a computer is a machine that manipulates

8/14/2019 A Computer is a Machine That Manipulates

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A computer is a machine that manipulates data according to a set of instructions .

Although mechanical examples of computers have existed through much of recordedhuman history, the first electronic computers were developed in the mid-20th century(19401945). These were the size of a large room, consuming as much power as several

hundred modern personal computers ( PCs ).[1]

Modern computers based on integratedcircuits are millions to billions of times more capable than the early machines, andoccupy a fraction of the space .[2] Simple computers are small enough to fit into awristwatch , and can be powered by a watch battery . Personal computers in their variousforms are icons of the Information Age and are what most people think of as"computers". The embedded computers found in many devices from MP3 players tofighter aircraft and from toys to industrial robots are however the most numerous.

The ability to store and execute lists of instructions called programs makes computersextremely versatile, distinguishing them from calculators . The ChurchTuring thesis is amathematical statement of this versatility: any computer with a certain minimum

capability is, in principle, capable of performing the same tasks that any other computer can perform. Therefore computers ranging from a mobile phone to a supercomputer areall able to perform the same computational tasks, given enough time and storagecapacity.

A microcontroller (also microcontroller unit, MCU or C) is a small computer on asingle integrated circuit consisting of a relatively simple CPU combined with supportfunctions such as a crystal oscillator , timers, watchdog timer , serial and analog I/O etc.Program memory in the form of NOR flash or OTP ROM is also often included on chip,

as well as a typically small amount of RAM. Microcontrollers are designed for small or dedicated applications. Thus, in contrast to the microprocessors used in personalcomputers and other high-performance or general purpose applications, simplicity isemphasized. Some microcontrollers may operate at clock rate frequencies as low as4 kHz, as this is adequate for many typical applications, enabling low power consumption(milliwatts or microwatts). They will generally have the ability to retain functionalitywhile waiting for an event such as a button press or other interrupt; power consumptionwhile sleeping (CPU clock and most peripherals off) may be just nanowatts, makingmany of them well suited for long lasting battery applications. Other microcontrollersmay serve performance-critical roles, where they may need to act more like a digitalsignal processor (DSP), with higher clock speeds and power consumption.

Microcontrollers are used in automatically controlled products and devices, such asautomobile engine control systems, remote controls, office machines, appliances, power tools, and toys. By reducing the size and cost compared to a design that uses a separatemicroprocessor, memory, and input/output devices, microcontrollers make it economicalto digitally control even more devices and processes. Mixed signal microcontrollers arecommon, integrating analog components needed to control non-digital electronicsystems.
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Silicon Graphics, Inc. (commonly initialised to SGI , historically sometimes referred toas Silicon Graphics Computer Systems or SGCS ) was a manufacturer of high-

performance computing solutions, including computer hardware and software , founded in1981 by Jim Clark and Abbey Silverstone . Its initial market was 3D graphics displayterminals, but its products, strategies and market positions evolved significantly over

time.

Initial systems were based on the Geometry Engine that Clark and Marc Hannah haddeveloped at Stanford University , and were derived from Clark's broader background incomputer graphics . The Geometry Engine was the first very-large-scale integration (VLSI) implementation of a geometry pipeline : specialized hardware that accelerated the"inner-loop" geometric computations needed to display three-dimensional images.

SGI was headquartered in Sunnyvale, California ; it was originally incorporated as aCalifornia corporation in November 1981, and reincorporated as a Delaware corporation in January 1990. In May 2006, SGI filed for Chapter 11 bankruptcy protection from

which it emerged a few months later, but on April 1, 2009 filed for Chapter 11 again andannounced that it would sell substantially all of its assets to Rackable Systems , a dealfinalized on May 11, 2009, with Rackable assuming the name "Silicon GraphicsInternational".

Columbia (supercomputer)

Named in honor of the crew who died in the Columbia disaster , Columbia is asupercomputer built by Silicon Graphics for NASA . Its main purpose was to simulate theviolent collision and merger of spiral galaxies that lead to the formation of ellipticalgalaxies. It is connected to the NASA Research and Engineering Network and wasinstalled at the NASA Advanced Supercomputing facility in 2004.

According to the TOP500 list of the fastest supercomputers, it entered the list in November 2004 at position 2, [1] running at 51.87 teraflops , or 51.87 trillion floating point calculations per second. By June 2007 it had dropped to position 13, and by June 2008was at position 25. [2] It is composed of twenty SGI Altix 3000 nodes running SUSE

Linux Enterprise Server 9[3]

each of which has 512 Intel Itanium 2 processors bringingthe total number of processors to 10,240. It has 20 terabytes of memory, 440 terabytes of storage, and 10 petabytes of archive storage. [4]

The SGI Altix platform was selected due to a positive experience with Kalpana , a singleAltix 512-CPU system operated by NASA Ames which was integrated into the Columbiasupercomputer system.
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The computers are connected together with a Voltaire InfiniBand ISR 9288 288 portswitch with transfer speeds of up to 10 gigabits (or 1250 megabytes ) per second, 10gigabit Ethernet and multiple 1 gigabit Ethernet nodes.

Kalpana is a supercomputer at NASA 's Ames Research Center operated by the NASAAdvanced Supercomputing Division (NAS). It is named in honor of the late astronaut Kalpana Chawla , who was killed in the Columbia disaster . It was dedicated on May 12 , 2004 .

The computer is a single SGI Altix 3000 , with 512 Itanium 2 processors .

Kalpana was purchased in November 2003 to work on computational fluid dynamics

problems for LANL , NCAR , and the ECCO Consortium . It was the first single Linux kernel 512-processor computer.

From July 2004 Kalpana was integrated, as the first node, into the 20-node ProjectColumbia supercomputer.

Electronic Delay Storage Automatic Calculator (EDSAC ) was an early Britishcomputer . The machine, having been inspired by John von Neumann 's seminal First

Draft of a Report on the EDVAC , was constructed by Maurice Wilkes and his team at the

University of Cambridge Mathematical Laboratory in England . EDSAC was the first practical stored-program electronic computer. [1]

Later the project was supported by J. Lyons & Co. Ltd. , a British firm, who wererewarded with the first commercially applied computer, LEO I , based on the EDSACdesign. EDSAC ran its first programs on 6 May 1949, when it calculated a table of squares [2] and a list of prime numbers.

A microprocessor incorporates most or all of the functions of a central processing unit

(CPU) on a single integrated circuit (IC).[1]

The first microprocessors emerged in theearly 1970s and were used for electronic calculators , using binary-coded decimal (BCD)arithmetic on 4-bit words . Other embedded uses of 4- and 8-bit microprocessors, such asterminals , printers , various kinds of automation etc, followed rather quickly. Affordable8-bit microprocessors with 16-bit addressing also led to the first general purposemicrocomputers in the mid-1970s.
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Computer processors were for a long period constructed out of small and medium-scaleICs containing the equivalent of a few to a few hundred transistors. The integration of thewhole CPU onto a single chip therefore greatly reduced the cost of processing capacity.From their humble beginnings, continued increases in microprocessor capacity haverendered other forms of computers almost completely obsolete (see history of computing

hardware ), with one or more microprocessor as processing element in everything fromthe smallest embedded systems and handheld devices to the largest mainframes andsupercomputers .

Since the early 1970s, the increase in capacity of microprocessors has been known togenerally follow Moore's Law , which suggests that the complexity of an integratedcircuit, with respect to minimum component cost, doubles every two years. [2] In the late1990s, and in the high-performance microprocessor segment, heat generation ( TDP ), dueto switching losses, static current leakage, and other factors, emerged as a leadingdevelopmental constraint. [3]

Input/output (I/O)Main article: Input/output

Hard disk drives are common I/O devices used with computers.

I/O is the means by which a computer exchanges information with the outside world .[27]

Devices that provide input or output to the computer are called peripherals .[28] On atypical personal computer , peripherals include input devices like the keyboard andmouse , and output devices such as the display and printer . Hard disk drives , floppy disk drives and optical disc drives serve as both input and output devices. Computer networking is another form of I/O.

Often, I/O devices are complex computers in their own right with their own CPU andmemory. A graphics processing unit might contain fifty or more tiny computers that

perform the calculations necessary to display 3D graphics [citation needed ]. Modern desktopcomputers contain many smaller computers that assist the main CPU in performing I/O.
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A computer network is a group of computers that are connected to each other for the purpose of communication. Networks may be classified according to a wide variety of characteristics. This article provides a general overview of some types and categories andalso presents the basic components of a network.

Connection method

Computer networks can also be classified according to the hardware and softwaretechnology that is used to interconnect the individual devices in the network, such asOptical fiber , Ethernet , Wireless LAN , HomePNA , Power line communication or G.hn .

Ethernet uses physical wiring to connect devices. Frequently deployed devices includehubs, switches, bridges and/or routers.

Wireless LAN technology is designed to connect devices without wiring. These devicesuse radio waves or infrared signals as a transmission medium.

ITU-T G.hn technology uses existing home wiring ( coaxial cable , phone lines and power lines ) to create a high-speed (up to 1 Gigabit/s) local area network.

Wired Technologies

Twisted-Pair Wire - This is the most widely used medium for telecommunication.Twisted-pair wires are ordinary telephone wires which consist of two insulated copper wires twisted into pairs and are used for both voice and data transmission. The use of twowires twisted together helps to reduce crosstalk and electromagnetic induction . Thetransmission speed range from 2 million bits per second to 100 million bits per second.

Coaxial Cable These cables are widely used for cable television systems, office buildings, and other worksites for local area networks. The cables consist of copper or aluminum wire wrapped with insulating layer typically of a flexible material with a highdielectric constant, all of which are surrounded by a conductive layer. The layers of insulation help minimize interference and distortion. Transmission speed range from 200

million to more than 500 million bits per second.

Fiber Optics These cables consist of one or more thin filaments of glass fiber wrappedin a protective layer. It transmits light which can travel over long distance and higher

bandwidths. Fiber-optic cables are not affected by electromagnetic radiation.Transmission speed could go up to as high as trillions of bits per second. The speed of fiber optics is hundreds of times faster than coaxial cables and thousands of times faster than twisted-pair wire.
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Wireless Technologies

Terrestrial Microwave Terrestrial microwaves use Earth-based transmitter and receiver.The equipment look similar to satellite dishes. Terrestrial microwaves use low-gigahertzrange, which limits all communications to line-of-sight. Path between relay stations

spaced approx. 30 miles apart. Microwave antennas are usually placed on top of buildings, towers, hills, and mountain peaks.

Communications Satellites The satellites use microwave radio as their telecommunications medium which are not deflected by the Earth's atmosphere. Thesatellites are stationed in space, typically 22,000 miles above the equator. These Earth-orbiting systems are capable of receiving and relaying voice, data, and TV signals.

Cellular and PCS Systems Use several radio communications technologies. Thesystems are divided to different geographic area. Each area has low-power transmitter or radio relay antenna device to relay calls from one area to the next area.

Wireless LANs Wireless local area network use a high-frequency radio technologysimilar to digital cellular and a low-frequency radio technology. Wireless LANS usespread spectrum technology to enable communication between multiple devices in alimited area. Example of open-standard wireless radio-wave technology is IEEE 802.11b.

Bluetooth A short range wireless technology. Operate at approx. 1Mbps with rangefrom 10 to 100 meters. Bluetooth is an open wireless protocol for data exchange over short distances.

The Wireless Web The wireless web refers to the use of the World Wide Web through

equipments like cellular phones, pagers,PDAs, and other portable communicationsdevices. The wireless web service offers anytime/anywhere connection.

[edit ] Scale

Networks are often classified as Local Area Network (LAN) , Wide Area Network (WAN) , Metropolitan Area Network (MAN) , Personal Area Network (PAN) , VirtualPrivate Network (VPN) , Campus Area Network (CAN) , Storage Area Network (SAN) ,etc. depending on their scale, scope and purpose. Usage, trust levels and access rightsoften differ between these types of network - for example, LANs tend to be designed for internal use by an organization's internal systems and employees in individual physical

locations (such as a building), while WANs may connect physically separate parts of anorganization to each other and may include connections to third parties.

[edit ] Functional relationship (network architecture)

Computer networks may be classified according to the functional relationships whichexist among the elements of the network , e.g., Active Networking , Client-server andPeer-to-peer (workgroup) architecture.
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[edit ] Network topology

Computer networks may be classified according to the network topology upon which thenetwork is based, such as bus network , star network , ring network , mesh network , star-

bus network , tree or hierarchical topology network . Network topology signifies the way

in which devices in the network see their logical relations to one another. The use of theterm "logical" here is significant. That is, network topology is independent of the"physical" layout of the network. Even if networked computers are physically placed in alinear arrangement, if they are connected via a hub, the network has a Star topology,rather than a bus topology. In this regard the visual and operational characteristics of anetwork are distinct; the logical network topology is not necessarily the same as the

physical layout. Networks may be classified based on the method of data used to conveythe data, these include digital and analog networks.

[ edit ] Types of networks

Below is a list of the most common types of computer networks in order of scale.

[edit ] Personal area network

A personal area network (PAN) is a computer network used for communication amongcomputer devices close to one person. Some examples of devices that are used in a PANare personal computers, printers, fax machines, telephones, PDAs, scanners, and evenvideo game consoles. Such a PAN may include wired and wireless connections betweendevices. The reach of a PAN is typically at least about 20-30 feet (approximately 6-9meters), but this is expected to increase with technology improvements.

[edit ] Local area network

A local Area Network (LAN) is a computer network covering a small physical area, likea home, office, or small group of buildings, such as a school, or an airport. Current wiredLANs are most likely to be based on Ethernet technology, although new standards likeITU-T G.hn also provide a way to create a wired LAN using existing home wires (coaxialcables, phone lines and power lines) [2].

For example, a library may have a wired or wireless LAN for users to interconnect localdevices (e.g., printers and servers) and to connect to the internet. On a wired LAN, PCs inthe library are typically connected by category 5 (Cat5) cable , running the IEEE 802.3

protocol through a system of interconnected devices and eventually connect to theInternet. The cables to the servers are typically on Cat 5e enhanced cable, which willsupport IEEE 802.3 at 1 Gbit/s. A wireless LAN may exist using a different IEEE

protocol, 802.11b, 802.11g or possibly 802.11n. The staff computers (bright green in thefigure) can get to the color printer, checkout records, and the academic network and theInternet. All user computers can get to the Internet and the card catalog. Each workgroupcan get to its local printer. Note that the printers are not accessible from outside their workgroup.
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Typical library network, in a branching tree topology and controlled access to resources

All interconnected devices must understand the network layer (layer 3), because they arehandling multiple subnets (the different colors). Those inside the library, which have only10/100 Mbit/s Ethernet connections to the user device and a Gigabit Ethernet connectionto the central router, could be called "layer 3 switches" because they only have Ethernetinterfaces and must understand IP. It would be more correct to call them access routers,where the router at the top is a distribution router that connects to the Internet andacademic networks' customer access routers.

The defining characteristics of LANs, in contrast to WANs (Wide Area Networks),include their higher data transfer rates, smaller geographic range, and lack of a need for leased telecommunication lines. Current Ethernet or other IEEE 802.3 LAN technologiesoperate at speeds up to 10 Gbit/s. This is the data transfer rate. IEEE has projectsinvestigating the standardization of 40 and 100 Gbit/s .[3]

[edit ] Campus area network

A campus area network (CAN) is a computer network made up of an interconnection of

local area networks (LANs) within a limited geographical area. It can be considered oneform of a metropolitan area network, specific to an academic setting.

In the case of a university campus-based campus area network, the network is likely tolink a variety of campus buildings including; academic departments, the university libraryand student residence halls. A campus area network is larger than a local area network

but smaller than a wide area network (WAN) (in some cases).

The main aim of a campus area network is to facilitate students accessing internet anduniversity resources. This is a network that connects two or more LANs but that islimited to a specific and contiguous geographical area such as a college campus,industrial complex, office building, or a military base. A CAN may be considered a typeof MAN (metropolitan area network), but is generally limited to a smaller area than atypical MAN. This term is most often used to discuss the implementation of networks for a contiguous area. This should not be confused with a Controller Area Network . A LANconnects network devices over a relatively short distance. A networked office building,school, or home usually contains a single LAN, though sometimes one building willcontain a few small LANs (perhaps one per room), and occasionally a LAN will span agroup of nearby buildings.
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[edit ] Metropolitan area network

A metropolitan area network (MAN) is a network that connects two or more local areanetworks or campus area networks together but does not extend beyond the boundaries of the immediate town/city. Routers, switches and hubs are connected to create a

metropolitan area network.

[edit ] Wide area network

A wide area network (WAN) is a computer network that covers a broad area (i.e. anynetwork whose communications links cross metropolitan, regional, or national

boundaries [1]). Less formally, a WAN is a network that uses routers and publiccommunications links. Contrast with personal area networks (PANs), local area networks(LANs), campus area networks (CANs), or metropolitan area networks (MANs), whichare usually limited to a room, building, campus or specific metropolitan area (e.g., a city)respectively. The largest and most well-known example of a WAN is the Internet. A

WAN is a data communications network that covers a relatively broad geographic area(i.e. one city to another and one country to another country) and that often usestransmission facilities provided by common carriers, such as telephone companies. WANtechnologies generally function at the lower three layers of the OSI reference model : the

physical layer , the data link layer , and the network layer .

[edit ] Global area network

A global area networks (GAN) (see also IEEE 802.20 ) specification is in development byseveral groups, and there is no common definition. In general, however, a GAN is amodel for supporting mobile communications across an arbitrary number of wireless

LANs, satellite coverage areas, etc. The key challenge in mobile communications is"handing off" the user communications from one local coverage area to the next. In IEEEProject 802, this involves a succession of terrestrial WIRELESS local area networks(WLAN) .[4]

[edit ] Virtual private network

A virtual private network (VPN) is a computer network in which some of the links between nodes are carried by open connections or virtual circuits in some larger network (e.g., the Internet) instead of by physical wires. The data link layer protocols of thevirtual network are said to be tunneled through the larger network when this is the case.

One common application is secure communications through the public Internet, but aVPN need not have explicit security features, such as authentication or contentencryption. VPNs, for example, can be used to separate the traffic of different user communities over an underlying network with strong security features.

A VPN may have best-effort performance, or may have a defined service level agreement(SLA) between the VPN customer and the VPN service provider. Generally, a VPN has atopology more complex than point-to-point.
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A VPN allows computer users to appear to be editing from an IP address location other than the one which connects the actual computer to the Internet.

[edit ] Internetwork

An Internetwork is the connection of two or more distinct computer networks or network segments via a common routing technology. The result is called an internetwork (oftenshortened to internet). Two or more networks or network segments connect using devicesthat operate at layer 3 (the 'network' layer) of the OSI Basic Reference Model, such as arouter. Any interconnection among or between public, private, commercial, industrial, or governmental networks may also be defined as an internetwork.

In modern practice, interconnected networks use the Internet Protocol. There are at leastthree variants of internetworks, depending on who administers and who participates inthem:

Intranet Extranet Internet

Intranets and extranets may or may not have connections to the Internet. If connected tothe Internet, the intranet or extranet is normally protected from being accessed from theInternet without proper authorization. The Internet is not considered to be a part of theintranet or extranet, although it may serve as a portal for access to portions of an extranet.

[edit ] Intranet

An intranet is a set of networks, using the Internet Protocol and IP-based tools such asweb browsers and file transfer applications, that is under the control of a singleadministrative entity. That administrative entity closes the intranet to all but specific,authorized users. Most commonly, an intranet is the internal network of an organization.A large intranet will typically have at least one web server to provide users withorganizational information.

[edit ] Extranet

An extranet is a network or internetwork that is limited in scope to a single organizationor entity but which also has limited connections to the networks of one or more other

usually, but not necessarily, trusted organizations or entities (e.g., a company's customersmay be given access to some part of its intranet creating in this way an extranet, while atthe same time the customers may not be considered 'trusted' from a security standpoint).Technically, an extranet may also be categorized as a CAN, MAN, WAN, or other typeof network, although, by definition, an extranet cannot consist of a single LAN; it musthave at least one connection with an external network.
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[edit ] Internet

The Internet consists of a worldwide interconnection of governmental, academic, public,and private networks based upon the networking technologies of the Internet ProtocolSuite . It is the successor of the Advanced Research Projects Agency Network

(ARPANET) developed by DARPA of the U.S. Department of Defense . The Internet isalso the communications backbone underlying the World Wide Web (WWW). The'Internet' is most commonly spelled with a capital 'I' as a proper noun, for historicalreasons and to distinguish it from other generic internetworks.

Participants in the Internet use a diverse array of methods of several hundreddocumented, and often standardized, protocols compatible with the Internet ProtocolSuite and an addressing system ( IP Addresses ) administered by the Internet Assigned

Numbers Authority and address registries . Service providers and large enterprisesexchange information about the reachability of their address spaces through the Border Gateway Protocol (BGP), forming a redundant worldwide mesh of transmission paths.

[ edit ] Basic hardware components

All networks are made up of basic hardware building blocks to interconnect network nodes , such as Network Interface Cards (NICs), Bridges, Hubs, Switches, and Routers. Inaddition, some method of connecting these building blocks is required, usually in theform of galvanic cable (most commonly Category 5 cable ). Less common are microwavelinks (as in IEEE 802.12 ) or optical cable (" optical fiber "). An Ethernet card may also berequired.

[edit ] Network interface cards

A network card , network adapter, or NIC (network interface card) is a piece of computer hardware designed to allow computers to communicate over a computer network. It

provides physical access to a networking medium and often provides a low-leveladdressing system through the use of MAC addresses .

[edit ] Repeaters

A repeater is an electronic device that receives a signal and retransmits it at a higher power level, or to the other side of an obstruction, so that the signal can cover longer distances without degradation. In most twisted pair Ethernet configurations, repeaters arerequired for cable which runs longer than 100 meters.

[edit ] Hubs

A network hub contains multiple ports. When a packet arrives at one port, it is copiedunmodified to all ports of the hub for transmission. The destination address in the frameis not changed to a broadcast address. [5]
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[edit ] Bridges

A network bridge connects multiple network segments at the data link layer (layer 2) of the OSI model . Bridges do not promiscuously copy traffic to all ports, as hubs do, butlearn which MAC addresses are reachable through specific ports. Once the bridge

associates a port and an address, it will send traffic for that address only to that port.Bridges do send broadcasts to all ports except the one on which the broadcast wasreceived.

Bridges learn the association of ports and addresses by examining the source address of frames that it sees on various ports. Once a frame arrives through a port, its sourceaddress is stored and the bridge assumes that MAC address is associated with that port.The first time that a previously unknown destination address is seen, the bridge willforward the frame to all ports other than the one on which the frame arrived.

Bridges come in three basic types:

1. Local bridges: Directly connect local area networks (LANs)2. Remote bridges: Can be used to create a wide area network (WAN) link between

LANs. Remote bridges, where the connecting link is slower than the endnetworks, largely have been replaced with routers.

3. Wireless bridges: Can be used to join LANs or connect remote stations to LANs

[edit ] Switches

A network switch is a device that forwards and filters OSI layer 2 datagrams (chunk of data communication) between ports (connected cables) based on the MAC addresses in

the packets.[6]

This is distinct from a hub in that it only forwards the packets to the portsinvolved in the communications rather than all ports connected. Strictly speaking, aswitch is not capable of routing traffic based on IP address (OSI Layer 3) which isnecessary for communicating between network segments or within a large or complexLAN. Some switches are capable of routing based on IP addresses but are still calledswitches as a marketing term. A switch normally has numerous ports, with the intention

being that most or all of the network is connected directly to the switch, or another switchthat is in turn connected to a switch. [7]

Switch is a marketing term that encompasses routers and bridges, as well as devices thatmay distribute traffic on load or by application content (e.g., a Web URL identifier).

Switches may operate at one or more OSI model layers, including physical , data link ,network , or transport (i.e., end-to-end) . A device that operates simultaneously at morethan one of these layers is called a multilayer switch .

Overemphasizing the ill-defined term "switch" often leads to confusion when first tryingto understand networking. Many experienced network designers and operatorsrecommend starting with the logic of devices dealing with only one protocol level, not allof which are covered by OSI. Multilayer device selection is an advanced topic that may
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lead to selecting particular implementations, but multilayer switching is simply not a real-world design concept.

[edit ] Routers

A router is a networking device that forwards packets between networks usinginformation in protocol headers and forwarding tables to determine the best next router for each packet. Routers work at the Network Layer of the OSI model and the InternetLayer of TCP/IP .

The Semi-Automatic Ground Environment (SAGE ) was an automated control systemfor tracking and intercepting enemy bomber aircraft used by NORAD from the late 1950s

into the 1980s. In later versions, the system could automatically direct aircraft to aninterception by sending instructions directly to the aircraft's autopilot .

By the time it was completely operational the Soviet bomber threat had been replaced bythe Soviet missile threat, for which SAGE was entirely inadequate. Nevertheless, SAGEwas extremely important; it led to great advances in online systems and interactivecomputing , real-time computing , and data communications using modems . It is generallyconsidered to be one of the most advanced and successful large computer systems ever developed.

Both MIT and IBM supported the project as contractors. IBM 's role in SAGE (the design

and manufacture of the AN/FSQ-7 computer, a vacuum tube computer with ferrite corememory based on the never-built Whirlwind II ) was an important factor leading to IBM'sdomination of the computer industry.

Packet-switched networksMain article: Lag

Latency in a packet-switched network is measured either one-way (the time from thesource sending a packet to the destination receiving it), or round-trip (the one-waylatency from source to destination plus the one-way latency from the destination back tothe source). Round-trip latency is more often quoted, because it can be measured from asingle point. Note that round trip latency excludes the amount of time that a destinationsystem spends processing the packet. Many software platforms provide a service called

ping that can be used to measure round-trip latency. Ping performs no packet processing;
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it merely sends a response back when it receives a packet (i.e. performs a no-op ), thus itis a relatively accurate way of measuring latency.

Where precision is important, one-way latency for a link can be more strictly defined asthe time from the start of packet transmission to the start of packet reception . The time

from the start of packet reception to the end of packet reception is measured separatelyand called "Serialization Delay". This definition of latency is independent of the link'sthroughput and the size of the packet, and is the absolute minimum delay possible withthat link.

However, in a non-trivial network, a typical packet will be forwarded over many links viamany gateways, each of which will not begin to forward the packet until it has beencompletely received. In such a network, the minimal latency is the sum of the minimumlatency of each link, plus the transmission delay of each link except the final one, plus theforwarding latency of each gateway. In practice, this minimal latency is further augmented by queuing and processing delays. Queuing delay occurs when a gateway

receives multiple packets from different sources heading towards the same destination.Since typically only one packet can be transmitted at a time, some of the packets mustqueue for transmission, incurring additional delay. Processing delays are incurred while agateway determines what to do with a newly received packet. The combination of

propagation, serialization, queuing, and processing delays often produces a complex andvariable network latency profile.
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a computer is a machine that manipulates

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