The basic elements of a Bluetooth system are the RU (radio unit) and the BU (base unit), in addition to the management and link interface unit which we will see later and which is actually software.rn
The protocol allows determining the types of services present and available in the piconet both in a server and client mode. In server mode, it will allow a device to query it about the services and protocols supported, thus making them available; in client mode, it will allow querying connected devices in the piconet to obtain information. In a connection, all connected Bluetooth devices are generally in standby mode, i.e., waiting, following a scan cycle at intervals of 1.28 seconds to check for the presence of any other devices; in this mode, all Bluetooth devices are low power consumption. As we have already seen, each unit contains several components internally: – the radio unit. – the link controller unit. – the link manager and interface unit. Connections between Bluetooth devices are controlled by the Link Manager (LM, software component that manages settings, authentication, configurations, etc.) and the Link Controller (LC, hardware component managing data sending/receiving, data settings, etc.). The Link Manager supervises the connection between different Bluetooth devices and verifies authentication procedures and connection configuration through contact with another Link Manager. Service information exchange happens via the LMP (Link Manager Protocol). LMP information can involve: data transmission and reception, authentication, scanning (page scan, inquiry scan, park hold sniff), identification, linking, communication channel determination, verification, and compression of exchanged data. The sent messages are called PDUs (Protocol Data Units) and consist of 55 types. A further higher control layer is the L2CAP (Logical Link Control and Adaptation Protocol), which acts once the connection between devices is established via LMP, managing segmentation and reassembly of data packets (up to 64 Kb max), multiplexing, QoS (Quality of Service) information. Bluetooth uses low-power radio waves for communication operating between 2.4 GHz and 2.5 GHz in the ISM band, the same band used by wireless LANs, and uses FHSS to avoid interference with other equipment. The FHSS specification uses a very interesting method to perform its task: it divides the spectral band into many channels and can change frequency more than 1500 times per second, hopping across 79 different frequencies called “hopping.” Data can be transmitted on synchronous (SCO) and asynchronous (ACL) channels with the only difference being that the former are typically used for circuit-switched point-to-point voice traffic and the latter for transmitting any other type of data (packet-switched point-to-multipoint).
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