Bluetooth is a wireless technology that enables any electrical device to wirelessly communicate in the 2.5 GHz ISM (license free) frequency band. Its primarily designed for devices such as mobile phones, headsets, PDA's and portable computers to communicate and send data to another Bluetooth enabled device. It has been specifically designed as a low cost, low power, radio technology, which is particularly suited to the short range Personal Area Network (PAN) application.
Bluetooth Features
- Operates in 2.4GHz ISM radio band
- Utilizes 79 channel FHSS (frequency-hopping spread spectrum) technology
- Communication channel can support both data (asynchronous) and voice (synchronous) communications with a total bandwidth of 1 Mb/sec.
- Provides a 128 encryption mode for security
- Supports 8 active and 255 inactive (parked) devices
- Automatic error correction and retransmission
- Well defined attachment profiles contained in SIG controlled specification
Architecture
Bluetooth devices can interact with other Bluetooth devices in several different ways. The simplest scheme is when only two devices are involved. This is referred to as point-to-point. One of the devices acts as the master and the other as a slave. This ad-hoc network is referred to as a piconet. Bluetooth networks can include one master and one or more slaves. There can be up to seven active slaves in a piconet. In the case of multiple slaves, the communication topology is referred to as point-to-multipoint. In this case, the channel (and bandwidth) is shared among all the devices in the piconet.
Supported channel configurations (and bandwidth) are as follows:
| Configuration |
Max. Data Rate Upstream |
Max. Data Rate Downstream |
| 3 Simultaneous Voice Channels |
64 kb/sec X 3 channels |
64 kb/sec X 3 channels |
| Symmetric Data |
433.9 kb/sec |
433.9 kb/sec |
| Asymmetric Data |
723.2 kb/sec or 57.6 kb/sec |
57.6 kb/sec or 723.2 kb/sec |
The synchronous voice channels are provided using circuit switching with a slot reservation at fixed intervals. A synchronous link is referred to as an SCO (synchronous connection-oriented) link. The asynchronous data channels are provided using packet switching utilizing a polling access scheme. An asynchronous link is referred to as an ACL (asynchronous connection-less) link. A combined data-voice SCO packet is also defined. This can provide 64 kb/sec voice and 64 kb/sec data in each direction.
Bluetooth devices are classified according to three different power classes, as shown in the following table.
| Power Class |
Maximum Output Power |
1
|
100 mW (20 dBm) |
2
|
2.5 mW (4 dBm) |
| 3 |
1 mW (0 dBm) |
Most portable Bluetooth devices will probably be in Power Class 1 or 2 (with a nominal output power of 0 dBm) due to cost and battery life issues. A Power Class 1 device requires that you utilize a power control to limit the transmitted power over 0 dBm. While a little more costly and power hungry, this will provide up to 100m of range, which should be sufficient for home networking and other applications that require a greater range. Bluetooth radio modules use Gaussian Frequency Shift Keying (GFSK) for modulation. A binary system is used where a one is signified by a positive frequency deviation and a zero is signified by a negative frequency deviation.
Bluetooth utilizes a Link Module, which is closely associated with its Link Manager software. This module and its software are responsible for the baseband protocols and some other low-level link functions. This includes; sending/receiving data, setting up connections, error detection and correction, data whitening, power management, and authentication.
The link module is also responsible for deriving the hop sequence. This is accomplished using the Bluetooth Device Address (BD_ADDR) of the master device. All Bluetooth devices are assigned a 48-bit IEEE 802 address. This 48-bit master device address is used by each of the devices in the piconet to derive the hop sequence.
The Link Module is also responsible for performing the three error correction schemes that are defined for Bluetooth:
- 1/3 rate FEC (forward error correction)
- 2/3 rate FEC
- ARQ scheme for the data (automatic retransmission request)
The purpose of the two FEC schemes is to reduce the number of retransmissions. The ARQ scheme will cause the data to be retransmitted until an acknowledgement is received indicating a successful transmission (or until a pre-defined time-out occurs). A CRC (cyclic redundancy check) code is added to each packet and used by the receiver to decide whether or not the packet has arrived error free. Note that the ARQ scheme is only used for data packets, not synchronous payloads such as voice.
In order to reduce highly redundant data and minimize DC bias, a data-whitening scheme is used to randomize the data. The data is scrambled by a data whitening word and then unscrambled using the same word at the receiver. This descrambling is done after the error detection/correction process. Bluetooth provides provisions for three low power modes to conserve battery life. These states, in decreasing order of power requirements are Sniff Mode, Hold Mode, and Park Mode. While in the Sniff mode, a device listens to the piconet at a reduced rate. The Sniff interval is programmable, providing flexibility for different applications. The Hold mode is similar to the Park mode, except that the Active Member address (AM_ADDR) is retained. In the Park mode, the device’s clock continues to run and remains synchronized to the master, but the device does not participate at all in the piconet.
High Speed Bluetooth
In early 2006, the Bluetooth Special Interest Group announced its selection of the WiMedia Alliance version of UWB for integration in future Bluetooth wireless technology. This initiative is intended to create a version of Bluetooth with a high-speed/high-data-rate option. This next generation of Bluetooth technology will meet the high-speed demands of synchronizing and transferring large amounts of data, as well as enabling high-quality video and audio applications for digital media devices including smart phones, multi-media projectors and television sets.
