Monday 26 March 2012
gigi biru?????????
Perkataan Bluetooth ialah versi anglicised Blåtand raja Denmark,sempena nama raja abad kesepuluh Harald Denmark, yang menyatukan bahagian- bahagian Norway dan suku-suku Denmark ke dalam sebuah negara tunggal (hasil menyatukan suku-suku yang sebelumnya berperang, termasuk suku dari wilayah yang sekarang bernama Norwegia dan Swedia).Dengan keupayaan raja itu sebagai perhubungan rakyat, Nama Bluetooth digunakan mirip dengan teknologi bluetooth sekarang yang boleh menghubungkan pelbagai peralatan seperti komputer personal dan telefon selular.Logo bluetooth berasal dari penyatuan dua huruf Jerman analog dengan huruf H dan B (singkatan dari Harald Bluetooth), iaitu (Hagall) dan (Blatand) yang kemudian digabungkan.
VSAT
A very-small-aperture terminal (VSAT), is a two-way satellite ground station or a stabilized maritime Vsat antenna with a dish antenna
that is smaller than 3 meters. The majority of VSAT antennas range from
75 cm to 1.2 m. Data rates typically range from 56 kbps up to 4 Mbps.
VSATs access satellite(s) in geosynchronous orbit to relay data from small remote earth stations (terminals) to other terminals (in mesh topology) or master earth station "hubs" (in star topology).
VSATs are most commonly used to transmit narrowband data (point of sale transactions such as credit card, polling or RFID data; or SCADA), or broadband data (for the provision of satellite Internet access to remote locations, VoIP or video). VSATs are also used for transportable, on-the-move (utilising phased array antennas) or mobile maritime communications.
VSATs are most commonly used to transmit narrowband data (point of sale transactions such as credit card, polling or RFID data; or SCADA), or broadband data (for the provision of satellite Internet access to remote locations, VoIP or video). VSATs are also used for transportable, on-the-move (utilising phased array antennas) or mobile maritime communications.
cellular network
A cellular network is a radio network distributed over land areas called cells, each served by at least one fixed-location transceiver known as a cell site or base station.
When joined together these cells provide radio coverage over a wide
geographic area. This enables a large number of portable transceivers
(e.g., mobile phones, pagers,
etc.) to communicate with each other and with fixed transceivers and
telephones anywhere in the network, via base stations, even if some of
the transceivers are moving through more than one cell during
transmission.
Cellular networks offer a number of advantages over alternative solutions:
n a cellular radio system, a land area to be supplied with radio service is divided into regular shaped cells, which can be hexagonal, square, circular or some other irregular shapes, although hexagonal cells are conventional. Each of these cells is assigned multiple frequencies (f1 - f6) which have corresponding radio base stations. The group of frequencies can be reused in other cells, provided that the same frequencies are not reused in adjacent neighboring cells as that would cause co-channel interference.
The increased capacity in a cellular network, compared with a network with a single transmitter, comes from the fact that the same radio frequency can be reused in a different area for a completely different transmission. If there is a single plain transmitter, only one transmission can be used on any given frequency. Unfortunately, there is inevitably some level of interference from the signal from the other cells which use the same frequency. This means that, in a standard FDMA system, there must be at least a one cell gap between cells which reuse the same frequency.
In the simple case of the taxi company, each radio had a manually operated channel selector knob to tune to different frequencies. As the drivers moved around, they would change from channel to channel. The drivers knew which frequency covered approximately what area. When they did not receive a signal from the transmitter, they would try other channels until they found one that worked. The taxi drivers would only speak one at a time, when invited by the base station operator (in a sense TDMA).
With FDMA, the transmitting and receiving frequencies used in each cell are different from the frequencies used in each neighbouring cell. In a simple taxi system, the taxi driver manually tuned to a frequency of a chosen cell to obtain a strong signal and to avoid interference from signals from other cells.
The principle of CDMA is more complex, but achieves the same result; the distributed transceivers can select one cell and listen to it.
Other available methods of multiplexing such as polarization division multiple access (PDMA) and time division multiple access (TDMA) cannot be used to separate signals from one cell to the next since the effects of both vary with position and this would make signal separation practically impossible. Time division multiple access, however, is used in combination with either FDMA or CDMA in a number of systems to give multiple channels within the coverage area of a single cell.
The reuse distance, D is calculated as
The frequency reuse factor is the rate at which the same frequency can be used in the network. It is 1/K (or K according to some books) where K is the number of cells which cannot use the same frequencies for transmission. Common values for the frequency reuse factor are 1/3, 1/4, 1/7, 1/9 and 1/12 (or 3, 4, 7, 9 and 12 depending on notation).
In case of N sector antennas on the same base station site, each with different direction, the base station site can serve N different sectors. N is typically 3. A reuse pattern of N/K denotes a further division in frequency among N sector antennas per site. Some current and historical reuse patterns are 3/7 (North American AMPS), 6/4 (Motorola NAMPS), and 3/4 (GSM).
If the total available bandwidth is B, each cell can only use a number of frequency channels corresponding to a bandwidth of B/K, and each sector can use a bandwidth of B/NK.
Code division multiple access-based systems use a wider frequency band to achieve the same rate of transmission as FDMA, but this is compensated for by the ability to use a frequency reuse factor of 1, for example using a reuse pattern of 1/1. In other words, adjacent base station sites use the same frequencies, and the different base stations and users are separated by codes rather than frequencies. While N is shown as 1 in this example, that does not mean the CDMA cell has only one sector, but rather that the entire cell bandwidth is also available to each sector individually.
Depending on the size of the city, a taxi system may not have any frequency-reuse in its own city, but certainly in other nearby cities, the same frequency can be used. In a big city, on the other hand, frequency-reuse could certainly be in use.
Recently also orthogonal frequency-division multiple access based systems such as LTE are being deployed with a frequency reuse of 1. Since such systems do not spread the signal across the frequency band, inter-cell radio resource management is important to coordinates resource allocation between different cell sites and to limit the inter-cell interference. There are various means of Inter-cell Interference Coordination (ICIC) already defined in the standard.[2] Coordinated scheduling, multi-site MIMO or multi-site beam forming are other examples for inter-cell radio resource management that might be standardized in the future.
Cellular telephone frequency reuse pattern. See U.S. Patent 4,144,411
Although the original 2-way-radio cell towers were at the centers of
the cells and were omni-directional, a cellular map can be redrawn with
the cellular telephone towers located at the corners of the hexagons
where three cells converge.[3]
Each tower has three sets of directional antennas aimed in three
different directions with 120 degrees for each cell (totaling 360
degrees) and receiving/transmitting into three different cells at
different frequencies. This provides a minimum of three channels (from
three towers) for each cell. The numbers in the illustration are
channel numbers, which repeat every 3 cells. Large cells can be
subdivided into smaller cells for high volume areas.[4]
The details of the process of paging vary somewhat from network to network, but normally we know a limited number of cells where the phone is located (this group of cells is called a Location Area in the GSM or UMTS system, or Routing Area if a data packet session is involved; in LTE, cells are grouped into Tracking Areas). Paging takes place by sending the broadcast message to all of those cells. Paging messages can be used for information transfer. This happens in pagers, in CDMA systems for sending SMS messages, and in the UMTS system where it allows for low downlink latency in packet-based connections.
In a cellular system, as the distributed mobile transceivers move from cell to cell during an ongoing continuous communication, switching from one cell frequency to a different cell frequency is done electronically without interruption and without a base station operator or manual switching. This is called the handover or handoff. Typically, a new channel is automatically selected for the mobile unit on the new base station which will serve it. The mobile unit then automatically switches from the current channel to the new channel and communication continues.
The exact details of the mobile system's move from one base station to the other varies considerably from system to system (see the example below for how a mobile phone network manages handover).
GSM network architecture
The most common example of a cellular network is a mobile phone (cell phone) network. A mobile phone is a portable telephone which receives or makes calls through a cell site (base station), or transmitting tower. Radio waves are used to transfer signals to and from the cell phone.
Modern mobile phone networks use cells because radio frequencies are a limited, shared resource. Cell-sites and handsets change frequency under computer control and use low power transmitters so that a limited number of radio frequencies can be simultaneously used by many callers with less interference.
A cellular network is used by the mobile phone operator to achieve both coverage and capacity for their subscribers. Large geographic areas are split into smaller cells to avoid line-of-sight signal loss and to support a large number of active phones in that area. All of the cell sites are connected to telephone exchanges (or switches) , which in turn connect to the public telephone network.
In cities, each cell site may have a range of up to approximately ½ mile, while in rural areas, the range could be as much as 5 miles. It is possible that in clear open areas, a user may receive signals from a cell site 25 miles away.
Since almost all mobile phones use cellular technology, including GSM, CDMA, and AMPS (analog), the term "cell phone" is in some regions, notably the US, used interchangeably with "mobile phone". However, satellite phones are mobile phones that do not communicate directly with a ground-based cellular tower, but may do so indirectly by way of a satellite.
There are a number of different digital cellular technologies, including: Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), Code Division Multiple Access (CDMA), Evolution-Data Optimized (EV-DO), Enhanced Data Rates for GSM Evolution (EDGE), 3GSM, Digital Enhanced Cordless Telecommunications (DECT), Digital AMPS (IS-136/TDMA), and Integrated Digital Enhanced Network (iDEN). Structure of the mobile phone cellular network
A simple view of the cellular mobile-radio network consists of the following:
Any phone connects to the network via an RBS (Radio Base Station) at a corner of the corresponding cell which in turn connects to the Mobile switching center (MSC). The MSC provides a connection to the public switched telephone network (PSTN). The link from a phone to the RBS is called an uplink while the other way is termed downlink.
Radio channels effectively use the transmission medium through the use of the following multiplexing schemes: frequency division multiplex (FDM), time division multiplex (TDM), code division multiplex (CDM), and space division multiplex (SDM). Corresponding to these multiplexing schemes are the following access techniques: frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and space division multiple access (SDMA).[5]
With CDMA, multiple CDMA handsets share a specific radio channel. The signals are separated by using a pseudonoise code (PN code) specific to each phone. As the user moves from one cell to another, the handset sets up radio links with multiple cell sites (or sectors of the same site) simultaneously. This is known as "soft handoff" because, unlike with traditional cellular technology, there is no one defined point where the phone switches to the new cell.
In IS-95 inter-frequency handovers and older analog systems such as NMT it will typically be impossible to test the target channel directly while communicating. In this case other techniques have to be used such as pilot beacons in IS-95. This means that there is almost always a brief break in the communication while searching for the new channel followed by the risk of an unexpected return to the old channel.
If there is no ongoing communication or the communication can be interrupted, it is possible for the mobile unit to spontaneously move from one cell to another and then notify the base station with the strongest signal.
Higher frequencies are a disadvantage when it comes to coverage, but it is a decided advantage when it comes to capacity. Pico cells, covering e.g. one floor of a building, become possible, and the same frequency can be used for cells which are practically neighbours.
Cell service area may also vary due to interference from transmitting systems, both within and around that cell. This is true especially in CDMA based systems. The receiver requires a certain signal-to-noise ratio, and the transmitter should not send with too high transmission power in view to not cause interference with other transmitters. As the receiver moves away from the transmitter, the power received decreases, so the power control algorithm of the transmitter increases the power it transmits to restore the level of received power. As the interference (noise) rises above the received power from the transmitter, and the power of the transmitter cannot be increased any more, the signal becomes corrupted and eventually unusable. In CDMA-based systems, the effect of interference from other mobile transmitters in the same cell on coverage area is very marked and has a special name, cell breathing.
One can see examples of cell coverage by studying some of the coverage maps provided by real operators on their web sites. In certain cases they may mark the site of the transmitter, in others it can be calculated by working out the point of strongest coverage.
Following table shows the dependency of frequency on coverage area of one cell of a CDMA2000 network:[6]
Cellular networks offer a number of advantages over alternative solutions:
- increased capacity
- reduced power use
- larger coverage area
- reduced interference from other signals
n a cellular radio system, a land area to be supplied with radio service is divided into regular shaped cells, which can be hexagonal, square, circular or some other irregular shapes, although hexagonal cells are conventional. Each of these cells is assigned multiple frequencies (f1 - f6) which have corresponding radio base stations. The group of frequencies can be reused in other cells, provided that the same frequencies are not reused in adjacent neighboring cells as that would cause co-channel interference.
The increased capacity in a cellular network, compared with a network with a single transmitter, comes from the fact that the same radio frequency can be reused in a different area for a completely different transmission. If there is a single plain transmitter, only one transmission can be used on any given frequency. Unfortunately, there is inevitably some level of interference from the signal from the other cells which use the same frequency. This means that, in a standard FDMA system, there must be at least a one cell gap between cells which reuse the same frequency.
In the simple case of the taxi company, each radio had a manually operated channel selector knob to tune to different frequencies. As the drivers moved around, they would change from channel to channel. The drivers knew which frequency covered approximately what area. When they did not receive a signal from the transmitter, they would try other channels until they found one that worked. The taxi drivers would only speak one at a time, when invited by the base station operator (in a sense TDMA).
[edit] Cell signal encoding
To distinguish signals from several different transmitters, frequency division multiple access (FDMA) and code division multiple access (CDMA) were developed.With FDMA, the transmitting and receiving frequencies used in each cell are different from the frequencies used in each neighbouring cell. In a simple taxi system, the taxi driver manually tuned to a frequency of a chosen cell to obtain a strong signal and to avoid interference from signals from other cells.
The principle of CDMA is more complex, but achieves the same result; the distributed transceivers can select one cell and listen to it.
Other available methods of multiplexing such as polarization division multiple access (PDMA) and time division multiple access (TDMA) cannot be used to separate signals from one cell to the next since the effects of both vary with position and this would make signal separation practically impossible. Time division multiple access, however, is used in combination with either FDMA or CDMA in a number of systems to give multiple channels within the coverage area of a single cell.
Frequency reuse
The key characteristic of a cellular network is the ability to re-use frequencies to increase both coverage and capacity. As described above, adjacent cells must use different frequencies, however there is no problem with two cells sufficiently far apart operating on the same frequency. The elements that determine frequency reuse are the reuse distance and the reuse factor.The reuse distance, D is calculated as
The frequency reuse factor is the rate at which the same frequency can be used in the network. It is 1/K (or K according to some books) where K is the number of cells which cannot use the same frequencies for transmission. Common values for the frequency reuse factor are 1/3, 1/4, 1/7, 1/9 and 1/12 (or 3, 4, 7, 9 and 12 depending on notation).
In case of N sector antennas on the same base station site, each with different direction, the base station site can serve N different sectors. N is typically 3. A reuse pattern of N/K denotes a further division in frequency among N sector antennas per site. Some current and historical reuse patterns are 3/7 (North American AMPS), 6/4 (Motorola NAMPS), and 3/4 (GSM).
If the total available bandwidth is B, each cell can only use a number of frequency channels corresponding to a bandwidth of B/K, and each sector can use a bandwidth of B/NK.
Code division multiple access-based systems use a wider frequency band to achieve the same rate of transmission as FDMA, but this is compensated for by the ability to use a frequency reuse factor of 1, for example using a reuse pattern of 1/1. In other words, adjacent base station sites use the same frequencies, and the different base stations and users are separated by codes rather than frequencies. While N is shown as 1 in this example, that does not mean the CDMA cell has only one sector, but rather that the entire cell bandwidth is also available to each sector individually.
Depending on the size of the city, a taxi system may not have any frequency-reuse in its own city, but certainly in other nearby cities, the same frequency can be used. In a big city, on the other hand, frequency-reuse could certainly be in use.
Recently also orthogonal frequency-division multiple access based systems such as LTE are being deployed with a frequency reuse of 1. Since such systems do not spread the signal across the frequency band, inter-cell radio resource management is important to coordinates resource allocation between different cell sites and to limit the inter-cell interference. There are various means of Inter-cell Interference Coordination (ICIC) already defined in the standard.[2] Coordinated scheduling, multi-site MIMO or multi-site beam forming are other examples for inter-cell radio resource management that might be standardized in the future.
Directional antennas
Broadcast messages and paging
Practically every cellular system has some kind of broadcast mechanism. This can be used directly for distributing information to multiple mobiles, commonly, for example in mobile telephony systems, the most important use of broadcast information is to set up channels for one to one communication between the mobile transceiver and the base station. This is called paging.The details of the process of paging vary somewhat from network to network, but normally we know a limited number of cells where the phone is located (this group of cells is called a Location Area in the GSM or UMTS system, or Routing Area if a data packet session is involved; in LTE, cells are grouped into Tracking Areas). Paging takes place by sending the broadcast message to all of those cells. Paging messages can be used for information transfer. This happens in pagers, in CDMA systems for sending SMS messages, and in the UMTS system where it allows for low downlink latency in packet-based connections.
Movement from cell to cell and handover
In a primitive taxi system, when the taxi moved away from a first tower and closer to a second tower, the taxi driver manually switched from one frequency to another as needed. If a communication was interrupted due to a loss of a signal, the taxi driver asked the base station operator to repeat the message on a different frequency.In a cellular system, as the distributed mobile transceivers move from cell to cell during an ongoing continuous communication, switching from one cell frequency to a different cell frequency is done electronically without interruption and without a base station operator or manual switching. This is called the handover or handoff. Typically, a new channel is automatically selected for the mobile unit on the new base station which will serve it. The mobile unit then automatically switches from the current channel to the new channel and communication continues.
The exact details of the mobile system's move from one base station to the other varies considerably from system to system (see the example below for how a mobile phone network manages handover).
[edit] Example of a cellular network: the mobile phone network
Modern mobile phone networks use cells because radio frequencies are a limited, shared resource. Cell-sites and handsets change frequency under computer control and use low power transmitters so that a limited number of radio frequencies can be simultaneously used by many callers with less interference.
A cellular network is used by the mobile phone operator to achieve both coverage and capacity for their subscribers. Large geographic areas are split into smaller cells to avoid line-of-sight signal loss and to support a large number of active phones in that area. All of the cell sites are connected to telephone exchanges (or switches) , which in turn connect to the public telephone network.
In cities, each cell site may have a range of up to approximately ½ mile, while in rural areas, the range could be as much as 5 miles. It is possible that in clear open areas, a user may receive signals from a cell site 25 miles away.
Since almost all mobile phones use cellular technology, including GSM, CDMA, and AMPS (analog), the term "cell phone" is in some regions, notably the US, used interchangeably with "mobile phone". However, satellite phones are mobile phones that do not communicate directly with a ground-based cellular tower, but may do so indirectly by way of a satellite.
There are a number of different digital cellular technologies, including: Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), Code Division Multiple Access (CDMA), Evolution-Data Optimized (EV-DO), Enhanced Data Rates for GSM Evolution (EDGE), 3GSM, Digital Enhanced Cordless Telecommunications (DECT), Digital AMPS (IS-136/TDMA), and Integrated Digital Enhanced Network (iDEN). Structure of the mobile phone cellular network
A simple view of the cellular mobile-radio network consists of the following:
- A network of Radio base stations forming the Base station subsystem.
- The core circuit switched network for handling voice calls and text
- A packet switched network for handling mobile data
- The Public switched telephone network to connect subscribers to the wider telephony network
Any phone connects to the network via an RBS (Radio Base Station) at a corner of the corresponding cell which in turn connects to the Mobile switching center (MSC). The MSC provides a connection to the public switched telephone network (PSTN). The link from a phone to the RBS is called an uplink while the other way is termed downlink.
Radio channels effectively use the transmission medium through the use of the following multiplexing schemes: frequency division multiplex (FDM), time division multiplex (TDM), code division multiplex (CDM), and space division multiplex (SDM). Corresponding to these multiplexing schemes are the following access techniques: frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and space division multiple access (SDMA).[5]
] Cellular handover in mobile phone networks
Main article: Handoff
As the phone user moves from one cell area to another cell whilst a
call is in progress, the mobile station will search for a new channel
to attach to in order not to drop the call. Once a new channel is
found, the network will command the mobile unit to switch to the new
channel and at the same time switch the call onto the new channel.With CDMA, multiple CDMA handsets share a specific radio channel. The signals are separated by using a pseudonoise code (PN code) specific to each phone. As the user moves from one cell to another, the handset sets up radio links with multiple cell sites (or sectors of the same site) simultaneously. This is known as "soft handoff" because, unlike with traditional cellular technology, there is no one defined point where the phone switches to the new cell.
In IS-95 inter-frequency handovers and older analog systems such as NMT it will typically be impossible to test the target channel directly while communicating. In this case other techniques have to be used such as pilot beacons in IS-95. This means that there is almost always a brief break in the communication while searching for the new channel followed by the risk of an unexpected return to the old channel.
If there is no ongoing communication or the communication can be interrupted, it is possible for the mobile unit to spontaneously move from one cell to another and then notify the base station with the strongest signal.
[edit] Cellular frequency choice in mobile phone networks
Main article: Cellular frequencies
The effect of frequency on cell coverage means that different
frequencies serve better for different uses. Low frequencies, such as
450 MHz NMT, serve very well for countryside coverage. GSM 900
(900 MHz) is a suitable solution for light urban coverage. GSM 1800
(1.8 GHz) starts to be limited by structural walls. UMTS, at 2.1 GHz is quite similar in coverage to GSM 1800.Higher frequencies are a disadvantage when it comes to coverage, but it is a decided advantage when it comes to capacity. Pico cells, covering e.g. one floor of a building, become possible, and the same frequency can be used for cells which are practically neighbours.
Cell service area may also vary due to interference from transmitting systems, both within and around that cell. This is true especially in CDMA based systems. The receiver requires a certain signal-to-noise ratio, and the transmitter should not send with too high transmission power in view to not cause interference with other transmitters. As the receiver moves away from the transmitter, the power received decreases, so the power control algorithm of the transmitter increases the power it transmits to restore the level of received power. As the interference (noise) rises above the received power from the transmitter, and the power of the transmitter cannot be increased any more, the signal becomes corrupted and eventually unusable. In CDMA-based systems, the effect of interference from other mobile transmitters in the same cell on coverage area is very marked and has a special name, cell breathing.
One can see examples of cell coverage by studying some of the coverage maps provided by real operators on their web sites. In certain cases they may mark the site of the transmitter, in others it can be calculated by working out the point of strongest coverage.
Following table shows the dependency of frequency on coverage area of one cell of a CDMA2000 network:[6]
| Frequency (MHz) | Cell radius (km) | Cell area (km2) | Relative Cell Count |
|---|---|---|---|
| 450 | 48.9 | 7521 | 1 |
| 950 | 26.9 | 2269 | 3.3 |
| 1800 | 14.0 | 618 | 12.2 |
| 2100 | 12.0 | 449 | 16.2 |
electromagnetic spectrum
electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation.[1]
The "electromagnetic spectrum" of an object is the characteristic
distribution of electromagnetic radiation emitted or absorbed by that
particular object.
The electromagnetic spectrum extends from low frequencies used for modern radio communication to gamma radiation at the short-wavelength (high-frequency) end, thereby covering wavelengths from thousands of kilometres down to a fraction of the size of an atom. It is for this reason that the electromagnetic spectrum is highly studied for spectroscopic purposes to characterize matter.[2] The limit for long wavelength is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length,[3] although in principle the spectrum is infinite and continuous.
Generally, EM radiation is classified by wavelength into radio wave, microwave, terahertz (or sub-millimeter) radiation, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays. The behavior of EM radiation depends on its wavelength. When EM radiation interacts with single atoms and molecules, its behaviour also depends on the amount of energy per quantum (photon) it carries.
Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400 nm to 700 nm. A common laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm. Detailed information about the physical properties of objects, gases, or even stars can be obtained from this type of device. Spectroscopes are widely used in astrophysics. For example, many hydrogen atoms emit a radio wave photon that has a wavelength of 21.12 cm. Also, frequencies of 30 Hz and below can be produced by and are important in the study of certain stellar nebulae[8] and frequencies as high as 2.9×1027 Hz have been detected from astrophysical sources.[9]
The electromagnetic spectrum
The types of electromagnetic radiation are broadly classified into the following classes:[2]
Also, the region of the spectrum of the particular electromagnetic radiation is reference frame-dependent (on account of the Doppler shift for light), so EM radiation that one observer would say is in one region of the spectrum could appear to an observer moving at a substantial fraction of the speed of light with respect to the first to be in another part of the spectrum. For example, consider the cosmic microwave background. It was produced, when matter and radiation decoupled, by the de-excitation of hydrogen atoms to the ground state. These photons were from Lyman series transitions, putting them in the ultraviolet (UV) part of the electromagnetic spectrum. Now this radiation has undergone enough cosmological red shift to put it into the microwave region of the spectrum for observers moving slowly (compared to the speed of light) with respect to the cosmos. However, for particles moving near the speed of light, this radiation will be blue-shifted in their rest frame. The highest-energy cosmic ray protons are moving such that, in their rest frame, this radiation is blueshifted to high-energy gamma rays, which interact with the proton to produce bound quark-antiquark pairs (pions). This is the source of the GZK limit.
Radio waves can be made to carry information by varying a combination of the amplitude, frequency, and phase of the wave within a frequency band. When EM radiation impinges upon a conductor, it couples to the conductor, travels along it, and induces an electric current on the surface of that conductor by exciting the electrons of the conducting material. This effect (the skin effect) is used in antennas.
Plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation.
The super-high frequency (SHF) and extremely high frequency (EHF) of microwaves come after radio waves. Microwaves are waves that are typically short enough to employ tubular metal waveguides of reasonable diameter. Microwave energy is produced with klystron and magnetron tubes, and with solid state diodes such as Gunn and IMPATT devices. Microwaves are absorbed by molecules that have a dipole moment in liquids. In a microwave oven, this effect is used to heat food. Low-intensity microwave radiation is used in Wi-Fi, although this is at intensity levels unable to cause thermal heating.
Volumetric heating, as used by microwave ovens, transfers energy through the material electromagnetically, not as a thermal heat flux. The benefit of this is a more uniform heating and reduced heating time; microwaves can heat material in less than 1% of the time of conventional heating methods.
When active, the average microwave oven is powerful enough to cause interference at close range with poorly shielded electromagnetic fields such as those found in mobile medical devices and cheap consumer electronics.
Electromagnetic radiation with a wavelength between 380 nm and 760 nm (790–400 terahertz) is detected by the human eye and perceived as visible light. Other wavelengths, especially near infrared (longer than 760 nm) and ultraviolet (shorter than 380 nm) are also sometimes referred to as light, especially when the visibility to humans is not relevant. White light is a combination of lights of different wavelengths in the visible spectrum. Passing white light through a prism splits it up in to the several colors of light observed in the visible spectrum between 400 nm and 780 nm.
If radiation having a frequency in the visible region of the EM spectrum reflects off an object, say, a bowl of fruit, and then strikes our eyes, this results in our visual perception of the scene. Our brain's visual system processes the multitude of reflected frequencies into different shades and hues, and through this not-entirely-understood psychophysical phenomenon, most people perceive a bowl of fruit.
At most wavelengths, however, the information carried by electromagnetic radiation is not directly detected by human senses. Natural sources produce EM radiation across the spectrum, and our technology can also manipulate a broad range of wavelengths. Optical fiber transmits light that, although not necessarily in the visible part of the spectrum, can carry information. The modulation is similar to that used with radio waves.
The amount of penetration of UV relative to altitude in Earth's ozone
Next in frequency comes ultraviolet (UV). The wavelength of UV rays is shorter than the violet end of the visible spectrum but longer than the X-ray.
Being very energetic, UV rays can break chemical bonds, making molecules unusually reactive. Sunburn, for example, is caused by the disruptive effects of UV radiation on skin cells, which is the main cause of skin cancer. UV rays can irreparably damage the complex DNA molecules in the cells producing thymine dimers making it a very potent mutagen. The sun emits a large amount of UV radiation, which could potentially turn Earth into a barren desert. However, most of it is absorbed by the atmosphere's ozone layer before it reaches the surface. The higher ranges of UV (vacuum UV) are absorbed by simple diatomic oxygen in the air. UV in this range (next to X-rays) is cabable even of ionizing atoms (see photoelectric effect), thus even more greatly changing their physical behavior.
Note that there are no precisely defined boundaries between the bands of the electromagnetic spectrum. Radiation of some types have a mixture of the properties of those in two regions of the spectrum. For example, red light resembles infrared radiation in that it can resonate some chemical bonds.
The electromagnetic spectrum extends from low frequencies used for modern radio communication to gamma radiation at the short-wavelength (high-frequency) end, thereby covering wavelengths from thousands of kilometres down to a fraction of the size of an atom. It is for this reason that the electromagnetic spectrum is highly studied for spectroscopic purposes to characterize matter.[2] The limit for long wavelength is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length,[3] although in principle the spectrum is infinite and continuous.
Legend[4][5][6]
| γ= Gamma rays | MIR= Mid infrared | HF= High freq. |
| HX= Hard X-rays | FIR= Far infrared | MF= Medium freq. |
| SX= Soft X-rays | Radio waves | LF= Low freq. |
| EUV= Extreme ultraviolet | EHF= Extremely high freq. | VLF= Very low freq. |
| NUV= Near ultraviolet | SHF= Super high freq. | VF/ULF= Voice freq. |
| Visible light | UHF= Ultra high freq. | SLF= Super low freq. |
| NIR= Near Infrared | VHF= Very high freq. | ELF= Extremely low freq. |
| Freq=Frequency |
Contents[hide] |
History
For most of history, light was the only know part of the electromagnetic spectrum. The ancient Greeks recognized that light traveled in straight lines and studied some of the properties of it, including reflection and refraction. Over the years the study of light continued and during the 16th and 17th centuries there were conflicting theories which regarded light as either a wave or a particle. It was first linked to electromagnetism in 1845 when Michael Faraday noticed that light responded to a magnetic field. The first discovery of electromagnetic waves other than light came in 1800, when William Herschel discovered infrared light. He was studying the temperature of different colours by moving a thermometer through light split by a prism. He noticed that the hottest temperature was beyond red. He theorized meant that there was 'light' that you could not see. The next year, Johann Ritter worked at the other end of spectrum and noticed that there were 'chemical rays' that behaved similar to, but were beyond, visible violet light rays. They were later renamed ultraviolet radiation. During the 1860s James Maxwell was studying electromagnetic field and realized that they traveled at around the speed of light. He developed four partial differential equations to explain this correlation. These equations predicted many frequencies of electromagnetic waves traveling at the speed of light. Attempting to prove Maxwell's equations, in 1886 Heinrich Hertz built an apparatus to generate and detect radio waves. He was able to observe that they traveled at the speed of light and could be both reflected and refracted. In a later experiment he similarly produced and measured microwaves. These new waves paved the way for inventions such as the wireless telegraph and the radio. In 1895 Wilhelm Röntgen noticed a new type of radiation emitted during an experiment. He called these x-rays and found they were able to travel through parts of the human body but were reflected by denser matter such as bones. Before long many uses were found for them in the field of medicine. The last portion of the electromagnetic spectrum was filled in with the discovery of gamma rays. In 1900 Paul Villard was studying radioactivity. He first thought they were particles similar to alpha and beta particles. However, in 1910 Ernest Rutherford measured their wave lengths and found that they were electromagnetic waves.Range of the spectrum
Electromagnetic waves are typically described by any of the following three physical properties: the frequency f, wavelength λ, or photon energy E. Frequencies range from 2.4×1023 Hz (1 GeV gamma rays) down to the local plasma frequency of the ionized interstellar medium (~1 kHz). Wavelength is inversely proportional to the wave frequency,[2] so gamma rays have very short wavelengths that are fractions of the size of atoms, whereas wavelengths can be as long as the universe. Photon energy is directly proportional to the wave frequency, so gamma rays have the highest energy (around a billion electron volts) and radio waves have very low energy (around a femto electron volts). These relations are illustrated by the following equations:
- c = 299,792,458 m/s is the speed of light in vacuum and
- h = 6.62606896(33)×10−34 J s = 4.13566733(10)×10−15 eV s is Planck's constant.[7]
Generally, EM radiation is classified by wavelength into radio wave, microwave, terahertz (or sub-millimeter) radiation, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays. The behavior of EM radiation depends on its wavelength. When EM radiation interacts with single atoms and molecules, its behaviour also depends on the amount of energy per quantum (photon) it carries.
Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400 nm to 700 nm. A common laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm. Detailed information about the physical properties of objects, gases, or even stars can be obtained from this type of device. Spectroscopes are widely used in astrophysics. For example, many hydrogen atoms emit a radio wave photon that has a wavelength of 21.12 cm. Also, frequencies of 30 Hz and below can be produced by and are important in the study of certain stellar nebulae[8] and frequencies as high as 2.9×1027 Hz have been detected from astrophysical sources.[9]
Rationale
Electromagnetic radiation interacts with matter in different ways in different parts of the spectrum. The types of interaction can be so different that it seems to be justified to refer to different types of radiation. At the same time, there is a continuum containing all these "different kinds" of electromagnetic radiation. Thus we refer to a spectrum, but divide it up based on the different interactions with matter.| Region of the spectrum | Main interactions with matter |
|---|---|
| Radio | Collective oscillation of charge carriers in bulk material (plasma oscillation). An example would be the oscillation of the electrons in an antenna. |
| Microwave through far infrared | Plasma oscillation, molecular rotation |
| Near infrared | Molecular vibration, plasma oscillation (in metals only) |
| Visible | Molecular electron excitation (including pigment molecules found in the human retina), plasma oscillations (in metals only) |
| Ultraviolet | Excitation of molecular and atomic valence electrons, including ejection of the electrons (photoelectric effect) |
| X-rays | Excitation and ejection of core atomic electrons, Compton scattering (for low atomic numbers) |
| Gamma rays | Energetic ejection of core electrons in heavy elements, Compton scattering (for all atomic numbers), excitation of atomic nuclei, including dissociation of nuclei |
| High-energy gamma rays | Creation of particle-antiparticle pairs. At very high energies a single photon can create a shower of high-energy particles and antiparticles upon interaction with matter. |
Types of radiation
- Gamma radiation
- X-ray radiation
- Ultraviolet radiation
- Visible radiation
- Infrared radiation
- Microwave radiation
- Radio waves
Also, the region of the spectrum of the particular electromagnetic radiation is reference frame-dependent (on account of the Doppler shift for light), so EM radiation that one observer would say is in one region of the spectrum could appear to an observer moving at a substantial fraction of the speed of light with respect to the first to be in another part of the spectrum. For example, consider the cosmic microwave background. It was produced, when matter and radiation decoupled, by the de-excitation of hydrogen atoms to the ground state. These photons were from Lyman series transitions, putting them in the ultraviolet (UV) part of the electromagnetic spectrum. Now this radiation has undergone enough cosmological red shift to put it into the microwave region of the spectrum for observers moving slowly (compared to the speed of light) with respect to the cosmos. However, for particles moving near the speed of light, this radiation will be blue-shifted in their rest frame. The highest-energy cosmic ray protons are moving such that, in their rest frame, this radiation is blueshifted to high-energy gamma rays, which interact with the proton to produce bound quark-antiquark pairs (pions). This is the source of the GZK limit.
Radio frequency
Radio waves generally are utilized by antennas of appropriate size (according to the principle of resonance), with wavelengths ranging from hundreds of meters to about one millimeter. They are used for transmission of data, via modulation. Television, mobile phones, wireless networking, and amateur radio all use radio waves. The use of the radio spectrum is regulated by many governments through frequency allocation.Radio waves can be made to carry information by varying a combination of the amplitude, frequency, and phase of the wave within a frequency band. When EM radiation impinges upon a conductor, it couples to the conductor, travels along it, and induces an electric current on the surface of that conductor by exciting the electrons of the conducting material. This effect (the skin effect) is used in antennas.
Microwaves
Main article: Microwaves
Volumetric heating, as used by microwave ovens, transfers energy through the material electromagnetically, not as a thermal heat flux. The benefit of this is a more uniform heating and reduced heating time; microwaves can heat material in less than 1% of the time of conventional heating methods.
When active, the average microwave oven is powerful enough to cause interference at close range with poorly shielded electromagnetic fields such as those found in mobile medical devices and cheap consumer electronics.
Terahertz radiation
Main article: Terahertz radiation
Terahertz radiation is a region of the spectrum between far infrared
and microwaves. Until recently, the range was rarely studied and few
sources existed for microwave energy at the high end of the band
(sub-millimetre waves or so-called terahertz waves),
but applications such as imaging and communications are now appearing.
Scientists are also looking to apply terahertz technology in the armed
forces, where high-frequency waves might be directed at enemy troops to
incapacitate their electronic equipment.[12]Infrared radiation
Main article: Infrared radiation
The infrared
part of the electromagnetic spectrum covers the range from roughly
300 GHz (1 mm) to 400 THz (750 nm). It can be divided into three parts:[2]- Far-infrared, from 300 GHz (1 mm) to 30 THz (10 μm). The lower part of this range may also be called microwaves. This radiation is typically absorbed by so-called rotational modes in gas-phase molecules, by molecular motions in liquids, and by phonons in solids. The water in Earth's atmosphere absorbs so strongly in this range that it renders the atmosphere in effect opaque. However, there are certain wavelength ranges ("windows") within the opaque range that allow partial transmission, and can be used for astronomy. The wavelength range from approximately 200 μm up to a few mm is often referred to as "sub-millimetre" in astronomy, reserving far infrared for wavelengths below 200 μm.
- Mid-infrared, from 30 to 120 THz (10 to 2.5 μm). Hot objects (black-body radiators) can radiate strongly in this range. It is absorbed by molecular vibrations, where the different atoms in a molecule vibrate around their equilibrium positions. This range is sometimes called the fingerprint region, since the mid-infrared absorption spectrum of a compound is very specific for that compound.
- Near-infrared, from 120 to 400 THz (2,500 to 750 nm). Physical processes that are relevant for this range are similar to those for visible light.
Visible radiation (light)
Main article: Visible spectrum
Above infrared in frequency comes visible light. This is the range in which the sun and other stars emit most of their radiation[citation needed] and the spectrum that the human eye is the most sensitive
to. Visible light (and near-infrared light) is typically absorbed and
emitted by electrons in molecules and atoms that move from one energy
level to another. The light we see with our eyes is really a very small
portion of the electromagnetic spectrum. A rainbow
shows the optical (visible) part of the electromagnetic spectrum;
infrared (if you could see it) would be located just beyond the red
side of the rainbow with ultraviolet appearing just beyond the violet end.Electromagnetic radiation with a wavelength between 380 nm and 760 nm (790–400 terahertz) is detected by the human eye and perceived as visible light. Other wavelengths, especially near infrared (longer than 760 nm) and ultraviolet (shorter than 380 nm) are also sometimes referred to as light, especially when the visibility to humans is not relevant. White light is a combination of lights of different wavelengths in the visible spectrum. Passing white light through a prism splits it up in to the several colors of light observed in the visible spectrum between 400 nm and 780 nm.
If radiation having a frequency in the visible region of the EM spectrum reflects off an object, say, a bowl of fruit, and then strikes our eyes, this results in our visual perception of the scene. Our brain's visual system processes the multitude of reflected frequencies into different shades and hues, and through this not-entirely-understood psychophysical phenomenon, most people perceive a bowl of fruit.
At most wavelengths, however, the information carried by electromagnetic radiation is not directly detected by human senses. Natural sources produce EM radiation across the spectrum, and our technology can also manipulate a broad range of wavelengths. Optical fiber transmits light that, although not necessarily in the visible part of the spectrum, can carry information. The modulation is similar to that used with radio waves.
Ultraviolet light
Main article: Ultraviolet
Being very energetic, UV rays can break chemical bonds, making molecules unusually reactive. Sunburn, for example, is caused by the disruptive effects of UV radiation on skin cells, which is the main cause of skin cancer. UV rays can irreparably damage the complex DNA molecules in the cells producing thymine dimers making it a very potent mutagen. The sun emits a large amount of UV radiation, which could potentially turn Earth into a barren desert. However, most of it is absorbed by the atmosphere's ozone layer before it reaches the surface. The higher ranges of UV (vacuum UV) are absorbed by simple diatomic oxygen in the air. UV in this range (next to X-rays) is cabable even of ionizing atoms (see photoelectric effect), thus even more greatly changing their physical behavior.
X-rays
Main article: X-rays
After UV come X-rays,
which, like the upper ranges of UV are also ionizing. However, due to
their higher energies, X-rays can also interact with matter by means of
the Compton effect.
Hard X-rays have shorter wavelengths than soft X-rays. As they can pass
through most substances, X-rays can be used to 'see through' objects,
the most notable use being diagnostic X-ray images in medicine (a
process known as radiography), as well as for high-energy physics and astronomy. Neutron stars and accretion disks around black holes emit X-rays, which enable us to study them. X-rays are given off by stars and are strongly emitted by some types of nebulae.Gamma rays
Main article: Gamma rays
After hard X-rays come gamma rays, which were discovered by Paul Villard in 1900. These are the most energetic photons, having no defined lower limit to their wavelength. They are useful to astronomers
in the study of high-energy objects or regions, and find a use with
physicists thanks to their penetrative ability and their production
from radioisotopes. Gamma rays are also used for the irradiation of food and seed for sterilization, and in medicine they are used in radiation cancer therapy and some kinds of diagnostic imaging such as PET scans. The wavelength of gamma rays can be measured with high accuracy by means of Compton scattering.Note that there are no precisely defined boundaries between the bands of the electromagnetic spectrum. Radiation of some types have a mixture of the properties of those in two regions of the spectrum. For example, red light resembles infrared radiation in that it can resonate some chemical bonds.
LAN....
A wireless local area network (WLAN) links two or more devices using some wireless distribution method (typically spread-spectrum or OFDM
radio), and usually providing a connection through an access point to
the wider internet. This gives users the mobility to move around within
a local coverage area and still be connected to the network. Most
modern WLANs are based on IEEE 802.11 standards, marketed under the Wi-Fi brand name.
Wireless LANs have become popular in the home due to ease of installation, and in commercial complexes offering wireless access to their customers; often for free. Large wireless network projects are being put up in many major cities: New York City, for instance, has begun a pilot program to provide city workers in all five boroughs of the city with wireless Internet access.[
Wireless LANs have become popular in the home due to ease of installation, and in commercial complexes offering wireless access to their customers; often for free. Large wireless network projects are being put up in many major cities: New York City, for instance, has begun a pilot program to provide city workers in all five boroughs of the city with wireless Internet access.[
speed up wireless
Wi-Fi networks are miraculous and devilish at the same time. They
can seem miraculous in their ability to deliver high-speed network and
Internet communication through walls and ceilings, and over long
distances. But they don't always deliver the speeds they promise. When
you're troubleshooting a slower-than-it-should-be network, the devil is
in the details.
The most common reason for an underperforming wireless network is interference,
which can crop up when competing wireless signals disrupt the
transmission of data on your network. Interference means bits have to
be resent or sent at a slower speed. It can also mean that network
throughput drops to zero or that devices lose their connections to your
base station.
In real-world usage, if you have a Mac using 802.11n Wi-Fi to connect to a computer or router with a wired Ethernet connection, and they're both in the same room, you should routinely see speeds of 20 to 70 Mbps in the 2.4 GHz band and 30 to 150 Mbps in the 5 GHz band. (Between two Wi-Fi devices, speeds will naturally be half, because the same data is being sent twice.) If you're regularly seeing speeds below 20 to 30 Mbps in either band, this article is for you. Here's how to find out what might be interfering with your wireless net and then to make sure you're getting the highest rates you can.
The 2.4 GHz band (which Wi-Fi equipment has been using since 1999) is called a junk band, because it’s shared by so many different kinds of equipment: medical monitors in hospitals; industrial sealers; home microwave ovens; Bluetooth devices; baby monitors; cordless phones; older wireless keyboards and mice; and on and on. So if your Wi-Fi equipment is using the 2.4 GHz band, its signals will have lots of competition.
There’s less competition in the 5 GHz band. The laws of physics demand that a 2.4 GHz signal travels farther than a 5 GHz signal using the same amount of power. The laws of nations put further restraints on devices that use the 5 GHz band, limiting signal strength often to a level lower than than allowed in 2.4 GHz. Those two factors can reduce interference (neighboring networks are less likely to get in the way because their signals don’t reach far enough to interfere), but they also means you can’t get the best speeds in 5 GHz more than a couple rooms away.
Interference doesn’t come just from competing sources; it also comes from overlap. The 2.4 and 5 GHz bands are both split into channels that are about 20 MHz wide. In the 2.4 GHz band, those channels overlap at 5 MHz intervals; that means that (in the United States) only three of the eleven available channels—1, 6, and 11—largely avoid crossing into adjacent channels. (Other countries and regulatory regions allow more or fewer channels.) Because there are fewer channels to choose from in 2.4 GHz, signals from nearby networks can bump into your network, reducing speed.
In the 5 GHz band, each 20 MHz channel butts right up against the next with no overlap. Apple 802.11n gear is designed to use eight non-overlapping channels. (There are 13 other channels in the 5 GHz band that Apple doesn’t use, because of regulatory and interference issues.) Additionally, Apple’s hardware can bind two of those channels together into a so-called wide channel, to double your bandwidth.
For starters, launch /Applications/Utilities/AirPort Utility and select your base station. Click Manual Setup at the bottom of the base station overview screen, then the Advanced view icon. Select Logs and Statistics at the bottom of the screen and then the Wireless Clients tab. There, you can see which devices are connected to your base station, as well as assessments of those connections.
Signal and noise are measured on a scale that uses negative numbers to denote the weakest levels; the further a number is below zero, the weaker the signal or noise. With signal, stronger is better, so you want a number closer to zero: -25 is far stronger (and a typical value to see) than -50. You want noise to be as weak as possible, so you'd rather have a noise measurement of -90 than -50. Both are measured on a logarithmic scale: an increase or decrease of 10 is a tenfold change.
The Rate field shows the current connected rate in Mbps; that can range from 5 to 270 depending on the band and device. Type shows the kind of connection that’s in use: 802.11b/g or b/g/n means the device is using the 2.4 GHz band; 802.11a/n means a 5 GHz connection is in use.
To find out what might be causing network slow-downs, start by checking to see how well each client is connecting to your base station.
When
you see a high amount of interference in the form of a high noise
number or a lower than expected rate for one of those connections, you
need to identify potential culprits. To find out if the problem is a nearby wireless network, download
and install iStumbler (donation requested); make sure you get the
correct release for your version of Mac OS X. That utility provides a
list of all the networks that your Mac’s Wi-Fi adapter can sense, and
the channel each one is using.
The easiest way to do so is to restart your router. When you first start an Apple base station (as well as Wi-Fi routers from many other companies), it automatically chooses the channel that’s least used at that time. Restarting it forces it to pick a new channel, which could solve the problem without making you muck about with settings.
But the choice your router makes may not always be best. And even if its initial choice is a good one, if you leave it on for months at a time, that choice may no longer be the right one. You could reboot the router again. Or, to make sure you get the channel you want, you could select it manually.
In AirPort Utility (/Applications/Utilities), select your base station, click Manual Setup, and click the Wireless tab in AirPort view. On a 2007 or 2008 AirPort Extreme, AirPort Express, or Time Capsule, select a channel from the Channel popup menu. On base stations released in 2009 or later, first select Manual from the Radio Channel Selection menu, then click the Edit button. From the 2.4 GHz Channel menu, select a different channel.
In the AirPort utility, you can manually select specific channels on the 5 GHz and 2.4 GHz bands.
The
specific channel you choose—with an exception in the 5 GHz band—isn't
as important as picking one that's not in use by a nearby network. In
the 2.4 GHz band, you want to pick from among channels 1, 6, or 11,
whichever one is being used by the fewest nearby networks. If that
channel still performs poorly, try one of the others. In the 5 GHz band, you can choose from any of the eight channels
Apple makes available, but there’s a big difference among them that's
worth noting. The four lower-numbered channels (36, 40, 44, and 48) use
just five percent of the signal power compared to the higher-numbered
channels (149, 153, 157, and 161). You can choose a low-numbered
channel to reduce interference and range. Or you could select a
high-numbered channel to boost range, even though that could cause
interference with other nearby networks. Reducing interference can
improve throughput if your base station is receiving signals from
competing devices. Increasing range can do the same thing, if the
problem is that you're trying to reach a distant device.
You could also try switching bands. If your base station can use only the 2.4 GHz band, you could upgrade to a new, dual-band router (such as the current AirPort Extreme () or Time Capsule ()). But before you do, make sure your other hardware can work in the 5 GHz band. All Macs with Wi-Fi released since October 2006—except for pre-2009 Mac minis—can; so can the iPad and the Apple TV. The iPhone and iPod touch work only over 2.4 GHz. (If you get a new simultaneous dual-band base station, you could plug in your old 2.4 GHz-only unit via Ethernet to extend your network's range in another part of the house or office.)
You could also try moving the base station. If you can locate your base station somewhere further from outside walls or other homes or apartments (the sources of interference), give it a try.
Finally, talk to your neighbors. It’s possible they’re having the same trouble. Perhaps you can agree to coordinate channel usage. Help them if they don’t know how to make changes; while you’re at it, you might turn on network security for them, if they haven’t done so already.
For example, are you using older wireless hardware, such as a baby monitor, old Bluetooth equipment (more than three or four years old), or a non-standard wireless keyboard with a USB dongle? Do you have a 2.4 GHz cordless phone? Even if you don't have any of those things, they could still cause problems if your neighbor does.
The first troubleshooting step is to eliminate as many possible causes as you can. Turn off every non-Wi-Fi wireless device you have, then check your network speed. If the problems go away, you may need to get rid of the offending item. If it’s your phone, note that new 1900 MHz DECT6 cordless phones are cheap, work well, and don’t interfere with either 2.4 GHz or 5 GHz Wi-Fi.
You could also try switching bands, as above. If the problem is with another device that uses the 2.4 GHz band, moving to 5 GHz could solve it.
Finally, if the problems persist, you could have a problem with your Wi-Fi radio itself. If you can’t duplicate the problems with another computer, look into warranty repair; document all the troubleshooting steps you’ve tried, because even Geniuses might not believe the radio is at fault.
Some of this clearly has to do with interference—in the 2.4 GHz or 5 GHz band—that’s outside of the control of the user or his neighbors. For example, I had a year or two when some massive unknown interference-source near my Seattle office was causing problems with Channel 1 in the 2.4 GHz band. No amount of tweaking would get more than a few kilobytes to dribble through per second.
If you suspect that you might have some kind of mysterious source like that, and if you don’t mind the investment, you could buy a device that scans the spectrum for signals—not just those produced by Wi-Fi, but all emissions. MetaGeek offers their 2.4 GHz Wi-Spy 2.4i software-plus-USB-dongle combo for $99.
Wi-Spy can help you find neighboring causes of wireless interference.
With
Wi-Spy installed, you could walk with a laptop and perhaps discover
that a neighbor’s hobby is microwave bag sealing, or that there’s a
high-powered amateur radio station in your area. (Hams have overlapping
licensed rights in part of the 2.4 GHz band, and can use vastly higher
power than Wi-Fi allows; switching to channel 11 could help you avoid
them.)
Complete Coverage
In real-world usage, if you have a Mac using 802.11n Wi-Fi to connect to a computer or router with a wired Ethernet connection, and they're both in the same room, you should routinely see speeds of 20 to 70 Mbps in the 2.4 GHz band and 30 to 150 Mbps in the 5 GHz band. (Between two Wi-Fi devices, speeds will naturally be half, because the same data is being sent twice.) If you're regularly seeing speeds below 20 to 30 Mbps in either band, this article is for you. Here's how to find out what might be interfering with your wireless net and then to make sure you're getting the highest rates you can.
Don't interfere
Wi-Fi uses two unlicensed spectrum bands (hunks of frequencies reserved by regulators for low-power devices) to send and receive data.The 2.4 GHz band (which Wi-Fi equipment has been using since 1999) is called a junk band, because it’s shared by so many different kinds of equipment: medical monitors in hospitals; industrial sealers; home microwave ovens; Bluetooth devices; baby monitors; cordless phones; older wireless keyboards and mice; and on and on. So if your Wi-Fi equipment is using the 2.4 GHz band, its signals will have lots of competition.
There’s less competition in the 5 GHz band. The laws of physics demand that a 2.4 GHz signal travels farther than a 5 GHz signal using the same amount of power. The laws of nations put further restraints on devices that use the 5 GHz band, limiting signal strength often to a level lower than than allowed in 2.4 GHz. Those two factors can reduce interference (neighboring networks are less likely to get in the way because their signals don’t reach far enough to interfere), but they also means you can’t get the best speeds in 5 GHz more than a couple rooms away.
Interference doesn’t come just from competing sources; it also comes from overlap. The 2.4 and 5 GHz bands are both split into channels that are about 20 MHz wide. In the 2.4 GHz band, those channels overlap at 5 MHz intervals; that means that (in the United States) only three of the eleven available channels—1, 6, and 11—largely avoid crossing into adjacent channels. (Other countries and regulatory regions allow more or fewer channels.) Because there are fewer channels to choose from in 2.4 GHz, signals from nearby networks can bump into your network, reducing speed.
In the 5 GHz band, each 20 MHz channel butts right up against the next with no overlap. Apple 802.11n gear is designed to use eight non-overlapping channels. (There are 13 other channels in the 5 GHz band that Apple doesn’t use, because of regulatory and interference issues.) Additionally, Apple’s hardware can bind two of those channels together into a so-called wide channel, to double your bandwidth.
Find the culprit
Given the above, the first step in troubleshooting a slow network is to find out more what might be interfering with it. (Note: For the purposes of this article, I'll provide instructions primarily for Apple's AirPort base stations. But the general principles can apply to any wireless router.)For starters, launch /Applications/Utilities/AirPort Utility and select your base station. Click Manual Setup at the bottom of the base station overview screen, then the Advanced view icon. Select Logs and Statistics at the bottom of the screen and then the Wireless Clients tab. There, you can see which devices are connected to your base station, as well as assessments of those connections.
Signal and noise are measured on a scale that uses negative numbers to denote the weakest levels; the further a number is below zero, the weaker the signal or noise. With signal, stronger is better, so you want a number closer to zero: -25 is far stronger (and a typical value to see) than -50. You want noise to be as weak as possible, so you'd rather have a noise measurement of -90 than -50. Both are measured on a logarithmic scale: an increase or decrease of 10 is a tenfold change.
The Rate field shows the current connected rate in Mbps; that can range from 5 to 270 depending on the band and device. Type shows the kind of connection that’s in use: 802.11b/g or b/g/n means the device is using the 2.4 GHz band; 802.11a/n means a 5 GHz connection is in use.
To find out what might be causing network slow-downs, start by checking to see how well each client is connecting to your base station.
Change channels
Assuming you do find a nearby wireless network that’s using the same channel as yours, you can try to change the channel your base station is using.The easiest way to do so is to restart your router. When you first start an Apple base station (as well as Wi-Fi routers from many other companies), it automatically chooses the channel that’s least used at that time. Restarting it forces it to pick a new channel, which could solve the problem without making you muck about with settings.
But the choice your router makes may not always be best. And even if its initial choice is a good one, if you leave it on for months at a time, that choice may no longer be the right one. You could reboot the router again. Or, to make sure you get the channel you want, you could select it manually.
In AirPort Utility (/Applications/Utilities), select your base station, click Manual Setup, and click the Wireless tab in AirPort view. On a 2007 or 2008 AirPort Extreme, AirPort Express, or Time Capsule, select a channel from the Channel popup menu. On base stations released in 2009 or later, first select Manual from the Radio Channel Selection menu, then click the Edit button. From the 2.4 GHz Channel menu, select a different channel.
In the AirPort utility, you can manually select specific channels on the 5 GHz and 2.4 GHz bands.
You could also try switching bands. If your base station can use only the 2.4 GHz band, you could upgrade to a new, dual-band router (such as the current AirPort Extreme () or Time Capsule ()). But before you do, make sure your other hardware can work in the 5 GHz band. All Macs with Wi-Fi released since October 2006—except for pre-2009 Mac minis—can; so can the iPad and the Apple TV. The iPhone and iPod touch work only over 2.4 GHz. (If you get a new simultaneous dual-band base station, you could plug in your old 2.4 GHz-only unit via Ethernet to extend your network's range in another part of the house or office.)
You could also try moving the base station. If you can locate your base station somewhere further from outside walls or other homes or apartments (the sources of interference), give it a try.
Finally, talk to your neighbors. It’s possible they’re having the same trouble. Perhaps you can agree to coordinate channel usage. Help them if they don’t know how to make changes; while you’re at it, you might turn on network security for them, if they haven’t done so already.
Other sources of trouble
If your wireless net is suffering from interference, but the Airport Utility or iStumbler have ruled out nearby networks as culprits, you need to consider other causes.For example, are you using older wireless hardware, such as a baby monitor, old Bluetooth equipment (more than three or four years old), or a non-standard wireless keyboard with a USB dongle? Do you have a 2.4 GHz cordless phone? Even if you don't have any of those things, they could still cause problems if your neighbor does.
The first troubleshooting step is to eliminate as many possible causes as you can. Turn off every non-Wi-Fi wireless device you have, then check your network speed. If the problems go away, you may need to get rid of the offending item. If it’s your phone, note that new 1900 MHz DECT6 cordless phones are cheap, work well, and don’t interfere with either 2.4 GHz or 5 GHz Wi-Fi.
You could also try switching bands, as above. If the problem is with another device that uses the 2.4 GHz band, moving to 5 GHz could solve it.
Finally, if the problems persist, you could have a problem with your Wi-Fi radio itself. If you can’t duplicate the problems with another computer, look into warranty repair; document all the troubleshooting steps you’ve tried, because even Geniuses might not believe the radio is at fault.
Stumped?
I receive hundreds of e-mails a year from Mac users about troubleshooting their Wi-Fi networks. In most cases, the advice above helps. But sometimes I encounter a case that no amount of reconfiguration will fix.Some of this clearly has to do with interference—in the 2.4 GHz or 5 GHz band—that’s outside of the control of the user or his neighbors. For example, I had a year or two when some massive unknown interference-source near my Seattle office was causing problems with Channel 1 in the 2.4 GHz band. No amount of tweaking would get more than a few kilobytes to dribble through per second.
If you suspect that you might have some kind of mysterious source like that, and if you don’t mind the investment, you could buy a device that scans the spectrum for signals—not just those produced by Wi-Fi, but all emissions. MetaGeek offers their 2.4 GHz Wi-Spy 2.4i software-plus-USB-dongle combo for $99.
Wi-Spy can help you find neighboring causes of wireless interference.
The Bottom Line
Getting a Wi-Fi network to run at something close to its top speed isn't quite a black art. But it is remarkable how often plug-connect-and-surf just doesn’t work. It's also remarkable how often Identifying, isolating, and working around a source of interference can solve the problem.Friday 23 March 2012
Fiber optik....
Fungsi fiber optik
Fiber optik atau dikenali juga sebagai gentian optik boleh digunakan sebagai perantaraan (medium) bagi telekomunikasi dan jaringan komputer kerana ia sangat mudah lentur dan boleh diikat sebagai kabel.
Walaupun gentian boleh diperbuat daripada plastik lutsinar atau kaca, gentian yang digunakan dalam telekomunikasi jarak jauh hampir selalunya menggunakan kaca, kerana penyerapan optik yang lebih rendah.
Cahaya yang dipancarkan melalui gentian disimpan disebabkan pantulan dalam penuh (total internal reflection) dalam bahan tersebut. Ini adalah ciri penting yang menyingkirkan silang isyarat antara gentian dalam kebal dan membenarkan pemasangan kabel dengan belokan dan pusingan.
Dalam kegunaan telekomunikasi, cahaya yang digunakan biasanya cahaya infra, pada gelombang cahaya menghampiri penyerapan jarak gelombang minimum bagi gentian yang digunakan.
Walaupun gentian boleh diperbuat daripada plastik lutsinar atau kaca, gentian yang digunakan dalam telekomunikasi jarak jauh hampir selalunya menggunakan kaca, kerana penyerapan optik yang lebih rendah.
Cahaya yang dipancarkan melalui gentian disimpan disebabkan pantulan dalam penuh (total internal reflection) dalam bahan tersebut. Ini adalah ciri penting yang menyingkirkan silang isyarat antara gentian dalam kebal dan membenarkan pemasangan kabel dengan belokan dan pusingan.
Dalam kegunaan telekomunikasi, cahaya yang digunakan biasanya cahaya infra, pada gelombang cahaya menghampiri penyerapan jarak gelombang minimum bagi gentian yang digunakan.
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