Bluetooth - An Overview

موضوع: Bluetooth - An Overview الخميس ديسمبر 11, 2008 7:44 pm

Bluetooth - An Overview

How does Bluetooth work?

Bluetooth - An Overview Masla5

The technical details of Bluetooth´s
function are very complex, and are best studied in detail in the
PDF-format documents that can be found on the Bluetooth website.But, superficially speaking, Bluetooth uses frequency hopping in timeslots.
Bluetooth has been designed to operate in noisy radio frequency
environments, and uses a fast acknowledgement and a frequency-hopping
scheme to make the communications link robust, communication-wise.
Bluetooth radio modules avoid interference from other signals by
hopping to a new frequency after transmitting or receiving a packet.Compared with other systems operating in
the same frequency band, the Bluetooth radio typically hops faster and
uses shorter packets. This is because short packages and fast hopping limit the impact of microwave ovens and other sources of disturbances. Use of Forward Error Correction (FEC) limits the impact of random noise on long-distance links.
Bluetooth - An Overview Connbar

2 transmission power levels

The Bluetooth radio is built into a small
microchip and operates in a globally available frequency band ensuring
communication compatibility worldwide. The Bluetooth specification has
two power levels defined;

a lower power level that covers the shorter personal area within a room, and

a higher power level that can cover a medium range, such as within a home.

Software controls and identity coding built into each microchip ensure that only those units preset by their owners can communicate.

Communication routes

One thing that can be noted from the figure
above is that, although Bluetooth works in an ad-hoc fashion (and not
server-based) all communication is done visavi the Master unit. There
is no direct communication between slave units. Nor is it intended for
the Master to route messages between slave units. Rather, if slave
units find that they want to talk directly to each other, they would
form a new piconet, with one of them acting as Master. This does not
mean that they have to leave the previous piconet. More likely,
they will be parked in the "old" net unless they decide to quit the
"old" net altogether. This is not a big decision for the slave units;
reconfiguration in Bluetooth is dynamic and very fast.
Bluetooth - An Overview Connbar

Technical prestanda

The Bluetooth baseband protocol is a
combination of circuit and packet switching. Time slots can be reserved
for synchronous packets. A frequency hop is done for each packet that
is transmitted. A packet nominally covers a single time slot, but can
be extended to cover up to five slots.Bluetooth can support;

an asynchronous data channel, or

up to 3 simultaneous synchronous voice channels, or

a channel which simultaneously supports asynchronous data and synchronous voice.

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Bluetooth definitions

Bluetooth - An Overview Masla4

It is well to aquaint oneself with the terminology used in Bluetooth,
if one is to understand the descriptions on these webpages. One could
say that there are 3 types of connections in Bluetooth, as shown to the
right:

a) Single-slave

b) Multi-slave (up to 7 ”slaves” on one master)

c) Scatternet

Piconet:

A collection of devices connected
via Bluetooth technology in an ad hoc fashion. A piconet starts with
two connected devices, such as a portable PC and a mobile phone. The
limit is set at 8 units in a piconet (that´s why the required
address-space is limited to 3 bits). All Bluetooth devices are peer units
and have identical implementations. However, when establishing a
piconet, one unit will act as a master for synchonization purposes, and
the other unit(s) will be slave(s) for the duration of the piconet
connection.

Scatternet:

Two or more independent and non-synchronized piconets that communicate with each other.
A slave as well as a master unit in one piconet can establish this connection by becoming a slave in the other piconet.
It will then relay communications between the piconets, if the need arises.

Master unit:

Bluetooth - An Overview Master

The
device in a piconet whose clock and hopping sequence are used to
synchronize all other devices in the piconet. The master also numbers
the communication channels.
Slave units:

Bluetooth - An Overview Slaves

All devices in a piconet that are not the master (up to 7 active units for each master).
Mac address:

A 3-bit Media Access Control address used to distinguish between units participating in the piconet.
Parked units:

Devices in a piconet which are regularly synchronized but do not have MAC addresses. They are woken up by the Master with a ”beacon signal”.
Sniff mode and hold mode:

Devices that are synchronized to a piconet,
and which have temporarily entered power-saving modes in which device
activity is lowered. They keep their MAC-addresses.
The Beacon-channel:

To
support slaves, the Master establishes a beacon channel when one or
more slaves are parked. This channel consists of one beacon slot, or a
train of equidistant beacon slots transmitted at constant time interval.

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Bluetooth - An Overview

What about radiation; is it dangerous?

Bluetooth - An Overview Print1

It is a matter of concern for some people
that the carrier waves used by Bluetooth´s transmitters use the same
frequency range as microwave owens (Bluetooth uses 2.402 GHz to 2.480
GHz). What does it feel like to get in the path of such waves?Actually, the transmitting power is far too
weak to be noticeable for humans. Moreover, the radiation is not
concentrated in a beam, but dispersed more or less in all directions.
When using a wireless phone or a Bluetooth device, some of the
emitted RF energy is absorbed by the body. The penetration depth is
about 1.5 cm at 2450 MHz (about 2.5 cm at 900 MHz), which means that
the absorption is very superficial. The main absorption mechanism is
fieldinduced rotation of polar molecules (for example H2O), which
generates heat through molecular "friction". Bluetooth - An Overview Laptops

Heating by means of radio frequencies is possible over a broad
frequency range. This is taken advantage of in microwave ovens at 2450
MHz using very high power levels (up to 1,000,000 times the power used
by Bluetooth devices). However, 2450 MHz is not a
resonance frequency of water. But does exposure to Bluetooth RF
emission heat the human body? No it does not. The output power of a
Bluetooth-enabled device is far too low to cause any detectable
temperature increase. Again, in comparison, the maximum increase from
handheld cellular phones is less than 0.1°C.There
is, however, another side to this; some people are demonstrably
over-sensitive to electromagnetic radiations. Long exposure to strong
fields make some individuals so sensitive, after a few years, that they
can no longer be near such fields without considerable discomfort.
Bluetooth fits into a general development pattern where antennas for
GSM-transmission and other sources of electromagnetic radiations become
more and more prevalent in our cities. The future will show whether
this is a healthy development.

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Bluetooth - An Overview

What kind of traffic can Bluetooth handle?

Bluetooth is specifically designed to provide low-cost, robust,
efficient, high capacity, ad hoc voice and data networking with the
following characteristics:

1 Mb/sec. transmission/reception rate exploits maximum available channel bandwidth.

Fast frequency hopping avoids interference.

Adaptive output power minimizes interference.

Short data packets maximize capacity during interference.

Fast acknowledge allows low coding overhead for links.

CVSD (Continuous Variable Slope Delta Modulation) voice coding enables operation at high bit-error rates.

Flexible packet types supports a wide application range.

Relaxed link budget supports low-cost single chip integration.

Transmission/reception interface tailored to minimize electric current consumption.

The Bluetooth technology was not planned to
be just a physical wireless medium offering merely a platform for
high-level protocols and applications. The aim is to provide something
more, with immediate device-interoperability as soon as the first
Bluetooth products hit the market. But this can only be achieved if all
the communication blocks, including radios, protocols and applications,
are accurately defined and can interoperate.

Sound Transmissions

Bluetooth uses either a 64 kb/s log PCM format (A-law or m-law) or a 64 kb/s CVSD (Continuous
Variable Slope Delta Modulation). The CVSD-format uses an adaptive
delta modulation algorithm with syl-labic companding.
The voice coding on the line interface should have a quality equal to
or better than the quality of 64 kb/s log PCM.LOG PCM CODEC
Since the voice channels on the air-interface can support a 64 kb/s
information stream, a 64 kb/s log PCM traffic can be used for
transmission, using either A-law or m-law compression. If the line
interface uses A-law and the air interface uses m-law or vice versa, a
conversion from A-law to m-law is performed. The compression method
follows ITU-T recommendations G. 711.CVSD CODEC
A more robust format for voice over the air interface is a delta
modulation. This modulation scheme follows the waveform where the
output bits indicate whether the prediction value is smaller or larger
then the input waveform. To reduce slope overload effects, syllabic
companding is applied: the step size is adapted according to the
average signal slope. The input to the CVSD encoder
is 64 ksamples/second linear PCM.

CVSD Waveform Coding Bluetooth - An Overview Cvsd

CVSD audio quality

For Bluetooth audio quality the
requirements are put on the transmitter side. The 64 ksamples/s linear
PCM input signal must have negligible spectral power density above 4
kHz. A set of reference input signals are encoded by the transmitter
and sent through a reference decoder (available on the website).The power spectral density in the 4-32 kHz
band of the decoded signal at the 64 ksample/s linear PCM output,
should be more than 20 dB below the maxi-mum in the 0-4 kHz range.

Fitting into the Environment

Connection speeds of up to 721kbps are possible, providing users with
network performance that it is not much slower than a shared Lan. In
theory, Bluetooth can be added to the network backbone to simplify
hot-desking. It can also allow users to synchronise address books and
e-mail messages with mobile devices without having to worry about
plugging in devices.But while Bluetooth will allow uses to
connect to mobile devices without needing to carry external cables,
current devices such as mobile phones will need adapters to work with
Bluetooth.

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What about Bluetooth´s security?

Bluetooth - An Overview Slaves

Security can mean two things in this context:

A) We want to be sure that transmitted data arrives in un-corrupted condition to the receiver.

B) We also want to feel that this data has not been eavesdropped by parties for whom it is not intended.

Both of these issues are (of course!) addressed by Bluetooth.Safer transmission of data

Are transmissions secure in a business and
home environment? Yes, they are supposed to be quite reliable.
Bluetooth has built in sufficient encryption and authentication and is
thus very secure in any environment. In addition to this, a
frequency-hopping scheme with 1600 hops/sec. is employed. This is far quicker than any other competing system. This, together with an automatic output power adaption to reduce the range exactly to requirement, makes the system extremely difficult to eavesdrop. Bluetooth - An Overview Connbar

Information Integrity in Bluetooth

Information Integrity is of vital importance. We don´t want outside parties to listen in. In Bluetooth, it has these components:

Random Number Generation

Encryption

Encryption Key Management

Authentication.

More information about these can be found on the Bluetooth website.

Bluetooth´s Error Correction Schemes

Bluetooth units often have to contend with
electro-magnetically noisy environments. Thus, the need for some kind
of error-detection and -correction. For error-detection, Bluetooth uses
various checksum-calculations. When errors are detected, there are 3
error-correction schemes defined for Bluetooth:

1/3 rate FEC (Forward Error Correction)

2/3 rate FEC

ARQ unnumbered scheme (Automatic Repeat Request).

The purpose of the FEC scheme on the
data payload is to reduce the number of re-transmissions. However, in a
reasonably error-free environment, FEC gives unnecessary overhead that
reduces the throughput. Therefore, the packet definitions have been
kept flexible to use FEC in the payload or not, resulting in

the DM and DH packets for the ACL link, and

the HV packets for the SCO link.

The packet header is always protected by a
1/3 rate FEC; it contains valuable link information and should be able
to sustain more bit errors. Bluetooth - An Overview Connbar

The ARQ-scheme is
illustrated at right. On 2 occasions, the transmitted data blocks get
corrupted, which is detected by the recipient. So the next time that
recipient get a chance to communicate with that sender (i.e. at the
next appropriate timeslot), the recipient sends a Negative
Acknowledgement (a NAK, depicted in red), which prompts the other party to re-transmit that data block.

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How timeslots are used

Bluetooth - An Overview Slots1

Transmission/reception takes place in timeslots that are only 625
mikroseconds in duration. The Master uses even-numbered slots to
address each slave in turn, and each addressed slave has the
opportunity to answer in the following odd-numbered timeslot. Or it can
wait for it turn next time around.
In addition to this, some timeslots are used for broadcasts and as
logical channels for synchronization and other control signals. Thus,
we get a rotating scheme, resembling the illustration above. The
slot-numbering proceeds to a very high number; it takes about a day for
the slot-numbering to start over again.The clock of the Master unit decides when
these slots start and end, and the slaves will thus need to be very
closely synchronized to this clock.

The Communications channels

The hopping sequence is unique for each
piconet, and is determined by the Bluetooth device address of the
Master. The channel is divided into 625 µs long timeslots. They are
cyclically numbered by the clock of the piconet Master. Bluetooth uses 5 Logical Channels:

LC Channel (Link Control)
Is mapped onto the packet header. Carries low-level link control
information, like flow control. Is carried on every packet except in
the ID packet, which has no header.

LM Channel (Link Manager)
Carries control information exchanged between the link managers of the master and slave(s).

UA/UI Channels (User Asynchronous/Isochronous Data)
Carries L2CAP transparent asynchronous user data.. May be transmitted in one or more baseband packets.

US Channel (User Synchronous Data)
Carries transparent synchronous user data. Is carried over the SCO link.

Multi-slot packets

A packet is normally contained in 1 slot,
as shown above, but it can cover up to 5 slots if needed. It is then
imprtant that the frequencies used in the following timeslots are those
that are assigned to those slots, and that they do not follow
the frequency sequence that should have normally applied. This is
illustrated below.
Bluetooth - An Overview Masla2

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What´s the advantage of frequency-hopping?

Bluetooth has been designed to operate in noisy radio frequency
environments, and uses a fast acknowledgement and frequency-hopping
scheme to make the link robust, communication-wise. Bluetooth radio
modules avoid interference from other signals by hopping to a new
frequency after transmitting or receiving a packet.Compared with other systems operating in
the same frequency band, the Bluetooth radio typically hops faster and
uses shorter packets. This is because short packages and fast hopping
limit the impact of microwave ovens and other sources of disturbances.
Use of Forward Error Correction (FEC) limits the impact of random noise
on long-distance links.Considering the example at right, one
realizes that hopping out-and-into a continous range of frequencies
that are subject to noise gives the communications link a better chance
to remedy transmission errors when out of the disturbed frequencies,
than would have been the case if the transmission had stayed for
several timeslots within the noisy frequency range. This argument does
not hold true if the noise is spread over the whole frequency range in
a random fashion, but sources of electro-magnetic noise usually give
the pattern illustrated at right.Whenever a connection (a "piconet") is
first established between 2 (or more) units, the Master-unit
establishes a frequency-hopping scheme, which is communicated to the
other units. This frequency selection scheme consists of two parts:

selecting a sequence;

mapping this sequence onto the hop frequencies.

The mapping from the input to a particular
hop frequency is performed in the selection box. Basically, the input
is the native clock and the current address. In the CONNECTION state,
the native clock is modified by an offset to equal the master clock.
Only the 27 MSBs of the clock are used. In the page and inquiry
substates, all 28 bits of the clock are used. However, in the PAGE substate the native clock will be modified to the master’s estimate of the paged unit.

The frequency-hopping schemes

10 types of frequency-hopping sequences are defined – 5 for the 79-hop and 5 for the 23-hop system. These sequences are:

A page hopping sequence with 32 (16)
unique wake-up frequencies, distributed equally over the 79 (23) MHz,
with a period length of 32 (16).

A page response sequence covering 32 (16) unique response
frequencies that are all in a one-to-one correspondence to the current
hopping sequence. Master & Slave use different rules to obtain the
same sequence.

An inquiry sequence with 32 (16) unique
wake-up frequencies, distributed equally over the 79 (23) MHz, with a
period length of 32 (16).

A inquiry response sequence covering 32 (16) unique
response frequencies that are all in a one-to-one correspondence to the
current inquiry hopping sequence.

A channel hopping sequence with very long period, which
does not show repetitive patterns over a short time interval, but which
distributes the hop frequencies equally

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Basic Bluetooth functions

Bluetooth - An Overview Diag01

The Bluetooth technology is quite complex.
This is not so surprising, considering the task it has to handle. It is
mainly based on the IEEE 802.11 standard, briefly described at
right. Of the 2 network modes described, Bluetooth uses the ad-hoc
mode. This means that each station must observe "netiqette" and give all other units fair access to the wireless media.The
above diagram shows the main building blocks. With today´s technology,
the transmitter/receiver-part for Bluetooth´s requirements could be
made as small as a thumbnail (!!), and the antenna could be more or
less hidden in the unit, much as it is in mobile telephones. Thus, the
connectors in corresponding older units would not be replaced by something of similar dimensions; the transceiver would just "disappear" among other circuits. Bluetooth - An Overview Connbar

Functionally, one talks about the three core protocols:
The logical link control and adaptation protocol (L2CAP), the service discovery protocol (SDP) and the RFCOMM protocol.L2CAP, which adapts upper layer
protocols over the Baseband, provides data services to the high layer
protocols with protocol multiplexing capability, segmentation and
reassembly operations, and group abstractions. Device information,
services and the characteristics of the services can be queried using
the SDP.
Like SDP, RFCOMM is layered on top of the
L2CAP. As a ‘cable replacement’ protocol, RFCOMM provides transport
capabilities for high-level services (e.g. OBEX protocol) that use
serial line as the transport mechanism.The Bluetooth air interface is based on a
nominal antenna power of 0 dBm. Spectrum spreading has been added to
facilitate optional operation at power levels up to 100 mW worldwide.
This is accomplished by frequency hopping; 79 hops displaced by 1 MHz, starting at 2.402 GHz and stopping at 2.480 GHz.Due to local regulations, the bandwidth is reduced in Japan, France and Spain. This is handled by an internal software switch.
The maximum frequency hopping rate is 1600 hops/s. The nominal link range is 10 centimeters to 10 meters, but can be
extended to more than 100 meters by increasing the transmit power. Bluetooth - An Overview Connbar

The Bluetooth "Channels"

"Channels" can mean 3 different things in this context:

It could refer to the 79 (or 23) RF-channels, on individual frequencies 1 MHz apart

It could also refer to the communications channels, consisting
of a pseudo-random hopping sequence through these 79 (or 23)
RF-channels. Such a channel could more be likened with what is called a
"session" in the OSI-model.

There are also 5 ”Logical Channels”, which are used for control purposes.

The IEEE 802.11 standard

The IEEE 802.11communications standard defines the protocol for two types of networks; Ad-hoc and client/server.The Ad-hoc network is a simple
network where communications are established between multiple stations
in a given coverage area without the use of an access point or server.
The 802.11- standard specifies the etiquette that each station must
observe so that all units have fair access to the wireless media. It
provides methods for arbitrating requests to use the media to ensure that throughput is maximized for all of the users in the base service set.The client/server network uses an access point that
controls the allocation of transmit time for all stations and allows
mobile stations to roam from cell to cell. The access point is used to
handle traffic from the mobile radio to the wired or wireless backbone
of the client/server network. This arrangement allows for point
coordination of all the stations in the basic service area and ensures
proper handling of the data traffic. The access point routes data
between the stations and other wireless stations or to and from the
network server.Typically, WLANs (i.e. wireless LANs) controlled by a central access point will provide better throughput performance than ad-hoc networks.

2 types of physical links are defined:

SCO (Synchronous Connection-Oriented)

ACL (Asynchronous Connection-Less)

The SCO link is point-to-point between
master and slave. The master maintains the link by using reserved
timeslots at regular intervals. Packet retransmissions are not allowed.

ACL provides
packet-switched connections between the master and all active slaves.
Packet retransmissions are usually applied to assure data integrity.For each of these link types, 12 different packet types have been defined. 4 control packets are common to both links.