You are here: > Documents > Bluetooth > How Bluetooth Short Range Radio Systems Work

How Bluetooth Short Range Radio Systems Work

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:

  • Many desktop computer systems have a CPU unit connected to a mouse, a keyboard, a printer and so on.
  • A personal digital assistant (PDA) will normally connect to the computer with a cable and a docking cradle.
  • Any TV is normally connected to a VCR and a cable box, with a remote control for all three components.
  • 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, the CD player, tape player and record player connect to the receiver, which connects to the speaker.

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 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.

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.

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. Cell phone 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.