Bluetooth protocol stack ➤ Radio and Baseband

The radio layer of Bluetooth utilizes the 2.4-GHz ISM band, the globally free available frequency band, for spread spectrum communication. About 79 MHz of bandwidth is used for frequency hopping, with 1-MHz carrier spacing. The modulation scheme is GFSK at a rate of 1 bit per Hz, providing a data rate of 1 Mbps. The frequency-hopping rate is 1600 hops per second with a dwell time of 625 μ sec. As a WPAN, the radio interface of Bluetooth imposes strict emitted power control. Three classes of transmitters are defined based on power and signal transmission range.

» Class 1 outputs a maximum of 100 mW and a minimum of 1 mW for the greatest distance (around 100 m without obstacles). Power control is mandatory.

» Class 2 outputs power between 0.25 and 2.4 mW for a range of about 10 m without obstacles. Power control is optional.

» Class 3 outputs around 1 mW with range of a few meters or less.

» The power control algorithm can be implemented in the link control protocol and controlled by the control component in the protocol stack.

The baseband layer controls transmission of frames in association with frequency hopping. The master in a piconet takes the channel to transmit in even-numbered hops, and slaves transmit in odd-numbered hops, reflecting a time-division duplex for all devices in a piconet. A single frame can be transmitted in the duration of one, three, or five hops. Depending on the nature of the logical link between a slave and the master, two types of links are offered. One is the asynchronous connectionless (ACL) link for best-effort packet data transmission. The other is synchronous connection oriented (SCO) for time-critical data such as voice. Frames sent on ACL links may have to be transmitted if lost, whereas frames sent over SCO links will never be retransmitted, necessitating upper layers for error correction.

The baseband layer has defined some types of frames that correspond to various purposes of the baseband frames. Different types of frames can carry different sizes of payload data and error-correction schemes. In particular, the access code fi eld in a baseband frame indicates the purpose of the frame in a special state. For example, a frame with the inquiry access code (IAC) will be sent when a device elects to scan for other devices within the radio range in a series of 32 frequency hops. Bluetooth devices can be configured to periodically hop according to the inquiry scan hopping sequence to scan inquires. When an inquiry is detected, the device, now the slave, will reply with its address and timing information to the master, then the master and the slave begin the paging process to determine a common hopping sequence to establish a connection. Eventually, both the master and the slave will hop on the same sequence of channels for the duration of the connection.

Source of Information : Elsevier Wireless Networking Complete 2010


Subscribe to Developer Techno ?
Enter your email address:

Delivered by FeedBurner