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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 |
