Bluetooth Technology Characteristics

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Bluetooth operates in the unlicensed 2.4 gigahertz (GHz) to 2.4835 GHz Industrial, Scientific, and Medical (ISM) frequency band. Numerous technologies operate in this band, including the IEEE 802.11b/g WLAN standard, making it somewhat crowded from the standpoint of the volume of wireless transmissions. Bluetooth employs frequency hopping spread spectrum (FHSS) technology for all transmissions. FHSS reduces interference and transmission errors and provides a limited level of transmission security. With FHSS technology, communications between Bluetooth devices use 79 different radio channels by hopping (i.e., changing) frequencies about 1600 times per second for data/voice links and 3200 times per second during page and inquiry scanning.

Bluetooth also provides for radio link power control, where devices can negotiate and adjust their radio power according to signal strength measurements. Each device in a Bluetooth network can determine its received signal strength indication (RSSI) and make a request of the other network device to adjust its relative radio power level (i.e., have the transmission power incrementally increased or decreased). This is performed to conserve power and/or to keep the received signal characteristics within a preferred range.

The combination of a frequency-hopping scheme and radio link power control provide Bluetooth with some additional, albeit limited, protection from eavesdropping and malicious access. If the Bluetooth power control feature is used appropriately, any potential adversary is forced to be in relatively close proximity to pose a threat to a Bluetooth piconet, especially if the Bluetooth devices are very close to each other.

Bluetooth versions 1.1 and 1.2 specify transmission speeds of up to 1 megabit per second (Mbps) and achieve throughput of approximately 720 kilobits per second (kbps). Bluetooth versions 2.0 + Enhanced Data Rate (EDR) and 2.1 + EDR specify data rates up to 3 Mbps and throughput of approximately 2.1 Mbps.

The range of Bluetooth devices is characterized by three classes that define power management. Table 2-summarizes the classes, including their power levels in milliwatts (mW) and decibels referenced to one milliwatt (dBm), and their operating ranges in meters (m). Most small, battery-powered devices are Class 2, while Class 1 devices are typically USB dongles for desktop and laptop computers, as well as access points and other AC-powered devices.

So that Bluetooth devices can find and establish communication with each other, discoverable and connectable modes are specified. A device in discoverable mode periodically listens on an inquiry scan physical channel (based on a specific set of frequencies) and will respond to an inquiry on that channel with its device address, local clock, and other characteristics needed to page and subsequently connect to it. A device in connectable mode periodically listens on its page scan physical channel and will respond to a page on that channel to initiate a network connection. The frequencies associated with the page scan.
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