SITI HAFIZAH ARIFFAH BT HAJI AHMAD MAHYUDDIN
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In 1994, Ericson invented Bluetooth to replace cable technology and its capability is becoming widespread in various device types which include all the intelligent devices such as PDAs and embedded applications for instance automobile power locks. Four year later, Bluetooth special Interest Group (SIG) is formed. Joint work by the SIG members endorsed the Bluetooth vision to change to open standards to ensure rapid recognition and compatibility in the marketplace. This technology has been developed by the Bluetooth SIG and is supported by over 2100 companies around the world. Furthermore, it is freely available at the official Bluetooth website.
Bluetooth Technology is one of wireless network types. It is a Radio Frequency specification for short-range, point-to-multipoint voice and data transfer. Apart from that, it is also a low-power short-range wireless standard for a wide range of devices which uses 2.45GHz band that is an ideal technology that enable various devices to communicate and available anywhere in the world.
Bluetooth devices group themselves for communication purposes. A Bluetooth Wireless Personal Area Network (BT-WPAN) comprises of piconets. It is a cluster of up to eight Bluetooth devices that is a master and slaves in the others for each piconet. Scatternet is formed when two piconets are connected through a getaway or a bridge. These interconnected piconets within the scatternet for a backbone for the Mobile Area Network (MANET) enables the device to exchange data through several hops in the scatternet even though they are out of range of another device or not directly communicate with each other.
Bluetooth depend primarily on point-to-point data links between Bluetooth devices that are within range. Apart from that, it can also be more complex networking topologies. The goal is to form Bluetooth scatternets that provide effective and efficient communication over multiple hops.
The Bluetooth protocol stack
It is divided into three logical groups that are Transport Protocol Stack, Middleware Protocol Stack and the Application Protocol Group. The Radio, Baseband, Link Manager, Logical Link Manager, Logical Link Control and Adaption layers are included In Transport Protocol Group that required to support communications between Bluetooth devices while third-party, industry-standard protocols as well as Bluetooth SIG developed protocols are included in the Middleware Protocol Group. These protocols allow existing and new applications to function over Bluetooth links. Lastly, the Application group consists of actual applications that use Bluetooth links which includes legacy application as well as Bluetooth-aware applications.
Communication
A Bluetooth transceiver is a frequency-hopping spread-spectrum (FHSS) device that uses 2.4GHz band but the nominal bandwidth for each channel is only 1MHz. The restrictions of this technology are that it is easily interrupt with other devices which used the same channel such as by 802.11b/g devices, HomeRF devices, portable phone and microwave ovens. When it is connected to other Bluetooth devices, it hops at the rate of 1600 times per second for typical use.
For standard transmissions, the Bluetooth transceiver uses all 79 channels and hops pseudo-randomly across all channels at 1600 hope per second. The range of each radio is approximately 10m but can be extended to 100m by increasing transmit power. This is because the transceiver has an extremely small footprint which easily embedded into physical devices making it a truly global radio link. This specification uses Time Division Duplexing and Time Division Multiple Access for device communication.
The access code holds the piconet address, usually 72bits in length. The header contains link control data encoded with a Forward Error-Correcting code with one-third rate for high reliability. Bluetooth device uses polling-based transmission but for all the communication between devices in a piconet use Time Division Duplex with only master to slave communication.
The master will poll each active slave to make sure it has data to transmit or not. The master transmits only in even-numbered time slots and oppositely in slaves. Inevery time slot, different frequency channel is used.
The piconet
Bluetooth facilitates ad-hoc connections for stationery and mobile connections which mean that any device that equipped with Bluetooth radio can establish instant connection to another Bluetooth radio as soon as it comes it comes into range. In piconet, a master can connect to 7 slaves simultaneously. These roles are temporary and exist as long as the piconet itself exists. The master device selects a frequency, the frequency-hopping sequence, the timing and the slaves polling order. It is also responsible to instruct the slave devices to switch to different device states for periods of inactivity. If there are more than 7 slaves, the rest must be “parked” which only limits to 255 parked slaves per piconet with direct addressing as defined by SIG. However, parked slave indirect addressing by own specific Bluetooth device address is also authorized which allows unlimited number of parked slaves. In order for it to reactive, active slave must be placed into a parked slave by the master.
Bluetooth devices can attempt to communicate with each other when it enters into communication range. In order for the slave to join the master’s piconet, master and slave must exchange and clock information as it have its own unique ID to create a hopping pattern. The master shares its ID and clock offset with slaves in its piconet providing the offset in it.A slave need to know which frequency to use and must synchronize with master’s clock to recreate the frequency-hopping sequence of joined piconet.
Bluetooth gateway or bridge device interconnects two or more piconets for multi-hop communication. The bridge can communicate with all the connected piconet which form scatternet by aligning itself with the clocking of each piconet when it is ready to communicate. This is because the bridge incurs overhead shifting from one clocking to another in order to communicate with each connected piconet which has a potential for a bottleneck to occur.
To fully realize the vision of Bluetooth, full networking of multiple Bluetooth devices is required that leads to the exploration of scatternets which address scatternet formation and reconfiguration, scheduling and routing issues.
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Reference
Patricia,W. McDermott, “What is Bluetooth”, IEEE Potentials, December 2004/January 2005, pp. 33-35.
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