Tuesday 12 June 2012
Monday 11 June 2012
internet explorer
Windows Internet Explorer (formerly Microsoft Internet Explorer, commonly abbreviated IE or MSIE) is a series of graphical web browsers developed by Microsoft and included as part of the Microsoft Windows line of operating systems, starting in 1995. It was first released as part of the add-on package Plus! for Windows 95 that year. Later versions were available as free downloads, or in service packs, and included in the OEM service releases of Windows 95 and later versions of Windows.
Internet Explorer is the second most widely used web browser, behind Google Chrome, which surpassed it in May 2012, attaining a peak of about 95% usage share during 2002 and 2003 with Internet Explorer 5 and Internet Explorer 6.[citation needed] Since its peak of popularity, its usage share has been declining in the face of renewed competition from other web browsers, and is 34.27% as of January 2012. It had been slightly higher, 43.55% as of February 2011, just prior to the release of the current version. Microsoft spent over US$100 million per year on Internet Explorer in the late 1990s,[1] with over 1000 people working on it by 1999.[2]
Since its first release, Microsoft has added features and technologies such as basic table display (in version 1.5); XMLHttpRequest (in version 5), which aids creation of dynamic web pages; and Internationalized Domain Names (in version 7), which allow Web sites to have native-language addresses with non-Latin characters. The browser has also received scrutiny throughout its development for use of third-party technology (such as the source code of Spyglass Mosaic, used without royalty in early versions) and security and privacy vulnerabilities, and both the United States and the European Union have alleged that integration of Internet Explorer with Windows has been to the detriment of other browsers.
The latest stable release is Internet Explorer 9, which is available as a free update for Windows 7, Windows Vista SP2, Windows Server 2008, and Windows Server 2008 R2. Internet Explorer was to be omitted from Windows 7 and Windows Server 2008 R2 in Europe, but Microsoft ultimately included it, with a browser option screen allowing users to select any of several web browsers (including Internet Explorer).[3][4][5][6]
Versions of Internet Explorer for other operating systems have also been produced, including an embedded OEM version called Pocket Internet Explorer, later rebranded Internet Explorer Mobile, which is currently based on Internet Explorer 9 and made for Windows Phone, Windows CE, and previously, based on Internet Explorer 7 for Windows Mobile. It remains in development alongside the desktop versions. Internet Explorer for Mac and Internet Explorer for UNIX (Solaris and HP-UX) have been discontinued.
fiber obtik
Light wave communication was first considered more than 100 years ago. The implementation of optical communication using light waveguides was restricted to very short distance prior to 1970. Corning Glass Company achieved a breakthrough in 1970 by producing a fused silica (SiO2) fiber with a loss approximately 20 dB/km. The development of semiconductor light source also started to mature at about that time, allowing the feasibility of transmission over a few kilometers to be demonstrated. Since 1970, the rate of technological progress has been phenomenal, and optical fibers are now used in transoceanic service. Besides the long-distance routes, fibers are used in the inter-CO (inter exchange) routes, and the subscriber loop in the final link in what will eventually be the global interconnection chain. Optical fibers are associated with high-capacity communications. A lot of attention is presently being given to optical fibers to provide a very extensive broadband ISDN.
Fiber optics is defined as that branch of optics that deals with the transmission of light through ultrapure fibers of glass, plastic, or some other form of transparent media. From a decorative standpoint, most of us are familiar with the fiber optic lamp, which uses bundles of thin optical fibers illuminated from the base end of the lamp by a light source.
The light source is made to vary in color, which can be seen at the opposite ends of the fiber as a tree of illuminating points radiating various colors of the transmitted light. Although the lamp is used for decorative purposes only, it serves as an excellent model of how light can be transmitted through the fiber.
2.1 Light
Light is a kind of electromagnetic radiation. The
basic characteristic of electromagnetic radiation
is its frequency or wavelength. Light frequencies fall between microwaves and x-rays, as shown in Figure 2.1.
the telefon
The Telephone
The telephone is one of the simplest devices we have in our house. It is so very
simple because the telephone connection to our house has not changed in
nearly a century.
The telephone only contains three parts and they are all simple as shown in figure 1.2.
A switch to connect and disconnect the phone from the network. This switch is generally called the hook switch. It connects when you lift the handset.
A speaker - It is generally in a small size, 8-ohm speaker of some sort.
A microphone - In the past, telephone microphones have been as simple as carbon granules compressed between two thin metal plates. Sound waves from our voice compress and decompress the granules, changing the resistance of the granules and modulating the current flowing through the microphone.
1.2 Telephone Bandwidth
In order to allow more long-distance calls to be transmitted, the frequencies transmitted are limited to a bandwidth of about 3000 hertz. All of the frequencies in our voice below 400 hertz and above 3,400 hertz are eliminated. That's why someone's voice on a phone has a distinctive sound.
1.3 Digital Telephone
The digital button is the latest technique of dialing. It uses the button to give signal for every one digit. The diagram in figure 1.3 shows the Dual Tone Multi Frequency (DTMF) type of dialing. The button on the phone is connected to a set of oscillators which produces a pair of tone on the local line whenever a button is being pressed. The tone will be detected at the main distributor and the digit will be confirmed. The detector circuits in the main distributor will confirm the tone within 33ms.
1.3.1 Progress Tones
The various types of tones generated by the exchange to guide the users are :
Dial Tone (DT). This is a 33 c/s continuous note and is applied to the line after the subscriber has lifted his handset and the switching equipment has allocated him an available outlet for this call to proceed. There would have been a physical limit on the number of calls an exchange could handle so if all equipment was already in use, the subscriber would not get a dial tone.
Busy Tone (BT). A higher pitched note of 400 c/s interrupts to give a cadence of 0.75 seconds on, 0.75 seconds off. Busy tone indicates either that the called subscriber is already off-hook (busy) or that the route to the called subscriber is congested. In later systems, a slightly different cadence was introduced in order to distinguish between these two scenarios. A busy tone is made up of a 480-hertz and a 620-hertz tone, with a cycle of one and a half second on and one and a half second off.
Number Unobtainable Tone (NUT). Identical pitch to the busy tone but continuous. This tone is used to indicate that a number is out of service, faulty or that a spare line has been dialed.
Ring Tone (RT). A tone of 133c/s which interrupts in the same cadence as the ring current which rings the telephone bell at the called party's end : 0.4 seconds on, 0.2 seconds off.
Lakaran Hidup: Wireless
Lakaran Hidup: Wireless: Rangkaian tanpa wayar adalah salah satu cara untuk menyambungkan komputer-komputer dalam sebuah rangkaian. la membina sebuah rangkaian ...
Asynchronous Transfer Mode (Atm)
• adalah satu teknik pensuisan standard, yang direka untuk menyatukan telekomunikasi dan rangkaian komputer.
• Ia menggunakan pembahagian masa tak segerak pemultipleksan dan mengekod data ke dalam sel-sel kecil, bersaiz tetap.
• Ini berbeza daripada pendekatan seperti Protokol Internet atau Ethernet yang menggunakan paket atau bingkai bersaiz ubah.
• ATM menyediakan perkhidmatan lapisan data link yang berjalan lebih pelbagai pautan Lapisan fizikal OSI.
• ATM mempunyai persamaan berfungsi dengan litar kedua-dua dihidupkan rangkaian dan paket kecil dihidupkan rangkaian.
• Ia direka untuk rangkaian yang dikendalikan oleh kedua-dua data lalu lintas tradisional tinggi throughput (contohnya, pemindahan fail), dan kandungan real-time,-kependaman rendah seperti suara dan video.
• ATM menggunakan model berorientasikan sambungan di mana litar maya perlu diwujudkan di antara dua titik hujung sebelum pertukaran data sebenar bermula.
• ATM adalah protokol utama yang digunakan ke atas tulang belakang SONET / SDH awam dihidupkan rangkaian telefon (PSTN) dan Rangkaian Digital Perkhidmatan Bersepadu (ISDN), tetapi penggunaannya semakin merosot memihak kepada IP Semua.
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.[
Subscribe to:
Posts (Atom)