Bluetooth Architecture

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Bluetooth permits devices to establish either ad hoc or infrastructure networks. Infrastructure networks use fixed Bluetooth access points (AP), which facilitate communication between Bluetooth devices. This document focuses on ad hoc piconets, which are much more common than infrastructure networks. Ad hoc networks provide easy connection establishment between mobile devices in the same physical area (e.g., the same room) without the use of any infrastructure devices. A Bluetooth client is simply a device with a Bluetooth radio and software incorporating the Bluetooth protocol stack and interfaces.

The Bluetooth specification provides separation of duties for performing stack functions between a host and a host controller. The host is responsible for the higher layer protocols, such as Logical Link Control and Adaptation Protocol (L2CAP) and Service Discovery Protocol (SDP). The host functions are performed by a computing device like a laptop or desktop computer. The host controller is responsible for the lower layers, including the Radio, Baseband, and Link Manager Protocol (LMP). The host controller functions are performed by an integrated or external (e.g., USB) Bluetooth dongle. The host and host controller send information to each other using the Host Controller Interface (HCI). In many cases, the host and host controller functions are integrated into a single device, with Bluetooth headsets being a prime example.

Figure 2-1 depicts the basic Bluetooth network topology. In a piconet, one device serves as the master, with all other devices in the piconet acting as slaves. Piconets can scale to include up to seven active slave devices and up to 255 inactive slave devices.

The master device controls and establishes the network (including defining the network’s frequency hopping scheme). Although only one device can serve as the master for each piconet, time division multiplexing (TDM) allows a slave in one piconet to act as the master for another piconet simultaneously, thus creating a chain of networks. This chain, called a scatternet, allows several devices to be networkedover an extended distance in a dynamic topology that can change during any given session. As a device moves toward or away from the master device, the topology, and therefore the relationships of the devices in the immediate network, may change. Figure 2-2 depicts a scatternet that connects three piconets.
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