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 bulk well 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 specification binary) 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.
Logical link control and adaptation
protocol (L2CAP)
L2CAP is
used within the Bluetooth protocol stack. It passes packets to either the Host
Controller Interface (HCI) or on a hostless system, directly to the Link
Manager/ACL link.
L2CAP's functions include:
•
Multiplexing data between different higher
layer protocols.
•
Segmentation and reassembly of packets.
•
Providing one-way transmission management
of multicast data to a group of other Bluetooth devices.
•
Quality of
service (QoS) management for higher layer
protocols.
L2CAP is used to communicate over the host
ACL link. Its connection is established after the ACL link has been set up.
In basic mode, L2CAP provides packets with
a payload configurable up to 64 kB, with 672 bytes as the default MTU, and
48 bytes as the minimum mandatory supported MTU. In retransmission and flow
control modes, L2CAP can be configured for reliable or asynchronous data per
channel by performing retransmissions and CRC checks. Reliability in either of
these modes is optionally and/or additionally guaranteed by the lower layer
Bluetooth BDR/EDR air interface by configuring the number of retransmissions
and flush timeout (time after which the radio will flush packets). In-order
sequencing is guaranteed by the lower layer.
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