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Bluetooth: Connectivity without wires

Bluetooth has been around for over two years now. What is this technology all about? How does it work? Is it the usual hype that follows any new technology? Here are some answers By Bhavish Sood

Heard of WPANs (wireless personal-area networks), WLANs (wireless local-area networks) and WWANs (wireless wide-area networks)? If you are interested in wireless networking, then you must have. Flexibility and mobility make wireless LANs an attractive alternative to wired networks. Wireless LANs provide all the functionality of wired LANs, without the physical constraints of the wire itself. Besides offering end-user mobility within a networked environment, wireless LANs enable portable networks, allowing LANs to move with the end users. All three types of networks are differentiated by range, data rates, power consumption and cost.

WPANs, such as Bluetooth piconets, provide short-range connectivity for devices such as laptops, PDAs, cell phones and even PCs in a network with small geographical spread, and support low data rates and

limited ranges to achieve low cost and minimal power drain. On the other hand, WLANs, such as 802.11b wireless Ethernet, offer higher speeds

and longer ranges in office buildings and homes. WWANs, such as cellular networks, work over a large area, but offer much lower data rates than WPANs and WLANs.

Bluetooth, named after Harold Bluetooth, a 10th century Danish king who united Norway and Denmark, is a short-range radio technology that allows voice and data connections to be made up to 10 meters (about 30 feet). The range can be extended to 100 meters with an amplifier. The first generation of the technology delivers performance up to 1Mbps. Subsequent versions are expected to deliver anywhere from 2 Mbps to 12 Mbps of throughput. Spanning telecommunications, personal computing, networking, and consumer electronic devices, Bluetooth free users from working with wires paving the way for a host of new conveniences.

Under the Hood: How it works?
Bluetooth uses the FHSS (Frequency Hopping Spread Spectrum) protocol as the wireless LAN standard. Most wireless LAN systems use spread-spectrum technology; a wideband radio frequency technique developed by the military for use in reliable, secure, mission-critical communications systems. Spread-spectrum is designed to trade off bandwidth efficiency for reliability, integrity, and security. Frequency hopping spread spectrum (FHSS) uses a narrowband carrier that changes frequency in a pattern known to both transmitter and receiver. Properly synchronized, the net effect is to maintain a single logical channel. To an unintended receiver, FHSS appears to be short-duration impulse noise. FHSS cannot avoid channels that have been obliterated by noise (It must hop through all 79 channels on a continuous cycle), which causes interference by appliances like Microwaves. Radios in Bluetooth can be master or slave, or be in simultaneous scenarios. Two possible Bluetooth networks exist: piconet and scatternet. When you bring Bluetooth radios within range of each other, they connect and form a piconet. One unit becomes a master, the other a slave. The master controls all the traffic in a piconet.

Bluetooth radios in a piconet frequency-hop together. Each piconet can have up to seven simultaneous or more than 200 active slaves.

Radios in a piconet can be in one of five states: standby, inquire, page, connect and park/hold. Standby is a radio waiting to join a piconet. Inquire is a radio seeking other radios to connect. Page is a master radio asking to connect to a specific radio. Connect is a radio active on a piconet as a master, slave or simultaneous. Park/hold is a low-power connected state. The master gives all the slaves in a piconet its clock-device ID and sets the unique hopping sequence based on the master's device address.

Scatternets occur when multiple masters exist in range of each other. A master radio may also be a slave radio on another piconet. Each piconet is hopping with a different sequence sharing the same 2.4-GHz band. Because of the different hopping sequences, there is very little chance that any master will hit a channel at the same time as another master.

Bluetooth Stack
A better understanding of Bluetooth's functionality would be to understand its underlying architecture. A Bluetooth stack has a physical layer (Baseband), data link layer (LMP) and an adaptation layer (L2CAP).

1. Baseband: Baseband is the physical layer of Bluetooth that manages physical channels and links apart from other services like error correction, data whitening, hop selection and Bluetooth security.

2. Bluetooth radio: Bluetooth radio is a transceiver, which transmits and receives modulated electrical signals from peer Bluetooth devices. The radio for compatibility reasons should have some defined transmitter and receiver characteristics.

3. ACL: ACL is Asynchronous Connection-Less physical link for transmitting data over physical channels. ACL link provides a packet switched connection between the master and all the active slaves.

4. SCO: SCO is Synchronous Connection-Oriented physical link for voice-like information. It is a symmetric, point-to-point link between the master and a specific slave. It behaves like a circuit-switched connection

5. Link Manager essentially handles link set-up, security and control. It provides services like authentication, encryption control, power control and provides QoS capabilities. It also manages devices in different modes (standby, inquire, page, connect and park/hold).

6. L2CAP is the Logical Link Control and Adaptation Layer protocol. It resides in the data link layer and provides connection-less and connection-oriented data services to upper layer protocols with protocol multiplexing capability, segmentation and reassembly operation and group abstractions. L2CAP permits higher-level protocols and applications to transmit and receive L2CAP data packets up to 64 KB in length.

7. SDP is Service Discovery Protocol for applications to discover which services are available and to determine the characteristics of these available services.

8. RFCOMM is a simple transport protocol. It supports up to 60 simultaneous connections between two Bluetooth devices.

Bluetooth and 802.11: Complementary or competing?
The wireless industry is full of opinions about why and how 802.11 might and might not compete with each other although the truth is that they can coexist. Bluetooth provides short-range connectivity for devices such as laoptops, PDAs and networks with a small spread, and support low data rates and limited ranges to achieve low cost and minimal power drain.

WLAN's, such as 802.11b wireless Ethernet, offer higher speeds and longer ranges in office buildings and homes. As I mentioned above they are not exactly competing technologies but complementary as both are servicing a particular set of objectives. However spectrum overlap battles between 802.11 and Bluetooth specifications since both of them operate in the unlicensed frequency would become a reality since. Already a host of devices have the potential o interfere with your Bluetooth device e.g. car remotes, and microwave ovens since they all operate in the same unlicensed frequency. Another scenario that is slowly catching on is the promotion of non-standardized solutions that are promoted by special interest groups or industry consortiums. In the wireless LAN space 802.11 promotion efforts are led by IEEE and Bluetooth has its own Special Interest Group, HiperLAN, which is now widely regarded as a powerful competition is promoted by an Industry consortium. What the world probably needs now is a slower innovation and a standard widely accepted.

Bhavish Sood can be reached at bhavishsood@netscape.net)

Technical Summary
Bluetooth technology supports both point-to-point and point-to-multipoint connections. Several piconets can be established and linked together ad hoc, and all devices in the same piconet are synchronized. The topology can best be described as a multiple piconet structure. The full duplex data rate within a multiple piconet structure with 10 fully loaded, independent piconets is more than 6 Mbps.

Normal range 10 m (0dBm)

Optimal range 100m (+20Bm)

Normal transmitting power 0dBm (1mW)

Optional transmitting power -30 to +20dBm (100mW)

Receiver sensitivity -70dBM

Frequency Band 2.4 GHz

Gross Data rate 1Mbits

Max Data Transfer 721+56 kbit/3 Voice Channels

Specification Comparison      
Specification Bluetooth IrDA Home RF
Data Rate (Kbps) 1000 4000 2000
Distance (m) 10 1 50
No. of Devices 8 2 127
Voice channels 3 1 6
Topology Point to Multipoint Point to Point Network


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