| Technology - BLUETOOTH TECHNOLOGY |
| By : Richa | Previous | Next |
| Posted on : 17 Oct, 2005 | Total Views : 9120 |
The convergence of computing and communications has led to the development of Bluetooth technology. Taking the short-range wireless data usage to a new level, this technology is predicted to dominate both the home and business markets.
Now no more complicated tangle of cables and wires are required to connect electronic equipment. A revolutionary technology called Bluetooth allows for the replacement of the cumbersome cables used today to connect a laptop to a cellular phone with one universal low-cost, short-range radio link. Printers, PDAs, desktops, fax machines, keyboards, and virtually any other digital device can be a part of the Bluetooth system. Bluetooth is faster than mobile phones, including the upcoming third-generation, or 3G, high-speed wireless data transmission.
The technology
Bluetooth is a universal radio interface in the 2.45GHz ISM frequency band to function on a worldwide basis. The interface consists of hardware using ICs, a radio frequency chip, and a baseband chip. These parts work together to put a radio signal into the air that can communicate to another chipset imbedded in other product. This makes it easier to do everything, from networking different types of computers to transferring data between digital telephones.
Bluetooth can transmit through solid, non-metal objects. Its nominal link range is from 10 cm to 10 m, which can be extended to 100 m by increasing the transmit power. Designed to operate in a noisy radio frequency environment, the Bluetooth radio uses a fast acknowledgement and frequency hopping scheme to make the link robust. Bluetooth radio modules avoid interference from other signals by hopping to a new frequency after transmitting or receiving packet.
Short range and fast hopping limit the impact of domestic and professional microwave ovens. Use of forward error correction (FEC) limits the impact of random noise on long-distance links. The encoding is optimised for an uncoordinated environment.
The Bluetooth baseband protocol is a combination of circuit and packet switching. Slots can be reserved for synchronous packets. Each packet is transmitted in a different hop frequency. A packet nominally covers a single slot, but can be extended to cover up to five slots.
The technology supports both isochronous and asynchroous services; easy integration of TCP/IP for networking. It can support an asynchronous data channel, up to three simultaneous synchronous voice channels, or a channel which simultaneously supports asynchronous data and synchronous voice. Each voice channel supports 64kbps synchronous (voice) link. The asynchronous channel can support an asymmetric link of maximally 721 kbps in either direction, while permitting 57.6 kbps in the return direction, or a 432.6kbps symmetric link.
How it works
A Bluetooth system comprises a radio unit, a link controller, link manager, and software.
The Bluetooth radio interface is based on a nominal antenna power of 0 dBm. The air interface complies with the FCC rules for the ISM band at power levels up to 0 dBm. Spectrum spreading facilitates optional operation at power levels up to 100 mW worldwide. It is accomplished by frequency hopping in 79 hops displaced by 1 MHz, starting at 2.402 GHz and stopping at 2.480 Hz. The bandwidth is controlled by an internal software switch. The maximum frequency hopping rate is 1,600 hops/sec.
The baseband describes the specifications of the digital signal processing part of the hardware—the Bluetooth link controller, which carries out the baseband protocols and other low-level link routines.
A piconet is a group of two to eight devices connected via Bluetooth technology. Before any connections in a piconet are created, all devices are in standby mode. In this mode, an unconnected unit periodically listens for messages every 1.28 seconds. Each time a device wakes up, it listens on a set of 32 hop frequencies defined for that unit. The number of hop frequencies is 32 for most countries, except Japan, Spain, and France.
Any of the devices becomes the master by initiating the connection procedure. If the address is already known, connection is made by a ‘page’ message. And if the address is unknown, connection is made by an ‘inquiry’ message followed by a subsequent ‘page’ message.
In the initial ‘page’ state, the master unit will send a train of identical page messages on 16 different hop frequencies defined for the device to be paged (slave unit). If there is no response, the master transmits a train of messages on the remaining 16 hop frequencies in the wake-up sequence.
The maximum delay before the master reaches the slave is twice the wake-up period (2.56 seconds) while the average delay is half the wake-up period (0.64 second).
The ‘inquiry’ message is used for finding Bluetooth devices, including public printers, fax machines, and similar devices with an unknown address. This message is very similar to the ‘page’ message, but may require one additional train period to collect all the responses.
A power-saving mode can be used for connected units in a piconet if no data needs to be transmitted. The master unit can put slave units into ‘hold’ mode, where only an internal timer is running. Slave units can also demand to be put into ‘hold’ mode. Data transfer restarts instantly when units transition out of ‘hold’ mode. The ‘hold’ mode is used when connecting several piconets or managing a low-power device such as a temperature sensor.
Two more low-power modes are available, the ‘sniff’ mode and the ‘park’ mode. In the ‘sniff’ mode, a slave device listens to the piconet at reduced rate, thus reducing its duty cycle. The ‘sniff’ interval is programmable and depends on the application. In the ‘park’ mode, a device is still synchronised to the piconet but does not participate in the traffic.
The Bluetooth baseband technology supports two types of link, synchronous connection oriented (SCO) and asynchronous connectionless (ACL). The SCO link is used primarily for voice, and the ACL for packet data.
Different master-slave pairs of the same piconet can use different link types, and the link type may change arbitrarily during a session. Each link type supports up to sixteen different packet types. Four of these are control packets, common to both the SCO and ACL links. Both of these link types use a time division duplex (TDD) scheme for full-duplex transmissions.
The SCO link is symmetric and typically supports time-bound voice traffic. SCO packets are transmitted over reserved intervals. Once the connection is established, both master and slave units may send SCO packets without being polled. One SCO packet type allows both voice and data transmission—with only the data portion being retransmitted when corrupted.
The ACL link is packet-oriented and supports both symmetric and asymmetric traffic. The master unit controls the link bandwidth and decides how much piconet bandwidth is given to each slave, and the symmetry of the traffic. Slaves must be polled before they can transmit data. The ACL link also supports broadcast messages from the master to all slaves in the piconet.
The three error-correction schemes defined for Bluetooth baseband controllers include 1/3 rate forward error correction code (FEC), 2/3 rate forward error correction code (FEC), and automatic repeat request (ARQ) scheme for data.
FEC scheme reduces the number of retransmissions in the data payload. However, in a reasonably error-free environment, FEC creates unnecessary overhead that reduces the throughput. Since the packet header contains valuable link information and should survive bit errors, it is always protected by a 1/3 rate FEC. An unnumbered ARQ scheme is applied in which data transmitted in one slot is directly acknowledged by the recipient in the next slot. For a data transmission to be acknowledged, both the header error check and the cyclic redundancy check must be okay; otherwise a negative acknowledge is returned.
The Link Manager (LM) software carries out link setup, authentication, link configuration, and other protocols. It discovers other remote LMs and communicates with them via the Link Manager Protocol (LMP). To perform its service provider role, the LM uses the services of the Link Controller (LC).
The LM offers services such as sending and receiving of data, link address inquiries, connection set-up, and authentication. It has an efficient means to inquire and report a name or device ID up to 16 characters in length. It negotiates on link mode and set-up. It decides the actual frame type on a packet-by-packet basis.
In ‘sniff’ mode, the duty cycle of the slave is reduced: it listens only M slots where M is negotiated at the LM. The master can only start transmission in specified time slots spaced at regular intervals.
In ‘hold’ mode, turning off the receiver for longer periods saves power. Any device can wake up the link again, with an average latency of 4 seconds. This is defined by the LM and handled by the LC.
The LM sets a device in ‘park’ mode when it does not need to participate on the channel and wants to stay synchronised. It wakes up at regular intervals to listen to the channel in order to resynchronise with the rest of the piconet, and to check for page messages.
Any device displaying the Bluetooth ‘logo’ should interoperate with other Bluetooth devices. Software interoperability begins with the Bluetooth link level protocol, responsible for protocol multiplexing, device and service discovery, and segmentation and reassembly.
Bluetooth devices must be able to recognise each other and load the appropriate software to discover the higher level abilities each device supports. Interoperability at the application level requires identical protocol stacks. Different Bluetooth devices (PCs, handhelds, headsets, cellular telephones) have different compliance requirements. To obtain more functionality, the Bluetooth software framework will reuse existing specifications such as OBEX, vCard/vCalendar, human interface device (HID), and TCP/IP.
Device compliance requires conformance to both the Bluetooth specification and existing protocols. The software framework is contemplating functions like configuration and diagnosis utility, device discovery, cable emulation, peripheral communication, audio communication and call control, object exchange for business cards and phone books, and networking protocol.
The Bluetooth specification contemplates interfaces where the radio modules may be integrated into notebook personal computers or attached using PC-card or USB.
Notebook PC usage models include remote networking using a Bluetooth cellular phone, speakerphone applications using a Bluetooth cellular phone, business card exchange between Bluetooth notebooks, handhelds, and phones, and calendar synchronisation between Bluetooth notebooks, handhelds, and phones.
Bluetooth technology is operating system independent and not tied to any specific operating system. Implementations of the Bluetooth for several commercial operating systems are in development. For notebook computers, the implementation of the Bluetooth in Microsoft Windows 98 and NT 5.0 using WDM and NDIS drivers is being contemplated. Customer-visible interoperability is promoted by requiring minimal levels of software functionality, such as speakerphone, on notebook computers.
Applications
Data exchange. Bluetooth’s ability to penetrate solid objects and its capability for maximum mobility within the piconet allow for many data exchange applications. For example, with Bluetooth a person could synchronise his phone to a PC without taking the phone out of his pocket or purse. The omnidirectional capability of Bluetooth allows synchronisation to start when the phone is brought into the range of the PC. This synchronisation does’t require the phone to be in a fixed location. LAN access. Because there are no line-of-sight requirements, Bluetooth is well suited to wirelessly connect a device to a wired network. Also, multipoint capability allows multiple devices to effectively share the media. Since Bluetooth’s aggregate bandwidth is limited to 1 Mbps, the performance is not very encouraging. Dial-up networking. Emulating an EIA/TIA232 connection between a portable computer and a mobile phone for establishing a dial-up connection to the Internet is also an application targeted by Bluetooth. The primary advantage of Bluetooth is that the user can leave the mobile phone clipped to his belt or in a pocket and walk around for the entire dial-up connection. Voice applications. A native feature of the Bluetooth specification is synchronous voice channels. Bluetooth has the ability to reserve bandwidth for carrying digital voice data. It can support as many as three simultaneous full-duplex voice conversations within a piconet. Security issues. Since Bluetooth is omnidirectional, it can be monitored by a snooping device from any direction, including hidden locations. Bluetooth makes it possible to prevent eavesdroping on a conversation by providing authentication and encryption in its baseband protocol. Authentication relies on a challenge-response protocol utilising a secret key (password). Both devices must contain the same secret key. The protocol allows each device to authenticate the other. After the devices are authenticated, it is possible to encrypt the transmission for added security.
Technology developers
Bluetooth technology is being developed through the combined contributions of the members of the Bluetooth Special Interest Group. Intel brings architectural knowledge, advanced silicon technology, manufacturing expertise, and software components to the group. Ericsson contributed the basic radio technology. Nokia contributed the radio technology and mobile handset software. IBM and Toshiba developed a common specification for integrating Bluetooth technology into mobile devices. Microsoft, Lucent technologies, Motorola, and 3Com have signed on as primary Bluetooth promoters. Many smaller companies worldwide have also signed up for royalty-free rights to the emerging standard.
The final word
Bluetooth technology is being developed as an open specification, and will be licensed royalty-free to all companies who sign up to become members of the Bluetooth initiative. This will allow a quick proliferation of the technology through a wide-range of mobile products and devices. Products integrated with Bluetooth radios are expected to hit the market shortly.
