July 4, 2011, 10:14 p.m.
posted by mv
Radio yields the most versatile connection system: no wires and no worries. Because radio waves can slip through most office walls, desktop ornaments, office supplies, and even employees, radio-based links eliminate the line-of-sight requirements of optical links such as IrDA. Linking devices with radio waves consequently yields the most convenient connection for workers to free their peripherals from the chains of interconnecting cables. A radio link can provide a reliable, cord-free connection that eliminates the snarl on the rear of every desktop PC. It also allows you to link your wireless devices—in particular, your cell phone—to your PC and keep everything wireless. Hardly a novel idea, of course, but one that has been a long time coming for practical connections.
Finally, a single standard may bring the wireless dream to life. To provide common ground and a standard for radio-based connections between PCs, their peripherals, and related communications equipment, several major corporations worked together to develop the Bluetooth specification. They designed the standard for the utmost in convenience, coupled with low cost but sacrificing range—Bluetooth is a short-range system suitable for linking devices in an office suite rather than across miles like a cell phone.
Originally conceived as a way to link cellular devices to PCs, the actual specification transcends its origins. Bluetooth makes possible not only cell phone connections but also could allow you to use your keyboard or mouse without a physical connection to your PC and without fretting about office debris blocking optical signals. But Bluetooth is more than a simple interface. It can become a small wireless network of intercommunicating devices, handling both voice and data with equal ease. Although not a rival to traditional networking systems—its speed limitations alone see to that—Bluetooth adds versatility that combines cell phone and PC technology.
The Bluetooth promoters refer to its multidevice links as a piconet (smaller than even a micronet), able to link up to eight devices. Bluetooth allows even greater assemblages of equipment by linking piconets together and accommodating temporarily inactive equipment within its reach.
On the other hand, the data speed of the Bluetooth system is modest. At most, Bluetooth can move bits at a claimed rate of about 723Kbps asymmetrically—that is, the high rate is in one direction; the return channel is slower, about one-fifth that rate. Moreover, Bluetooth slows to accommodate bidirectional data and phone conversations. Despite the modest data rate, however, the Bluetooth bit-rate is high enough to handle three simultaneous telephone conversations or a combination of voice and data simultaneously.
Good as it sounds, Bluetooth currently has a number of handicaps to overcome. It is not supported by any version of Microsoft Windows in current release (including the initial release of Windows XP). Microsoft, however, promises to add its own native support for Bluetooth in subsequent releases of Windows.
Certainly Bluetooth was not the first attempt at creating radio-based data links for computers. Wireless schemes for exchanging data have been around longer than personal computers. But Bluetooth differs from any previous radio-based data-exchange system in that it was conceived as an open standard for the computer and communications industries to facilitate the design of compatible wireless hardware.
As with so many modern standards, Bluetooth represents the work of an industry consortium. In May 1998, representatives from five major corporations involved with PCs, office equipment, and cellular telephones jointly conceived the idea of Bluetooth and began working toward creating the standard. The five founders were Ericsson (Telefonaktiebolaget LM Ericsson), International Business Machines Corporation, Intel Corporation, Nokia Corporation, and Toshiba Corporation. Together they formed the Bluetooth Special Interest Group (SIG) and started work on the standard and the technologies needed to make it a reality. The SIG released the first version of the specification, Bluetooth 1.0, on July 24, 1999. A slightly revised version was released in December 1999.
Membership in the Bluetooth SIG grew to nine on December 1, 1999, when 3Com Technologies, Lucent Technologies, Microsoft Corporation, and Motorola, Inc., joined the group. In addition, over 1,200 individuals and companies have adopted the technology by entering an agreement with the SIG that allows them to use the standard and share the intellectual property required to implement it.
Although support for Bluetooth has been slow in coming, manufacturers have adapted the technology for low-speed computer peripherals (such as wireless keyboards and mice). Owing to the success of other wireless technologies, most successful applications of Bluetooth are in communications products.
Bluetooth is a wireless packetized communications system that allows multiple devices to share data in a small network. Heir to both cell phone and digital technologies, it nestles between several existing standards, embracing them. It can link to your PC using a USB connection, and it shares logical layers with IrDA. It not only handles data like a traditional serial port but also can carry more than 60 RS-232C connections.
In theory, a Bluetooth system operates entirely transparently. Devices link themselves together without you having to do anything. All you need to do is turn on your Bluetooth devices and bring them within range of one another. For example, available devices should automatically pop up on your Windows desktop—at least once Windows gains Bluetooth support. You can then drag files to and from the device as if it were a local Windows resource.
Behind the scenes, however, things aren't quite so simple. The Bluetooth system must accommodate a variety of device and data types. It needs to keep in constant contact with each device. It must be able to detect when a new device appears and when other devices get switched off or venture out of range. It has to moderate the conversations between units, ensuring they don't all try to talk at the same time and interfere with one another.
As an advanced interface, Bluetooth heavily processes the raw data it transmits. It repackages serial data bits into packets with built-in error control. It then combines a series of packets of related serial data into links. It further processes the links through multiplexing so that several serial streams can simultaneously share a single Bluetooth connection.
Bluetooth packetizes data, breaking a serial input stream into small pieces, each containing address and optional error-correction information. A series of packets that starts as a single data stream and is later reconstructed into a replica of that stream is a link.
The Bluetooth standard supports two kinds of data links: synchronous and asynchronous. Synchronous data is typically voice information, such as audio from telephone conversations. Asynchronous data is typically computer data. The chief difference is that synchronous data is time dependent, so synchronous packets get transmitted once without regard to their reception. If a synchronous packet gets lost during transmission, it is forever lost.
Synchronous links between Bluetooth devices provide a full-duplex channel with an effective data rate of 64Kbps in each direction. In effect, a synchronous link is a standard digital telephone channel with eight-bit resolution and an 8-KHz sampling rate. The Bluetooth standard allows for two devices to simultaneously share three such synchronous links, the equivalent of three real-time telephone conversations. All links begin asynchronously because commands can only be sent in asynchronous packets. After the link is established, the master and slave can negotiate to switch over to a synchronous link for voice transfers or to move data asynchronously.
Each piconet has a single master and, potentially, multiple slaves. Each piconet shares a single communications channel with all the devices (master and slave) locked together on a common frequency and using a common clock, as discussed later. The single channel is subdivided into one or more links—asynchronous and/or synchronous.
To handle contention between multiple links on its single channel, Bluetooth uses time-division multiplexing. That is, each separate packet of a link gets a time period for transmission. The standard divides the communications channel into time slots, each 625 microseconds long. The shortest single packet fits a single slot with room to spare, although Bluetooth allows packets to stretch out for up to five slots. Bluetooth allows a maximum length of single-slot packets of 366 microseconds. The system accommodates larger packets by letting them extend through up to five slots, filling the entire time of four of the slots and part of the fifth.
In the Bluetooth system, each packet also defines a hop. That is, after each packet is sent, the Bluetooth system switches to (or hops to) another carrier frequency. As noted later, frequency-hopping helps ensure the integrity of Bluetooth transmissions. The minimum hop length corresponds to a single slot, although a hop can last for up to five slots to accommodate a long packet.
The time division duplexing of the Bluetooth system works by assigning even-numbered slots to the master and odd-numbered slots to the slaves. Masters can begin their transmissions only in even-numbered slots. If a packet lasts for an even number of slots (two or four), no slave can begin until the next odd-numbered slot. In effect, then, packets use an odd number of slots even if they use only a shorter, even number of slots.
Bluetooth hardware provides the connection that carries the processed and packetized data. Although in that Bluetooth makes a wireless connection, its hardware is essentially invisible—the system requires a collection of circuits to transmit and receive the data properly.
As with all radio systems, Bluetooth starts with a carrier wave and modulates it with data. Unlike most common radio systems, however, Bluetooth does not use a single fixed carrier frequency but rather hops to different frequencies more than a thousand times each second. As a serial transmission system, time is important to Bluetooth to sort out data bits. Each Bluetooth device maintains a clock that helps it determine when each bit in its serial stream appears. Bluetooth cleverly combines these necessary elements to make a wireless communications network.
For the Bluetooth signals to be effectively demodulated, the clocks of the master and slaves must be synchronized. The master device sets the frequency for all the slaves with which it communicates. The slaves determine the exact frequency of the clock from the packet data. The preamble of each packet contains a predetermined pattern of several cycles, which the slaves can use to achieve synchrony. The Bluetooth system does not alter the operation of the clock of the slaves, however. Instead, it stores the difference between the master and slave clocks and uses this difference value to maintain its lock on the master.
Bluetooth is designed to handle a variety of connection types. The basic link is point-to-point, two devices communicating only with one another. In such a link, one device operates as the master and the other as the slave. In a piconet configuration, a single master can communicate with up to seven active slaves (a total of eight devices intercommunicating). In addition, other slaves may lock on to the master's signal and be ready to communicate without sending out active signals. Such inactive slaves are said to be in a parked state.
The master in the piconet determines which devices can communicate (that is, which slaves are active or parked). In addition, several piconets can be linked together into a scatternet, with the master of one piconet communicating to a master or slave in another.
Bluetooth operates at radio frequencies assigned to industrial, scientific, and medical devices; a range termed the ISM band. This range of frequencies in the UHF (Ultra High Frequency) band has been set aside throughout most of the world for unlicensed, low-power electronic equipment. Near the top of the UHF range, the ISM band uses frequencies about twice that of the highest UHF television channel.
The exact frequencies available vary somewhat in North America, Europe, and Japan. In addition, France and Spain differ from the rest of Europe (although both countries are working on moving to the standards used throughout the rest of Europe).
Bluetooth uses channels one megahertz wide for its signals. Rather than operating on a single channel, a Bluetooth system uses them all. It uses the channels one at a time but switches between them to help minimize interference and fading. It can also help keep communications secure. Only the devices participating in a piconet know which channel they will hop to next.
In Europe (except France and Spain) and North America, the Bluetooth system can hop between 79 different channels. Elsewhere, the choices are limited to 23 channels. The available frequencies and number of channels available are summarized in the Figure.
A given Bluetooth system does not operate on one frequency but rather uses them all, hopping from one channel to another, up to 1,600 times per second. If a given asynchronous packet does not get through on one frequency due to interference (and is therefore not acknowledged), the next hop will send out a duplicate packet at a different frequency.
Unfortunately, Bluetooth does not have the entire 2.4GHz band to itself. The IEEE 802.11 wireless-networking standard currently uses the same frequencies, and interference between the two systems (where both are active) is inevitable. Although in the long term IEEE 802.11 will migrate to the 5GHz frequency range, at present the only way to entirely prevent interference between the two systems is to use one or the other, not both.
The Bluetooth specification defines three classes of equipment based on transmitter power. Class 1 devices are the most powerful and can transmit with up to 100 milliwatts of output power. Class 3 devices transmit with less than 1 milliwatt. Figure lists the maximum and minimum output powers for each power class.
As with any radio-based system, greater power increases the coverage area, so a Class 1 device will have greater range than a Class 3 device (about 10 times greater because radio propagation follows an inverse-square law). On the downside, greater output power means the need for greater input power, which directly translates into battery drain. That 100 mW of output power will require about 100 times the battery power as a 1-mW device. Fortunately, even Class 1 devices are modest power consumers compared to other facets of notebook computers. For example, the power needs of a Class 1 device are less than one-tenth the demand of a typical display screen.
Bluetooth uses Gaussian frequency shift keying (FSK)—that is, the presence of a data bit alters (or shifts) the frequency of the carrier wave. Bluetooth specifies the polarity of the FSK modulation. It represents a binary one with a positive deviation of the carrier wave and a binary zero with a negative deviation. The raw data rate is 1Mbps (one million symbols per second).
Because of how the digital code affects the frequency shift keying modulation, the information content of the modulation affects the deviation of the signal. The Bluetooth standard specifies that the minimum deviation should never be smaller than 115KHz. The maximum deviation will be between 140 and 175KHz.
Bluetooth architecture builds a system from three parts: a radio unit, a link control unit, and a support unit that provides link management and the host terminal interface. These are functional divisions, and all will be integrated into most handheld Bluetooth devices. In your PC, all three will likely reside on a Bluetooth interface card that installs like any other expansion board in a standard PCI slot.
The radio unit implements the hardware aspects of Bluetooth described earlier. It determines the power and coverage of the Bluetooth device, and its circuitry creates the carrier wave (altering its frequency for each hop), modulates it, amplifies it, and radiates it through an antenna. The ultra-high frequencies used by the Bluetooth system have a short wavelength that allows the antenna to be integrated invisibly into the cases of many mobile devices.
The link control unit is the mastermind of the Bluetooth system. It implements the various control and management protocols for setting up and maintaining the wireless connection. It searches out and identifies new devices wanting to join the piconet, tracks the frequency hopping, and controls the operating state of the device.
The support unit provides the actual interface between the logic of the host device and the Bluetooth connection. It adapts the signals of the host to match the Bluetooth system, both electrically and logically. For example, in a PC-based Bluetooth interface card, the support unit adapts the parallel bus signals of the PCI connection into the packetized serial form used by Bluetooth. It also checks data coming in from the wireless connection for errors and requests retransmission when necessary.
Standards and Coordination
The Bluetooth Special Interest Group promulgates the Bluetooth specifications. It also facilitates the licensing of Bluetooth intellectual property. You can obtain the complete specification from the SIG at www.bluetooth.com.