an amplifier or simply amp, is any device that changes, usually increases, the amplitude of a signal. The relationship of the input to the output of an amplifier—usually expressed as a function of the input frequency—is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain.

In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually voltage or current. In audio applications, amplifiers operate loudspeakers used in PA systems to make the human voice louder or play recorded music. Amplifiers may be classified according to the input (source) they are designed to amplify (such as a guitar amplifier, to perform with an electric guitar), the device they are intended to drive (such as a headphone amplifier), the frequency range of the signals (Audio, IF, RF, and VHF amplifiers, for example), whether they invert the signal (inverting amplifiers and non-inverting amplifiers), or the type of device used in the amplification (valve or tube amplifiers, FET amplifiers, etc.).

A related device that emphasizes conversion of signals of one type to another (for example, a light signal in photons to a DC signal in amperes) is a transducer, a transformer, or a sensor. However, none of these amplify power.

Figures of merit
The quality of an amplifier can be characterized by a number of specifications, listed below.

Gain
The gain of an amplifier is the ratio of output to input power or amplitude, and is usually measured in decibels. (When measured in decibels it is logarithmically related to the power ratio: G(dB)=10 log(Pout /(Pin)). RF amplifiers are often specified in terms of the maximum power gain obtainable, while the voltage gain of audio amplifiers and instrumentation amplifiers will be more often specified (since the amplifier's input impedance will often be much higher than the source impedance, and the load impedance higher than the amplifier's output impedance).

Example: an audio amplifier with a gain given as 20dB will have a voltage gain of ten (but a power gain of 100 would only occur in the unlikely event the input and output impedances were identical).

Bandwidth
The bandwidth (BW) of an amplifier is the range of frequencies for which the amplifier gives "satisfactory performance". The "satisfactory performance" may be different for different applications. However, a common and well-accepted metric are the half power points (i.e. frequency where the power goes down by half its peak value) on the power vs. frequency curve. Therefore bandwidth can be defined as the difference between the lower and upper half power points. This is therefore also known as the −3 dB bandwidth. Bandwidths (otherwise called "frequency responses") for other response tolerances are sometimes quoted (−1 dB, −6 dB etc.) or "plus or minus 1dB" (roughly the sound level difference people usually can detect).

A full-range audio amplifier will be essentially flat between 20 Hz to about 20 kHz (the range of normal human hearing). In minimalist amplifier design, the amp's usable frequency response needs to extend considerably beyond this (one or more octaves either side) and typically a good minimalist amplifier will have −3 dB points < 10 and > 65 kHz. Professional touring amplifiers often have input and/or output filtering to sharply limit frequency response beyond 20 Hz-20 kHz; too much of the amplifier's potential output power would otherwise be wasted on infrasonic and ultrasonic frequencies, and the danger of AM radio interference would increase. Modern switching amplifiers need steep low pass filtering at the output to get rid of high frequency switching noise and harmonics.

Efficiency
Efficiency is a measure of how much of the input power is usefully applied to the amplifier's output. Class A amplifiers are very inefficient, in the range of 10–20% with a max efficiency of 25%. Class B amplifiers have a very high efficiency but are impractical because of high levels of distortion (See: Crossover distortion). In practical design, the result of a tradeoff is the class AB design. Modern Class AB amps are commonly between 35–55% efficient with a theoretical maximum of 78.5%. Commercially available Class D switching amplifiers have reported efficiencies as high as 90%. Amplifiers of Class C-F are usually known to be very high efficiency amplifiers. The efficiency of the amplifier limits the amount of total power output that is usefully available. Note that more efficient amplifiers run much cooler, and often do not need any cooling fans even in multi-kilowatt designs. The reason for this is that the loss of efficiency produces heat as a by-product of the energy lost during the conversion of power. In more efficient amplifiers there is less loss of energy so in turn less heat.

In RF Power Amplifiers, such as cellular base stations and broadcast transmitters, specialist design techniques are used to improve efficiency. Doherty designs, which use a second transistor, can lift efficiency from the typical 15% up to 30-35% in a narrow bandwidth. Envelope Tracking designs are able to achieve efficiencies of up to 60%, by modulating the supply voltage to the amplifier in line with the envelope of the signal.

Linearity
An ideal amplifier would be a totally linear device, but real amplifiers are only linear within certain practical limits. When the signal drive to the amplifier is increased, the output also increases until a point is reached where some part of the amplifier becomes saturated and cannot produce any more output; this is called clipping, and results in distortion.

Some amplifiers are designed to handle this in a controlled way which causes a reduction in gain to take place instead of excessive distortion; the result is a compression effect, which (if the amplifier is an audio amplifier) will sound much less unpleasant to the ear. For these amplifiers, the 1 dB compression point is defined as the input power (or output power) where the gain is 1 dB less than the small signal gain.

Linearization is an emergent field, and there are many techniques, such as feedforward, predistortion, postdistortion, EER, LINC, CALLUM, cartesian feedback, etc., in order to avoid the undesired effects of the non-linearities.

Noise
This is a measure of how much noise is introduced in the amplification process. Noise is an undesirable but inevitable product of the electronic devices and components. The metric for noise performance of a circuit is Noise Factor. Noise Factor is the ratio of input signal to that of the output signal.

Output dynamic range
Output dynamic range is the range, usually given in dB, between the smallest and largest useful output levels. The lowest useful level is limited by output noise, while the largest is limited most often by distortion. The ratio of these two is quoted as the amplifier dynamic range. More precisely, if S = maximal allowed signal power and N = noise power, the dynamic range DR is DR = (S + N ) /N.[1]

Slew rate
Slew rate is the maximum rate of change of output variable, usually quoted in volts per second (or microsecond). Many amplifiers are ultimately slew rate limited (typically by the impedance of a drive current having to overcome capacitive effects at some point in the circuit), which may limit the full power bandwidth to frequencies well below the amplifier's small-signal frequency response.

Rise time
The rise time, tr, of an amplifier is the time taken for the output to change from 10% to 90% of its final level when driven by a step input. For a Gaussian response system (or a simple RC roll off), the rise time is approximated by:

tr * BW = 0.35, where tr is rise time in seconds and BW is bandwidth in Hz.

Settling time and ringing
Time taken for output to settle to within a certain percentage of the final value (say 0.1%). This is called the settle time, and is usually specified for oscilloscope vertical amplifiers and high accuracy measurement systems. Ringing refers to an output that cycles above and below its final value, leading to a delay in reaching final value quantified by the settling time above.

Overshoot
In response to a step input, the overshoot is the amount the output exceeds its final, steady-state value.

Stability factor
Stability is a major concern in RF and microwave amplifiers. The degree of an amplifier's stability can be quantified by a so-called stability factor. There are several different stability factors, such as the Stern stability factor and the Linvil stability factor, which specify a condition that must be met for the absolute stability of an amplifier in terms of its two-port parameters.

Electronic amplifiers
Main article: Electronic amplifier
There are many types of electronic amplifiers, commonly used in radio and television transmitters and receivers, high-fidelity ("hi-fi") stereo equipment, microcomputers and other electronic digital equipment, and guitar and other instrument amplifiers. Critical components include active devices, such as vacuum tubes or transistors. A brief introduction to the many types of electronic amplifier follows.

Power amplifier
The term "power amplifier" is a relative term with respect to the amount of power delivered to the load and/or sourced by the supply circuit. In general a power amplifier is designated as the last amplifier in a transmission chain (the output stage) and is the amplifier stage that typically requires most attention to power efficiency. Efficiency considerations lead to various classes of power amplifier: see power amplifier classes.

Operational amplifiers (op-amps)
An operational amplifier is an amplifier circuit with very high open loop gain and differential inputs which employs external feedback for control of its transfer function or gain. Although the term is today commonly applied to integrated circuits, the original operational amplifier design was implemented with valves.

Fully differential amplifiers (FDA)
A fully differential amplifier is a solid state integrated circuit amplifier which employs external feedback for control of its transfer function or gain. It is similar to the operational amplifier but it also has differential output pins.

[edit] Video amplifiers
These deal with video signals and have varying bandwidths depending on whether the video signal is for SDTV, EDTV, HDTV 720p or 1080i/p etc.. The specification of the bandwidth itself depends on what kind of filter is used and which point (-1 dB or -3 dB for example) the bandwidth is measured. Certain requirements for step response and overshoot are necessary in order for acceptable TV images to be presented.

Oscilloscope vertical amplifiers
These are used to deal with video signals to drive an oscilloscope display tube and can have bandwidths of about 500 MHz. The specifications on step response, rise time, overshoot and aberrations can make the design of these amplifiers extremely difficult. One of the pioneers in high bandwidth vertical amplifiers was the Tektronix company.

Distributed amplifiers
These use transmission lines to temporally split the signal and amplify each portion separately in order to achieve higher bandwidth than can be obtained from a single amplifying device. The outputs of each stage are combined in the output transmission line. This type of amplifier was commonly used on oscilloscopes as the final vertical amplifier. The transmission lines were often housed inside the display tube glass envelope.

Microwave amplifiers:
Travelling wave tube (TWT) amplifiers
Used for high power amplification at low microwave frequencies. They typically can amplify across a broad spectrum of frequencies; however, they are usually not as tunable as klystrons.

Klystrons
Very similar to TWT amplifiers, but more powerful and with a specific frequency "sweet spot". They generally are also much heavier than TWT amplifiers, and are therefore ill-suited for light-weight mobile applications. Klystrons are tunable, offering selective output within their specified frequency range.

Musical instrument (audio) amplifiers
An audio amplifier is usually used to amplify signals such as music or speech.

Other amplifier types:

Carbon microphone
One of the first devices used to amplify signals was the carbon microphone (effectively a sound-controlled variable resistor). By channeling a large electric current through the compressed carbon granules in the microphone, a small sound signal could produce a much larger electric signal. The carbon microphone was extremely important in early telecommunications; analog telephones in fact work without the use of any other amplifier. Before the invention of electronic amplifiers, mechanically coupled carbon microphones were also used as amplifiers in telephone repeaters for long distance service.

Magnetic amplifier
Main article: magnetic amplifier
A magnetic amplifier is a transformer-like device that makes use of the saturation of magnetic materials to produce amplification. It is a non-electronic electrical amplifier with no moving parts. The bandwidth of magnetic amplifiers extends to the hundreds of kilohertz.

Rotating electrical machinery amplifier
A Ward Leonard control is a rotating machine like an electrical generator that provides amplification of electrical signals by the conversion of mechanical energy to electrical energy. Changes in generator field current result in larger changes in the output current of the generator, providing gain. This class of device was used for smooth control of large motors, primarily for elevators and naval guns.

Johnsen-Rahbek effect amplifier
The earliest form of audio power amplifier was Edison's "electromotograph" loud-speaking telephone, which used a wetted rotating chalk cylinder in contact with a stationary contact. The friction between cylinder and contact varied with the current, providing gain. Edison discovered this effect in 1874, but the theory behind the Johnsen-Rahbek effect was not understood until the semiconductor era.

Mechanical amplifiers
Mechanical amplifiers were used in the pre-electronic era in specialized applications. Early autopilot units designed by Elmer Ambrose Sperry incorporated a mechanical amplifier using belts wrapped around rotating drums; a slight increase in the tension of the belt caused the drum to move the belt. A paired, opposing set of such drives made up a single amplifier. This amplified small gyro errors into signals large enough to move aircraft control surfaces. A similar mechanism was used in the Vannevar Bush differential analyzer.

Optical amplifiers

Optical amplifiers amplify light through the process of stimulated emission.

Miscellaneous types
There are also mechanical amplifiers, such as the automotive servo used in braking.
Relays can be included under the above definition of amplifiers, although their transfer function is not linear (that is, they are either open or closed).
Also purely mechanical manifestations of such digital amplifiers can be built (for theoretical, didactical purposes, or for entertainment), see e.g. domino computer.
Another type of amplifier is the fluidic amplifier, based on the fluidic triode.

Thapar University, (a private university) previously known as Thapar Institute of Engineering and Technology (before 2007)- was founded in 1956 by Karam Chand Thapar of the Thapar Group of companies. Situated in the city of Patiala in Punjab, it is one of north India's premier institutes of engineering and technology. Thapar University offers courses at the bachelors, masters and Ph. D., level in several disciplines of engineering and technology. Over 10,000 students have graduated from Thapar University. In a survey conducted by DataQuest, it was rated among the top 15 engineering schools in India.

Thaparian culture is based on overall growth. The students keep themselves busy in some activity or the other. These include various academic, cultural and literary activities. At the same time discipline is maintained by the institute to maintain focus on professional growth. The annual college festival "Aranya" has been quite a success among all colleges in Punjab. The institute also has an literary-cultural festival by the name "Saturnalia" held every year in March.

Campus
Thapar Institute of Engineering & Technology (TIET) is the major Institute located in the 250-acre (1.0 km2) Thapar Technology Campus (TTC) in the historic city of Patiala(Punjab). Other prominent institute is the L.M.Thapar School of Management. It is one of the premier institute of management in India which facilitate young minds having traits of management and technology blended together to form a perfect suit for current scenario.Other than Thapar Insitute of Engineering and Technology, the Thapar Technology campus also has Thapar Polytechnic, Thapar Centre for Industrial Research and Development


Academics
The Institute offers undergraduate programmes leading to Bachelor of Engineering (B.E.) degree and postgraduate programmes leading to Master of Engineering (M.E.), Master of Software Engineering, Master of Technology (M.Tech.), Master of Science (M.Sc.), Master of Computer Applications (M.C.A.), Doctor of Philosophy (Ph.D.), and Doctor of Science (D.Sc.) degrees. In addition, industry oriented and practice based Master's Degree programmes in various disciplines are offered in collaboration with TTTI, Chandigarh.Students have to take the AIEEE Exam in order to get admission into Thapar University for the undergraduate program and students take the MET exam for admission into graduate/postgraduate programs.

Recently, the institute has started MBA as well as dual MBA+BE five year integrated programme.

A number of Thapar graduates have made it to the IES. D.S.Kapur, a civil graduate of 1978 batch was all India IES Topper. Apart from the technical field, Thaparians have excelled in the administrative services.IAS and other Central Services also caught the fancy of the Thaparians, after a 1983-batch electronics and telecommunications graduate K. B. S. Sidhu set the trend, finishing among the IAS toppers in the 1984 batch.

The Management school i.e. L.M.Thapar School of Management has also marched its steps towards excellence. The students, most of whom have prior professional work experience, have been crossing the boundaries and are frequently visiting top institutes like IIM Bangalore and IIM Ahmedabad for project work and dissertation purposes.

LMTSOM is also hugely benefited by the presence of Science and Technology Entrepreneurs' Park which is a part of Thapar University. Management Research and Innovation Forum (MRIF) has been set up specifically for promoting management based research. It is looking forward for collaboration with prominent research centers across the world.


Activities
The most pronounced aspect of education at TIET is the extensive participation of students in extracurricular activities. These are carried out by various societes headed by one member of the faculty. MUDRA (Music and Dramatics Society) conducts its cultural nite and an inter-year cultural competition IZHAAR. LITSOC (Literary Society) is another popular society which conducts the annual inter-hostel literary festival ACUMEN. Other societes include ACID, CCS, IEEE (TIET chapter), SAE (TIET chapter), etc and various departmental societies.

The institute also organises a literary-cultural festival by the name Saturnalia held every year in March. This is complemented by a technical festival ARANYA, normally held in October-November each year. These events involve large participation from prestigious institutes from all over India and is graced by the presence of artists like JAL, KAILASH KHER, EUPHORIA etc. The students publish their own magazines, AVANT GARDE, which represents the views of the students along with addressing issues of the youth and nation, and ECLECTIZA, which emphasizes on the latest advancements and applications in the field of science, thus catering to the thirst of all the techees in the institute. The theatrical society is also one of the most active societies on campus.


History
Thapar Institute of Engineering & Technology (TIET) was established in 1956 with the goal of providing undergraduate and postgraduate education in Engineering and Technology, a close interaction with industry, and a strong emphasis on research. TIET has grown steadily in size and activities during the last four decades of its existence. Over 10,000 engineers have graduated from TIET. In recognition of its achievements, TIET was granted full autonomy and the status of a Deemed University in 1985 by the University Grants Commission.


The Thapar Group
Ballarpur Industries Limited, Bilt Paper Holdings Limited, Orient Engineering and Commercial Company Limited, The Pioneer, Sohna Stud Farm Pvt Limited, TT & G Trading Pvt Limited, iBilt Technologies Limited, Bilt Middle East Pvt Limited, Himalayan Hideaways Pvt Limited, Karam Chand Thapar & Bros Limited, Lavasa Corporation Limited and KCT Papers Limited are some of the Thapar group companies. Other Thapar group companies and concerns in India are, the Pioneer newspaper, Thapar house on Janpath lane, Tiger Bay restaurant, Global Green. The Crompton Greaves, Greaves Cotton and JCT group is with MM Thapar. His son Samir Thapar runs JCT Mills and JCT Limited and his son Arjun Thapar runs JCT electronics. Bilt Chemicals and Bharat Starch was vested with BM Thapar and hence would be under his sons Gautam and Karan Thapar's control. Gautam has also been named by L.M. Thapar as the successor of his group of companies. IM Thapar's son Vikram Thapar looks after his group which includes the Calcutta based coal trading business K.C Thapar started out with and prawn farms for prawn exports as well as the Tiger Bay restaurant chain. Thapar Institute of Engineering and Tech. is a group project funded by the Thapar's. JCT Mills FC is a Football club of JCT Mills.


Future Expansions
Patiala-based Thapar University (formerly Thapar Institute of Engineering and Technology) plans to set up two campuses over the next five years — one on a 14-acre campus near Chandigarh by 2010, and another in Chhattisgarh.It plans to offer courses in management and engineering at its proposed Chandigarh campus and will invest around Rs 30-35 crore.

The existing 250-acre campus in Patiala has a student strength of 4,600 and can accommodate around 6,000 students, according to Abhijit Mukherjee, director, Thapar University.

“The initial student strength at the new campus would be 200 but would go up to 2,000 eventually. Also, we will need 15-20 faculty members for the Chandigarh campus. The Patiala campus’ management school will be shifted to Chandigarh and the existing engineering courses will also be expanded. As of now, there are 300 students in management and 3,000 in engineering,” said Mukherjee.

As for a campus in central India, Thapar University has been invited by the government of Chhattisgarh to set up a large campus with residential facilities like the Patiala one .

The university is also keen on introducing new inter-disciplinary courses. Recently, it added courses in biology and biotechnology.

On the foreign-alliances front, it already has a tie-up with the University of Waterloo in Canada, Western Ontario and Virginia Tech, and is in talks with more institutes to have exchange programmes and research collaboration

A router is a networking device whose software and hardware are usually tailored to the tasks of routing and forwarding information. For example, on the Internet, information is directed to various paths by routers.
Routers connect two or more logical subnets, which do not necessarily map one-to-one to the physical interfaces of the router. The term "layer 3 switch" often is used interchangeably with router, but switch is a general term without a rigorous technical definition. In marketing usage, it is generally optimized for Ethernet LAN interfaces and may not have other physical interface types. In comparison, a network hub does not do any routing, instead every packet it receives on one network line gets forwarded to all the other network lines.

Routers operate in two different planes:
Control plane, in which the router learns the outgoing interface that is most appropriate for forwarding specific packets to specific destinations,
Forwarding plane, which is responsible for the actual process of sending a packet received on a logical interface to an outbound logical interface.

Control plane
Control plane processing leads to the construction of what is variously called a routing table or routing information base (RIB). The RIB may be used by the Forwarding Plane to look up the outbound interface for a given packet, or, depending on the router implementation, the Control Plane may populate a separate forwarding information base (FIB) with destination information. RIBs are optimized for efficient updating with control mechanisms such as routing protocols, while FIBs are optimized for the fastest possible lookup of the information needed to select the outbound interface.
The Control Plane constructs the routing table from knowledge of the up/down status of its local interfaces, from hard-coded static routes, and from exchanging routing protocol information with other routers. It is not compulsory for a router to use routing protocols to function, if for example it was configured solely with static routes. The routing table stores the best routes to certain network destinations, the "routing metrics" associated with those routes, and the path to the next hop router.
Routers do maintain state on the routes in the RIB/routing table, but this is quite distinct from not maintaining state on individual packets that have been forwarded.

Forwarding plane

For the pure Internet Protocol (IP) forwarding function, router design tries to minimize the state information kept on individual packets. Once a packet is forwarded, the router should no longer retain statistical information about it. It is the sending and receiving endpoints that keeps information about such things as errored or missing packets.
Forwarding decisions can involve decisions at layers other than the IP internetwork layer or OSI layer 3. Again, the marketing term switch can be applied to devices that have these capabilities. A function that forwards based on data link layer, or OSI layer 2, information, is properly called a bridge. Marketing literature may call it a layer 2 switch, but a switch has no precise definition.
Among the most important forwarding decisions is deciding what to do when congestion occurs, i.e., packets arrive at the router at a rate higher than the router can process. Three policies commonly used in the Internet are Tail drop, Random early detection, and Weighted random early detection. Tail drop is the simplest and most easily implemented; the router simply drops packets once the length of the queue exceeds the size of the buffers in the router. Random early detection (RED) probabilistically drops datagrams early when the queue exceeds a configured size. Weighted random early detection requires a weighted average queue size to exceed the configured size, so that short bursts will not trigger random drops.

Types of routers

Routers may provide connectivity inside enterprises, between enterprises and the Internet, and inside Internet Service Providers (ISP). The largest routers (for example the Cisco CRS-1 or Juniper T1600) interconnect ISPs, are used inside ISPs, or may be used in very large enterprise networks. The smallest routers provide connectivity for small and home offices.

Routers for Internet connectivity and internal use
Routers intended for ISP and major enterprise connectivity will almost invariably exchange routing information with the Border Gateway Protocol (BGP). RFC 4098[4] defines several types of BGP-speaking routers:
Edge Router: Placed at the edge of an ISP network, it speaks external BGP (eBGP) to a BGP speaker in another provider or large enterprise Autonomous System (AS).
Subscriber Edge Router: Located at the edge of the subscriber's network, it speaks eBGP to its provider's AS(s). It belongs to an end user (enterprise) organization.
Inter-provider Border Router: Interconnecting ISPs, this is a BGP speaking router that maintains BGP sessions with other BGP speaking routers in other providers' ASes.
Core router: A router that resides within the middle or backbone of the LAN network rather than at its periphery.
Within an ISP: Internal to the provider's AS, such a router speaks internal BGP (iBGP) to that provider's edge routers, other intra-provider core routers, or the provider's inter-provider border routers.
"Internet backbone:" The Internet does not have a clearly identifiable backbone, as did its predecessors. See default-free zone (DFZ). Nevertheless, it is the major ISPs' routers that make up what many would consider the core. These ISPs operate all four types of the BGP-speaking routers described here. In ISP usage, a "core" router is internal to an ISP, and used to interconnect its edge and border routers. Core routers may also have specialized functions in virtual private networks based on a combination of BGP and Multi-Protocol Label Switching (MPLS).
Routers are also used for port forwarding for private servers.

[edit] Small Office Home Office (SOHO) connectivity
Main article: Residential gateway
Residential gateways (often called routers) are frequently used in homes to connect to a broadband service, such as IP over cable or DSL. A home router may allow connectivity to an enterprise via a secure Virtual Private Network.
While functionally similar to routers, residential gateways use port address translation in addition to routing. Instead of connecting local computers to the remote network directly, a residential gateway makes multiple local computers appear to be a single computer.

Enterprise routers
All sizes of routers may be found inside enterprises. The most powerful routers tend to be found in ISPs and academic & research facilities. Large businesses may also need powerful routers.
A three-layer model is in common use, not all of which need be present in smaller networks.

Access
Access routers, including SOHO, are located at customer sites such as branch offices that do not need hierarchical routing of their own. Typically, they are optimized for low cost.

Distribution
Distribution routers aggregate traffic from multiple access routers, either at the same site, or to collect the data streams from multiple sites to a major enterprise location. Distribution routers often are responsible for enforcing quality of service across a WAN, so they may have considerable memory, multiple WAN interfaces, and substantial processing intelligence.
They may also provide connectivity to groups of servers or to external networks. In the latter application, the router's functionality must be carefully considered as part of the overall security architecture. Separate from the router may be a Firewalled or VPN concentrator, or the router may include these and other security functions.
When an enterprise is primarily on one campus, there may not be a distinct distribution tier, other than perhaps off-campus access. In such cases, the access routers, connected to LANs, interconnect via core routers.

A full list of Computer networking devices are units that mediate data in a computer network. Computer networking devices are also called network equipment, Intermediate Systems (IS) or InterWorking Unit (IWU). Units which are the last receiver or generate data are called hosts or data terminal equipment.

List of computer networking devices
Common basic networking devices:
Gateway: device sitting at a network node for interfacing with another network that uses different protocols. Works on OSI layers 4 to 7.
Router: a specialized network device that determines the next network point to which to forward a data packet toward its destination. Unlike a gateway, it cannot interface different protocols. Works on OSI layer 3.
Bridge: a device that connects multiple network segments along the data link layer. Works on OSI layer 2.
Switch: a device that allocates traffic from one network segment to certain lines (intended destination(s)) which connect the segment to another network segment. So unlike a hub a switch splits the network traffic and sends it to different destinations rather than to all systems on the network. Works on OSI layer 2.
Hub: connects multiple Ethernet segments together making them act as a single segment. When using a hub, every attached device shares the same broadcast domain and the same collision domain. Therefore, only one computer connected to the hub is able to transmit at a time. Depending on the network topology, the hub provides a basic level 1 OSI model connection among the network objects (workstations, servers, etc). It provides bandwidth which is shared among all the objects, compared to switches, which provide a dedicated connection between individual nodes. Works on OSI layer 1.
Repeater: device to amplify or regenerate digital signals received while setting them from one part of a network into another. Works on OSI layer 1.
Some hybrid network devices:
Multilayer Switch: a switch which, in addition to switching on OSI layer 2, provides functionality at higher protocol layers.
Protocol Converter: a hardware device that converts between two different types of transmissions, such as asynchronous and synchronous transmissions.
Bridge Router(Brouter): Combine router and bridge functionality and are therefore working on OSI layers 2 and 3.
Digital media receiver: Connects a computer network to a home theatre
Hardware or software components that typically sit on the connection point of different networks, e.g. between an internal network and an external network:
Proxy: computer network service which allows clients to make indirect network connections to other network services
Firewall: a piece of hardware or software put on the network to prevent some communications forbidden by the network policy
Network Address Translator: network service provide as hardware or software that converts internal to external network addresses and vice versa
Other hardware for establishing networks or dial-up connections:
Multiplexer: device that combines several electrical signals into a single signal
Network Card: a piece of computer hardware to allow the attached computer to communicate by network
Modem: device that modulates an analog "carrier" signal (such as sound), to encode digital information, and that also demodulates such a carrier signal to decode the transmitted information, as a computer communicating with another computer over the telephone network
ISDN terminal adapter (TA): a specialized gateway for ISDN
Line Driver: a device to increase transmission distance by amplifying the signal. Base-band networks only.
Network Device Connectivity

The United States dollar (sign: $; code: USD) is the unit of currency of the United States The U.S. dollar is normally abbreviated as the dollar sign, $, or as USD or US$ to distinguish it from other dollar-denominated currencies and from others that use the $ symbol. It is divided into 100 cents (200 half-cents prior to 1857).
The U.S. dollar is the currency most used in international transactions.Although U.S. dollar is a fiat currency, several countries use it as their official currency, and in many others it is the de facto currency.

Overview
The U.S. dollar bill uses the decimal system, consisting of 100 equal cents (symbol ¢). In another division, there are 1,000 mills or ten dimes to a dollar, or 4 quarters to a dollar. However, only cents are in everyday use as divisions of the dollar; "dime" is used solely as the name of the coin with the value of 10¢, while "eagle" and "mill" are largely unknown to the general public, though mills are sometimes used in matters of tax levies and gasoline prices. When currently issued in circulating form, denominations equal to or less than a dollar are emitted as U.S. coins while denominations equal to or greater than a dollar are emitted as Federal Reserve notes (with the exception of gold, silver and platinum coins valued up to $100 as legal tender, but worth far more as bullion). Both one-dollar coins and notes are produced today, although the note form is significantly more common. In the past, "paper money" was occasionally issued in denominations less than a dollar (fractional currency) and gold coins were issued for circulation up to the value of $20 (known as the "double eagle," discontinued in the 1930s). The term eagle was used in the Coinage Act of 1792 for the denomination of ten dollars, and subsequently was used in naming gold coins. In 1854, James Guthrie, then Secretary of the Treasury, proposed creating $100, $50 and $25 gold coins, which were referred to as a "Union," "Half Union," and "Quarter Union," thus implying a denomination of 1 Union = $100.
Today, USD notes are made from cotton fiber paper, unlike most common paper, which is made of wood fiber.

U.S. coins are produced by the United States Mint. U.S. dollar banknotes are printed by the Bureau of Engraving and Printing, and, since 1914, have been issued by the Federal Reserve. The "large-sized notes" issued before 1928 measured 7.42 inches (188 mm) by 3.125 inches (79.4 mm); small-sized notes, introduced that year, measure 6.14 inches (156 mm) by 2.61 inches (66 mm) by 0.0043 inches (0.11 mm).

Etymology
The name Thaler (from German thal, or nowadays usually Tal, "valley", cognate with "dale" in English) came from the German coin Guldengroschen ("great guilder", being of silver but equal in value to a gold guilder), minted from the silver from a rich mine at Joachimsthal (St. Joachim's Valley, now Jáchymov) in Bohemia (then part of the Holy Roman Empire, now part of the Czech Republic).

The colloquialism buck (much like the British term "quid") is often used to refer to dollars of various nations, including the U.S. dollar. This term, dating to the 18th century, may have originated with the colonial fur trade. Greenback is another nickname originally applied specifically to the 19th century Demand Note dollars created by Abraham Lincoln to finance the costs of the Civil War for the North. The original note was printed in black and green on the back side. It is still used to refer to the U.S. dollar (but not to the dollars of other countries), other well known names of the dollar as a whole in denominations include "greenmail", "green", and "Dead Presidents", (the last one due to the nature of late presidents being the most represented on the bills).
Grand, sometimes shortened to simply G, is a common term for the amount of $1,000. The suffix k (from "kilo-") is also commonly used to denote this amount (such as "$10k" to mean $10,000). In street slang, when someone refers to a denomination "large", they are usually referring to any amount of $10,000, such as "fifty large", meaning $50,000. Banknotes' nicknames are the same as their values (such as five, twenty, etc.) The $5 bill has been referred to as a "fin" or a "fiver" or a "five-spot;" the $10 bill as a "sawbuck," a "ten-spot," or a "Hamilton"; the $20 bill as a "double sawbuck," or a "Jackson"; the $1 bill is sometimes called a "single," or a "buck," the $2 bill a "deuce," "Jefferson," or a "T.J." and the $100 bill is nicknamed a "Benjamin," "Benji," or "Franklin" (after Benjamin Franklin, who is pictured on the note), C-note (C being the Roman numeral for 100), Century Note, or "bill" ("two bills" being $200, etc.). The dollar has also been referred to as a "bone" or "bones" (i.e. twenty bones is equal to $20) or a "bean". Occasionally these will be referred to as "dead presidents," although neither Hamilton ($10) nor Franklin ($100) was President. $1000 notes are occasionally referred to as 'large' in banking ("twenty large" being $20,000, etc.). The newer designs are sometimes referred to as "Bigface" bills, or "Monopoly Money".
In Panama, the translation of buck is palo (lit. stick); a nickname for the balboa. For example: "Esto vale 20 palos" ("This is worth 20 bucks"). In Puerto Rico (as well as by Puerto Ricans living in the continental U.S.), the dollar may be referred to as a peso. In French-speaking areas of Louisiana, the dollar is referred to as a piastre which is pronounced "pee-as", and cents by the French holdhover of sous, pronounced "soo." In Mexico, prices in dollars are referred in some places to as "en americano" ("in American"): one would ask "¿Cuánto cuesta en americano?" ("How much does it cost 'in American'?") and would receive the U.S. dollar price in the Spanish language. (In Mexico, peso is used primarily for the Mexican peso.) In Peru, a nickname for the U.S. dollar is coco, which is a pet name for Jorge (George in Spanish), a reference to the portrait of George Washington on the $1 note.

Yahoo! Mail (shortened as Y! Mail) is a web-based email service created by Yahoo! and is also owned by them. It was inaugurated in 1997. Yahoo! Mail serves over 280 million users as of 2009. It is currently the largest web-based email service, after its competitor Windows Live Hotmail.
Currently, Yahoo! offers two versions of Yahoo! Mail: an Outlook-like Ajax interface powered by Yahoo!'s Zimbra introduced in 2007, as well as the traditional static-web-page "Yahoo! Mail Classic," which continues the availability of the simpler 1997-2006 interface for the brand's considerable installed base of users. In early 2008, Yahoo! started offering unlimited mail storage even to its non-paying users, in response to heated competition in the free-webmail market segment.
In June 27, 2009, Yahoo! Messenger was integrated with Yahoo! Mail Beta. This means that Messenger and Mail are together at one place. You can connect instantly with friends that are online, switch seamlessly between email and chat, and no download or setup required. This new feature is not available for Mail Classic.

History
The history of Yahoo! Mail began with JoJo Healy, Yahoo!'s resident investment banker since November 1996, who has dealt with every Yahoo! acquisition since it was created. "No one knows your business like your employees," he said. According to him, the main question was always to consider whether to "build, buy or rent." The answer really depended on the growth of competitors and the current position of the company. The main reason to buy things was to gain speed to market.
The growth in the number of Internet users eventually boosted the e-mail technology, but also created a very competitive environment where the winner was the first company to launch a successful e-mail service and attract potential users. E-mail became one of the most important features of a Web company as it would mean regular visits from e-mail users to the website.
When Hotmail and Mirabilis (the creator of the instant messenger ICQ) were looking to be bought, Yahoo! was the first company to which both made offers. Yahoo!, however, passed on both companies as they were too expensive for Yahoo! at that time. In the end, Microsoft ended up buying Hotmail for $400 million and AOL bought Mirabilis for $288 million.
Later there was also to be another battle to acquire the online communications company Four11. Yahoo! made a deal with the company for co-branded white pages. Marvin Gavin, who worked at Four11 as director of international business development said, "We always had a bias about being acquired by Yahoo! They were more entrepreneurial than Microsoft. We had a great cultural fit – it made a lot of sense." The real point in acquiring Four11 was that in March 1997, the company had launched Rocketmail, a webmail system that could be offered to users. In the end, Yahoo! concluded a deal with Four11 for $96 million. Yahoo! announced the acquisition on October 8, 1997, very close to the time that Yahoo! Mail was launched. Yahoo! Mail resulted from an acquisition rather than internal platform development because, as Healy said, "Hotmail was growing at thousands and thousands users per week. We did an analysis. For us to build, it would have taken four to six months, and by then, so many users would have taken an e-mail account. The speed of the market was critical."
The transition to Yahoo! Mail was not easy for many Rocketmail users at first. Yahoo! released various help pages to try and help these users. Soon after, on March 21, 2002, Yahoo! cut free software client access and introduced the $29.99 per year Mail Forwarding Service. Mary Osako, a Yahoo! Spokeswoman, told CNET, "For-pay services on Yahoo!, originally launched in February 1999, have experienced great acceptance from our base of active registered users, and we expect this adoption to continue to grow."
During the summer of 2002, the Yahoo! network was gradually redesigned. On July 2, Yahoo.com was redesigned and it was announced that other services like Yahoo! Mail would enter the same process. Along with this new design, new features were to be implemented, including new navigation tools, such as drop-down menus in DHTML and different category tabs, and a new user-customizable color scheme.
In November of the same year, Yahoo! launched another paid service: Yahoo! Mail Plus.[9] Yahoo! Mail Plus offered a number of new features, including:
25 megabytes of e-mail storage
10 megabyte message size limit
Ability to send up to 10 attachments per e-mail
POP Access and Forwarding
Archiving of e-mail messages to a hard drive for offline access
Ability to send messages from Yahoo! Mail using other e-mail domains
200 blocked addresses and 50 filters to help screen unsolicited e-mails
No promotional taglines in messages
No account expiration.
The launch of Yahoo! Mail Plus is part of Yahoo!'s strategic initiative to offer premium services that deliver innovative, reliable and relevant services to consumers", said Geoff Ralston, senior vice president, Yahoo! Network Services, and creator of the original Yahoo! Mail technology in 1997. "In just five years, Yahoo! Mail has grown from one million to tens of millions of users, illustrating how consumers have made e-mail an essential part of their daily lives. Through Yahoo! Mail Plus, Yahoo! continues to demonstrate leadership and innovation by offering consumers the industry's most complete and powerful e-mail solution.
On April 1, 2004, Google announced a free webmail with 1 gigabyte of storage. Though Gmail, Google's e-mail service, offered a large amount of storage, its invitation-only accounts kept the other webmail services at the forefront. Most of the major webmail providers like Yahoo! Mail, Hotmail, and AOL followed Google's lead and increased their mailbox storage considerably. Yahoo! was the first provider to announce 100 MB of storage for basic accounts and 2 GB of storage for premium users. Determined not to lose customers, Yahoo! Mail then countered Hotmail and Google by increasing the storage quota of its free email accounts to 1 GB, and eventually removing a storage limit altogether and allowing unlimited storage.
On July 9, 2004, Yahoo! acquired Oddpost, a strong webmail offering that simulated a desktop email client like Microsoft Outlook. Oddpost had new innovative features such as drag-and-drop support, right-click menus, RSS feeds, and a preview pane, but it also had incredible speed, using e-mail caching to shorten response time, and many of these features were incorporated into an updated Yahoo! Mail service.
On August 30, 2007, Walter Mossberg wrote in the Wall Street Journal that Yahoo! will be releasing the new version over the next few weeks.

New Yahoo! Mail
The New Yahoo! Mail has a revised interface which contains mostly Ajax (JavaScript and XML). It is very similar to a heavy mail client like Outlook (Drag and drop capabilities, Mail search, tabs). The new Yahoo! Mail comes with an integrated Messenger; as a result, users are able to chat with Yahoo! Messenger and Windows Live Messenger contacts while reading their email.
As of August 26, 2007, the new Yahoo! Mail came out of Beta period. The development of what has now become the new interface, started since July 2004, although it is possible other prototypes were in development before then. It is currently compatible with Internet Explorer 7, Firefox and Safari as well as Camino and other Gecko based browsers. (As part of Yahoo!'s plan to eventually upgrade all of their sites to be compatible with Firefox). Although it is usable under Opera, there are slight visual problems regarding the layout.[citation needed]
In September 2005 Yahoo! began beta testing a significantly enhanced version of their e-mail service, based on Ajax scripting acquired from Oddpost, along with new philosophical approaches to email, including the Oddpost design philosophy (which Google made heavy use of in Gmail):
Given that the All-New Yahoo! Mail is based on Oddpost, it features the same underlying code base, including the implementation of this design concept to minimize the amount of data sent during an email session by creating a JavaScript UI engine on the client side and sending "Datapacks" instead of reloading the whole interface on every click like a traditional webmail service (e.g. traditional Yahoo! Mail). This makes the service much faster than its counterparts.
However, unlike Oddpost, All-New Yahoo! Mail runs on a variety of OSes and browsers, and runs perfectly under Internet Explorer 7 and Firefox. On February 13, 2008, Yahoo! announced on the Yahoo! Mail blog that it would begin supporting version 3 of the Mac OS X default browser Safari, as well as the Mac versions of Firefox.
The look and feel of the new version is designed to mimic a desktop e-mail client, and it offers unlimited storage space, tabbed emails, RSS feeds, drag-and-drop capabilities, advanced keyboard shortcuts, advanced search, integration with Yahoo! Calendar and Messenger, Domain Keys, address auto-complete and more.
The All-New Yahoo! Mail also has a mascot called Liam, who is basically a little boy, who is shown in the "Help" panel as well as in the loading screen. Liam is "mail" spelled backward.

Introduction
Wide-scale beta release of the new version was introduced in late 2006. In November 2006, the Mail Beta team announced plans of integrating Yahoo! Messenger functionality with the Mail Beta interface. This service was deployed in March, 2007.
Public and critical reaction to the new product has been positive,although a number of users have encountered speed issues, which can render the client very difficult to use, especially on older hardware. Each update to the Beta has included significant speed improvements, and the Mail Beta team is still focused on improving speed issues. For example, the earlier versions of Mail Beta featured a loading screen every time you access mail. However, with various updates Mail Beta now appears immediately (no loading screen).
On August 26, 2007, Yahoo! Mail left beta and now the two versions which exist are known as Yahoo! Mail (the default interface) and Yahoo! Mail Classic (an updated version of the current interface). The announcement was made on the Yahoo! Mail blog by John Kremer, Vice President, Yahoo! Mail and the major improvements relate to intelligent shortcuts, SMS Messaging, and improved search.
The final version was released on October 9, 2007.

Features
Some of the new or improved features included in the New Yahoo! Mail are:
Yahoo! Calendar integration
Yahoo! Messenger integration
Windows Live Messenger integration
SMS Messaging in certain countries
Keyboard shortcuts
Unlimited storage
Improved search facilities
Applications(Piknic, Flickr, Wordpress and more)
In addition, an Easter egg was added called a Subject-O-Matique. This hidden feature displays a random message in the subject line when the subject button is clicked. The messages range from cultural references ("I AM the walrus") to sarcastic ("If you don't tell lies, at least you don't have to remember what you have said.") to unusual and outlandish ("the art of driving a giant, nuclear powered duck").
On December 15, 2008, Yahoo! Mail introduced features to make it more social.

Yahoo! Mail Classic

Mail Classic is, since the launch of the new Yahoo! Mail, an optional secondary User Interface for Yahoo! Mail. Although the AJAX interface is the default one, Classic is available for users who prefer to use the legacy style interface. In March 2009, Yahoo! Mail Classic integrated Yahoo! Messenger is its interface so people can chat and email at the same time.

Yahoo! Mail has the following features:

Free version
Unlimited Mail Storage.
10 MB attachments (20 MB for the Plus version)
100 filters to automatically sort incoming messages (200 filters for the Plus version)
Protection against spam and viruses. (See: DomainKeys)
Advertising is displayed on the screen while working with the e-mail account, but text ads are not within the emails themselves, and they are no longer added to the footer of outgoing messages.
POP3 support, Mail Forwarding facility, and SMTP support in some countries (but not in the US).
Accounts not logged into for four months get deactivated (The account can be retrieved but all stored data, such as emails, are lost).
Early in 2006, Yahoo! Mail introduced aliases to its repertoire of features. Users could now add a (single) alias username containing a dot character for a pre-existing account.
The Chinese version of Yahoo! Mail offers 3.5 GB quota and 20 MB attachments. [3]
Some users get features added if they are long time users.
Latest beta of Yahoo! Zimbra desktop allows for all Yahoo! users to use the software
Users can get around the Web browser access restriction by using software that simulates a POP3 server to which the e-mail application connects. YPOPs! and FreePOPs are examples of free software applications that allow e-mail clients access to webmail (including Yahoo! Mail) services through POP3.
Another way of getting POP3 access without signing up for the paid mail plans is signing up for Yahoo! Delivers, a service which sends the user promotional email messages. According to the Yahoo! Mail help pages,"Yahoo! offers POP access to Yahoo! Mail as a free feature exclusively for Yahoo! Delivers members".

Business
Yahoo! Business E-mail is a combination of all of their e-mail services with 10 distinct accounts each with the same features of the plus version and personalized domain name and e-mail address. Accounts can be managed by an administrator. There is $25 set-up fee and $9.99 monthly fee to use this service. Yahoo! is still working out how to allow their business mail clients to access their email accounts from certain mobile smart phones and PDAs. According to Yahoo!'s website, for now, you can access your Business Email only from a BlackBerry. Yahoo! says it plans to make business email compatible with more devices in the future. Yahoo!'s Business mail is also not compatible with the latest releases of Leopard/Safari. It is compatible with the latest releases of Safari - but only when running on Tiger.
Unlimited Mail Storage
10 E-mail quota.
Additionally, a user can pay $35 per year to have up to five custom e-mail addresses and a domain name.
Yahoo! Mail underlines addresses and phone numbers in emails and allows the user to add them to the address book.

Ymail and Rocketmail
Yahoo! Mail Vice President John Kremer on June 19, 2008 announced the tripling of the size of its free online e-mail service with the launching of 2 domains as options for its 266 million users of "@yahoo.com" addresses: the new, simpler e-mail addresses ending in ymail.com and rocketmail.com. Rocketmail has a "hip retro feel" since it is a resurrected email address of a 1997 Yahoo service.[20] E-mail under the ymail and rocketmail domains will offer all the same features as the Yahoo domain, with an unlimited amount of storage capacity, with ability to instant message from within their e-mail inbox and spam and virus protection.

Aircel is a mobile phone service provider in India. It offers both prepaid and postpaid GSM cellular phone coverage throughout Tamil Nadu (including Chennai), Andhra Pradesh, Assam, North East India, Orissa, West Bengal (including Kolkata), Jammu and Kashmir, Bihar, Kerala, Himachal Pradesh & Mumbai. Aircel was founded by NRI businessman C Sivasankaran. [1] Aircel is a joint venture between Maxis Communications Berhad of Malaysia and Apollo Hospital Enterprise Ltd of India. It is India’s seventh largest GSM mobile service provider with a subscriber base of over 18 million. It has a market share of 6.3% among the GSM operators in the country. As on date, Aircel is present in 17 telecom circles (including Assam, Bihar, Himachal Pradesh, Jammu & Kashmir, Kerala, North East, Orissa, Tamil Nadu, Karnataka, West Bengal, Delhi) and with licences secured for the remaining 10 of the 23 telecom circles, the company plans to become a pan-India operator by 2009. Additionally, Aircel has also obtained permission from Department of Telecommunications (DoT) to provide International Long Distance (ILD) and National Long Distance (NLD) telephony services. It is also a category A ISP.
Aircel Business Solutions (ABS), part of Aircel, is an ISO 9000 certified company. ABS is a registered member of WiMAX forum – both in the Indian and International Chapters. ABS’ product range includes enterprise solutions such as Multi Protocol Label Switching Virtual Private Networks (MPLS VPNs), Voice over Internet Protocol (VoIP) and Managed Video Services on wireless platform including WiMAX.
Aircel has won many awards for its services. Aircel got the highest rating for overall customer satisfaction and network quality in 2006 by Voice and Data. Aircel was rated as the top mid-size utility company in Business World’s ‘List of Best Mid-Size Companies’ in 2007. Moreover, Aircel has been selected as the best regional operator in 2008 by Tele.net.
Aircel are one of the sponsors of the Indian Premier League Cricket Team Chennai Super Kings captained by Mahendra Singh Dhoni. It is also the sponsor for Professional Golf Tour of India from 2009 onwards.

Competitors
Aircel is the seventh largest carrier in India. Its competitors are (in order of customer totals):
Airtel
Reliance Communications
Vodafone Essar
BSNL
Idea Cellular
Tata Teleservices
Mts India