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How Bluetooth Works

This section seeks to give you an overview of the technology and specification that will provide context for the Bluetooth implementation on Mac OS X. If you’re already familiar with the Bluetooth specification and how Bluetooth devices work, you might choose to skip ahead to “Bluetooth on Mac OS X.”

Bluetooth devices operate at 2.4 GHz in the license-free, globally available ISM (Industrial, Scientific, and Medical) radio band. The advantage of operating in this band is worldwide availability and compatibility. A potential disadvantage is that Bluetooth devices must share this band with many other RF emitters. These include automobile security systems, other wireless communications standards (such as 802.11), and ordinary noise sources (such as microwave ovens).

To overcome this challenge, Bluetooth employs a fast frequency-hopping scheme and uses shorter packets than other standards in the ISM band. This scheme makes Bluetooth communication more robust and more secure.

In this section:

Frequency Hopping
Power Consumption
Security

Frequency Hopping

Frequency hopping is literally jumping from frequency to frequency within the ISM band. After a Bluetooth device sends or receives a packet, it and the Bluetooth device or devices it is communicating with “hop” to another frequency before the next packet is sent. This scheme has three advantages:

• It allows Bluetooth devices to use the entirety of the available ISM band, while never transmitting from a fixed frequency for more than a very short time This ensures that Bluetooth conforms to the ISM restrictions on transmission quantity per frequency.

• It ensures that any interference will be short-lived. Any packet that doesn't arrive safely at its destination can be resent at the next frequency.

• It provides a base level of security because it's very difficult for an eavesdropping device to predict which frequency the Bluetooth devices will use next.

Of course, the connected devices must agree upon the next frequency to use. The Bluetooth specification ensures this in two ways. First, it defines a master-slave relationship between Bluetooth devices. Second, it specifies an algorithm that uses device-specific information to calculate frequency-hop sequences.

A Bluetooth device operating in master mode can communicate with up to seven slave devices. To each of its slaves, the master Bluetooth device sends its own unique device address (similar to an ethernet address) and the value of its internal clock. This information is used to calculate the frequency-hop sequence. Because the master device and all its slaves use the same algorithm with the same initial input, the connected devices always arrive together at the next frequency.

Power Consumption

As a cable-replacement technology, it’s not surprising that Bluetooth devices are usually battery-powered devices, such as wireless mice and mobile phones. To conserve power, most Bluetooth devices operate as low-power, 1 mW radios (Class 3 radio power). This gives Bluetooth devices a range of about 5–10 meters. This range is far enough for comfortable wireless peripheral communication but close enough to avoid drawing too much power from the device’s power source.

Security

Security is a challenge faced by every communications standard. Wireless communications present special security challenges. Bluetooth builds security into its model on several different levels, beginning with the security inherent in its frequency-hopping scheme (described in “Frequency Hopping”).

At the lowest levels of the protocol stack, Bluetooth uses the publicly available cipher algorithm known as SAFER+ to authenticate a device’s identity. The generic-access profile depends on this authentication for its device-pairing process. This process involves creating a special link to create and exchange a link key. Once verified, the link key is used to negotiate an encryption mode the devices will use for their communication.

The topic of Bluetooth security is beyond the scope of this document. For references that contain more information on Bluetooth’s encryption and authentication processes, see “See Also.”

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