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Introduction to Bluetooth

Bluetooth is an always-on, short-range radio hookup that resides on a microchip. It was initially developed by Swedish mobile phone maker Ericsson in 1994 as a way to let laptop computers make calls over a mobile phone. Since then, several thousand companies have signed on to make Bluetooth the low-power short-range wireless standard for a wide range of devices. Industry observers expect Bluetooth to be installed in billions of devices by 2005.

The Bluetooth standards are published by an industry consortium known as the Bluetooth SIG (special interest group).

The concept behind Bluetooth is to provide a universal short-range wireless capability. Using the 2.4 GHz band, available globally for unlicensed low-power uses, two Bluetooth devices within 10 m of each other can share up to 720 Kbps of capacity. Bluetooth is intended to support an open-ended list of applications, including data (such as schedules and telephone numbers), audio, graphics, and even video. For example, audio devices can include headsets, cordless and standard phones, home stereos, and digital MP3 players. Following are some examples of the capabilities that Bluetooth can provide consumers:

  • Make calls from a wireless headset connected remotely to a cell phone;
  • Eliminate cables linking computers to printers, keyboards, and the mouse;
  • Hook up MP3 players wirelessly to other machines to download music;
  • Set up home networks so that a couch potato can remotely monitor air conditioning, the oven, and children's Internet surfing;
  • Call home from a remote location to turn appliances on and off, set the alarm, and monitor activity.

Bluetooth Applications

Bluetooth is designed to operate in an environment of many users. Up to eight devices can communicate in a small network called a piconet. Ten of these piconets can coexist in the same coverage range of the Bluetooth radio. To provide security, each link is encoded and protected against eavesdropping and interference.

Bluetooth provides support for three general application areas using short-range wireless connectivity:

  • Data and voice access points - Bluetooth facilitates real-time voice and data transmissions by providing effortless wireless connection of portable and stationary communications devices;
  • Cable replacement - Bluetooth eliminates the need for numerous, often proprietary cable attachments for connection of practically any kind of communications device. Connections are instant and are maintained even when devices are not within line of sight. The range of each radio is approximately 10 m, but can be extended to 100 m with an optional amplifier;
  • Ad hoc networking - A device equipped with a Bluetooth radio can establish instant connection to another Bluetooth radio as soon as it comes into range.

Bluetooth Standards Documents

The Bluetooth standards present a formidable bulkówell over 1,500 pages, divided into two groups: core and profile. The core specifications describe the details of the various layers of the Bluetooth protocol architecture, from the radio interface to link control. Related topics are also covered, such as interoperability with related technologies, testing requirements, and a definition of various Bluetooth timers and their associated values.

The profile specifications are concerned with the use of Bluetooth technology to support various applications. Each profile specification discusses the use of the technology defined in the core specifications to implement a particular usage model. The profile specification includes a description of which aspects of the core specifications are mandatory, optional, and not applicable. The purpose of a profile specification is to define a standard of interoperability, so that products from different vendors that claim to support a given usage model will work together. In general terms, profile specifications fall into one of two categories: cable replacement or wireless audio. The cable replacement profiles provide a convenient means for logically connecting devices in proximity to one another and for exchanging data. For example, when two devices first come within range of one another, they can automatically query each other for a common profile. This might then cause the end users of the device to be alerted, or cause some automatic data exchange to take place. The wireless audio profiles are concerned with establishing short-range voice connections.

The Bluetooth developer must wade through the many documents with a particular application in mind. The reading list begins with coverage of some essential core specifications plus the general access profile. This profile is one of a number of profiles that serve as a foundation for other profiles and don't specify independently usable functionality. The general access profile specifies how the Bluetooth baseband architecture, defined in the core specifications, is to be used between devices that implement one or multiple profiles. Following a basic set of documents, the reading list splits along two lines, depending on whether the reader's interest is in cable replacement or wireless audio.

Protocol Architecture

Bluetooth is defined as a layered protocol architecture consisting of core protocols, cable replacement and telephony control protocols, and adopted protocols.

The core protocols form a five-layer stack consisting of the following elements:

  • Radio - Specifies details of the air interface, including frequency, the use of frequency hopping, modulation scheme, and transmit power.
  • Baseband - Concerned with connection establishment within a piconet, addressing, packet format, timing, and power control.
  • Link manager protocol (LMP) - Responsible for link setup between Bluetooth devices and ongoing link management. This includes security aspects such as authentication and encryption, plus the control and negotiation of baseband packet sizes.
  • Logical link control and adaptation protocol (L2CAP) - Adapts upper-layer protocols to the baseband layer. L2CAP provides both connectionless and connection-oriented services.
  • Service discovery protocol (SDP) - Device information, services, and the characteristics of the services can be queried to enable the establishment of a connection between two or more Bluetooth devices.

Protocol Architecture

Bluetooth is defined as a layered protocol architecture consisting of core protocols, cable replacement and telephony control protocols, and adopted protocols.

The core protocols form a five-layer stack consisting of the following elements:

  • Radio - Specifies details of the air interface, including frequency, the use of frequency hopping, modulation scheme, and transmit power.
  • Baseband - Concerned with connection establishment within a piconet, addressing, packet format, timing, and power control.
  • Link manager protocol (LMP) - Responsible for link setup between Bluetooth devices and ongoing link management. This includes security aspects such as authentication and encryption, plus the control and negotiation of baseband packet sizes.
  • Logical link control and adaptation protocol (L2CAP) - Adapts upper-layer protocols to the baseband layer. L2CAP provides both connectionless and connection-oriented services.
  • Service discovery protocol (SDP) - Device information, services, and the characteristics of the services can be queried to enable the establishment of a connection between two or more Bluetooth devices.

RFCOMM is the cable replacement protocol included in the Bluetooth specification. RFCOMM presents a virtual serial port that is designed to make replacement of cable technologies as transparent as possible. Serial ports are one of the most common types of communications interfaces used with computing and communications devices. Hence, RFCOMM enables the replacement of serial port cables with the minimum of modification of existing devices. RFCOMM provides for binary data transport and emulates EIA-232 control signals over the Bluetooth baseband layer. EIA-232 (formerly known as RS-232) is a widely used serial port interface standard.

Bluetooth specifies a telephony control protocol. TCS BIN (telephony control specificationóbinary) is a bit-oriented protocol that defines the call control signaling for the establishment of speech and data calls between Bluetooth devices. In addition, it defines mobility-management procedures for handling groups of Bluetooth TCS devices.

The adopted protocols are defined in specifications issued by other standards-making organizations and incorporated into the overall Bluetooth architecture. The Bluetooth strategy is to invent only necessary protocols and use existing standards whenever possible. These are the adopted protocols:

  • PPP - The point-to-point protocol is an Internet standard protocol for transporting IP datagrams over a point-to-point link;
  • TCP/UDP/IP - These are the foundation protocols of the TCP/IP protocol suite;
  • OBEX - The object exchange protocol is a session-level protocol developed by the Infrared Data Association (IrDA) for the exchange of objects. OBEX provides functionality similar to that of HTTP, but in a simpler fashion. It also provides a model for representing objects and operations. Examples of content formats transferred by OBEX are vCard and vCalendar, which provide the format of an electronic business card and personal calendar entries and scheduling information, respectively;
  • WAE/WAP - Bluetooth incorporates the wireless application environment and the wireless application protocol into its architecture.

Usage Models

A number of usage models are defined in Bluetooth profile documents. In essence, a usage model is a set of protocols that implement a particular Bluetooth-based application. Each profile defines the protocols and protocol features supporting a particular usage model. Following are the highest-priority usage models:

  • File transfer - The file transfer usage model supports the transfer of directories, files, documents, images, and streaming media formats. This usage model also includes the capability to browse folders on a remote device;
  • Internet bridge - With this usage model, a PC is wirelessly connected to a mobile phone or cordless modem to provide dial-up networking and fax capabilities. For dial-up networking, AT commands are used to control the mobile phone or modem, and another protocol stack (such as PPP over RFCOMM) is used for data transfer. For fax transfer, the fax software operates directly over RFCOMM;
  • LAN access - This usage model enables devices on a piconet to access a LAN. Once connected, a device functions as if it were directly connected (wired) to the LAN;
  • Synchronization - This model provides a device-to-device synchronization of PIM (personal information management) information, such as phone book, calendar, message, and note information. IrMC (Ir mobile communications) is an IrDA protocol that provides client/server capability for transferring updated PIM information from one device to another;
  • Three-in-one phone - Telephone handsets that implement this usage model may act as a cordless phone connecting to a voice base station, as an intercom device for connecting to other telephones, and as a cellular phone;
  • Headset - The headset can act as a remote device's audio input and output interface.