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Section 11.2 presents the Bluetooth specification and discusses, among others,the way Bluetooth devices establish connections and exchange data.. The aim of the stack shownin Figure 11.2

Personal Area Networks (PANs)

11.1 Introduction to PAN Technology and Applications

11.1.1 Historical Overview

The concept of a Personal Area Network (PAN) differs from that of other types of datanetworks (e.g LAN, MAN, WAN) in terms of size, performance and cost (Figure 11.1).PANs are the next step down from LANs and target applications that demand short-rangecommunications inside the Personal Operating Space (POS) of a person or device The termPOS is used to define the space in the near vicinity of a person or device and can be thought of

as a bubble that surrounds him As the person goes through his regular daily activities, hisPOS changes to include a number of different devices (such as cellular phones, pagers,headphones, PC interfaces, etc.) with whom the ability for easy and transparent informationexchange would be useful PANs aim to provide such ability in an efficient manner.There exist a number of different communication mediums to choose for implementing aPAN, such as electric and magnetic fields, radio and optical signal transmission One of thefirst research concepts for PANs dates back to an IBM research project in 1996 and is known

Figure 11.1 The various kinds of wireless networks

ISBN: 0-470-84529-5

as ‘Near-field Intra-body Communication PAN (NIC-PAN)’ [1] This approach uses thehuman body as the communication medium, which can conduct electricity due to its naturalcontent of salt According to this approach, NIC-PAN-compliant devices worn by a user cancommunicate with each other through the user’s body, thus no wires are needed Furthermore,

a user wearing such a device can initiate communication with another device located onanother user by means of a simple handshake In order to transmit data between devicesattached to the users’ bodies or clothing, the NIC-PAN device transmitter charges anddischarges the human body, thus resulting in an oscillating potential appearing betweenthe body and the environment These changes of potential are picked up by the receivingNIC-PAN device and thus a communication channel is established The electrical currentused in this approach is approximately 1 nA, which is much lower than the natural electricalcurrent of the human body In the context of this research, a small prototype was built, whichachieved data rates around 2.4 kbps

However, the NIC-PAN approach did not evolve into something more than a researchproject, whereas the true revolution in the area of PANs was brought about by the use ofwireless transmission-based PANs (WPANs1) The first attempt to define a standard for PANsdates back to an Ericsson project in 1994, which aimed to find a solution for wirelesscommunication between mobile phones and related accessories (e.g hands-free kits) Thisproject was named Bluetooth, after the king that united the Viking tribes Bluetooth was apromising approach and as a result, Nokia, Intel, Toshiba and IBM joined Ericsson to formthe Bluetooth Special Interest Group (SIG) in May 1998 The purpose of the Bluetooth SIG is

to develop a de facto standard for PANs that meets the communication needs of all mobilecomputing and communication devices located in a reduced geographical space regardless oftheir size or power budget The size of the SIG has grown from the initial five members tonearly 2000 others at the present day Any interested company is allowed to join the SIGprovided that it lets all other members of the SIG use its patents royalty-free, in an effort tokeep the standard open Version 1.0 of the Bluetooth specification was released by the SIG in

1999, followed by version 1.1 in 2001 Both these versions support 64 kbps voice channelsand asynchronous data channels either asymmetric, with a maximum data rate of 721 kbps inone direction and 57.6 in the other or symmetric with a 432 kbps maximum rate in bothdirections Bluetooth uses Frequency Hopping Spread Spectrum (FHSS) modulation in the2.4 GHz ISM band The supported range is 10 m with the possibility of extending this to 100

m [2]

Another initiative from industry members to develop a PAN standard was made in 1997 bythe formation of the HomeRF Working Group The primary goal of this group is to enableinteroperable wireless voice and data networking within the home Version 1.0 of HomeRFwas published in 1999 It supported four 32 kbps voice connections and data rates up to 1.6Mbps at ranges up to 50 m Version 2.0 of HomeRF was released in 2001 and increased thesenumbers to eight channels and 10 Mbps, respectively, making HomeRF more suitable thanBluetooth for transmitting music, audio, video and other high data applications However,Bluetooth seems to have more industry backing Like Bluetooth, HomeRF also supportsvoice and asynchronous data channels using FHSS modulation in the 2.4 GHz ISM band.After the appearance of the Bluetooth and HomeRF initiatives, IEEE also decided to jointhe area of developing specifications for PANs Thus the 802.15 Working Group [2–5] was

1

Since all PAN technology alternatives today employ wireless transmission, the terms PAN and WPAN are used

in this book synonymously.

formed in March 1999, with the responsibility of defining physical and MAC layer tions for PANs having low implementation complexity and low power consumption.Although Working Group 802.11, which deals with Wireless LANs, already existed, itwas decided that a new Working Group was needed for PAN standardization This is attrib-uted to the fact that there is a much greater concern over power consumption, size, andproduct cost in PANs stemming from the demands for PAN devices compared to WLANdevices:

specifica-† Less size and weight, in order to be easily carried or worn for long periods of time On theother hand, the size and weight of WLAN cards is a matter of secondary importance This

is because WLAN devices are typically either attached to portable computers, which ofcourse are not carried by users all of the time, or to fixed desktop PCs

† Lower cost, in order not to burden the total cost of the device PAN devices aim to providewireless connectivity to commercial electronic appliances In order to enable small devicesize, PAN functionality will be integrated within the devices To earn market acceptance,the cost burden of PAN functionality on the total cost of the device should be small Users,

on the other hand, can buy WLAN cards separately, thus the impact of their cost is lessimportant

There are four Task Groups (TGs) inside Working Group 802.15:

† TG1 This group is working on a PAN standard based on Bluetooth

† TG2 This group aims to facilitate coexistence of PAN and WLAN networks

† TG3 This group aims to produce a PAN standard with data rates exceeding 20 Mbps,while maintaining low cost, power consumption and interoperability with industry stan-dards

† TG4 This group aims to produce a PAN standard that will enable low-rate operation whileachieving levels of power consumption so low, that a battery life of months or years will bepossible

Due to the fact that industry consortia initiatives for PAN standards development precededthe initiative of IEEE, a key mission of the 802.15 Working Group will be to work closelywith such consortia, such as Bluetooth and HomeRF, in order to achieve interoperability forPANs coexisting in a shared wireless medium

11.1.2 PAN Concerns

There are certain issues that need to be taken into account when designing a PAN The mostobvious one affects all types of wireless networks and concerns the increased Bit Error Rate(BER) of the wireless medium As has been mentioned, the primary reason for the increasedBER is atmospheric noise, physical obstructions found in the signal’s path, multipath propa-gation and interference from other systems The primary source of interference in the 2.4 GHzISM band in which PANs operate, comes both from narrowband and wideband sources, such

as microwave ovens Apart from the good interference avoidance properties of SS tion, which is employed for unlicensed transmission in the 2.4 GHz ISM band, AutomaticRepeat Requests (ARQ) and Forward Error Control (FEC) techniques can also be used in thisdirection Furthermore, PANs have to deal with interference from collocated PANs andWLANs, although this should not be a big problem due to the use of FHSS modulation

modula-PANs should provide full communication capability between all devices in the POS of aperson However, two PAN devices that initiate communication inside the POS should notinterfere with other devices when this is not wanted Consider for example the case of aperson entering a conference room After sitting in a chair, a PAN interface nearby couldcommunicate with a handheld device of the person and deliver to him the information hewants However, automatic initiation of communication with the devices of nearby confer-ence attendants may not be desirable for privacy reasons.

A person carrying a PAN-enabled device could find him-/herself in a diverse range ofsituations, whether personal or professional, that demand information delivery through thePAN device Therefore, PAN devices should be compatible to enable information exchange

in all cases Returning to the conference example, imagine two attendants wanting toexchange information, discovering that their systems are not compatible and thus theyneed to exchange the information on paper In such case, the market community wouldsee PANs as a waste of time and money Compatibility not only involves following a specificPAN standard but also software compatibility For example, the file formats on each of theabove devices should be able to be read by both devices

PAN-enabled devices will be typically be carried by people for long periods of time Thus,they need to be small enough in order to be carried around without burdening PAN devicesshould therefore be as small and light as possible Their energy efficiency should be enough inorder not to trouble the user with frequent recharging of batteries, while maintaining a lowdevice weight Therefore, both low power consumption devices and high capacity batteriesare desirable Furthermore, the efficiency in terms of size and weight should not come at anexpense over the price of PAN devices, in order to enable market acceptance

Security issues are also crucial in PANs Communications should be secure and difficult toeavesdrop Consider the case of a malicious user approaching pedestrians carrying PANdevices The unpleasant ‘Big Brother’ scenario of Orwell’s 1984 is obvious here and should

be as difficult to achieve as possible It should be very hard for the malicious user to obtaininformation regarding the unsuspected pedestrians, such as their names, home addresses, etc.Therefore, robust authentication and encryption schemes should be developed in an effort toprevent unauthorized initiation of communication and eavesdropping These schemes should

be developed while keeping in mind the relatively low processing and power capabilities ofPAN devices which stem from the requirements for reduced cost, size and weight

Finally, as in the case of all wireless networks human safety issues are of great concern APAN device will typically be very close to the user for long periods of time and therefore evensmall dangers could potentially have some impact on the user over time The good thing here

is that PAN (like WLAN) devices typically transmit at power levels up to 0.5 W Despite thefact that a final answer to the question of radiation threats to human health has yet to be given,

it is reassuring for the consumers to know that the operating power levels of a PAN device aresubstantially lower than the 600-mW to 3-W range of common cellular phones

11.1.3 PAN Applications

The main goal of PANs is freedom from cables and easy sharing of information between allkinds of wireless devices The number of different possible applications can be very large Inthe following we outline a representative set:

† Personal device synchronization Automatic data synchronization between mobile less equipment such as a mobile phone, notebook PC, etc that execute similar applica-tions.

wire-† Ad hoc connectivity Transferring files, and other information to another user’s enabled device

PAN-† Cordless computer Wireless interfacing of devices like mice, keyboards, game pads to thecomputer

† Cordless peripherals Access to a variety of wireless peripherals including printers, ners, fax, copier, storage systems, etc

scan-† Localized wireless LAN access PAN-enabled devices can gain access to services offered

by wired LANs through PAN-compatible Access Points (APs)

† Internet access Downloads of email or browsing a web page using a PAN-enabled device,such as a mobile phone

† Wireless synchronization Synchronization of portable devices with the stationary serversvia PAN APs

† Cordless telephony/headset A user selects a contact name from a handheld, the handheldwirelessly prompts the mobile phone in its proximity to dial the number and the audiofrom the call is wirelessly forwarded to the user’s headset

† Home automation Seamless transfer of commands to PAN-enabled home devices Forexample, automatic unlocking of the door upon the arrival of the user at his home, orautomatic tuning of the television to the user’s favorite channel upon his entrance to theroom

† Electronic purchases/reservations PAN devices can be used to electronically book ets For example, the PAN device of a user can be programmed to instantly initiate arequest for booking a ticket for a specific flight when the user enters the airport, thusavoiding the long queues and waiting times of the traditional booking procedures

tick-† Emergency situations Medical devices with PAN interfaces can be used in order toincrease the safety of patients For example, pacemakers could be monitored andcontrolled remotely through PAN interfaces, or can be programmed to immediately call

an ambulance while also transmitting the patient’s medical condition in the case of a heartattack or other serious health problem

11.1.4 Scope of the Chapter

The remainder of this chapter provides a detailed presentation of technological alternatives inthe PAN area Section 11.2 presents the Bluetooth specification and discusses, among others,the way Bluetooth devices establish connections and exchange data Furthermore, Blue-tooth’s provisions on security and power management are discussed Section 11.3 is a similardiscussion on the HomeRF standard The chapter ends with a brief summary Section 11.4

11.2 Commercial Alternatives: Bluetooth

11.2.1 The Bluetooth Specification

The Bluetooth specification 1.1 [6–9] comprises two parts: core and profiles The core

specification defines the layers of the Bluetooth Protocol stack The aim of the stack (shown

in Figure 11.2) is to provide a common data link and physical layer to applications and level protocols that communicate over the Bluetooth wireless link and maximize reuse ofexisting protocols at the higher layers The protocols that run at different layers of the stackcan be categorized into three groups: Bluetooth core, cable replacement protocols(RFCOMM), telephony control protocols (TCS and AT commands) and adopted protocols(such as IP, PPP, etc.) From these protocols, only the core protocols, RFCOMM and TCS areBluetooth-specific protocols (those run at the shaded layers of Figure 11.2) The layers of thestack are summarized below:

high-† The radio layer provides the electrical specifications in order to send and receivebitstreams over the wireless channel These specifications are discussed in Section 11.2.2

† The baseband layer enables the operation of the Bluetooth links (described in Section11.2.4) over the wireless medium This layer is also responsible for framing, flow controland timing operations and it also manages the links between communicating Bluetoothdevices

† The Link Management (LM) layer runs the Link Management Protocol (LMP) This is anentity responsible for managing connection states, ensuring access fairness, performingpower management and providing authentication and encryption services to upper layers.Power management issues are discussed in Section 11.2.7 while security is discussed inSection 11.2.8

† The Logical Link And Adaptation Layer (L2CAP) provides connection-oriented andconnectionless data services to upper layer protocols with protocol multiplexing capabil-ity, segmentation and reassembly (SAR) operations, and group abstractions L2CAPpermits higher-level protocols and applications to transmit and receive L2CAP packets

up to 64 kilobytes in length L2CAP supports only data traffic As can be seen from Figure11.2, audio data is not conveyed through L2CAP but is mapped directly to the basebandlayer Thus, data for audio connections is exchanged directly between the baseband layers

of Bluetooth devices

† The service discovery layer runs the Service Discovery Prototcol (SDP), which is used in

Figure 11.2 The Bluetooth protocol stack Shaded layers implement Bluetooth-specific protocols

order for a Bluetooth device to learn about services on offer and neighboring deviceinformation Using SDP, neighbors of a device can be queried and if some requirementsare met, a connection can be established.

† The RFCOMM layer runs a serial line RS-232 control and data signal emulation protocol

It is used for cable replacement, offering transport capabilities over the wireless link toapplications that use serial lines as a transport mechanism

† The TCS layer defines call control signaling procedures for the establishment of voice anddata calls between Bluetooth devices

† The Host Controller Interface (HCI) is not a stack layer but an interface that provides themeans for accessing the Bluetooth hardware capabilities

† Layers that implement non-Bluetooth specific protocols (OBEX, WAP, etc.) are used toenable high-layer application functionality

The profiles part of the specification is used to classify Bluetooth applications into nineapplication profiles, with each profile implementing only a certain set of the stack’s protocols.This approach has received some criticism, which supports that the Bluetooth specification isessentially a set of nine standards instead of one, with the number likely to rise as newapplication profiles are added However, the existence of application profiles aims to ensureinteroperability between Bluetooth devices In order for a device to be certified for a specificBluetooth application, it has to follow the corresponding profile Furthermore, the production

of devices for a specific application means that the device could support only some of theapplication profiles, thus reducing its overall cost Apart from the nine application profiles,version 1.1 of the Bluetooth specification also supports four system profiles, which includefunctionality common to one or more application profiles The thirteen profiles are summar-ized below Some of the profiles can exist only if they implement other profiles, as shown inFigure 11.3

Figure 11.3 The Bluetooth profiles

† Generic access profile This system profile is responsible for link maintenance betweendevices This profile is not useful for supporting any useful application by itself, however,

it needs to be supported in every Bluetooth device since it includes the functionalityneeded to use all the other system profiles

† Service discovery application profile This is another system profile that enables users toaccess the Service Discovery Protocol (SDP) in order to find out which applications aresupported by a specific device The support of the service discovery application profile isoptional If, however, this system profile is not supported, only applications can access theSDP, not users

† Intercom profile This application profile supports direct voice communication betweentwo Bluetooth devices within range of each other

† Cordless telephony profile This application profile is designed in order to support the in-1 phone’ concept, meaning that a Bluetooth-compliant telephone can be used either as

‘3-an intercom (communicating directly with ‘3-another Bluetooth device), cordless location) or mobile phone

(fixed-† Serial port profile This system profile emulates RS232 and USB serial ports in order toallow applications to exchange data over a serial link

† Headset profile This application profile uses the serial port profile to provide connectionsbetween Bluetooth-enabled computers or mobile phones and Bluetooth-enabled wirelessheadset microphones

† Dial-up networking profile This application profile uses the serial port profile to providedial-up connections via Bluetooth-enabled cellular phones

† Fax profile This application profile uses the serial port profile to enable computers to send

a fax via a Bluetooth-enabled cellular phone

† LAN access profile This application profile enables Bluetooth devices either to form small

IP networks among themselves or connect to traditional LANs through Access Points (APs)

† Generic object exchange profile This system profile defines the functionality needed forBluetooth devices to support object exchanges

† Object push profile This application profile defines the functionality needed to support

‘pushed’ data Examples of such information are advertisements and news distribution

† File transfer profile This application profile enables file transfers between Bluetoothdevices

† Synchronization profile This application profile enables automatic data synchronizationbetween Bluetooth devices For example, it can be used to synchronize address booksbetween a desktop computer and a portable

11.2.2 The Bluetooth Radio Channel

The Bluetooth radio channel [7], enabled by the radio layer, provides the electrical interfacefor the transfer of Bluetooth packets over the wireless medium The radio channel operates atthe 2.4 GHz ISM band by performing frequency hopping through a set of 79 (US and Europe)

or 23 (Spain, France and Japan) RF channels spaced 1 MHz apart The wireless linkcomprises time slots of 0.625 ms length each, with each slot corresponding to a hopfrequency The nominal hop rate is 1600 hops/s At each hop, the transmitted signal ismodulated using GFSK with a binary one being represented by a positive frequency shift

and a binary zero by a negative frequency shift Despite the fact that this configurationprovides link speeds up to 1 Mbps, the effective data transfer speeds offered are lower.This is because different protocol layers use parts of the packet data payload to add headerinformation for purposes of communication with peer layers More efficient modulationschemes could obviously achieve higher speeds, however, the use of GFSK is preferred as

it allows for low-cost device implementations

Depending on the transmitted power, Bluetooth devices can be classified into three classes,

as shown in Figure 11.4 Power control mechanisms can be used to optimize the transmitter’spower output This is done by measuring the received signal strength and relaying LMPcommands to the respective transmitter indicating whether power should be increased ordecreased Power control is required for class 1 equipment, whereas it is optional for equip-ment of classes 2 and 3 Using power class 1 a range of 100 m can be achieved in a Bluetoothsystem [2]

11.2.3 Piconets and Scatternets

When two Bluetooth devices want to connect, the one requesting the connection, is known asthe master, whereas the other is known as the slave The master always controls the linkcreated between the two devices The master–slave relationship is good only for a specificlink establishment, since any Bluetooth unit can be either master or slave A given master canmaintain up to seven connections to active slaves As a result, several very small networks,called piconets, can be established However, if the master wants to open connections to morethan seven slaves it can instruct one or more active slaves to ‘sleep’ for a specified period oftime by putting them in the low-power PARK mode (described in Section 11.2.7) Then themaster can admit new slaves into the piconet for the period the old set of slaves were put tosleep Thus, piconets of many devices can be formed

A piconet is shown in Figure 11.5 Devices inside a piconet hop together according to themaster’s clock value and its 48-bit device ID The way the hopping sequence is used and thestarting point within that sequence is selected is shown in Figure 11.6 The first parameterdefines the hopping sequence used for FHSS transmission inside the piconet The secondparameter is derived from the native clock of the master and defines the phase within thatsequence Slave to slave transmission is not supported inside a piconet If two slaves need tocommunicate, they either have to form a separate piconet in which one of them is the master,

or use a higher layer protocol, such as IP in order to relay messages through the piconet’smaster

A number of piconets can coexist in the same area This coexistence is enabled due to theuse of FHSS transmission Since the hopping sequences used in Bluetooth are pseudorandom,different coexisting piconets will use different hopping sequences, resulting in a low prob-

Figure 11.4 Power classes for Bluetooth devices

ability of hop collisions Still, if such a case occurs, a Bluetooth device can recognize and thusignore packets originating from collocated networks by checking the access code field on thereceived packets which is different for every piconet, as we explain later However, when alarge number of piconets coexist in the same area, this probability rises and the performance

of each piconet degrades

A collection of overlapping piconets is called a scatternet A scatternet will typicallycontain devices that participate in one or more piconets Devices participating in two ormore piconets are known as bridge devices and participate in each piconet in a time-sharingmanner After spending its time inside a piconet, a bridge device will change its hoppingsequence to that of the new piconet in order to join it Furthermore, it will select the startingpoint within that sequence based on the clock value of the master of the new piconet Byalternating among several piconets and buffering packets, bridge nodes can forward packetsfrom one piconet to another A bridge node that participates in several piconets can be either:

† Slave in all the piconets In this case, when leaving the old piconet, the slave has to informthe master for the duration of its absence

† Master in one piconet and slave in all others In this case, all traffic in the old piconet is

Figure 11.6 Combination of the master’s device ID and native clock values to select the hoppingsequence to be used and its starting point

Figure 11.5 A piconet formed by six devices

suspended until the master returns to the piconet The suspension of traffic is achieved byputting the piconet’s slaves into the low-power HOLD mode (described in Section 11.2.7).

A bridge node cannot obviously be master in more than one piconet, since this would makethese piconets use the same hopping sequence and thus collide

Figure 11.7 shows a scatternet, where the participating nodes fall into four categories.Nodes in category A are masters within a single piconet, nodes in category B are slaves within

a single piconet, nodes in category C are participating in two piconets as slaves and nodes incategory D are participating in two piconets as slaves in the one and master in the other.Obviously, nodes in categories C and D are bridge nodes between the piconets in which theyparticipate Techniques for efficient interpiconet communication and efficient formation ofscatternets from overlapping piconets are under development [10–12]

11.2.4 Inquiry, Paging and Link Establishment

Bluetooth devices can communicate as soon as they are within range of one another.However, since the in-range neighbors of a Bluetooth device change with time, a procedurethat informs a device about its neighbors is needed This procedure is carried out by issuinginquiries Although an inquiry is a fairly simple procedure, it becomes complicated due to theuse of FHSS at the physical layer Assume that device A is within range of device B andwants to acquire knowledge about its neighbors Since the hopping sequence of a link isdefined by the master’s device ID and clock values, A and B cannot exchange messages untilthey agree to a common hopping sequence as well as a common phase within that sequence

In order for two devices to exchange messages during the inquiry procedure, Bluetoothdefines a specific hopping sequence to be used for inquiries by all devices Furthermore,since there may be a phase uncertainty between A and B since those devices may not becompletely synchronized (meaning that they start from a different hop inside the hoppingsequence due to the differing Clock values of A and B), the sender hops faster than thelistener, by transmitting a signal on each hop and listening between successive transmissionsfor an answer The term Frequency Synchronization delay (or FS delay) refers to the timeelapsed until the sender (A in this case) transmits at the frequency the receiver (B) is currently

Figure 11.7 A scatternet formed by four piconets

listening on Upon reception of the inquiry, B waits for a randomly distributed period of time,called the random backoff delay After the random backoff delay has elapsed, B replies to thenext inquiry received by A by sending a FHSS packet containing its 48-bit device ID andclock values The RB delay obviously aims to reduce the chance of collisions between two ormore devices that are listening for inquiries at the same time.

The inquiry procedure provides a means for a master to gather neighborhood information.Two Bluetooth devices wanting to connect defines the paging procedure Assuming that A isthe master, B is the slave and that A does not participate in a piconet, the paging procedure is

as follows: A is supposed to possess B’s device IDand an estimate of its clock parameter fromthe inquiry procedure To connect to B, A pages B using the hop sequence defined by B’sdevice ID However, since some time has elapsed from the inquiry procedure, A cannotpossess the exact value of the native clock of B and thus the phase within the hoppingsequence defined by B’s device ID Therefore, A transmits the page message not only inthe hop in which it expects B to be, but also in neighboring hops, for a period of 10 ms Uponreception of the page request, B responds by sending its device ID to A Finally, A transmits

to B a packet containing the master’s device ID and clock values Upon receiving thisinformation, B creates a variable that contains its clock value, adds an offset to this variable

in order to synchronize with A’s clock and connects to A as slave Information exchangebetween A and B can now initiate

The above procedure becomes slightly more complicated when the master has alreadyformed a piconet In order for new slaves to join the piconet, the master needs to periodicallysuspend the traffic inside the piconet in order to scan for new slaves or accept slave requests.This traffic suspension obviously results in capacity reduction within the piconet Therefore,when selecting the time period a master will spend searching for new slaves, a tradeoff has to

be made between the latency of accepting a new slave to the piconet and overall piconetcapacity

11.2.5 Packet Format

As mentioned, each slot in Bluetooth corresponds to a single hop and lasts 0.625 ms For apair of communicating Bluetooth devices, the master always starts transmission at evennumbered slots while slave transmission is set to be initiated only at odd numbered slots.Inside each piconet one packet can be transmitted in each slot The format of packetsexchanged over a Bluetooth link is shown in Figure 11.8 It comprises the following parts:

† A 72-bit access code, which is defined by the master and is unique for the piconet Thispart is used by Bluetooth devices to identify incoming packets If a Bluetooth deviceconcludes that the access code of an incoming packet does not match the code of itspiconet, the packet is discarded Furthermore, this field is used by receiving units forsynchronization purposes

† A 54-bit packet header, which contains information related to MAC addressing, packettype, flow control, Automatic Repeat Request (ARQ) and Header Error Correction (HEC)

Figure 11.8 The format of a transmitted Bluetooth packet

MAC addresses are 3-bit fields, with address 000 specifying a broadcast packet The 3-bitMAC field is the reason for the fact that up to seven active slaves are supported inside thesame piconet and that further slaves can be admitted only if some others enter the PARKmode The fields of a Bluetooth packet header are shown in Figure 11.9.

† A variable length payload may trail the header Since the hop duration is 0.625 ms, 625bits can be transmitted at a single hop over a 1 Mbps link However, the Bluetoothspecification states that the length of a packet transmitted at a single hop is 366 bits, inorder to give transmitters and receivers enough time to hop to the next frequency andstabilize Thus, the effective payload of a packet transmitted over a single hop equals 366bits minus the access code and packet header size (126 bits), which results in 240 bits (30bytes) The multislot packets, described later, can have a larger payload

The 4-bit packet type field in the packet header defines 16 types of packets Of these, fourare control packets:

† The ID packet, which is used for signaling purposes

† The NULL packet, which only contains the access code and packet header This is used incase no payload has to be transmitted, but information in the packet header is needed forlink management

† The POLL packet, which is used by the master to poll slaves in an ACL link

† The FHSS packet, which is used to exchange synchronization information between units,such as clock values of Bluetooth devices

The remaining 12 types codes are divided into segments that define kinds of data packets In

an effort to improve efficiency, Bluetooth supports multislot packets, either three or five slotslong, which are transmitted in consecutive slots Thus, segment 1 specifies single-slot pack-ets, segment 2 specifies three-slot packets and segment 3 specifies five-slot packets Multislotpackets are always sent on a single frequency, which is that used at the beginning of themultislot transmission The transmission following that of the multislot packet occurs at thehop that would be used when the multislot packet was replaced by single hop packets Forexample, consider four consecutive slots k, k 1 1, k 1 2, k 1 3.In the case of three single hoptransmissions, these would take place in frequencies fk, fk11, fk12 In the case of a three-slotpacket, the entire packet would be transmitted at frequency fkwith the next transmission onthe channel beginning in slot k 1 3 and using frequency fk13 The fact that multislot packetsare transmitted on the same frequency results in a capacity increase, which comes, however,

at an expense over the hopping rate of the system and thus lowering the system’s interferenceavoidance

11.2.6 Link Types

The baseband layer provides two types of links for Bluetooth: Synchronous

Connection-Figure 11.9 The fields of a Bluetooth packet header