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Bluetooth communication occurs between a master radio and a slave radio. Bluetooth radios are symmetric in that the same device may operate as a master and also the slave. Each radio has a 48-bit unique device address (BD_ADDR) that is fixed.

Two or more radio devices together form ad-hoc networks called piconets. All units within a piconet share the same channel. Each piconet has one master device and one or more slaves. There may be up to seven active slaves at a time within a piconet. Thus, each active device within a piconet is identifiable by a 3-bit active device address. Inactive slaves in unconnected modes may continue to reside within the piconet.

A master is the only one that may initiate a Bluetooth communication link. However, once a link is established, the slave may request a master/slave switch to become the master. Slaves are not allowed to talk to each other directly. All communication occurs within the slave and the master. Slaves within a piconet must also synchronize their internal clocks and frequency hops with that of the master. Each piconet uses a different frequency hopping sequence. Radio devices used Time Division Multiplexing (TDM). A master device in a piconet transmits on even numbered slots and the slaves may transmit on odd numbered slots.

Fig 1: Bluetooth Scatternets and Piconets

Multiple piconets with overlapping coverage areas form a scatternet. Each piconet may have only one master, but slaves may participate in different piconets on a time-division multiplex basis. A device may be a master in one piconet and a slave in another or a slave in more than one piconet.

The main principle in mind when developing the Bluetooth Protocol Architecture has been the maximization and the re-use of existing protocols for different purposes at the higher layers. The one main advantage is that existing (legacy) applications can be adapted to work with the Bluetooth Technology. The Bluetooth Protocol Architecture also allows for the use of commonly used application protocols on top of the Bluetooth-Specific protocols. In simpler terms, this permits new applications to take full advantage of the capabilities of the Bluetooth technology and for many applications that are already developed by vendors; they can take immediate advantage of hardware and software systems, which are also compliant with the Specification.

Fig 1:The Bluetooth Protocol Stack Model

In addition to the above protocol layers, the Specification also defines a Host Controller Interface (HCI). This provides a command interface to the baseband controller, link manager, and access to hardware status and control registers.

Three-in-One Phone

Telephone handsets built to this profile may connect to three different service providers. First, telephones may act as cordless phones connecting to the public switched telephone network (PSTN) at home or the office and incurring a fixed line charge. This scenario includes making calls via a voice base station, making direct calls between two terminals via the base station and accessing supplementary services provided by an external network. Second, telephones can connect directly to other telephones for the purpose of acting as a ~Swalkie-talkie~T or handset extension. Referred to as the intercom scenario , the connection incurs no additional charge. Third, the telephone may act as a cellular phone connecting to the cellular infrastructure and incurring cellular charges. The cordless and intercom scenarios use the same protocol stack, which is shown in Figure 5. The audio stream is directly connected to the Baseband protocol indicated by the L2CAP bypassing audio arrow.

Fig 3:Protocol Stack for Cordless Phone and Intercom Scenarios

Ultimate Headset

The headset can be wirelessly connected for the purpose of acting as a remote device~Rs audio input and output interface. The headset increases the user~Rs freedom of movement while maintaining call privacy. A common example is a scenario where a headset is used with either a cellular handset, cordless handset, or personal computer for audio input and output. The protocol stack for this usage model is depicted in Figure 6. The audio stream is directly connected to the Baseband protocol indicated by the L2CAP bypassing audio arrow. The headset must be able to send AT-commands (Attention commands) and receive result codes. This ability allows the headset to answer incoming calls and then terminate them without physically manipulating the telephone handset.

Fig 4: Ultimate Headset Protocol Stack