting to
a computer modem, for example -- Bluetooth can transmit up to 721
Kilo bits per second 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.
Computer manufacturers are planning to include Bluetooth in a
number of products. Cellphone and entertainment product designers
are also making plans for the standard because Bluetooth includes
three voice channels in each piconet. Since Bluetooth doesn't
require a physical adapter, manufacturers are planning cellular
modems, headsets and other devices that tie telephone and data
together.
Details
Here are some details from The
Bluetooth Specification
- The devices in a piconet share a common communication data
channel. The channel has a total capacity of 1 Megabit/second.
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 RF channels. In practice, the range
is 2,402 MHz to 2,480 MHz. In Japan the frequency range is 2,472
to 2,497MHz 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 (asynch
connection-less) links.
- ln 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 3 SCO links with 1, 2 or 3 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 point-to-point (master to one slave) or broadcast to
all the slaves.
- ACL slaves can only transmit when requested by master.