Bluetooth Technology

Bluetooth Basics
Bluetooth is a wireless personal area network (WPAN) communication system standard that allows for wireless data connections to be dynamically added and removed between nearby devices. Each Bluetooth wireless network can contain up to 8 active devices and is called a Piconet. Piconets can be linked to each other (overlap) to form larger area Scatternets.

The system control for Bluetooth requires one device to operate as the coordinating device (a master) and all the other devices are slaves. This is very similar to the structure of a universal serial bus (USB) system that is commonly used in personal computers and devices such as digital cameras. However, unlike USB connections, most Bluetooth devices can operate as either a master (coordinator) or slave (follower) and Bluetooth devices can reverse their roles if necessary.

The characteristics of Bluetooth include an unlicensed frequency band that ranges from 2.4 GHz to 2.483 GHz. This frequency band was chosen because it is available for use in most countries throughout the world. While the standard frequency band for Bluetooth is in the 2.400 GHz to 2.483 GHz (83 MHz) frequency band, the original Bluetooth specification had an optional smaller frequency band 23 MHz version for use in some countries. The use of a smaller frequency band does not change the data transmission rate, however, these devices will be more sensitive to interference (such as other Bluetooth device transmission) and this interference may cause a lower overall data transmission rate.

Every Bluetooth device has a unique 48-bit address BD_ADDR (pronounced “B-D-Adder”). In addition to identifying each Bluetooth device, this address is used to determine the frequency hopping pattern that is used by the Bluetooth device.

Bluetooth devices may have different power classification levels. The 3 power versions for Bluetooth include; 1 mW (class 3), 2.5 mW (class 2) and 100 mWatts (class 1). Devices that have an extremely low power level of 1 milliwatt have a very short range of approximately 1 meter. Bluetooth devices have a power level of up to 100 milliwatts can provide a transmission range of approximately to 100 meters. The high power version (class 1) is required to use adjustable (dynamic) power control that automatically is reduced when enough signal strength is available between Bluetooth devices.

Because on of the objectives of Bluetooth is low power and low complexity, the simple modulation type of Gaussian frequency shift keying (GFSK) is used. This modulation technology represents a logical 1 or 0 with a shift of 115 kHz above or below the carrier signal. The data transmission rate of the RF channel is 1 Mbps.

The smallest packet size in the Bluetooth system is the Bluetooth packet data unit (PDU). Bluetooth PDUs are transmitted between master and slave devices within a Bluetooth Piconet. Each PDU contains the address code of the Piconet, device identifier, and a payload of data. When the PDUs are used to carry logical channels, part of the data payload includes a header, which includes logical channel identifiers. The length of the PDU can vary to fit within 1, 3 or 5 time slot period (625 usec per time slot). Control message PDUs (e.g. link control) always fit within 1 time slot.

Figure that the structure of a Bluetooth contains the address code of the piconet (local system), device identifier (specific device within the piconet), logical channel identifiers (to identify ports), and a payload of data. If a specific protocol is used (such as a wireless RS-232 communication port - RFCOMM), an additional protocol service multiplexer (PSM) field is included at the beginning of the payload data. This diagram also shows that the PDU size can have a 1, 3 or 5 slot length.



Figure : Bluetooth Packet Structure
The Bluetooth system uses time division duplex (TDD) operation. TDD operation permits devices to transmit in either direction, but not at the same time.

Figure shows the basic radio transmission process used in the Bluetooth system. This diagram shows that the frequency range of the Bluetooth system ranges from 2.4 GHz to 2.483 GHz and that the basic radio transmission packet time slot is 625 usec. It also shows that one device in a Bluetooth piconet is the master (controller) and other devices are slaves to the master. Each radio packet contains a local area piconet ID, device ID, and logical channel identifier. This diagram also shows that the hopping sequence is normally determined by the master’s Bluetooth device address. However, when a device is not under control of the master, it does not know what hop- ping sequence to use to it listens for inquiries on a standard hopping sequence and then listens for pages using its own Bluetooth device address.


Figure : Bluetooth Radio Operation

Temporary Small Networks (Piconets)
Bluetooth forms temporary small networks of Bluetooth communication devices of up to 8 active devices called Piconets. The Bluetooth system allows for wireless data connections within the Piconet to be dynamically added and removed between nearby devices. Because the Bluetooth system hops over 79 channels, the probability of interfering (overlapping) with another Bluetooth system is less than 1.5%. This allows several Bluetooth Piconets can operate in the same area at the same time with minimal interference.

Bluetooth communication always designates on of the Bluetooth devices as a main controlling unit (called the master unit). This allows Bluetooth system to be non-contention based. This means that after a Bluetooth device has been added to the temporary network (the Piconet), each device is assigned specific time period to transmit and they do not collide or overlap with other units operating within the same Piconet.

Multiple Piconets can be linked to each other to form Scatternets. Scatternets allow the master in one Piconet to operate as a slave in another Piconet. While this allows Bluetooth devices in one Piconet to communicate with devices in another Piconet (cross-Piconet communication), the use of Scatternets require synchronization (and sharing of data transmission Bandwidth) making them inefficient.

Data Transmission Rates
The basic (gross) radio channel data transmission rate for a single Bluetooth radio channel is 1 Mbps with over 723.2 kbps available to a single user. The data rate available to each user is less than the radio channel data transmission rate because some of the data transmission is used for control and channel management purposes. The users in each Piconet split the remaining data transmission rate.

An example of how the data transmission rate is shared, a Bluetooth Piconet that provides for headset operation, which uses 64 kbps channels in both directions, uses a total data transmission rate of 128 kbps. This is approximately 25% of the total available data transmission bandwidth.

The Bluetooth system allows for different rates in different directions (asynchronous) or for equal data rate (symmetrical rate) transmission.

Figure shows how the radio channel data transmission rate for Bluetooth devices is divided between transmission directions and between multiple devices. In example 1, a PDA is transferring a large file to a laptop computer using asymmetrical transmission. During the transfer, it uses the 5-slot packet size to reach the maximum data transmission rate of 723.2 kbps from the PDA to the laptop. This only allows a data transmission rate of 57.6 kbps from the laptop to the PDA. Example 2 shows a symmetrical data transmission rate of 433.9 kbps between two video conferencing stations. Example 3 shows how the data transmission rate from a laptop is shared between a wireless headset and a PDA. This example shows that the headset uses a symmetrical data transmission rate is 64 kbps from device 1 (the master coordinator) to device 2 and a 57.6 kbps asynchronous data transmission rate between the Laptop and the PDA.


Figure : Bluetooth Data Transmission Rates