Communication can take many forms—audio, visual, written, electronic and so on. In the realm of electronics, analog and digital communications are so pervasive in modern society that they are largely taken for granted. The exchange of data using these forms of communication has led to the use of terms such as "the information industry" and "the information age." From telephones to computers to televisions, communication in many respects "makes the world go around." Bluetooth wireless technology is one form of electronic communication, and in this chapter we examine some of its fundamental and specific characteristics, including relationships to other forms of communication. We begin with a brief general discussion of wireless communications and then progress through more specific forms relevant to the Bluetooth technology. There are many other types of wireless communication; our intent here is to touch upon those that provide background for the Bluetooth technology rather than to provide a primer on wireless technologies in general.
A great deal of data is carried over wired networks—many telephones, coaxial cable systems, local area networks and parts of the Internet communicate via wires and cables. Many televisions are connected to cable systems, most networked computers are connected to telephone lines or wired networks such as Ethernet networks, and even cordless and mobile telephones rely on wired "landline" telephone systems to carry and route calls between endpoints.
Communicating without wires is not a new concept. Broadcast radio and television are two common examples of wireless communications; others include satellites, cordless and cellular telephones and remotely controlled televisions, garage door openers and automobile door locks. While most of these examples employ communication via radio waves, the use of infrared, a nonvisible spectrum of light, is also relatively common. Bluetooth wireless communication employs radio frequency (or RF) technology, using radio waves to communicate through the air in a manner fundamentally similar to broadcast radio or television.
RF technologies employ transmitters and receivers tuned to produce and consume, respectively, radio waves of a given frequency range. The transmitter's power and the receiver's sensitivity help to determine the distance over which they can communicate. High transmission power output is used for long-range communications such as broadcast television while short-range communications generally require much less power; thus technologies that are designed to communicate across only a few meters could be employed in small, mobile battery powered devices. Another characteristic that is relevant for communication applications is the ability of radio waves to penetrate many objects. Obstacles reflect light waves used in technologies such as infrared, but radio waves used in RF technologies in general can (with certain limitations) penetrate many obstacles (although in some cases radio waves can diffract or go around objects too). Thus RF technologies can permeate many obstacles such as clothing, bodies, walls, doors and the like. This means that there is no requirement for a "line of sight" between the transmitter and the receiver.
RF technologies use frequency modulation to generate radio waves within a certain frequency spectrum, which encode information and can be intercepted by receivers tuned to the corresponding frequency. FM radio broadcasts, for example, operate in the 88 megahertz (MHz) to 108 MHz frequency spectrum; some cordless telephones operate in the 900 MHz frequency spectrum; Bluetooth wireless communications and other technologies operate in the 2.4 gigahertz (or GHz; one gigahertz equals one billion cycles per second) spectrum. Because the usable radio frequency space is finite, most governments regulate its use, partitioning frequency ranges and granting licenses to transmit at those frequencies at specified power levels. In the United States, for example, a federal license is required to transmit in the FM radio frequency spectrum except at extremely low power levels that limit the range to no more than about 30 meters. Some frequencies are reserved for use without a license under certain conditions. For example, in the United States unlicensed operation is permitted, with some restrictions, in the 900 MHz and 2.4 GHz frequency ranges (the latter being where Bluetooth wireless communication operates). In fact, through multinational agreement, the 2.4 GHz spectrum requires no license for its use anywhere in the world.[1] The SIG together with other organizations such as the IEEE 802.11 standards body is working with regulatory authorities in some countries to pursue harmonization of the frequency assignments for unlicensed use within the 2.4 GHz spectrum and of the approval process for wireless communications. In general the chosen frequency spectrum can be used globally without license so long as the rules for operating within this spectrum are followed.
While the 2.4 GHz spectrum is globally unlicensed, there are regulatory requirements and other considerations for its use. These include:
The spectrum is divided into 79 channels (although in some countries, notably Spain and France, only 23 channels were available for use in the year 2000. Japan began using all 79 channels in 2000).
Bandwidth is limited to 1 MHz per channel.
Frequency hopping spread spectrum communications (described below) must be employed.
Interference must be anticipated and appropriately handled.
Other RF communications technologies also use this spectrum; notable among them are HomeRF™ (an open industry specification for RF communications in home environments; see http://www.homerf.org) and the Institute of Electrical and Electronics Engineers (IEEE) 802.11 specification for wireless local area networks (see http://www.ieee.org). Microwave ovens also operate within this frequency range. Because this spectrum is unlicensed, new uses for it are to be expected (for example, a new generation of cordless telephones also uses the 2.4 GHz frequency) and as the spectrum becomes more widely used, radio interference is more likely. Thus the requirement to anticipate and address interference in the 2.4 GHz range is important for all technologies that operate in it.
Each technology using this spectrum has made design choices within the spectrum's constraints that optimize that technology for particular applications or domains. Bluetooth wireless communication is designed to take maximum advantage of the available channel bandwidth and to minimize RF interference and its effects while operating at very low power.
Within RF communications, spread spectrum refers to dividing the available spectrum based upon frequency, time, a coding scheme or some other method. Messages to be sent are then divided into various parts (packets) that are transmitted across the divided spectrum. Frequency division spread spectrum (or frequency hopping), which is the method employed with Bluetooth wireless communication, divides the spectrum into different frequencies, or channels.[2] A single message packet is transmitted on a selected channel, then the radio selects a new channel (a process called hopping to a new frequency) to transmit the next packet, and the process repeats, thereby spreading the message across the available frequency spectrum. Each technology specifies its own method for establishing the frequency hopping pattern. Obviously the receiver(s) of the message must know the hopping pattern to tune to the correct channels in succession to receive each packet and assemble the complete message. This process is called frequency hopping spread spectrum, or FHSS.
FHSS introduces additional complexity as compared to using a single statically selected frequency, yet it also supplies some benefits. First, RF interference can be reduced since all radios hop (often randomly or at least pseudorandomly, and often rapidly) from one frequency to another. When all of the participants in the spectrum employ FHSS, interference caused by colliding transmissions on the same frequency is less likely than it would be if each radio used a single channel for a long duration. In addition, when collisions do occur, their effects are lessened, since only a single packet is lost and that packet could be retransmitted at a new frequency, where again it is less likely to encounter interference. Second, FHSS can provide a degree of security for communications in that only a receiver that knows the frequency hopping pattern can receive and assemble all the packets of a message. Because the hopping pattern for a given spectrum could be constructed in a number of ways, it could be difficult to deduce and follow an unknown hopping pattern, especially when the spectrum is heavily utilized with many radios. Thus FHSS can be employed to hinder eavesdropping. In fact, this latter characteristic led to the invention of FHSS, usually attributed to George Antheil and Hedy Lamarr (the latter is more famous as an American actress). Their 1942 patent of the frequency hopping concept was motivated by an attempt to find a "secret communication system" using radio waves to control torpedoes during World War II.[3]
As previously noted, the use of spread spectrum is required in the 2.4 GHz range, largely to minimize interference problems because the spectrum is unlicensed. The design for Bluetooth wireless communication employs relatively rapid frequency hopping (nominally 1,600 times per second) and is described more fully below and in Chapter 6.
RF is not the only form of wireless communication. Infrared technology is used with devices such as notebook computers, personal digital assistants and electronic remote controls. Infrared wireless communication makes use of the invisible spectrum of light just beyond red in the visible spectrum.
In particular, one standard method for infrared communication is specified by the Infrared Data Association (IrDA; see http://www.irda.org); this method is commonly used with mobile phones and notebook and handheld computers. IrDA technology is relevant when discussing Bluetooth technology because IrDA is also designed for short-range, low-power unlicensed communications. IrDA also defines a physical layer and a software protocol stack designed to promote interoperable communications, as does the Bluetooth specification.
Despite the differences between IrDA and Bluetooth wireless communications, such as data transmission speeds and signal paths (infrared largely requires line-of-sight paths while RF can penetrate many objects), the similarities are such that the SIG worked with the IrDA in developing the specification. Because there is overlap in the application spaces of IrDA and Bluetooth wireless communications, the specification includes an IrDA interoperability layer in which some protocols defined by IrDA are incorporated. This helps to promote interoperability among wireless applications no matter which communications transport is being used. IrDA interoperability in the Bluetooth specification is further detailed in Chapter 9.
The preceding discussion forms the basis for understanding the Bluetooth design, which:
in the lower layers centers around wireless RF communications in the 2.4 GHz spectrum;
is optimized for short-range communication, low power consumption and low cost; and
in the higher layers reuses transport and application protocols already developed for similar domains such as those used with infrared wireless communication.
The result is a wireless communication technology that is especially appropriate for cable replacement and for use with portable devices in pervasive computing applications. Some of the fundamental principles for Bluetooth RF communication are described here; details of the radio and baseband operation are given in Chapter 6.
At the baseband level, when two devices establish a Bluetooth link, one acts in the role of master and the other in the role of slave. The specification permits any Bluetooth radio to assume either role, and a device may act as a master for one communication link and as a slave for another link.[4] The role of master does not imply special privileges or authority; instead it governs the synchronization of the FHSS communications between the devices. The master device determines the frequency hopping pattern (based upon its Bluetooth device address) and the phase for the hopping sequence (based upon its clock). All slaves communicating with a given master hop together in unison with the master. The master role generally is assumed by the device that initiates the communication.[5] Part 2 of this book provides more details about establishing communications links.
A given master may communicate with multiple slaves—up to 7 active slaves and up to 255 parked slaves[6] (active and parked slaves are described more fully below); all slaves communicating with a single master form what the specification calls a piconet (also described more fully below). There can be only one master in a single piconet.
The master-slave relationship is necessary in Bluetooth low level communications but in general devices operate as peers. When one device establishes a point-to-point link with another device, the role that each device assumes (master or slave) is often unimportant and is irrelevant to higher-level protocols and to the user of the device. In some usage scenarios it may be advantageous or even necessary for a given device to assume a particular role, but in many cases it is not critical to establish a single specific role for each device; some scenarios work equally well with device roles reversed. It is important to understand the master-slave relationship for low-level communications while at the same time understanding that in general devices operate as peers to each other. Figure 2.1 shows the master and slave roles in a simple piconet.
As noted above, a piconet can include up to seven active slaves and many more parked slaves. The specification includes a definition for this parked baseband mode, as well as for other modes called sniff and hold. The various baseband modes facilitate energy conservation by allowing the radios to enter low-power states. These low-power modes are really just three different methods for entering and exiting a low-power state, and the mode applies to a given Bluetooth connection (rather than to the device as a whole). These baseband modes also permit a greater number of devices to be co-located in the same proximity sphere, since not all devices need to have active communication links at the same time. All four of these baseband modes (active, sniff, hold and park) apply when the baseband is in a connection state; when not connected, the baseband is in a standby state, which should not be confused with any of the connected state modes. That is, the baseband states are connected and standby; within the connected state there are four modes (active, sniff, hold and parked). These states and modes are described in more detail in Chapter 6.
In active mode a slave essentially always listens for transmissions from the master. Active slaves receive packets that enable them to remain synchronized with the master and that inform them when they can transmit packets back to the master. An active slave must listen for all packets coming from the master, although one optimization is permitted in which active slaves need not listen for entire packets (rather, just the packet headers) coming from the master when it is known that some other active slave is communicating with the master during that time. The active state typically provides the fastest response time but also typically consumes the most power, since it is always receiving packets and is always prepared to transmit packets.
Sniff mode is one method for reducing power consumption. In sniff mode a slave essentially becomes active periodically. The master agrees to transmit packets destined for a particular slave only at certain regular intervals (although it may not transmit packets at every interval). The slave then needs to listen for packets from the master only at the start of each interval (subject to some timing tolerances). If the slave receives packets at the start of the interval it continues to listen and receive packets; otherwise it can "sleep" until the next interval. Sniff mode could permit reduced power consumption by reducing the average duty cycle of the radio but is likely to be less responsive than active mode. The power consumption and responsiveness in sniff mode depend upon the length of the sniff interval.
In hold mode a slave may stop listening for packets entirely for a specified time interval.[7] The master and slave agree upon a hold time, and the communication link is quiesced for that amount of time. During the hold time the slave need not listen for packets from the master and could be doing other things such as establishing links to other devices, or the slave could just sleep during the hold time. At the end of the hold period the slave resumes listening for packets from the master. Hold mode may be less responsive than sniff mode and could permit greater power savings than sniff mode, although this depends upon the hold time duration and upon what the slave does during the hold time (sleeps versus communicates on other links).
In parked mode a slave maintains synchronization with the master but is no longer considered active (slaves in active, sniff and hold modes are considered active). Since there can be only seven active slaves in a piconet at one time, the use of parked mode allows the master to orchestrate communications within a piconet of more than seven devices by exchanging active and parked slaves to maintain up to seven active connections while the remaining slaves in the piconet are parked. A parked slave still needs to maintain synchronization with the master and does so by listening to the master periodically, using a beaconing scheme described in Chapter 6. Parked mode is typically the least responsive of the connected modes, since the slave must make the transition to become an active member of the piconet before resuming general communications, but parked mode may permit greater power conservation.
Figure 2.2 shows a typical relationship of the connected state modes in terms of their relative responsiveness versus power consumption. However, both power consumption and responsiveness in these modes is highly dependent upon factors such as the amount of communications traffic and the hold and sniff periods, which can affect the duty cycle of the radios. As a general rule, active slaves will consume the most power but will be the most responsive, while parked slaves will typically be the least responsive. The figure illustrates the general trend, although these relationships may vary in specific cases.
Figure 2.2. Typical relative responsiveness versus power consumption for connected state baseband modes (generalized; may not apply in all cases).
In addition to the baseband modes which permit energy conservation, another power-saving feature is adaptive transmission power. This feature allows slaves to inform the master when the master's transmission power is not appropriate, so that the master can adjust its transmission power. This is accomplished through the use of a received signal strength indicator (RSSI). When the RSSI value is outside some determined boundaries, the slave can ask the master to adjust the power. This is especially useful when two devices are in close proximity and maximum transmission power is not required (analogous to two people standing next to each other, with one person shouting and the second person asking the shouter to speak more quietly). Of course the converse is also true: transmission power increases also could be requested when the RSSI value indicates too weak a signal but the primary motivation for adaptive transmission power is to reduce power consumption when a lower transmission power is sufficient. The master maintains transmission power settings for each slave so that a change in transmission power for one slave does not affect other slaves in the piconet. Like other energy-conservation features, adaptive transmission power could also allow a greater number of devices to be co-located in the same proximity sphere, since it could further reduce the possibility of RF interference with other devices.
The Bluetooth network model is one of peer-to-peer communications based upon proximity networking. When two devices come within range of each other,[8] they could automatically establish a communications link. Devices will not necessarily begin to communicate spontaneously when they encounter each other, as the baseband could be configured to accept only certain connections, or even none at all. The process of initiating communications links is explored in detail in Chapters 6 and 7.
Proximity networking without wires enables the formation of personal area networks, or federations of personal devices such as mobile telephones, pagers, notebook computers and personal digital assistants. When these devices can communicate seamlessly, their overall utility is enhanced. Another application for proximity networking is the interaction of mobile devices with fixed devices such as kiosks, printers, network access points and vending machines—a person could establish communication between his personal device and a fixed device just by approaching it. This topology enables other usage models, too; these are explored more fully in Chapter 3.
Piconet topology, introduced earlier, can now be further explored given the foregoing discussion of master and slave roles and baseband modes. A piconet consists of a single master and all slaves in proximity that are connected to (in communication with) that master. The slaves may be in active, sniff, hold or park modes at any given time. All of the devices in the piconet are synchronized, all hopping together. There may be other devices in proximity that are not connected to (not in communication with) the master and thus are not part of the piconet, including devices in standby state. Figure 2.3 shows this more general view of a piconet; note that there could be up to seven active slaves and any number of parked slaves and standby devices (although most typical piconets, especially those that are formed to support the version 1.0 profiles, are expected to have only a few devices).
Figure 2.3. General Bluetooth piconet including active and parked slaves. Standby devices which are in proximity but are not part of the piconet are also illustrated.
As described and illustrated above, a device may be an active or parked participant in a piconet or it may not be part of any piconet. In addition, it is possible for a device to take part in more than one piconet. When two or more piconets at least partially overlap in time and space a scatternet is formed. All of the same principles of piconets also apply for scatternets; each piconet has a single master and a set of slaves which may be active or parked. Each piconet has its own hopping pattern determined by its master. A slave could participate in multiple piconets by in turn establishing connections with and synchronizing to different masters in proximity. In fact, a single device might act as a slave in one piconet but assume the master role in another piconet. The scatternet topology provides a flexible method by which devices could maintain multiple connections. This could be especially useful for mobile devices which frequently move into and out of proximity to other devices. Figure 2.4 shows one example of a scatternet using the same representations as in Figure 2.3; other examples of scatternets are possible. In this example slave A2/B3 is a member of both piconet A and piconet B as an active slave.
[1] In some countries there are restrictions and only part of this spectrum is available for unlicensed use; these restrictions are discussed elsewhere in the book, notably in Chapter 6.
[2] Contrast frequency hopping spread spectrum with direct sequence spread spectrum, which is not examined here. Direct sequence is another form of spread spectrum RF communication employed in other technologies such as wireless LANs and is outside the scope of this book.
[3] The complete story of this invention is fascinating but is outside the scope of this book. Interested readers are referred to, for example, [IAL99] or other accounts easily found via World Wide Web search engines. Furthermore, any rationale or implications of the choice of naming the Bluetooth technology after a Danish king rather than an American actress are not explored here.
[4] Some devices might be configured to act in only one role, but most Bluetooth devices are expected to include radios that can assume either role, depending upon the usage case being performed.
[5] The device initiating communication assumes the master role at the outset, although the master and slave roles can be switched, as detailed in Chapter 6.
[6] Actually more than 255 parked slaves are possible. The Bluetooth specification defines "direct" addressing for up to 255 parked slaves via a parked slave address but also permits "indirect" addressing of parked slaves by their specific Bluetooth device address, thus effectively allowing any number of parked slaves, although from a practical perspective it would be unusual to have more than 256 devices in a single piconet. This topic is explored more fully in Chapter 6.
[7] Or at least certain types of packets. Since packet types have not yet been introduced, this section describes the fundamental concept of hold mode. A more complete description can be found in Chapter 6.
[8] Nominal range for the standard 0 dBm Bluetooth radio is approximately 10 meters; power-amplified 20 dBm radios with a range of about 100 meters are also possible. The Bluetooth version 1.0 specification focuses primarily on the standard radio and thus deals mostly with communication within a 10-meter range.