Chapter 1. Bluetooth

Idea for Bluetooth development originates to Ericsson. The main idea was to develop short distance wireless connection that would replace cable connection. The Bluetooth Special Interest Group (SIG) was founded in 1998 to implement the technology and define common de facto standard. The aim of the standard was to implement short distance wireless communication link between two or more equipments.

Technology is based on chips that have relatively low transmission power. Nevertheless the radio connection can penetrate thin walls. Connection range for class 1 chips are up to 100 m. Most of the current chips belongs to class 3 which range is up to 10 m.

When Bluetooth nodes are in range they can establish an ad hoc connection. This is called piconet. One piconet has one master node and from one to seven slaves. Individual piconets can be linked into a scatternet. In these net some nodes are member in more than one piconet.

Figure 1-1. Illustration of the scatternet

Bluetooth uses frequency 2.4GHz for communicating. The frequency spectrum is used also by other devices like wireless telephones and microwave ovens, that might otherwise disturb the transmission. To improve the quality of the connection Bluetooth uses frequency hopping technique. Bluetooth nodes are able to transmit 1 Mb/s. This is also the capacity of the piconet.

Bluetooth uses wireless LAN standard IEEE 802.11 in data transmission. This is why Bluetooth equipments can be easily integrated in TCP/IP network.

Transmission between nodes is whether synchronous or asynchronous. Synchronous is used mainly for transmitting voice and asynchronous connection is used to transmit data. Synchronous connection is symmetric and after establishing the connection both the master and the slave nodes can send packets without opening a connection again. Asynchronous connection is whether asymmetrical or symmetrical. The master node define the used bandwidth. In both cases the connection is established by the master. Connections between nodes can be whether point-to-point or point-to-multipoint connections. Special characteristics of piconets is that rather than connecting to network as in traditional LANs Bluetooth devices connect directly to another device. Connections are dynamic and can change quickly.

1.1. Protocols

Bluetooth standard uses existing protocols as much as possible. Still it defines couple of new protocols. These are L2CAP (Link Layer Control and Adaptation Protocol), SDP (Service Discovery Protocol), RFCOMM and TCS (Telephony, Control Protocol).

L2CAP operates on session control layer (in OSI model) and it makes it possible to use several different protocols on the top of itself. It is considered with segmentation and assembling the sent and received datagrams. It provides the upper layers with quality of service management. L2CAP transfers only data not audio and uses asynchronous connectionless (ACL) transfer.

SDP is used to find services near you. It is used to query services and the information how to connect with them. Services around you might change rapidly which is different from traditional networks. SDP makes this dynamic service discovery possible. It operates on the top of L2CAP protocol. SDP is also used establishing some of the (non-SDP) connections to the service.

RFCOMM provides ordinary RS-232 connection emulation. It provides both point-to-point and point-to-multipoint connections. It relies on L2CAP protocol that takes care of multiplexing the several connections over one L2CAP connection. TCS protocol defines how telephone calls are transmitted over a Bluetooth link. It is used to carry voice and data calls between Bluetooth devices.