Bluetooth is an omnidirectional wireless technology that provides limited-range voice and data transmission over the unlicensed 2.4-GHz frequency band, allowing connections with a wide variety of fixed and portable devices that normally would have to be cabled together. Up to eight devices—one master and seven slaves—can communicate with one another in a socalled piconet at distances of up to 30 feet. Table B-1 summarizes the performance characteristics of Bluetooth products that operate at 1 Mbps in the 2.4-GHz range.
Among the many things users can do with Bluetooth is swap data and synchronize files merely by having the devices come within range of one another. Images captured with a digital camera, for example, can be dropped off at a personal computer (PC) for editing or a color printer for output on photo-quality paper—all without having to connect cables, load files, open applications, or click buttons.
The technology is a combination of circuit switching and packet switching, making it suitable for voice as well as data. Instead of fumbling with a cell phone while driving, for example, the user can wear a lightweight headset to answer a call and engage in a conversation even if the phone is tucked away in a briefcase or purse.
While useful in minimizing the need for cables, wireless local area networks (LANs) are not intended for interconnecting the range of mobile devices people carry around everyday between home and office. For this, Bluetooth is needed. And in the office, a Bluetooth portable device can be in motion while connected to the LAN access point as long as the user stays within the 30-foot range.
Bluetooth can be combined with other technologies to offer wholly new capabilities, such as automatically lowering the ring volume of cell phones or shutting them off as users enter quiet zones such as churches, restaurants, theaters, and classrooms. On leaving the quiet zone, the cell phones are returned to their original settings.
The devices within a piconet play one of two roles: that of master or slave. The master is the device in a piconet whose clock and hopping sequence are used to synchronize all other devices (i.e., slaves) in the piconet. The unit that carries out the paging procedure and establishes a connection is by default the master of the connection. The slaves are the units within a piconet that are synchronized to the master via its clock and hopping sequence.
The Bluetooth topology is best described as a multiplepiconet structure. Since Bluetooth supports both point-topoint and point-to-multipoint connections, several piconets can be established and linked together in a topology called a “scatternet” whenever the need arises.
Piconets are uncoordinated, with frequency hopping occurring independently. Several piconets can be established and linked together ad hoc, where each piconet is identified by a different frequency-hopping sequence. All users participating on the same piconet are synchronized to this hopping sequence. Although synchronization of different piconets is not permitted in the unlicensed ISM band, Bluetooth units may participate in different piconets through Time Division Multiplexing (TDM). This enables a unit to sequentially participate in different piconets by being active in only one piconet at a time.
With its service discovery protocol, Bluetooth enables a much broader vision of networking, including the creation of personal area networks, where all the devices in a person’s life can communicate and work together. Technical safeguards ensure that a cluster of Bluetooth devices in public places, such as an airport lounge or train terminal, would not suddenly start talking to one another.
Two types of links have been defined for Bluetooth in support of voice and data applications: an asynchronous connectionless (ACL) link and a synchronous connection-oriented (SCO) link. ACL links support data traffic on a best-effort basis. The information carried can be user data or control data. SCO links support real-time voice and multimedia traffic using reserved bandwidth. Both data and voice are carried in the form of packets, and Bluetooth devices can support active ACL and SCO links at the same time.
ACL links support symmetric or asymmetric packetswitched point-to-multipoint connections, which are typically used for data. For symmetric connections, the maximum data rate is 433.9 kbps in both directions, send and receive. For asymmetric connections, the maximum data rate is 723.2 kbps in one direction and 57.6 kbps in the reverse direction. If errors are detected at the receiving device, a notification is sent in the header of the return packet so that only lost or corrupt packets need to be retransmitted.
SCO links provide symmetric circuit-switched point-topoint connections, which are typically used for voice. Three synchronous channels of 64 kbps each are available for voice. The channels are derived through the use of either Pulse Code Modulation (PCM) or Continuous Variable Slope Delta (CVSD) Modulation. PCM is the standard for encoding speech in analog form into the digital format of ones and zeros.
CVSD is another standard for analog-to-digital encoding but offers more immunity to interference and therefore is better suited than PCM for voice communication over a wireless link. Bluetooth supports both PCM and CVSD; the appropriate voice-coding scheme is selected after negotiations between the link managers of each Bluetooth device before the call takes place.
Voice and data are sent as packets. Communication is handled with Time Division Duplexing (TDD), which divides the channel into time slots, each 625 microseconds (μs) in length. The time slots are numbered according to the clock of the piconet master. In the time slots, master and slave can transmit packets. In the TDD scheme, master and slave alternatively transmit.
The master starts its transmission in even-numbered time slots only, and the slave starts its transmission in odd-numbered time slots only. The start of the packet is aligned with the slot start. Packets transmitted by the master or the slave may extend over as many as five time slots. With TDD, bandwidth can be allocated on an as-needed basis, changing the makeup of the traffic flow as demand warrants.
For example, if the user wants to download a large data file, as much bandwidth as is needed will be allocated to the transfer. Then, at the next moment, if a file is being uploaded, that same amount of bandwidth can be allocated to that transfer. No matter what the application—voice or data—making connections between Bluetooth devices is as easy as powering them up.
In fact, one advantage of Bluetooth is that it does not need to be set up—it is always on, running in the background, and looking for other devices that it can communicate with. When Bluetooth devices come within range of one another, they engage in a service discovery procedure, which entails the exchange of messages to become aware of each other’s service and feature capabilities.
Having located available services within the vicinity, the user may select from any of them. After that, a connection between two or more Bluetooth devices can be established. The radio link itself is very robust, using frequencyhopping spread-spectrum technology to overcome interference and fading. Spread spectrum is a digital coding technique in which the signal is taken apart or “spread” so that it sounds more like noise as it is sent through the air.
With the addition of frequency hopping—having the signals skip from one frequency to another—wireless transmissions are made even more secure. Bluetooth specifies a rate of 1600 hops per second among 79 frequencies. Since only the sender and receiver know the hopping sequence for coding and decoding the signal, eavesdropping is virtually impossible. For enhanced security, Bluetooth also supports device authentication and encryption.
Other frequency-hopping transmitters in the vicinity will be using different hopping patterns and much slower hop rates than Bluetooth devices. Although the chance of Bluetooth devices interfering with non-Bluetooth devices that share the same 2.4-GHz band is minimal, should non-Bluetooth transmitters and Bluetooth transmitters coincidentally attempt to use the same frequency at the same moment, the data packets transmitted by one or both devices will become garbled in the collision, and a retransmission of the affected data packets will be required. Anew data packet will be sent again on the next hopping cycle of each transmitter. Voice packets, because of their sensitivity to delay, are never retransmitted.
Points of Convergence
In some ways, Bluetooth competes with infrared, and in other ways, the two technologies are complementary. With both infrared and Bluetooth, data exchange is considered to be a fundamental function. Data exchange can be as simple as transferring business card information from a mobile phone to a palmtop or as sophisticated as synchronizing personal information between a palmtop and desktop PC. In fact, both technologies can support many of the same applications, raising the question: Why would users need both technologies?
The answer lies in the fact that each technology has its advantages and disadvantages. The very scenarios that leave infrared falling short are the ones where Bluetooth excels, and vice versa. Take the electronic exchange of business card information between two devices. This application usually will take place in a conference room or exhibit floor where a number of other devices may be attempting to do the same thing. This is the situation where infrared excels.
The shortrange and narrow angle of infrared—30 degrees or less— allow each user to aim his or her device at the intended recipient with point-and-shoot ease. Close proximity to another person is natural in a business card exchange situation, as is pointing one device at another. The limited range and angle of infrared allow other users to perform a similar activity with ample security and no interference.
In the same situation, a Bluetooth device would not perform as well as an infrared device. With its omnidirectional capability, the Bluetooth device must first discover the intended recipient. The user cannot simply point at the intended recipient—a Bluetooth device must perform a discovery operation that probably will reveal several other Bluetooth devices within range, so close proximity offers no advantage here.
The user will be forced to select from a list of discovered devices and apply a security mechanism to prevent unauthorized access. All this makes the use of Bluetooth for business card exchange an awkward and needlessly time-consuming process.
However, in other data-exchange situations, Bluetooth might be the preferred choice. Bluetooth’s ability to penetrate solid objects and its ability to communicate with other devices in a piconet allow for data-exchange opportunities that are very difficult or impossible with infrared.
For example, Bluetooth allows a user to synchronize a mobile phone with a notebook computer without taking the phone out of a jacket pocket or purse. This would allow the user to type a new address at the computer and move it to the mobile phone’s directory without unpacking the phone and setting up a cable connection between the two devices. The omnidirectional capability of Bluetooth allows synchronization to occur instantly, assuming that the phone and computer are within 30 feet of each other.
Using Bluetooth for synchronization does not require that the phone remain in a fixed location. If a phone is carried about in a briefcase, the synchronization can occur while the user moves around. This is not possible with infrared because the signal is not able to penetrate solid objects, and the devices must be within a few feet of each other. Furthermore, the use of infrared requires that both devices remain stationary while the synchronization occurs.
When it comes to data transfers, infrared does offer a big speed advantage over Bluetooth. While Bluetooth moves data between devices at an aggregate rate of 1 Mbps, infrared offers 4 Mbps of data throughput. Ahigher -speed version of infrared is now available that can transmit data between devices at up to 16 Mbps—a four times improvement over the previous version.
The higher speed is achieved with the Very Fast Infrared (VFIR) Protocol, which is designed to address the new demands of transferring large image files between digital cameras, scanners, and PCs. Even when Bluetooth is enhanced for higher data rates in the future, infrared is likely to maintain its speed advantage for many years to come.
Bluetooth complements infrared’s point-and-shoot ease of use with omnidirectional signaling, longer-distance communications, and capacity to penetrate walls. For some users, having both Bluetooth and infrared will provide the optimal short-range wireless solution. For others, the choice of adding Bluetooth or infrared will be based on the applications and intended usage.
Communicator platforms of the future will combine a number of technologies and features in one device, including mobile Internet browsing, messaging, imaging, location-based applications and services, mobile telephony, personal information management, and enterprise applications. Bluetooth will be a key component of these platforms.
Since Bluetooth radio transceivers operate in the globally available ISM (industrial, scientific, and medical) radio band of 2.4 GHz, products do not require an operator license from a regulatory agency, such as the FCC in the United States. The use of a generally available frequency band means that Bluetooth-enabled devices can be used virtually anywhere in the world and link up with one another for ad hoc networking when they come within range.