THE MAGAZINE OF WIRELESS COMMUNICATIONS AND NETWORKING
`
`Connectivity and Application
`aR wMOLL)a |
`Computing and
`OTTT@ALECEKS
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`YVEEEPersonal “==
`Communications
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`SONYExhibit 1008 - 0001
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`SONY Exhibit 1008 - 0001
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`FEBRUARY 2000 VOL. 7 NO. 1
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`aR
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`oO 7 2000
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`THE MAGAZINE OF VWvireiS- COMMUNICATIONS AND NETWORKING
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`ww
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`-
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`aus
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`* =IEEEPersonal
`Communications
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`i
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`Guest Editorial:
`Connectivity and Application Enablers for
`Ubiquitous Computing and Communications
`Chatschik Bisdikian, Jaap C. Haartsen, and Parviz Kermani
`8
`Remembering Mark Weiser: Chief Technologist, Xerox PARC
`Roy Want
`11
`IrDA:Past, Present and Future
`Stuart Williams
`20
`Tein &
`netwo
`HomeRF: Wireless Networking for the Connected Home
`asynchronous <|
`~/O\,/peer-peer devices
`Kevin J. Negus, Adrian P. Stephens, and Jim Lansford
`oO
`28
`The Bluetooth Radio System
`Jaap C. Haartsen
`37
`Paving the Way for Personal Area Network Standards:
`An Overview ofthe IEEE P802.15 Working Group for
`Wireless Personal Area Networks
`Thomas M. Siep, lan C. Gifford, Richard C. Braley, and Robert F. Heile
`
`i
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`|
`
`44
`System Support for Mobile, Adaptive Applications
`Brian Noble
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`lgpeel
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`
`
`Coverillustration: The Stock
`Market
`
`Editor’s Note — 4
`Index to Articles: 1999 — 50
`
`iz
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`IEEE Personal Communications * February 2000
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`SONYExhibit 1008 - 0002
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`SONY Exhibit 1008 - 0002
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`

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`
`aT t is my utmost honorandprivilege
`
`EDITOR’S NOTE
`
`MAHMOUD
`NAGHSHINEH
`
`tion ofthe possibilities and potentials wire-
`less and mobile technologies will bring to us.
`It is expected that by 2003 the numberof
`cellular subscribers will be equal to the
`numberof wired phone subscribers. Obvi-
`ously, new evolving architectures and stan-
`dards, such as GPRS and EDGE,and
`third-generation systemswill play a major
`role in this transition, enabling a high-speed,
`ubiquitous wireless Internet. After all, it will
`be a totally new world when one billion peo-
`ple carry a personal gateway to the Internet
`with integrated voice and data support.
`Today, wireless technologies have crossed
`the cost, integration, and power consump-
`tion barriers, and are a key factor in defining new directions
`not only in enterprise but also in consumer markets. Sec-
`ond-generation cellular technologies are leading this front,
`
`to serve our JEEE Personal Communications
`community starting with this issue of our
`magazine. At first, on behalf of our Editorial
`Board and the IEEE Communications Soci-
`ety. | would like to extend my sincere grati-
`tude to Tom La Porta, who served as
`Editor-in-Chief from 1997 to 1999, Tom will
`continue to help and guideus as a senior
`advisor of the magazine. It will be hard to
`match the high level of leadership quality he
`has established, and I count on him, our
`founding Editor-in-Chief, Hamid Ahmadi,
`our Advisory Board, technical editors, and
`the IEEEeditorial team to help me carry my
`responsibility as the new Editor-in-Chief.
`In myview, this is the most exiting time in our area;
`we are starting a new millennium with tremendousanticipa-
`
`
`
`
`IEEE Personal Communications — The Maga-
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`Networking (ISSN 1070-9916) is published
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`IEEEPersonal
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`2000 Communications Society
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`JEEE Personal Communications * February 2000
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`SONY Exhibit 1008 - 0003
`
`SONY Exhibit 1008 - 0003
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`

`

`This material may be protected by Copyright law (Title 17 U.S. Code)
`
`Abstract
`A few years ago it was recognized that the vision of a truly low-cost, low-power radio-based cable replacement was feasible. Such a ubiquitous
`link would provide the basis for portable devices to communicate together in an ad hoe fashion by creating personal area networks which have
`similar advantagesto their office environment counterpart, the local area network. Bluetooth™ is an effort by a consortium of companies to
`design a royalty-free technology specification enabling this vision. Thisarticle describes the radio system behind the Bluetooth concept. Designing
`an ad hoe radio system for worldwide usage poses several challenges. Thearticle describesthecritical system characteristics and motivates the
`design choices that have been made.
`
`The Bluetooth Radio System
`
`
`
`JAAP C. HAARTSEN, ERICSSON RADIO SYSTEMS B.V.
`
`n the last decades, progress in
`microelectronics and very large scale integration (VLSI) tech-
`nology has fostered the widespread use of computing and
`communication devices for commercial usage. The success of
`consumer products like PCs, laptops, personaldigital assis-
`tants (PDAs), cell phones, cordless phones, and their periph-
`erals has been based on continuouscost and size reduction.
`Information transfer between these devices has been cumber-
`some, mainly relying on cables. Recently, a new universal
`radio interface has been developed enabling electronic devices
`to communicate wirelessly via short-range ad hoe radio con-
`nections. The Bluetooth technology — whichhas gained the
`support of leading manufacturers like Ericsson, Nokia, IBM,
`Toshiba, Intel, and many others — eliminates the need for
`wires, cables, and the corresponding connectors between cord-
`less or mobile phones, modems, headsets, PDAs, computers,
`printers, projectors, and so on, and paves the way for new and
`completely different deyices and applications. The technology
`enables the design of low-power, small-sized, low-cost radios
`that can be embeddedin existing (portable) devices. Eventual-
`ly, these embedded radios will lead toward ubiquitous connec-
`tivity and truly connect everything to everything. Radio
`technology will allow this connectivity to occur without any
`explicit user interaction.
`This article describes the basic design and technology
`trade-offs which have led to the Bluetooth radio system. We
`describe some fundamental issues regarding ad hoc radio sys-
`tems. We give an overview of the Bluetooth system itself with
`the emphasis on the radio architecture. It explains howthe
`system has been optimized to support ad hoe connectivity. We
`also describe the Bluetooth specification effort.
`Ad Hoc Radio Connectivity
`The majority of radio systems in commercial use today are
`based on a cellular radio architecture. A mobile network estab-
`lished on a wired backboneinfrastructure uses one or more
`basestations placed at strategic positions to provide localcell
`coverage; users apply portable phones, or more generic mobile
`terminals, to access the mobile network; the terminals main-
`tain a connection to the network via a radio link to the base
`stations. Thereis a strict separation betweenthe base stations
`and the terminals. Once registered to the network, the termi-
`nals remain locked to the control channels in the network, and
`connections can be established and released according to the
`control channel protocols. Channel access, channel allocation,
`traffic control, and interference minimization are neatly con-
`
`trolled by the base stations. Examples of these conventional
`radio systems are the public cellular phone systems like Glob-
`al System for Mobile Communications (GSM), D-AMPS,and
`IS-95 [1-3], but also private systemslike wireless local area
`network (WLAN) systems based on 802.11 or HIPERLAN I
`and HIPERLAN II [4-6]. and cordless systems like Digital
`Enhanced Cordless Telecommunications (DECT) and Person-
`al Handyphone System (PHS)[7, 8].
`In contrast, in truly ad hoc systems, there is no difference
`between radio units; that is, there are no distinctive base sta-
`tions or terminals. Ad hoc connectivity is based on peer com-
`munications. There is no wired infrastructure to support
`connectivity between portable units; there is no central con-
`troller for the units to rely on for making interconnections; nor
`is there support for coordination of communications. In addi-
`tion, there is no intervention of operators. For the scenarios
`envisioned by Bluetooth,it is highly likely that a large number
`of ad hoc connectionswill coexist in the same area without any
`mutual coordination; that is, tens of ad hoc links must share
`the same medium at the same location in an uncoordinated
`fashion. This is different from ad hoc scenarios considered in
`the past, where ad hoc connectivity focused on providing a sin-
`gle (or very few) network(s) between the units in range [4,5].
`For the Bluetooth applications, typically many independent
`networks overlap in the same area. This will be indicated as a
`seatter ad hoc environment. Scatter ad hoc environments con-
`sist of multiple networks, each containing only a limited num-
`ber of units. The difference between a conventionalcellular
`environment, a conventional ad hoc environment, and a scatter
`ad hoc environmentis illustrated in Fig. 1. The environmental
`characteristics the ad hoc radio system has to operate in have a
`major impacton the following fundamental issues:
`* Applied radio spectrum
`* Determining which units are available to connect to
`* Connection establishment
`* Multiple access scheme
`* Channelallocation
`* Medium access control
`* Service prioritization (i.e., voice before data)
`* (Mutual) interference
`* Power consumption
`Ad hoe radio system have been in use for some time, for
`example, walky-talky systems used by the military, police, fire
`departments, and rescue teams in general. However, the Blue-
`tooth system is the first commercial ad hoe radio system envi-
`sioned to be used on a large seale and widely available to the
`public.
`
`;
`
`1070-9916/00/$10.00 © 2000 IEEE
`28
`—_—_—_—_—_—_—_—_—_—_—
`
`IEEE Personal Communications « February 2000
`SONY Exhibit 1008 - 0004 _
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`SONY Exhibit 1008 - 0004
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`

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`Bluetooth Radio System Architecture
`
`In this section the technical background of the Bluetooth
`radio system is presented. [t describes the design trade-offs
`made in order to optimize the ad hoc functionality and
`addresses the issues listed above.
`
`Radio Spectrum
`The choice of radio spectrum is first determined by the lack
`of operator interaction. The spectrum must be opento the
`public without the need for licenses. Second, the spectrum
`must be available worldwide. The first Bluetooth applications
`are targeted at the traveling businessperson who connects
`his/her portable devices wherever he/she goes. Fortunately,
`there is an unlicensed radio band thatis globally available.
`This band, the Industrial, Scientific, Medical (ISM) band,is
`centered around 2.45 GHz and was formerly reserved for
`some professional user groups but has recently been opened
`worldwide for commercial use. In the United States, the band
`ranges from 2400 to 2483.5 MHz,and the FCC Part 15 regu-
`lations apply. In most parts of Europe,! the same band is
`available under the ETS-300328 regulations. In Japan, recent-
`ly the band from 2400 to 2500 MHz has been allowed for
`commercial applications and has been harmonized with the
`rest of the world. Summarizing, in most countries of the
`world, free spectrum is available from 2400 MHz to 2483.5
`MHz, and harmonization efforts are ongoing to have this
`radio band available truly worldwide.
`The regulations in different parts of the world differ. How-
`ever, their scope is to enable fair access to the radio band by
`an arbitrary user. The regulations generally specify the
`spreading of transmitted signal energy and maximum allow-
`able transmit power. For a system to operate globally, a radio
`concept has to be found thatsatisfies all regulations simulta-
`neously. The result will therefore be the minimum denomina-
`tor of all the requirements.
`Interference Immunity
`Since the radio bandis free to be accessed by any radio trans-
`mitter as long as it satisfies the regulations, interference
`immunity is an important issue. The extent and nature of the
`interference in the 2.45 GHz ISM band cannotbe predicted.
`Radio transmitters may range, for example, from 10 dBm
`baby monitors to 30 dBm WLAN access points. With high
`probability, the different systems sharing the same bandwill
`not be able to communicate. Coordination is therefore not
`possible. More of a problem are the high-power transmitters
`covered by the FCC part 18 rules which include, for example,
`microwave ovensand lighting devices. These devices fall out-
`side the power and spreading regulations of part 15, butstill
`coexist in the 2.45 GHz ISM band.In addition to interference
`from external sources, co-user interference must be taken into
`account, which results from other Bluetooth users.
`Interference immunity can be obtained by interference
`suppression or avoidance. Suppression can be obtained by
`coding or direct-sequence spreading. However, the dynamic
`range of the interfering and intended signals in an ad hoc,
`uncoordinated radio environment can be huge. Taking into
`account the distance ratios and power differences of uncoordi-
`nated transmitters, near-far ratios in excess of 50 dB are no
`exception. With desired user rates on the order of 1 Mb/s and
`beyond, practically attained coding and processing gains are
`inadequate. Instead, interference avoidance is more attractive
`
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`§ Figure 1. Topologiesfor: a) cellular radio systems with squares
`representing stationary base stations; b) conventional ad hoc
`systems; and ¢) scatter ad hac systems.
`
`since the desired signal is transmitted at points in frequency
`and/or time where interference is low or absent. Avoidance in
`time can be an alternative if the interference concerns a
`pulsed jammer and the desired signal can be interrupted.
`Avoidance in frequencyis more practical. Since the 2.45 GHz
`band provides about 80 MHz of bandwidth and most radio
`systems are band-limited, with high probability a part of the
`radio spectrum can be found where there is no dominant
`interference. Filtering in the frequency domain provides the
`suppression of the interferers at other parts of the radio band.
`The filter suppression can easily arrive at 50 dB or more.
`Multiple Access Scheme
`The selection of the multiple access scheme for ad hoc radio
`systems is driven by the lack of coordination and the regula-
`tions in the ISM band. Frequency-division multiple access
`(FDMA)is attractive for ad hoc systems since channel orthog-
`onality only relies on the accuracy of the crystal oscillators in
`the radio units. Combined with an adaptive or dynamic chan-
`nel allocation scheme, interference can be avoided. Unfortu-
`nately, pure FDMA does not
`fulfill
`the spreading
`requirements set in the ISM band. Time-division multiple
`access (TDMA)requiresstrict time synchronization for chan-
`nel orthogonality. For multiple collocated ad hoe connections,
`maintaining a common timing reference becomes rather cum-
`bersome. Code-division multiple access (CDMA)offers the
`best properties for ad hoc radio systemssince it provides
`spreading and can deal with uncoordinated systems. Direct
`sequence (DS)-CDMAisless attractive because of the near-
`far problem which requires coordinated power control or
`excessive processing gain. In addition, as in TDMA, DS-
`CDMA channel orthogonality requires a commontiming ref-
`erence. Finally, for higher user rates, rather high chiprates
`are required, which is less attractive because of the wide
`bandwidth (interference immunity) and higher current con-
`sumption. Frequency-hopping (FH)-CDMA combines a num-
`ber properties which makeit the best choice for ad hoc radio
`systems. On average the signal can be spread overa large fre-
`quency range, but instantaneously only a small bandwidth is
`occupied, avoiding most of the potential interference in the
`ISM band. The hop carriers are orthogonal, and the interfer-
`"In France and Spain the exact locationofthe band differs, and the band
`is smaller.
`ence on adjacent hops can effectively be suppressed byfilter-
`
`TEBE Personal Communications * February 2000
`i
`
`29
`
`SONY Exhibit 1008 - 0005
`
`SONY Exhibit 1008 - 0005
`
`

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`ing. The hop sequenceswill not be orthogonal (coordination
`of hop sequencesis not allowed by the FCC rules anyway),
`but narrowband and co-user interference is experienced as
`short interruptions in the communications, which can be over-
`come with measuresat higher-layer protocols,
`Bluetooth is based on FH-CDMA. In the 2.45 GHz ISM
`band, a set of 79 hop carriers have been defined at a 1 MHz
`spacing.? The channelis a hopping channel with a nominal
`hop dwell time of 625 is. A large number of pseudo-random
`hopping sequences have been defined. The particular
`sequenceis determined by the unit that controls the FH chan-
`nel, which is called the master. The native clock of the master
`unit also defines the phase in the hopping sequence.All other
`participants on the hopping channel areslaves; they use the
`master identity to select the same hopping sequence and add
`time offsets to their respective native clocks to synchronize to
`the frequeney hopping. In the time domain, the channelis
`divided into slots. The minimum dwell time of 625 us corre-
`spondsto a single slot. To simplify implementation, full-
`duplex communicationsis achieved by applying time-division
`duplex (TDD). This means that a unit alternately transmits
`and receives. Separation of transmission and reception in time
`effectively prevents crosstalk between the transmit and receive
`operationsin the radio transceiver, which is essential if a one-
`chip implementation is desired, Since transmission and recep-
`tion take place at different time slots, transmission and
`reception also take place at different hop carriers. Figure 2
`illustrates the FH/TDD channelapplied in Bluetooth. Note
`that multiple ad hoc links will make use of different hopping
`channels with different hopping sequences and may have mis-
`aligned slottiming,
`The Modulation Scheme
`In the ISM band,the signal bandwidth of FH systemsis limit-
`ed to 1 MHz. For robustness, a binary modulation scheme was
`chosen. With the above-mentioned bandwidth restriction, the
`data rates are limited to about 1 Mb/s. For FH systems and
`support for bursty data traffic, a noncoherent detection
`scheme is most appropriate. Bluetooth uses Gaussian-shaped
`frequency shift keying (FSK) modulation with a nominal mod-
`ulation index of k = 0.3. Logical onesare sentas positive fre-
`quency deviations, logical zeroes as negative frequency
`deviations. Demodulation can simply be accomplished by a
`limiting FM discriminator. This modulation scheme allows the
`implementation of low-cost radio units,
`
`Medium Access Control
`Bluetooth has been optimized to allow a large numberof
`uncoordinated communications to take place in the same
`area. Unlike other ad hoc solutions whereall units in range
`es
`
`30
`
`
`
`IEEE Personal Communications « February 2000
`
`SONYExhibit 1008-0006
`
`‘
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`share the same channel, Bluetooth has been
`designed to allow a large numberof independent
`channels, each channel serving only a limited num-
`ber of participants. With the considered modulation
`scheme, a single FH channel in the ISM band only
`supports a gross bit rate of 1 Mb/s. This capacity has
`to be shared by all participants on the channel. The-
`oretically, the spectrum with 79 carriers can support
`25 tis
`79 Mb/s. In the user scenarios targeted by Blue-
`tooth, it is highly unlikely that all units in range
`@ Figure 2. An illustration ofthe FH/TDD channel applied in Bluetooth.
`need to share information among all of them. By
`using a large numberof independent 1 Mb/s chan-
`nels to which only the units are connectedthat real-
`ly want to exchange information, the 80 MHzis exploited
`much more effectively. Due to nonorthogonality of the hop
`sequences, the theoretical capacity of 79 Mb/s cannot be
`reached, butis at least much larger than 1 Mb/s.
`An FH Bluetooth channelis associated with a piconet. As
`mentioned earlier, the piconet channelis defined by the identi-
`ty (providing the hop sequence) and system clock (providing
`the hop phase) of a master unit, All other units participating in
`the piconet are slaves. Each Bluetooth radio unit has a free-
`running system or native clock, There is not a commontiming
`reference, but when a piconetis established, the slaves add off-
`sets to their native clocks to synchronize to the master. These
`offsets are released again when the piconetis cancelled, but can
`be stored for later use. Different channels have different mas-
`ters and therefore also different hopping sequences and phases.
`The numberof units that can participate on a common channel
`is deliberately limited to eight (one master and seven slaves) in
`order to keep a high-capacity link betweenall the units. It also
`limits the overhead required for addressing. Bluetooth is based
`on peer communications. The master/slaverole is only attribut-
`ed to a unit for the duration of the piconet. When the piconet
`is cancelled, the master and slave roles are cancelled. Each unit
`can become a master or slave. By definition, the unit that estab-
`lishes the piconet becomes the master,
`In addition to defining the piconet, the master also controls
`the traffic on the piconet and takes care of access control.
`Access is completely contention free. The short dwell time of
`625 js only allows the transmission of a single packet. A con-
`tention-based access scheme would provide too much over-
`head and is not efficient in the short dwell time Bluetooth
`applies. In Bluetooth, the master implements centralized con-
`trol; only communication between the master and one or more
`slaves is possible. The timeslots are alternately used for mas-
`ter transmission and slave transmission. In the master trans-
`mission, the master includes a slave address of the unit for
`which the information is intended. In order to preventcolli-
`sions on the channel due to multiple slave transmissions, the
`master applies a polling technique: for each slave-to-master
`slot, the master decides which slave is allowed to transmit. This
`decision is performed on a per-slot basis: only the slave
`addressed in the master-to- slave slot directly preceding the
`slave-to-masterslot is allowed to transmit in this slave-to-mas-
`ter slot. If the master has information to send to a specific
`slave, this slave is polled implicitly and can return information.
`If the master has no information to send,it has to poll the
`slave explicitly with a short poll packet. Since the master
`Schedules the traffic in both the uplink and downlink,intelli-
`gent scheduling algorithms have to be used that take into
`account the slave characteristics. The master control effectively
`prevents collisions between theparticipants on the piconet
`channel. Independentcollocated piconets may interfere when
`they occasionally use the same hop carrier. A type of ALOHA
`? Currently, for France and Spain a reduced set af23 hop carriers has been
`is applied: information is transmitted without checking for a
`defined at a 1 MHz carrier spacing.
`clear carrier (no listen-before-talk). If the information is
`
`
`SONY Exhibit 1008 - 0006
`
`

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`72bits 54 bits
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`0-2745 bits
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`received incorrectly, it is retransmitted at the next
`transmission opportunity (for data only), Due to the
`short dwell time, collision avoidance schemes are
`less appropriate for FH radio. For each hop, differ-
`ent contenders are encountered. Backoff mecha-
`nisms are therefore less efficient.
`
`Payload
`Packet
`Access
`
`code header
`
`M Figure 3. The format ofpackets applied in Bluetooth.
`
`Physical Link Definition
`The Bluetooth link supports both synchronousservices such
`as voice traffic, and asynchronous services such as bursty data
`traffic. Two physical link types have been defined:
`* The synchronous connection-oriented (SCO)link
`* The asynchronous connectionless (ACL) link
`The SCOlinkis a point-to-point link between the master
`and a single slave. The link is established by reservation of
`duplex slots at regular intervals. The ACLlink is a point-to-
`multipoint link between the master and all the slaves on the
`piconet. The ACLlink can use all of the remainingslots on
`the channel not used for SCO links. The traffic over the
`ACL link is scheduled by the master. The slotted structure
`of the piconet channel allows effective mixing of the syn-
`chronous and asynchronouslinks. An example of a channel
`
`
`Packet-Based Communications
`have been defined, as will be further explained later. The
`The Bluetooth system uses packet-based transmission: the
`interpretation of packettype is different for synchronous and
`information stream is fragmented into packets. In each slot,
`asynchronous links. Currently, asynchronous links support
`only a single packet can be sent. All packets have the same
`payloads with or without a 2/3-rate FEC coding scheme. In
`format, starting with an access code, followed by a packet
`addition, on these links single-slot, three-slot, and five-slot
`header, and ending with the user payload (Fig.3),
`packets are available. The maximum user rate that can be
`The access code has pseudo-random properties andis used
`obtained over the asynchronous link is 723.2 kb/s. In that
`as a direct-sequence code in certain access operations. The
`case, a return link of 57.6 kb/s can still be supported. Link
`access code includes the identity of the piconet master. All pack-
`adaptation can be applied on the asynchronouslink by chang-
`ets exchanged on the channel are identified by this master iden-
`ing the packet length and FEC coding depending on link con-
`tity. Only if the access code matches the access code
`ditions. The payload length is variable and depends on the
`correspondingto the piconet masterwill the packet be accepted
`available user data. However, the maximum length is limited
`by the recipient. This prevents packets sent in one piconetfalse-
`by the minimum switching time between RX and TX, which
`ly being accepted by units of another piconet that happens to
`is specified at 200 us. This switching time seems large, but
`land on the same hopcarrier. In the receiver, the access code is
`allows the use of open-loop voltage controlled oscillators
`matched against the anticipated code inasliding correlator,
`(VCOs) for direct modulation and provides time for packet
`This correlator provides the direct-sequence processing gain.
`processing between RX and TX;
`this is also discussedlater.
`The packet header contains link control information: a 3-bit
`For synchronous links, only single-slot packets have been
`slave address to separate the slaves on the piconet, a 1-bit
`defined, The payload length is fixed. Payloads with 1/3-rate
`acknowledgment/negative acknowledgmennt (ACK/NACK)for
`FEC, 2/3-rate, or no FEC are supported. The synchronous
`the automatic repeat request (ARQ) scheme, a 4-bit packet type
`link supports a full-duplex link with a user rate of 64 kb/s in
`both directions.
`code to define 16 different payload types, and an 8-bit header
`error check (HEC) code which is a cyclic redundancy check
`(CRC) code to detect errors in the header. The packet headeris
`limited to 18 information bits in orderto restrict the overhead.
`The headeris further protected by 1/3 rate forward error correc-
`tion (FEC) coding. Bluetooth defines four control packets:
`* The ID or identification packet: Only consists of the access
`code; used for signaling
`* The NULL packet: Only has an access code and a packet
`header; used if link control information carried by the
`packet header has to be conveyed
`* The POLL packet: Similar to the NULL packet; used by the
`master to force slaves to return a response
`* The FHS packet: An FH-synchronization packet; used to
`exchange real-time clock and identity information between
`the units; contains all the information to get two units hop
`synchronized
`T