This whitepaper provides information about Bluetooth - an upcoming technology using radio links. Bluetooth standard is defined by a joint group of industries (telecommunication and computing) known as Bluetooth special Interest Group (Bluestooth SIG).
Bluetooth is an open standard which utilises unlicensed 2.4GHz Industrial-Scientific-Medical (ISM) Radio Band. Bluetooth promises one megabyte (1MB) data transfer rate without wire connection. Bluetooth is primarily intended to make it easier to attach a peripheral device to a computer and perform data transfer operation wirelessly.
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Wireless
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Bluetooth peripherals communicate with each other using 2.5GHz ISM Band. The user need not manage ( and worry) about the cable to connect each other. |
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Globally
Available
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The 2.4GHz ISM band is specially allotted worldwide, to short range personal devices. Maximum range allowed to such devices is 10-100metre. This allows to create smaller (and many) local networks. |
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Automatic
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Bluetooth users need not configure/reconfigure devices while connected. The devices automatically detect each other in the proximity and configure themselves accordingly. |
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Low power
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Bluetooth enabled devices need require very less power to transmit over the specifed range, in order of mW. Hence such devices can work off the battery, making mobility a stronger feature. |
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Simultaneous
Voice/Data
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Bluetooth offers simultaneous voice and data transfers using either 1data/3voice or 1data/voice channel. Bluetooth can trasnfer data at almost 1Mbps. |
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Omni-directional
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Unlike other wireless signals, bluetooth signals can pass through walls. This offers greater convenience to user. Devices are not limited by the line-of-sight restrictions as in case of IrDA. |
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Secure
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Bluetooth uses frequency hopping spread-spectrum mechanism which is hard to intercept and spoof. Alongwith this stronger application level security mechanisms provide secure channel for communication. |
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Promising
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Bluetooth manufacturers claim that cost of embedding and pushing this technology are lower. Also, considering the features and advantages provided, it seems that the technology will be accepted as a mainstream technology in near future. |
A Bluetooth enabled device has four major blocks, as shown in the figure.
The radio unit is the actual physical trasciever that handles transmission and reception of radio waves. This is the physical layer of the specification. Link controller is anologous to the Data link layer of ISO-OSI Model. The protocol residing at this layer is called L2CAP ( Logical Link Control and Adaptation Protocol). L2CAP provides connectionless and connection-oriented services to upper layers alongwith protocol multiplexing capabalities, segmentation and reasembly. Link Management is performed by a software module within a Bluetooth enabled device. Tasks that are performed by this layer include link setup, authentication, link configuration and other management functions. It also adds Encryption control, power control and quality of service (QoS) capabilities. Host implies the application layer of the bluetooth device stack. It contains all the application level primitives offered by a particular device.
Each Bluetooth device is assigned a worldwide unique 48bit number (BD_ADDR) at the time of manufacturing. This number is used to identify it when joined in a Piconet. Bluetooth SIG takes care that the addresses allotment is done in a standard manner so that duplications will not occur. This number is also used for intra-device communication, which will be clear from subsequent discussion.
A typical bluetooth environment is composed of different Bluetooth enabled devices. Each device can transfer its wireless signals over a circular range of approx 100mts. When more than one such devices are brought together, their ranges overlap and form a communcation link. A collection of such devices is said to form a PicoNet. A PicoNet can consist of upto 8 Bluetooth enabled devices. Each of the devices has same Bluetooth implementations. One of the device's clock signal is chosen as a reference for all the activities to synchronise activites in a Piconet.
When a PicoNet is established, one of the device acts as a Master and rest of them act as Slaves. However this assignement occurs only during a single connection setup and may change from time to time. Any Device that initiates an action or requests a service is generally chosen as the master. All the slaves use the master's clock to synchronise their activities during the operations. Also, the frequency hopping pattern (explained later) of the master is also followed by the slaves. The frequency hopping pattern is derived from the master's BD_ADDR.
As said above, slave devices are the devices which respond to the requests of a Master device. They use Master's clock as reference to synchronise all piconet activites.
Bluetooth devices communicate using Radio waves. But they differ from conventional radio devices in a manner that, they do not use a single frequency to communicate. Bluetooth devices use a technique called Frequency Hopping Spread Spectrum (FHSS).
Spread Spectrum is the technology where communication is carried over a group of frequencies. Along with the actual data, special "spreading" codes are transmitted called "Psuedo Random Noise" codes. These signals are similar to Gausian noise singals that arise due to actual noise interference. However they differ because inherantly they are not random (Pseudo Random). The extra data (or noise) helps to recover desired data even if part of signal is destroyed. Existance of this noise like code signals in the data also makes the overall communication appear like wideband noise signals to an unauthorised user. A device that knows the predetermined Noise Code can only recover actual data properly. This code can also change at each transaction, which makes spying the communication more difficult.
Frequency Hopping is a mode of Spread Spectrum transmission where the carrier frequency of transmission changes after a certain period. Total bandwidth is divided into 'N' separate channels, each having bandwidth 'b'. At any given instant 't', any one of the available channel is used for transmission. This change of frequency is predetermined before begining each transaction. To be specific, this signal change is a function of a Bluetooth device id (BD_ADDR). Only legitimate users (devices) know the sequence in advance and thus can recover desired signals from the air.
Frequency Hopping provides many advantages, some of them are as follows
| Secure | Inherant nature of Spread Spectrum technology is security, due to the noise codes in the signal. Frequency hopping provides more security by providing hard to detect frequency changes. |
| Low Power | Spread Spectrum Devices need not use total available bandwidth at a time. Thus power requirements of such devices remain low. |
| Tolerance | When the frequency change takes place, one or more channel may experience interference from other radio devices. In this case only a part of data is lost. Actual data can be recovered using various available techniques. |
| Co-existance | Many transactions can take place in vicinity because they can use different channels. Channel usage may collide but they will not repeat indefinitely. Also lost data can be recovered in this case with a greater probability. Frequency change sequence which do not collide at all are said to be "Orthogonal". |
| Anti-jamming | Entire Bluetooth communication is hard to "jamm" (overtaken by a strong source) since it is meant for low range and also not relied upon a single frequency. |
The figure shows the various layers of Bluetooth Protocols. Following is a brief explanation of each layer.
This is the actual transciever section of a Bluetooth device. Operating frequency is 2.4GHz. This section typically operates at 10-100mW of powe level. Spectrum spreading is accomplished by frequency hopping in 79 jops displaced by 1MHz (2.402GHz - 2.480GHz). Maximum frequency hopping rate is 1600 hops/s. Transmission and reception is performed using Guassian Frequency Shift Keying.
Baseband is the digital signal processing part of the Bluetooth hardware. It provides communication channel to the upper layers. Both data and voice can be sent using the provided channels. The channles can be either Synchronous or Asynchronous. Synchronous channels are typically utilized for voice communication, where timely delivery is required without any need of retransmissions. On the other hand, Asynchronous channel can be used for data communication where reliabality is the prime concern.
The Link Manager (LM) is the software entity in the Baseband layer. It communicated with remote peer layer using Link Management Protocol (LMP). Services provided by this layers include
Host Controller Interface helps the upper software layers to access lower levels through standard calls. It encapsulated the hardware so that Bluetooth applications can access it without knowledge of lower implementataion details.
This is a software entity which provides following services on the logical level...
L2CAP operates over links provided by the baseband layer and creates logical channels for the use of upper layers. Data is exchanged using packets. L2CAP supports packet segmentation and reassembly. Protocol multiplexing is also supported by this layer.
Service Discovery Protocol is used to determine what offered by other devices. Only information about the services is conveyed using this protocol. However the layer doesn't provide automatic notification about the availability of a service. Client must explicitly poll a server for it using SDP.
RFCOMM protocol provides emulation of serial ports over L2CAP. This protocol is a subset of ETSI-TS 07.10 standard. It supports upto 60 simultaneous connections between two Bluetooth devices.
OBEX is a upper layer of service protocol for RFCOMM. It provides easy and spontaneous mechanism to transfer objects over the channel provided by RFCOMM. OBEX is nased on IrOBEX specifications given by IrDA for object exchange over infrared chanels.
TCS provides bit level interface to transfer voice signals over the channel. This part of the specification is called TCS-BIN. It also provides a set of commands called TCS-AT. The commands are used to control telephone or modem present in Bluetooth devices.
Bluetooth protocol stack also supports existing application protocols such as TCP/IP and WAP. This allows migration from existing application to bluetooth in an easy manner.
Bluetooth is meant mainly for communication between personal devices within a small distnace. But it is not limited to household use. Bluetooth devices are expected to be present everywhere. Much of the data transfer is supposed to automatic, exp. a personal diary is automatically supposed to synchronise its data with a computer. Now it may be the case that the nearest computer may not belong to the owner of the diary. In this case the synchronisation is not desired. It may also mean that privacy of information could be at risk. Thankfully, bluetooth consideres this issue and security is given due concern. Security is provided by both hardware and software levels.
At the hardware level, inherant security comes through Frequency hopping. It is difficult to know exact frequency hopping sequency unless the sniffer knows master's BD_ADDR. Which is determined only at the begining of a transaction. Also, bluetooth devices can be grouped by a PIN (Personal Identification Number). This is given by the user. Devices having this can only be contacted for any service. The link is authenticated before establishment using assymetric 128bit Keys with Challage-Handshake Protocol, for which, keys are never transmitted over the link. The authentication keys are based on PIN, BD_ADDR and an embedded key in the device. Dependance on these parameters makes the scheme much stronger.
At the software level, the applications themselves may use data encryption. TCP/IP can use SSL (Secured socket Layer). WAP stack may use WTLS (Wireless Transport Layer Security).
These multiple levels of security mechanisms helps to build a much secure system altogether.
So far so good. Bluetooth promises various new applications in the real world. It also promises new trends in the existing ones. Finally we'll have a look at few of them...
Bluetooth is technically sound. Now the only hurdle in the widespread use is the availability of the devices and their cost-effeciveness. Manufacturers claim that the cost of embedding Bluetooth in the existing devices would be lower that $10 per device, and it may come down further. With companies making big forecasts, there is equal delay in making the technology available in the market on big scale.
If the manufacturers are able to provide the technology in a cheap and innovative manner, Bluetooth is certainly going to be the future.