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Therefore other devices which are actively scanningwill be able to discover it.There are actually two types of ‘discoverability’ – limited and general. Ifa device is in ‘limited discoverable’ mode then it can be found by devicesperforming ‘limited’ or ‘general’ discoveries. If a device performs a ‘limited’ discovery, it will find only devices in ‘limited discoverable’ mode.Devices should only stay in limited discoverable mode for a restrictedperiod of time: the corollary of which is that the user of the devicebeing discovered would have to perform some specific action in theUI to activate the mode; typically when they are expecting some Bluetooth activity – for example, if they are preparing to receive a vCard viaBluetooth.The difference between limited and general discoveries is that thedevices taking part in the discovery procedure are using a different‘inquiry access code’ (IAC) in the Bluetooth hardware.ConnectabilityConnectability is somewhat orthogonal to discoverability within theBluetooth specification.
If a device can receive a Bluetooth physicalconnection then it is said to be ‘connectable’. Note that a device need notbe discoverable (in either the limited or general forms) to be connectable.However, it rarely makes sense for a device to be discoverable but notconnectable.PairabilityAs part of the process of forming a connection to a service on a remoteBluetooth device, the devices may wish to authenticate each otherand form a long-term security relationship. This allows the devices toauthenticate each other in future without further input from the user.2Consider the alternative ‘out-of-band’ method, where, for example, devices could bediscovered and paired using another technology, such as some form of embedded RFIDchip, by touching them together.68BLUETOOTHThe initial authentication process requires that the same ‘passkey’ beentered into each device when prompted by the UI.
The passkey willultimately yield a security token (the ‘link key’) shared between the twodevices. If a long-term security relationship is to be established this linkkey will be retained in persistent storage.A Bluetooth device is said to be ‘pairable’ if it allows the input ofa passkey.Class of deviceWhen a device responds to discovery, it sends a three-byte value calledthe ‘Class of Device’, or ‘CoD’. In the present specification this comprisestwo fields describing the broad nature of the Bluetooth device and a thirdfield which outlines the services it provides.
The first two fields are the‘major’ and ‘minor’ device class, and the third is the ‘major service class’.The major and minor device classes are often collectively referred to(perhaps unhelpfully!) as the ‘device class’, and describe, in loose terms,the physical appearance of the device: for example whether the device isa computer, a phone, a printer or a headset.The service class (since there is no ‘minor’ service class, the ‘major’is usually dropped – but care is needed to distinguish this from ‘serviceclass’ in SDP, which is discussed later) is a bit field with individual bitsrepresenting – at a high level – services available on the device at thetime it responded, such as ‘networking’, ‘rendering’ and ‘object transfer’.Major service class only provides hints as to the services available,especially as more than one service may map onto a single service classbit – for example, both Bluetooth A2DP headsets and Bluetooth printerswould set the ‘rendering’ bit: for conclusive results as to the availabilityof services on a Bluetooth device, the Service Discovery Protocol (SDP)should be used.
See section 4.1.6 for more on SDP.The service class bits are shown in Table 4.1.The semantics of each are quite loose, and generally up to the application to choose, or a profile specification to advise, of the appropriatebits to set for a given service.The class of device information for a remote device is also obtainedby the Bluetooth stack when it receives an inbound connection. Unfortunately there is no other way to ascertain the class of device of a remotedevice than during the inquiry phase, or at the time of an inbound connection at the physical link level (physical links are discussed in more detaillater).
This means that in certain situations, such as a new service startingon an already-connected device, the current class of device informationfor a remote device may not be known.4.1.3 Physical LinksOnce a device has discovered another device, it may form a physicallink between them. The device creating the connection is, by default,BLUETOOTH TECHNOLOGY OVERVIEW69Table 4.1 Class of device service classesService class bitExample of useLimited discoverable mode This bit is not really a ‘service class’, despite itbeing in the service class field of the class ofdevice. Applications cannot control thisPositioningBluetooth GPS device providing positioncoordinatesNetworkingBluetooth modem providing access to a networkRenderingBluetooth printerCapturingBluetooth web-cam that captures images andforwards them to a displayObject transferA Bluetooth device that implements OBEXAudioBluetooth headsetTelephonyBluetooth-enabled mobile phoneInformationBluetooth-enabled advertising displaythe master of that link; the receiver of the connection is, by default,the slave.
At most one physical link exists between a master deviceand a slave device in a piconet (see Figure 4.3). The master and slaveroles can be swapped. However, physical links always emanate from amaster – Bluetooth is a star topology.A piconet is formed when a master is connected with at least one slave;all slaves connected to a single master are coordinating their use of theRF channels so they are all tuned to the same RF channel at the same time(which changes, in a piconet, 1600 times per second). The piconet is onetype of ‘physical channel’; others are used when a device is waiting to bediscovered, and another is used when a device is expecting a physical link.It is also worth noting that slaves in a piconet cannot communicatewith each other – each slave can only connect to the master device.
Theonly way for slaves in a piconet to communicate is using a higher levelservice, such as the virtual Ethernet network provided by PAN profile.Two different piconets can be brought together to form a scatternet.No direct Bluetooth connection is made between the two piconets; allthat happens is that a device can switch its radio between two frequencyhopping sequences, allowing it to operate in two piconets simultaneously.By definition, it is not possible, using one Bluetooth radio, to be a masterin one piconet whilst simultaneously being a master in another piconet.It is possible for a device to be a master of one piconet and a slave in70BLUETOOTH‘Slave’device‘Slave’device‘Slave’device‘Master’device‘Slave’device‘Slave’device‘Slave’device‘Slave’deviceFigure 4.3 Bluetooth piconet topologyanother; however, due to the time division multiplexing, the bandwidthavailable to the device in each piconet can become unacceptable. Itis also – just – possible for a device to be a slave in two piconets.
Theability to participate in multiple piconets is entirely dependant on thecapabilities of the controller – the only time the host knows about thisis either when explicitly examining and manipulating the piconet roles,or when link creation fails because the controller cannot meet themaster/slave requirements of that new connection.3The practical consequence of this is that it is not always possible toconnect to a device that is in range, as it may be participating in otherpiconets and not be able to accept another connection.
In these cases,it is best just to inform the user the remote device is busy rather than tryand manipulate roles in order to achieve a connection, especially as nosingle device has enough information to work out if there is a suitable setof roles that will allow every other device involved to connect to eachother in the desired topology.The properties of the physical link that are of interest include:• Piconet roles – whether the local device is a master, or a slave• Power – whether the link is in the correct mode for the traffic theservice is consuming or producing3This can occur with controllers which do not support scatternet and therefore requirespecific roles when connecting, or in cases where both remote devices are trying to enforceusage of a certain role (e.g, both devices insist on being master).
Both these problems havebeen observed when performing Bluetooth interoperability testing.BLUETOOTH TECHNOLOGY OVERVIEW71• Security – whether the physical link is, for example, authenticatedand/or encrypted.These properties pertain to the physical link, and are important tothe application programmer, particularly the power and security aspects.However, a physical link doesn’t directly carry data itself: instead itsupports logical transports that can carry data – logical transports aredescribed in section 4.1.4.The power consumed by each link depends on the mode in which thatlink operates. Reducing the time for which the receiver in the controlleris active can reduce the power consumption of the link. This is possiblebecause, in the time domain, ‘slots’ are created between the master andslaves in a physical channel, and it is possible to specify that we onlyneed to listen for traffic in certain slots.
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