科技英文
Liquid crystals are substances that exhibit a phase of matter that has
properties between those of a conventional liquid, and those of a solid crystal.
For instance, a liquid crystal (LC) may flow like a liquid, but have the
molecules in the liquid arranged and/or oriented in a crystal-like way. There
are many different types of LC phases, which can be distinguished based on
their different optical properties (such as birefringence). When viewed under a
microscope using a polarized light source, different liquid crystal phases will
appear to have a distinct texture. The contrasting areas in the texture each
correspond to a domain where the LC molecules are oriented in a different
direction. Within a domain, however, the molecules are well ordered. LC
materials may not always be in an LC phase (just as water is not always in the
liquid phase: it may also be found in the solid and gaseous phase).
Liquid crystals can be divided into thermotropic and lyotropic LCs.
Thermotropic LCs exhibit a phase transition into the LC phase as temperature
is changed, whereas lyotropic LCs exhibit phase transitions as a function of
concentration of the mesogen in a solvent (typically water) as well as
temperature.
Liquid crystal display
A liquid crystal display (LCD) is an electronically modulated optical
amplification shaped into a thin, flat display device made up of any number of
color or monochrome pixels arrayed in front of a light source (backlight) or
reflector. It is often utilized in battery-powered electronic devices because it
uses very small amounts of electric power.
A comprehensive classification of the various types and electro-optical modes
of LCDs is provided in the article LCD classification.
Reflective twisted nematic liquid crystal display.
1. Polarizing filter film with a vertical axis to polarize light as it enters.
2. Glass substrate with ITO electrodes. The shapes of these electrodes will
determine the shapes that will appear when the LCD is turned ON.
Vertical ridges etched on the surface are smooth.
3. Twisted nematic liquid crystal.
4. Glass substrate with common electrode film (ITO) with horizontal
ridges to line up with the horizontal filter.
5. Polarizing filter film with a horizontal axis to block/pass light.
6. Reflective surface to send light back to viewer. (In a backlit LCD, this
layer is replaced with a light source.)
Each pixel of an LCD typically consists of a layer of molecules aligned between
two transparent electrodes, and two polarizing filters, the axes of transmission
of which are (in most of the cases) perpendicular to each other. With no actual
liquid crystal between the polarizing filters, light passing through the first
filter would be blocked by the second (crossed) polarizer.
The surface of the electrodes that are in contact with the liquid crystal
material are treated so as to align the liquid crystal molecules in a particular
direction. This treatment typically consists of a thin polymer layer that is
unidirectionally rubbed using, for example, a cloth. The direction of the liquid
crystal alignment is then defined by the direction of rubbing. Electrodes are
made of a transparent conductor called Indium Tin Oxide (ITO).
Before applying an electric field, the orientation of the liquid crystal molecules
is determined by the alignment at the surfaces. In a twisted nematic device
(still the most common liquid crystal device), the surface alignment directions
at the two electrodes are perpendicular to each other, and so the molecules
arrange themselves in a helical structure, or twist. Because the liquid crystal
material is birefringent, light passing through one polarizing filter is rotated
by the liquid crystal helix as it passes through the liquid crystal layer, allowing
it to pass through the second polarized filter. Half of the incident light is
absorbed by the first polarizing filter, but otherwise the entire assembly is
reasonably transparent.
LCD with top polarizer removed from device and placed on top,
such that the top and bottom polarizers are crossed.
When a voltage is applied across the electrodes, a torque acts to align the
liquid crystal molecules parallel to the electric field, distorting the helical
structure (this is resisted by elastic forces since the molecules are constrained
at the surfaces). This reduces the rotation of the polarization of the incident
light, and the device appears grey. If the applied voltage is large enough, the
liquid crystal molecules in the center of the layer are almost completely
untwisted and the polarization of the incident light is not rotated as it passes
through the liquid crystal layer. This light will then be mainly polarized
perpendicular to the second filter, and thus be blocked and the pixel will
appear black. By controlling the voltage applied across the liquid crystal layer
in each pixel, light can be allowed to pass through in varying amounts thus
constituting different levels of gray.
LCD with top polarizer removed from device and placed on top,
such that the top and bottom polarizers are parallel.
The optical effect of a twisted nematic device in the voltage-on state is far less
dependent on variations in the device thickness than that in the voltage-off
state. Because of this, these devices are usually operated between crossed
polarizers such that they appear bright with no voltage (the eye is much more
sensitive to variations in the dark state than the bright state). These devices
can also be operated between parallel polarizers, in which case the bright and
dark states are reversed. The voltage-off dark state in this configuration
appears blotchy, however, because of small variations of thickness across the
device.
Both the liquid crystal material and the alignment layer material contain ionic
compounds. If an electric field of one particular polarity is applied for a long
period of time, this ionic material is attracted to the surfaces and degrades the
device performance. This is avoided either by applying an alternating current
or by reversing the polarity of the electric field as the device is addressed (the
response of the liquid crystal layer is identical, regardless of the polarity of the
applied field).
When a large number of pixels are needed in a display, it is not technically
possible to drive each directly since then each pixel would require
independent electrodes. Instead, the display is multiplexed. In a multiplexed
display, electrodes on one side of the display are grouped and wired together
(typically in columns), and each group gets its own voltage source. On the
other side, the electrodes are also grouped (typically in rows), with each group
getting a voltage sink. The groups are designed so each pixel has a unique,
unshared combination of source and sink. The electronics, or the software
driving the electronics then turns on sinks in sequence, and drives sources for
the pixels of each sink.
Color displays
A subpixel of a color LCD
In color LCDs each individual pixel is divided into three cells, or subpixels,
which are colored red, green, and blue, respectively, by additional filters
(pigment filters, dye filters and metal oxide filters). Each subpixel can be
controlled independently to yield thousands or millions of possible colors for
each pixel. CRT monitors employ a similar 'subpixel' structures via phosphors,
although the electron beam employed in CRTs do not hit exact 'subpixels'.
Color components may be arrayed in various pixel geometries, depending on
the monitor's usage. If the software knows which type of geometry is being
used in a given LCD, this can be used to increase the apparent resolution of
the monitor through subpixel rendering. This technique is especially useful for
text anti-aliasing.
To reduce smudging in a moving picture when pixels do not respond quickly
enough to color changes, so-called pixel overdrive may be used.
Organic light-emitting diode
An Organic Light Emitting Diode (OLED), also Light Emitting
Polymer (LEP) and Organic Electro Luminescence (OEL), is any Light
Emitting Diode (LED) whose emissive electroluminescent layer is composed
of a film of organic compounds. The layer usually contains a polymer
substance that allows suitable organic compounds to be deposited. They are
deposited in rows and columns onto a flat carrier by a simple "printing"
process. The resulting matrix of pixels can emit light of different colors.
Such systems can be used in television screens, computer displays, small,
portable system screens such as cell phones and PDAs, advertising,
information and indication. OLEDs can also be used in light sources for
general space illumination, and large-area light-emitting elements. OLEDs
typically emit less light per area than inorganic solid-state based LEDs which
are usually designed for use as point-light sources.
A significant benefit of OLED displays over traditional liquid crystal displays
(LCDs) is that OLEDs do not require a backlight to function. Thus they draw
far less power and, when powered from a battery, can operate longer on the
same charge. Because there is no need for a backlight, an OLED display can be
much thinner than an LCD panel. Degradation of OLED materials has limited
their use.
A typical OLED is composed of an emissive layer, a conductive layer, a
substrate, and anode and cathode terminals. The layers are made of organic
molecules that conduct electricity. The layers have conductivity levels ranging
from insulators to conductors, so OLEDs are considered organic
semiconductors.
The first, most basic OLEDs consisted of a single organic layer, for example
the first light-emitting polymer device synthesised by Burroughs et al.
involved a single layer of poly(p-phenylene vinylene). Multilayer OLEDs can
have more than two layers to improve device efficiency. As well as conductive
properties, layers may be chosen to aid charge injection at electrodes by
providing a more gradual electronic profile, or block a charge from reaching
the opposite electrode and being wasted.
Schematic of a 2-layer OLED: 1. Cathode (−), 2. Emissive Layer, 3. Emission
of radiation, 4. Conductive Layer, 5. Anode (+)
A voltage is applied across the OLED such that the anode is positive with
respect to the cathode. This causes a current of electrons to flow through the
device from cathode to anode. Thus, the cathode gives electrons to the
emissive layer and the anode withdraws electrons from the conductive layer;
in other words, the anode gives electron holes to the conductive layer.
Soon, the emissive layer becomes negatively charged, while the conductive
layer becomes rich in positively charged holes. Electrostatic forces bring the
electrons and the holes towards each other and they recombine. This happens
closer to the emissive layer, because in organic semiconductors holes are more
mobile than electrons (unlike in inorganic semiconductors). The
recombination causes a drop in the energy levels of electrons, accompanied by
an emission of radiation whose frequency is in the visible region. That is why
this layer is called emissive.
The device does not work when the anode is put at a negative potential with
respect to the cathode. In this condition, holes move to the anode and
electrons to the cathode, so they are moving away from each other and do not
recombine.
Indium tin oxide is commonly used as the anode material. It is transparent to
visible light and has a high work function which promotes injection of holes
into the polymer layer. Metals such as aluminium and calcium are often used
for the cathode as they have low work functions which promote injection of
electrons into the polymer layer.
Just like passive-matrix LCD versus active-matrix LCD, OLEDs can be
categorized into passive-matrix and active-matrix displays. Active-matrix
OLEDs (AMOLED) require a thin film transistor backplane to switch the
individual pixel on or off, and can make higher resolution and larger size
displays possible.
Artificial intelligence (AI) is the intelligence of machines and the branch of
computer science which aims to create it. Major AI textbooks define the field
as "the study and design of intelligent agents," where an intelligent agent is a
system that perceives its environment and takes actions which maximize its
chances of success. John McCarthy, who coined the term in 1956, defines it as
"the science and engineering of making intelligent machines."
The field was founded on the claim that a central property of human beings,
intelligence—the sapience of Homo sapiens—can be so precisely described
that it can be simulated by a machine. This raises philosophical issues about
the nature of the mind and limits of scientific hubris, issues which have been
addressed by myth, fiction and philosophy since antiquity. Artificial
intelligence has been the subject of breathtaking optimism, has suffered
stunning setbacks and, today, has become an essential part of the technology
industry, providing the heavy lifting for many of the most difficult problems in
computer science.
AI research is highly technical and specialized, so much so that some critics
decry the "fragmentation" of the field. Subfields of AI are organized around
particular problems, the application of particular tools and around long
standing theoretical differences of opinion. The central problems of AI include
such traits as reasoning, knowledge, planning, learning, communication,
perception and the ability to move and manipulate objects. General
intelligence (or "strong AI") is still a long term goal of (some) research.
ASIMO uses sensors and intelligent algorithms to avoid obstacles and navigate
stairs.
Mass spectrometry is an analytical technique that identifies the chemical
composition of a compound or sample based on the mass-to-charge ratio of
charged particles. A sample undergoes chemical fragmentation, thereby
forming charged particles (ions). The ratio of charge to mass of the particles is
calculated by passing them through electric and magnetic fields in a mass
spectrometer.
The design of a mass spectrometer has three essential modules: an ion source,
which transforms the molecules in a sample into ionized fragments; a mass
analyzer, which sorts the ions by their masses by applying electric and
magnetic fields; and a detector, which measures the value of some indicator
quantity and thus provides data for calculating the abundances of each ion
fragment present. The technique has both qualitative and quantitative uses,
such as identifying unknown compounds, determining the isotopic
composition of elements in a compound, determining the structure of a
compound by observing its fragmentation, quantifying the amount of a
compound in a sample, studying the fundamentals of gas phase ion chemistry
(the chemistry of ions and neutrals in a vacuum), and determining other
physical, chemical, or biological properties of compounds.
Main steps of measuring with a mass spectrometer
Simplified example
The following example describes the operation of a spectrometer mass
analyzer, which is of the sector type. (Other analyzer types are treated below.)
Consider a sample of sodium chloride (table salt). In the ion source, the
sample is vaporized (turned into gas) and ionized (transformed into
electrically charged particles) into sodium (Na+) and chloride (Cl-) ions.
Sodium atoms and ions are monoisotopic, with a mass of about 23 amu.
Chloride atoms and ions come in two isotopes with masses of approximately
35 amu (at a natural abundance of about 75 percent) and approximately 37
amu (at a natural abundance of about 25 percent). The analyzer part of the
spectrometer contains electric and magnetic fields, which exert forces on ions
traveling through these fields. The speed of a charged particle may be
increased or decreased while passing through the electric field, and its
direction may be altered by the magnetic field. The magnitude of the
deflection of the moving ion's trajectory depends on its mass-to-charge ratio.
By Newton's second law of motion, lighter ions get deflected by the magnetic
force more than heavier ions. The streams of sorted ions pass from the
analyzer to the detector, which records the relative abundance of each ion type.
This information is used to determine the chemical element composition of
the original sample (i.e. that both sodium and chlorine are present in the
sample) and the isotopic composition of its constituents (the ratio of 35Cl to
37Cl).
Schematics of a simple mass spectrometer with sector type mass analyzer
Instrumentation
Ion source technologies
The ion source is the part of the mass spectrometer that ionizes the material
under analysis (the analyte). The ions are then transported by magnetic or
electric fields to the mass analyzer.
Techniques for ionization have been key to determining what types of samples
can be analyzed by mass spectrometry. Electron ionization and chemical
ionization are used for gases and vapors. In chemical ionization sources, the
analyte is ionized by chemical ion-molecule reactions during collisions in the
source. Two techniques often used with liquid and solid biological samples
include electrospray ionization (invented by John Fenn) and matrix-assisted
laser desorption/ionization (MALDI, developed by K. Tanaka and separately
by M. Karas and F. Hillenkamp). Inductively coupled plasma (ICP) sources
are used primarily for metal analysis on a wide array of sample types. Others
include glow discharge, field desorption (FD), fast atom bombardment (FAB),
thermospray, desorption/ionization on silicon (DIOS), Direct Analysis in Real
Time (DART), atmospheric pressure chemical ionization (APCI), secondary
ion mass spectrometry (SIMS), spark ionization and thermal ionisation
(TIMS). Ion Attachment Ionization is a newer soft ionization technique that
allows for fragmentation free analysis.
Mass analyzer technologies
Mass analyzers separate the ions according to their mass-to-charge ratio. The
following two laws govern the dynamics of charged particles in electric and
magnetic fields in vacuum:
(Lorentz force law)
(Newton's second law of motion)
where F is the force applied to the ion, m is the mass of the ion, a is the
acceleration, Q is the ion charge, E is the electric field, and v x B is the vector
cross product of the ion velocity and the magnetic field
Equating the above expressions for the force applied to the ion yields:
This differential equation is the classic equation of motion for charged
particles. Together with the particle's initial conditions, it completely
determines the particle's motion in space and time in terms of m/Q. Thus
mass spectrometers could be thought of as "mass-to-charge spectrometers".
When presenting data, it is common to use the (officially) dimensionless m/z,
where z is the number of elementary charges (e) on the ion (z=Q/e). This
quantity, although it is informally called the mass-to-charge ratio, more
accurately speaking represents the ratio of the mass number and the charge
number, z.
There are many types of mass analyzers, using either static or dynamic fields,
and magnetic or electric fields, but all operate according to the above
differential equation. Each analyzer type has its strengths and weaknesses.
Many mass spectrometers use two or more mass analyzers for tandem mass
spectrometry (MS/MS). In addition to the more common mass analyzers
listed below, there are others designed for special situations.
Sector
A sector field mass analyzer uses an electric and/or magnetic field to
affect the path and/or velocity of the charged particles in some way. As shown
above, sector instruments bend the trajectories of the ions as they pass
through the mass analyzer, according to their mass-to-charge ratios,
deflecting the more charged and faster-moving, lighter ions more. The
analyzer can be used to select a narrow range of m/z or to scan through a
range of m/z to catalog the ions present.
Time-of-flight
The time-of-flight (TOF) analyzer uses an electric field to accelerate the ions
through the same potential, and then measures the time they take to reach the
detector. If the particles all have the same charge, the kinetic energies will be
identical, and their velocities will depend only on their masses. Lighter ions
will reach the detector first.
Quadrupole
Quadrupole mass analyzers use oscillating electrical fields to selectively
stabilize or destabilize ions passing through a radio frequency (RF)
quadrupole field. A quadrupole mass analyzer acts as a mass selective filter
and is closely related to the Quadrupole ion trap, particularly the linear
quadrupole ion trap except that it operates without trapping the ions and is
for that reason referred to as a transmission quadrupole. A common variation
of the quadrupole is the triple quadrupole.
Quadrupole ion trap
The quadrupole ion trap works on the same physical principles as the
quadrupole mass analyzer, but the ions are trapped and sequentially ejected.
Ions are created and trapped in a mainly quadrupole RF potential and
separated by m/Q, non-destructively or destructively.
There are many mass/charge separation and isolation methods but most
commonly used is the mass instability mode in which the RF potential is
ramped so that the orbit of ions with a mass a > b are stable while ions with
mass b become unstable and are ejected on the z-axis onto a detector.
Ions may also be ejected by the resonance excitation method, whereby a
supplemental oscillatory excitation voltage is applied to the endcap electrodes,
and the trapping voltage amplitude and/or excitation voltage frequency is
varied to bring ions into a resonance condition in order of their mass/charge
ratio.
The cylindrical ion trap mass spectrometer is a derivative of the quadrupole
ion trap mass spectrometer.
Linear quadrupole ion trap
A linear quadrupole ion trap is similar to a quadrupole ion trap, but it traps
ions in a two dimensional quadrupole field, instead of a three dimensional
quadrupole field as in a quadrupole ion trap. Thermo Fisher's LTQ ("linear
trap quadrupole") is an example of the linear ion trap.
Fourier transform ion cyclotron resonance
A FT-ICR mass spectrometer
Fourier transform mass spectrometry, or more precisely Fourier transform
ion cyclotron resonance MS, measures mass by detecting the image current
produced by ions cyclotroning in the presence of a magnetic field. Instead of
measuring the deflection of ions with a detector such as an electron multiplier,
the ions are injected into a Penning trap (a static electric/magnetic ion trap)
where they effectively form part of a circuit. Detectors at fixed positions in
space measure the electrical signal of ions which pass near them over time,
producing a periodic signal. Since the frequency of an ion's cycling is
determined by its mass to charge ratio, this can be deconvoluted by
performing a Fourier transform on the signal. FTMS has the advantage of high
sensitivity (since each ion is "counted" more than once) and much higher
resolution and thus precision.
Ion cyclotron resonance (ICR) is an older mass analysis technique similar to
FTMS except that ions are detected with a traditional detector. Ions trapped in
a Penning trap are excited by an RF electric field until they impact the wall of
the trap, where the detector is located. Ions of different mass are resolved
according to impact time.
Very similar nonmagnetic FTMS has been performed, where ions are
electrostatically trapped in an orbit around a central, spindle shaped electrode.
The electrode confines the ions so that they both orbit around the central
electrode and oscillate back and forth along the central electrode's long axis.
This oscillation generates an image current in the detector plates which is
recorded by the instrument. The frequencies of these image currents depend
on the mass to charge ratios of the ions. Mass spectra are obtained by Fourier
transformation of the recorded image currents.
Similar to Fourier transform ion cyclotron resonance mass spectrometers,
Orbitraps have a high mass accuracy, high sensitivity and a good dynamic
range.
Detector
A continuous dynode particle multiplier detector.
The final element of the mass spectrometer is the detector. The detector
records either the charge induced or the current produced when an ion passes
by or hits a surface. In a scanning instrument, the signal produced in the
detector during the course of the scan versus where the instrument is in the
scan (at what m/Q) will produce a mass spectrum, a record of ions as a
function of m/Q.
Typically, some type of electron multiplier is used, though other detectors
including Faraday cups and ion-to-photon detectors are also used. Because
the number of ions leaving the mass analyzer at a particular instant is typically
quite small, considerable amplification is often necessary to get a signal.
Microchannel Plate Detectors are commonly used in modern commercial
instruments. In FTMS and Orbitraps, the detector consists of a pair of metal
surfaces within the mass analyzer/ion trap region which the ions only pass
near as they oscillate. No DC current is produced, only a weak AC image
current is produced in a circuit between the electrodes. Other inductive
detectors have also been used.
Tandem mass spectrometry
A tandem mass spectrometer is one capable of multiple rounds of mass
spectrometry, usually separated by some form of molecule fragmentation. For
example, one mass analyzer can isolate one peptide from many entering a
mass spectrometer. A second mass analyzer then stabilizes the peptide ions
while they collide with a gas, causing them to fragment by collision-induced
dissociation (CID). A third mass analyzer then sorts the fragments produced
from the peptides. Tandem MS can also be done in a single mass analyzer over
time, as in a quadrupole ion trap. There are various methods for fragmenting
molecules for tandem MS, including collision-induced dissociation (CID),
electron capture dissociation (ECD), electron transfer dissociation (ETD),
infrared multiphoton dissociation (IRMPD) and blackbody infrared radiative
dissociation (BIRD). An important application using tandem mass
spectrometry is in protein identification.
Tandem mass spectrometry enables a variety of experimental sequences.
Many commercial mass spectrometers are designed to expedite the execution
of such routine sequences as single reaction monitoring (SRM), multiple
reaction monitoring (MRM), and precursor ion scan. In SRM, the first
analyzer allows only a single mass through and the second analyzer monitors
for a single user defined fragment ion. MRM allows for multiple user defined
fragment ions. SRM and MRM are most often used with scanning instruments
where the second mass analysis event is duty cycle limited. These experiments
are used to increase specificity of detection of known molecules, notably in
pharmacokinetic studies. Precursor ion scan refers to monitoring for a specific
loss from the precursor ion. The first and second mass analyzers scan across
the spectrum as partitioned by a user defined m/z value. This experiment is
used to detect specific motifs within unknown molecules.
An important type of Tandem mass spectrometry is Accelerator Mass
Spectrometry (AMS), which uses very high voltages, usually in the mega-volt
range, to accelerate negative ions into a type of tandem mass spectrometer.
One of the most important applications of this technique is radiocarbon
dating.
Common mass spectrometer configurations
and techniques
When a specific configuration of source, analyzer, and detector becomes
conventional in practice, often a compound acronym arises to designate it,
and the compound acronym may be more well known among
nonspectrometrists than the component acronyms. The epitome of this is
MALDI-TOF, which simply refers to combining a Matrix-assisted laser
desorption/ionization source with a Time-of-flight mass analyzer. The
MALDI-TOF moniker is more widely recognized by the non-mass
spectrometrist scientist than MALDI or TOF individually. Other examples
include inductively coupled plasma-mass spectrometry (ICP-MS), accelerator
mass spectrometry (AMS), Thermal ionization-mass spectrometry (TIMS) and
spark source mass spectrometry (SSMS). Sometimes the use of the generic
"MS" actually connotes a very specific mass analyzer and detection system, as
is the case with AMS, which is always sector based.
Certain applications of mass spectrometry have developed monikers that
although strictly speaking they would seem to refer to a broad application, in
practice have come instead to connote a specific or a limited number of
instrument configurations. An example of this is isotope ratio mass
spectrometry (IRMS), which refers in practice to the use of a limited number
of sector based mass analyzers; this name is used to refer to both the
application and the instrument used for the application.
Data representations
Mass spectrum of a peptide showing the isotopic distribution
Mass spectrometry produces various types of data. The most common data
representation is the mass spectrum.
Certain types of mass spectrometry data are best represented as a mass
chromatogram. Types of chromatograms include selected ion monitoring
(SIM), total ion current (TIC), and selected reaction monitoring
chromatogram (SRM), among many others.
Other types of mass spectrometry data are well represented as a three
dimensional contour map. In this form, the mass-to-charge, m/z is on the
x-axis, intensity the y-axis, and an additional experimental parameter, such as
time, is recorded on the z-axis.
Bluetooth is a wireless protocol for exchanging data over short distances
from fixed and mobile devices, creating personal area networks (PANs). It was
originally conceived as a wireless alternative to RS232 data cables. It can
connect several devices, overcoming problems of synchronization.
Bluetooth uses a radio technology called frequency-hopping spread spectrum,
which chops up the data being sent and transmits chunks of it on up to 79
frequencies. In its basic mode, the modulation is Gaussian frequency-shift
keying (GFSK). It can achieve a gross data rate of 1 Mb/s. Bluetooth provides a
way to connect and exchange information between devices such as mobile
phones, telephones, laptops, personal computers, printers, Global Positioning
System (GPS) receivers, digital cameras, and video game consoles through a
secure, globally unlicensed Industrial, Scientific, and Medical (ISM) 2.4 GHz
short-range radio frequency bandwidth. The Bluetooth specifications are
developed and licensed by the Bluetooth Special Interest Group (SIG). The
Bluetooth SIG consists of companies in the areas of telecommunication,
computing, networking, and consumer electronics.
Bluetooth is a standard and communications protocol primarily designed for
low power consumption, with a short range (power-class-dependent: 1 meter,
10 meters, 100 meters) based on low-cost transceiver microchips in each
device. Bluetooth makes it possible for these devices to communicate with
each other when they are in range. Because the devices use a radio
communications system, they do not have to be in line of sight of each other;
they can even be far apart if the transmission has sufficient power.
Class
Maximum Permitted Power
Range
mW (dBm)
(approximate)
Class 1 100 mW (20 dBm)
~100 meters
Class 2 2.5 mW (4 dBm)
~10 meters
Class 3 1 mW (0 dBm)
~1 meter
In most cases the effective range of class 2 devices is extended if they connect
to a class 1 transceiver, compared to a pure class 2 network. This is
accomplished by the higher sensitivity and transmission power of Class 1
devices.
Version
Version 1.2
Data Rate
1 Mbit/s
Version 2.0 + EDR 3 Mbit/s
WiMedia Alliance
(proposed)
53 - 480 Mbit/s
Bluetooth profiles
In order to use Bluetooth, a device must be compatible with certain Bluetooth
profiles. These define the possible applications and uses of the technology.
List of applications
A typical Bluetooth mobile phone headset.
Nokia BH-208 headset internals.
More prevalent applications of Bluetooth include:
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Wireless control of and communication between a mobile phone and a
hands-free headset. This was one of the earliest applications to become
popular.
Wireless networking between PCs in a confined space and where little
bandwidth is required.
Wireless communication with PC input and output devices, the most
common being the mouse, keyboard and printer.
Transfer of files between devices with OBEX.
Transfer of contact details, calendar appointments, and reminders
between devices with OBEX.
Replacement of traditional wired serial communications in test
equipment, GPS receivers, medical equipment, bar code scanners, and
traffic control devices.
For controls where infrared was traditionally used.
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Sending small advertisements from Bluetooth-enabled advertising
hoardings to other, discoverable, Bluetooth devices.
Two seventh-generation game consoles, Nintendo's Wii[3] and Sony's
PlayStation 3, use Bluetooth for their respective wireless controllers.
Dial-up internet access on personal computers or PDAs using a
data-capable mobile phone as a modem.
Communication: how
bluetooth works
How bluetooth works, with an explanation of its wireless
capabailities.
In today’s world of high technology the ability to communicate has become
more and more important – but also more and difficult due to the variety of
electronic devices we use in our everyday life. Who hasn’t sat on the floor of
their living room, surrounded by a multitude of cables and wires, staring at
the new DVD player and wondering how you were going to hook it into your
present entertainment system
? And just when you figure out the confusing mess of black cable yet another
invention arrives on the scene, along with yet another remote control
! But wouldn’t it be so much easier if all these devices could talk to each other?
What if you could sit on your couch and program your VCR using your laptop
or your PDA while transferring data between the two without a plethora of
extra cable coiling around your feet?
In a nutshell, Bluetooth provides the means to do exactly that – provide a way
for all these devices to communicate without a thousand wires and cables
strung around your house.
First, you have to understand some of the basics about how your electronic
devices communicate right now. While you might use a remote control for
your television set or perhaps a slew of wires to hook your home
entertainment system up to your DVD or VHS player, all of them use the same
basic principles. Information is exchanged using a variety of protocols, or
programs that help your computer talk to your printer and vice versa. The
problem comes in when your printer, for example, tries to talk to your PDA. If
the protocol isn’t the same on both ends a confusing mishmash of numbers
will be sent and nothing will happen. While this wasn’t perceived as a major
problem in the past, it’s become a major concern for companies as more and
more businesses go high-tech and their employees need more and more
electronic access and devices to get the job done.
The manufacturers of these devices realized that in the long term they were
losing customers or not gaining new ones because of this problem of
incompatibility. But what could they do? Thus Bluetooth was born – a single
protocol that would work for all the devices in your home, from the VCR to
your laptop and to your PDA. All without miles of cables, jacks and confusing
instructions as to what should be plugged in where.
Bluetooth is a small computer card that can be installed in any electronic
device and which communicates on a special radio frequency that all your
other items can receive, translate and understand on the same level. But
wait – how can you send and receive without, say, opening all of your
neighbors’ garage doors? Or intercept other cell phone calls while leaving your
own open to such misuse? Will that new baby monitor program your VCR to
tape wrestling instead of your favorite soap opera?
Everyone’s been in a situation where he or she’s been driving along, listening
to his or her favorite radio station and suddenly the signal cuts out, only to be
replaced by another station and probably a very different music program.
Local radio stations only have the frequency they’ve been assigned for as far as
they can transmit; meaning that if you go out of range of the towers set up to
sends the signal you will lose the radio station. But it’s likely that the same
frequency has been assigned to another radio station in another area, which is
why often you don’t have to make any adjustment to your radio to have the
programming change abruptly as you go from one transmitting area to the
next. But you don’t have to worry about this sort of problem with Bluetooth.
The frequency that Bluetooth uses is 2.45 gigahertz, which is a bit higher on
the radio band than television and radio but below that used by satellite dishes.
This frequency has been designated for Bluetooth alone by an international
agreement, ensuring that there will be no conflict with any other devices. No
one else can create a product that transmits on the same frequency.
One of the pros and cons of Bluetooth is the limited range. A cell phone sends
and receives at 3 watts while Bluetooth works at only 1 milliwatt – a tiny
fraction compared to your cell phone. Because of this the range is severely
limited for Bluetooth devices, usually only fifteen feet or so. But this also
makes it ideal for home use, where the signal can easily pass through walls
and help you coordinate your devices. By boosting the signal you can reach up
to 300 feet, enabling more and more computers
to be connected via the one signal. This works great for a home office or a
small company where data exchange is often necessary between cell phones,
PDAs and laptop computers as well as desktops – and all without wires or
cables clogging up the environment!
More and more small businesses and private residences are considering
Bluetooth as a way to make their lives easier and to increase either their
productivity or just making life easier for the homeowner. Larger companies
are creating Bluetooth groups within their own buildings or smaller area so
that the employees can exchange information quicker and easier, without
having to deal with protocol conflict and worrying about compatibility with
each and every electronic device. For the homeowner dealing with a large
entertainment system it provides a great relief where you don’t have to sit
there amid a tangled pyramid of wires and cables to rival the Space Shuttle’s
insides. True, Bluetooth has some limitations, but in the future there will be
more and more advancements in this area in order to make life easier for
those of us who have to deal with a plethora of electronic devices.
Communication: how wifi
works
Learn about wi-fi technology, how it functions in your computer
and where to find hotspots.
Wi-fi refers to wireless networking, and is sometimes mentioned as 802.11.
802.11 is the standard designation of wireless networking
. There are often letters added on after 802.11, and refer to the type of wireless
network it is. The 'g' added on is the faster form of wireless networking, while
'b', more widespread, is the slower of the two. 'A' exists in between these.
The basic concept is often compared to that of walkie talkies. Wi-fi uses radio
waves to send signals to various devices. They convert the data into 1s and 0s
so that the computer can understand what is being sent, and use a much
higher frequency than that of the standard radio. This allows data to be sent
very quickly, and makes it a good choice in a number of situations. Unlike
wireless devices like cordless phones, which can sometimes have interference
problems with similar devices, wi-fi is able to change it's frequency through a
large range. This gets around any transmission interference and lets many
wi-fi enabled devices co-exist in the same space. The typical range of a
wireless network is about 100 feet.
The technology sounds a lot more complicated than it really is. All you really
need to set up the typical wi-fi network is a sender and a receiver. The typical
sender is a wireless router that hooks up to your modem. The receiver in this
case would be the wireless network card on your desktop or laptop. There can
be certain obstacles in signal strength, such as furniture and thick walls. Most
of the setup time will consist of you finding the optimal positioning for the
router.
There are also many hotspots scattered around the country. These are publicly
accessible wireless networks. Depending on where it is, it could either be a
free public access spot, someone's personal unsecured network, or a place
where you pay per minute/hour/day/etc for wireless access. They are
cropping up just about everywhere around the country, with some major
metropolitan cities creating huge amounts of hotspots to bring Internet access
to many people who were unable to get it any other way.
There are also many websites available that offer hotspot search engines
. Going on a trip and worried about Internet access? Just look through one of
these databases and visit the hotspot. Most wi-fi equipment will detect the
settings of the network automatically. The only problem you will have in
connecting is if the network requires you to enter a WEP key. This key would
be given to you by the owner of the network so that you could log on.
This is a very useful and user friendly technology, and is gaining ground in
more than just computers. Wi-fi is used in many forms of consumer
electronics, and is enabling many devices to become wired in to already
existing networks. Not only that, but it also cuts down on the amount of cables
cluttering your workspace. With all the cables your computer already has,
wouldn't you love to cut down on at least one?
Bluetooth vs. Wi-Fi in networking
Bluetooth and Wi-Fi have many applications in today's offices, homes, and on
the move: setting up networks, printing, or transferring presentations and
files from PDAs to computers. Both are versions of unlicensed wireless
technology. Wi-Fi differs from Bluetooth in that it provides higher throughput
and covers greater distances, but requires more expensive hardware and may
present higher power consumption.
They use the same frequency range, but employ different modulation
techniques. While Bluetooth is a replacement for cabling in a variety of
small-scale applications, Wi-Fi is a replacement for cabling for general local
area network access.
Bluetooth devices
Bluetooth exists in many products, such as telephones, the Wii, PlayStation 3
and recently in some high definition watches, modems and headsets. The
technology is useful when transferring information between two or more
devices that are near each other in low-bandwidth situations. Bluetooth is
commonly used to transfer sound data with telephones (i.e. with a Bluetooth
headset) or byte data with hand-held computers (transferring files).
Bluetooth protocols simplify the discovery and setup of services between
devices. Bluetooth devices can advertise all of the services they provide. This
makes using services easier because more of the security, network address and
permission configuration can be automated than with many other network
types
Wi-Fi
Wi-Fi is more like a traditional Ethernet network, and requires configuration
to set up shared resources, transmit files, and to set up audio links (for
example, headsets and hands-free devices). Wi-Fi uses the same radio
frequencies as Bluetooth, but with higher power, resulting in a stronger
connection. Wi-Fi is sometimes called "wireless Ethernet." This description is
accurate, as it also provides an indication of its relative strengths and
weaknesses. Wi-Fi requires more setup but is better suited for operating
full-scale networks; it enables a faster connection, better range from the base
station, and better security than Bluetooth.