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U. Glaeser

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each with one common node. Theoretically, this would work, but the rest of the original Bluetooth<br />

specification is not designed for making Bluetooth a wireless LAN. It is likely the newer SIG work group<br />

on personal area networking will be interested in expanding the definition and capability of Bluetooth<br />

scatternet capability. Currently there is lots of interest in forming location aware ad hoc wireless networks<br />

[7]. NASA has already approached this author for ideas for use of Bluetooth for ad hoc small area networks<br />

in space missions. The appeal of a wireless link made up of five dollar, very small, low-power, selfconfiguring,<br />

parts capable of connecting various sensors is irresistible for complex space missions where<br />

power and payload weight is at a premium.<br />

Bluetooth SIG Working Groups<br />

To understand the Bluetooth specification it is important to understand how the very large Bluetooth<br />

SIG is organized. The actual work in producing the various specifications is done by the various SIG<br />

working groups. Given that the Bluetooth specification is thousands of pages of detailed technical documentation,<br />

it is not practical to just sit down and read the specification sheet. Briefly, five major groups<br />

compose the SIG including the air interface group, the software group, the interoperability group, the<br />

legal group, and the marketing group [3].<br />

The software group contains three working subgroups primarily responsible for the Bluetooth protocol<br />

stack. These are the lower Transport Protocol Group, the Middleware Protocol Group, and the Application<br />

Group. The protocol stack follows the international origination of standardization (ISO) seven-layer<br />

reference model for open system interconnection [8].<br />

The Transport Protocol Group<br />

The Transport Protocol Group includes ISO layers one and two, which are the Bluetooth radio, the link<br />

controller baseband, the link manager, the logical link controller and application protocol (L2CAP) layer,<br />

and the host controller interface. Collectively this set of protocol groups form a virtual pipe to move<br />

voice and data from one Bluetooth device to another. Audio applications bypass all of the higher level<br />

layers to move voice from one user to another [6].<br />

The L2CAP layer prevents higher level layers from having to deal with any of the complexity of the<br />

frequency hopping Bluetooth radio and its complex control or special packets used over the Bluetooth<br />

air radio interface. The responsibility of the L2CAP layer is to coordinate and maintain the desired level<br />

of service requested and coordinate new incoming traffic. The L2CAP layer’s concern is with asynchronous<br />

information (ACL packet) transmission [6]. This layer does not know about the details of the<br />

Bluetooth air interface such as master, slave, polling, frequency hopping, and such. Its job is to support<br />

the higher layer protocol multiplexing so multiple applications can establish connectivity over the same<br />

Bluetooth link simultaneously [9].<br />

Device authentication is based on an interactive transaction from the link manager. When an unknown<br />

Bluetooth device request connectivity, the device requested ask the requester to send back a 16 byte<br />

random number key, which is similar to the familiar bank ATM PIN code procedure. Once a device is<br />

authenticated it is necessary for the device to store the authentication codes so this process can be automatic<br />

in future connections. Link encryption up to 128 bytes is supported and is controlled by desirability and<br />

governing legal issues of the area. Encryption applies only to the data payload and is symmetric.<br />

Power management of connected devices is also handled at this level. In sniff mode the slave must<br />

wake up and listen at the beginning of each even-numbered slot to see if the master intends to transmit [6].<br />

In hold mode the slave is suspended for a specified time. The API for hold mode puts the master in<br />

charge but provisions are available to negotiate the time. In Park mode, the slave dissociates itself from<br />

the piconet while still maintaining synchronization of the hopping sequence. Before going in to park<br />

mode the master informs the slave of a low-bandwidth beacon channel the master can use to wake the<br />

parked slave if there not already seven active slaves.<br />

Paging schemes allow for a more repaid reconnection of Bluetooth devices. For example, paging is<br />

used in the event a master and a slave need to switch rolls to solve some problem such as forming some<br />

© 2002 by CRC Press LLC

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