Cisco HDLC

Cisco HDLC (aussi appelé cHDLC) est un protocole de transmission synchrone sur lien série. Il est au niveau 2 du modèle OSI. C'est une extension du protocole High-Level Data Link Control (HDLC) développé par Cisco. Souvent décrit comme propriétaire car non standardisé, il est, en fait, ouvert et largement distribué. D'autres constructeurs de matériel que Cisco utilisent ce protocole.

Trame HDLC

 

Cette table décrit l'organisation de la trame cHDLC.

Address	Control	Protocol Code	Information	Frame Check Sequence (FCS)	Flag
8 bits	8 bits	16 bits	Variable length, 0 or more bits, in multiples of 8	16 bits	8 bits

• Le champ address est utilisé pour indiquer le type de paquet : 0x0F Unicast et 0x8F Broadcast.
• Le champ control est toujours à zéro (0x00).
• Le champ protocol indique le type de protocole encapsulé dans la trame cHDLC (par exemple 0x0800 pour Internet Protocol).

Source: http://fr.wikipedia.org/wiki/Cisco_HDLC

File Transfer Protocol

File Transfer Protocol (FTP) is a standard network protocol used to transfer files from one host or to another host over a TCP-based network, such as the Internet.
FTP is built on a client-server architecture and uses separate control and data connections between the client and the server.^[1] FTP users may authenticate themselves using a clear-text sign-in protocol, normally in the form of a username and password, but can connect anonymously if the server is configured to allow it. For secure transmission that hides (encrypts) the username and password, and encrypts the content, FTP is often secured with SSL/TLS ("FTPS"). SSH File Transfer Protocol ("SFTP") is sometimes also used instead.
The first FTP client applications were command-line applications developed before operating systems had graphical user interfaces, and are still shipped with most Windows, Unix, and Linux operating systems.^[2][3] Dozens of FTP clients and automation utilities have since been developed for desktops, servers, mobile devices, and hardware, and FTP has been incorporated into hundreds of productivity applications, such as Web page editors.
Source: http://en.wikipedia.org/wiki/File_Transfer_Protocol

Internet protocol suite

The Internet protocol suite is the set of communications protocols used for the Internet and similar networks, and generally the most popular protocol stack for wide area networks. It is commonly known as TCP/IP, because of its most important protocols: Transmission Control Protocol (TCP) and Internet Protocol (IP), which were the first networking protocols defined in this standard. It is occasionally known as the DoD model due to the foundational influence of the ARPANET in the 1970s (operated by DARPA, an agency of the United States Department of Defense).
TCP/IP provides end-to-end connectivity specifying how data should be formatted, addressed, transmitted, routed and received at the destination. It has four abstraction layers which are used to sort all Internet protocols according to the scope of networking involved.^[1][2] From lowest to highest, the layers are:

1. The link layer contains communication technologies for a local network.
2. The internet layer (IP) connects local networks, thus establishing internetworking.
3. The transport layer handles host-to-host communication.
4. The application layer contains all protocols for specific data communications services on a process-to-process level. For example, HTTP specifies the web browser communication with a web server.

The TCP/IP model and related protocols are maintained by the Internet Engineering Task Force (IETF).
Source: http://en.wikipedia.org/wiki/Internet_protocol_suite

Internet

 

The Internet (or internet) is a global system of interconnected computer networks that use the standard Internet protocol suite (TCP/IP) to serve billions of users worldwide. It is a network of networks that consists of millions of private, public, academic, business, and government networks, of local to global scope, that are linked by a broad array of electronic, wireless and optical networking technologies. The Internet carries an extensive range of information resources and services, such as the inter-linked hypertext documents of the World Wide Web (WWW) and the infrastructure to support email.
Most traditional communications media including telephone, music, film, and television are being reshaped or redefined by the Internet, giving birth to new services such as Voice over Internet Protocol (VoIP) and Internet Protocol Television (IPTV). Newspaper, book and other print publishing are adapting to Web site technology, or are reshaped into blogging and web feeds. The Internet has enabled and accelerated new forms of human interactions through instant messaging, Internet forums, and social networking. Online shopping has boomed both for major retail outlets and small artisans and traders. Business-to-business and financial services on the Internet affect supply chains across entire industries.
The origins of the Internet reach back to research of the 1960s, commissioned by the United States government to build robust, fault-tolerant, and distributed computer networks. The funding of a new U.S. backbone by the National Science Foundation in the 1980s, as well as private funding for other commercial backbones, led to worldwide participation in the development of new networking technologies, and the merger of many networks. The commercialization of what was by the 1990s an international network resulted in its popularization and incorporation into virtually every aspect of modern human life. As of June 2012, more than 2.4 billion people—over a third of the world's human population—have used the services of the Internet.^[1]
The Internet has no centralized governance in either technological implementation or policies for access and usage; each constituent network sets its own standards. Only the overreaching definitions of the two principal name spaces in the Internet, the Internet Protocol address space and the Domain Name System, are directed by a maintainer organization, the Internet Corporation for Assigned Names and Numbers (ICANN). The technical underpinning and standardization of the core protocols (IPv4 and IPv6) is an activity of the Internet Engineering Task Force (IETF), a non-profit organization of loosely affiliated international participants that anyone may associate with by contributing technical expertise.
Source: http://en.wikipedia.org/wiki/Internet

Website

 

A website, also written as Web site,^[1] web site, or simply site,^[2] is a set of related web pages containing content such as text, images, video, audio, etc. A website is hosted on at least one web server, accessible via a network such as the Internet or a private local area network through an Internet address known as a Uniform Resource Locator. All publicly accessible websites collectively constitute the World Wide Web.
A webpage is a document, typically written in plain text interspersed with formatting instructions of Hypertext Markup Language (HTML, XHTML). A webpage may incorporate elements from other websites with suitable markup anchors.
Webpages are accessed and transported with the Hypertext Transfer Protocol (HTTP), which may optionally employ encryption (HTTP Secure, HTTPS) to provide security and privacy for the user of the webpage content. The user's application, often a web browser, renders the page content according to its HTML markup instructions onto a display terminal.
The pages of a website can usually be accessed from a simple Uniform Resource Locator (URL) called the web address. The URLs of the pages organize them into a hierarchy, although hyperlinking between them conveys the reader's perceived site structure and guides the reader's navigation of the site which generally includes a home page with most of the links to the site's web content, and a supplementary about, contact and link page.
Some websites require a subscription to access some or all of their content. Examples of subscription websites include many business sites, parts of news websites, academic journal websites, gaming websites, file-sharing websites, message boards, web-based email, social networking websites, websites providing real-time stock market data, and websites providing various other services (e.g., websites offering storing and/or sharing of images, files and so forth).
Source: http://en.wikipedia.org/wiki/Website

How to Create a Software Startup

 There is a glut of overly optimistic literature giving advice and encouragement on entrepreneurship, innovation, and fearlessness in the face of risk. The advice can be quite misleading due to its authors' success bias, because they represent the tiny percentage of people who have succeeded and are given a voice among the infinitely many more people who have failed. Since chances are against most of us who try, the goal of this article is to offer suggestions on how to build a company and give it a chance to stand on its feet, while spending nothing more than the legal fee to register as a company and a domain name. Hopefully it will afford you more time to try to build your company well.

Bootstrapping: What You Can Expect to Achieve

During the bootstrapping stage, as many as possible of the below milestones have to be accomplished in order to ensure the company is able to survive moving forward:

* A small but solid team has to be gelled and chugging along
* An early version of the product has to be put on the market
* Figure out a way to earn a little bit of revenue in the early stages
* The product has to be gaining some traction

The more of the above a company can accomplish, the better. If none of these are established, there is little chance that the business will become a viable one and the entrepreneur has to take a look at what he or she has accomplished. If, on the other hand, everything falls nicely into place and you are able to accomplish all four of the goals, you may have an incredible new business on your hands that might one day be very successful. If the business is somewhere in the middle, it may be a good candidate for an investment and still have great chances to be successful in the future. Now we'll go through all the necessary steps, pitfalls and considerations to get on the road to accomplishing the above milestones.

The Beginning: The Value of the Original Idea

You probably had a little laugh at the title of this section. Many first-time entrepreneurs and dreamers live in the hopes of a "million-dollar idea." In reality, it is very rare for an idea to be even remotely close to being that valuable. Only years of top-notch execution, continuous innovation, and very hard work will create value. No matter your previous background, there are some common pitfalls. Engineers tend to overdo and business people tend to over-plan; not because they are incompetent, but rather because they are hard workers who are excited about the prospects of their ideas and do what they can before the other parts of the business come along. This is often crippling to the company. One of the first tasks is to build a well-rounded partner team and validate your idea. I won't get into specifics of idea-validation here, as Steve Blank does an amazing job of it in his book "Four Steps to Epiphany" which is a must-read for every entrepreneur, but I will discuss how to build a superstar founding team, starting with the differences in approaches in case your background is technical or in business. What I often hear business people say is, "I am not very technical ..." If this sounds familiar, don't worry, that is understood. Just find a partner who is technical, communicates well, and in whose professionalism you can trust. They will get you up to speed to a level where you need to be. Engineers tend to jump in and do-do-do. But without a clear vision and direction, they often run themselves in circles, over-building unnecessary or overly-complicated features. For their part, engineers might say something similar to "I don't understand business." I myself have been guilty of such a quote. If I could go back in time, I'd advise myself to listen and learn as much and as fast as I can because after a short while, the business environment becomes second nature, fun and fascinating.

Why You Need Partners

Sometimes engineers who have become confident in the business world can pull of a start-up by themselves, but most people need partners. Business people always need engineers for the obvious reason that someone has to actually build the product. Engineers need business people and additional engineers because it is just too difficult for one person to write code, go to business meetings, network, deal with legal issues, do marketing, branding, social media, a slew of other tasks, while keeping a clear mind to maintain focus on their entrepreneurial vision.
Source: http://www.devx.com/webdev/Article/44068

Read-only memory

Read-only memory (ROM) is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware (software that is very closely tied to specific hardware, and unlikely to need frequent updates).
In its strictest sense, ROM refers only to mask ROM (the oldest type of solid state ROM), which is fabricated with the desired data permanently stored in it, and thus can never be modified. Despite the simplicity, speed and economies of scale of mask ROM, field-programmability often make reprogrammable memories more flexible and inexpensive. As of 2007, actual ROM circuitry is therefore mainly used for applications such as microcode, and similar structures, on various kinds of processors.
Other types of non-volatile memory such as erasable programmable read only memory (EPROM) and electrically erasable programmable read-only memory (EEPROM or Flash ROM) are sometimes referred to, in an abbreviated way, as "read-only memory" (ROM); although these types of memory can be erased and re-programmed multiple times, writing to this memory takes longer and may require different procedures than reading the memory.^[1] When used in this less precise way, "ROM" indicates a non-volatile memory which serves functions typically provided by mask ROM, such as storage of program code and nonvolatile data.
Source: http://en.wikipedia.org/wiki/Read-only_memory

Random-access memory

Random-access memory (RAM) is a form of computer data storage. A random-access device allows stored data to be accessed in very nearly the same amount of time for any storage location, so data can be accessed quickly in any random order. In contrast, other data storage media such as hard disks, CDs, DVDs and magnetic tape, as well as early primary memory types such as drum memory, read and write data only in a predetermined order, consecutively, because of mechanical design limitations. Therefore the time to access a given data location varies significantly depending on its physical location.
Today, random-access memory takes the form of integrated circuits. Strictly speaking, modern types of DRAM are not random access, as data is read in bursts, although the name DRAM / RAM has stuck. However, many types of SRAM, ROM, OTP, and NOR flash are still random access even in a strict sense. RAM is often associated with volatile types of memory (such as DRAM memory modules), where its stored information is lost if the power is removed. Many other types of non-volatile memory are RAM as well, including most types of ROM and a type of flash memory called NOR-Flash. The first RAM modules to come into the market were created in 1951 and were sold until the late 1960s and early 1970s.
source: http://en.wikipedia.org/wiki/Random-access_memory

Types of operating systems

Types of operating systems

Real-time

A real-time operating system is a multitasking operating system that aims at executing real-time applications. Real-time operating systems often use specialized scheduling algorithms so that they can achieve a deterministic nature of behavior. The main objective of real-time operating systems is their quick and predictable response to events. They have an event-driven or time-sharing design and often aspects of both. An event-driven system switches between tasks based on their priorities or external events while time-sharing operating systems switch tasks based on clock interrupts.

Multi-user

A multi-user operating system allows multiple users to access a computer system at the same time. Time-sharing systems and Internet servers can be classified as multi-user systems as they enable multiple-user access to a computer through the sharing of time. Single-user operating systems have only one user but may allow multiple programs to run at the same time.

Multi-tasking vs. single-tasking

A multi-tasking operating system allows more than one program to be running at a time, from the point of view of human time scales. A single-tasking system has only one running program. Multi-tasking can be of two types: pre-emptive and co-operative. In pre-emptive multitasking, the operating system slices the CPU time and dedicates one slot to each of the programs. Unix-like operating systems such as Solaris and Linux support pre-emptive multitasking, as does AmigaOS. Cooperative multitasking is achieved by relying on each process to give time to the other processes in a defined manner. 16-bit versions of Microsoft Windows used cooperative multi-tasking. 32-bit versions of both Windows NT and Win9x, used pre-emptive multi-tasking. Mac OS prior to OS X used to support cooperative multitasking.

Distributed

Further information: Distributed system

A distributed operating system manages a group of independent computers and makes them appear to be a single computer. The development of networked computers that could be linked and communicate with each other gave rise to distributed computing. Distributed computations are carried out on more than one machine. When computers in a group work in cooperation, they make a distributed system.

Embedded

Embedded operating systems are designed to be used in embedded computer systems. They are designed to operate on small machines like PDAs with less autonomy. They are able to operate with a limited number of resources. They are very compact and extremely efficient by design. Windows CE and Minix 3 are some examples of embedded operating systems.

Source: http://en.wikipedia.org/wiki/Operating_system

 

Operating system

An operating system (OS) is a collection of software that manages computer hardware resources and provides common services for computer programs. The operating system is a vital component of the system software in a computer system. Application programs usually require an operating system to function.
Time-sharing operating systems schedule tasks for efficient use of the system and may also include accounting for cost allocation of processor time, mass storage, printing, and other resources.
For hardware functions such as input and output and memory allocation, the operating system acts as an intermediary between programs and the computer hardware,^[1][2] although the application code is usually executed directly by the hardware and will frequently make a system call to an OS function or be interrupted by it. Operating systems can be found on almost any device that contains a computer—from cellular phones and video game consoles to supercomputers and web servers.
Examples of popular modern operating systems include Android, BSD, iOS, Linux, Mac OS X, Microsoft Windows,^[3] Windows Phone, and IBM z/OS. All these, except Windows and z/OS, share roots in UNIX.

source: http://en.wikipedia.org/wiki/Operating_system

Microprocessor

A microprocessor incorporates the functions of a computer's central processing unit (CPU) on a single integrated circuit (IC),^[1] or at most a few integrated circuits.^[2] It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and provides results as output. It is an example of sequential digital logic, as it has internal memory. Microprocessors operate on numbers and symbols represented in the binary numeral system.
The advent of low-cost computers on integrated circuits has transformed modern society. General-purpose microprocessors in personal computers are used for computation, text editing, multimedia display, and communication over the Internet. Many more microprocessors are part of embedded systems, providing digital control of a myriad of objects from appliances to automobiles to cellular phones and industrial process control.
source: http://en.wikipedia.org/wiki/Microprocessor

Stack (abstract data type)

In computer science, a stack is a particular kind of abstract data type or collection in which the principal (or only) operations on the collection are the addition of an entity to the collection, known as push, and removal of an entity, known as pop. The relation between the push and pop operations is such that the stack is a Last-In-First-Out (LIFO) data structure. In a LIFO data structure, the last element added to the structure must be the first one to be removed. This is equivalent to the requirement that, considered as a linear data structure, or more abstractly a sequential collection, the push and pop operations occur only at one end of the structure, referred to as the top of the stack. Often a peek or top operation is also implemented, returning the value of the top element without removing it.
A stack may be implemented to have a bounded capacity. If the stack is full and does not contain enough space to accept an entity to be pushed, the stack is then considered to be in an overflow state. The pop operation removes an item from the top of the stack. A pop either reveals previously concealed items or results in an empty stack, but if the stack is empty then it goes into underflow state (It means no items are present in stack to be removed).
A stack is a restricted data structure, because only a small number of operations are performed on it. The nature of the pop and push operations also means that stack elements have a natural order. Elements are removed from the stack in the reverse order to the order of their addition: therefore, the lower elements are those that have been on the stack the longest
Source: http://en.wikipedia.org/wiki/Stack_%28abstract_data_type%29

 

Database

Databases rank among the most significant structural elements of the World Wide Web today. Lying in the basis of the majority of the content-driven websites and applications, databases serve a special mission - to provide a well-organized mechanism for data manipulation. The database approach in website/application development now rules the web by offering a quick and automated way for the information to be stored, managed, deleted or retrieved. Databases' powerful set of capabilities has determined the introduction of dynamic websites, which has opened a new page in the evolution history of the web.


source:http://www.ntchosting.com/databases/database.html

What is program?

What is program?

(n) An organized list of instructions that, when executed, causes the computer to behave in a predetermined manner. Without programs, computers are useless.
A program is like a recipe. It contains a list of ingredients (called variables) and a list of directions (called statements) that tell the computer what to do with the variables. The variables can represent numeric data, text, or graphical images.
There are many programming languages -- C, C++, Pascal, BASIC, FORTRAN, COBOL, and LISP are just a few. These are all high-level languages. One can also write programs in low-level languages called assembly languages, although this is more difficult. Low-level languages are closer to the language used by a computer, while high-level languages are closer to human languages.
Eventually, every program must be translated into a machine language that the computer can understand. This translation is performed by compilers, interpreters, and assemblers.
When you buy software, you normally buy an executable version of a program. This means that the program is already in machine language -- it has already been compiled and assembled and is ready to execute.
source:http://www.webopedia.com/TERM/P/program.html

Computer programming

Computer programming (often shortened to programming, scripting, or coding) is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages (such as Java, C++, C#, Python, etc.). The purpose of programming is to create a set of instructions that computers use to perform specific operations or to exhibit desired behaviors. The process of writing source code often requires expertise in many different subjects, including knowledge of the application domain, specialized algorithms and formal logic.
Within software engineering, programming (the implementation) is regarded as one phase in a software development process.
There is an ongoing debate on the extent to which the writing of programs is an art form, a craft, or an engineering discipline.^[1] In general, good programming is considered to be the measured application of all three, with the goal of producing an efficient and evolvable software solution (the criteria for "efficient" and "evolvable" vary considerably). The discipline differs from many other technical professions in that programmers, in general, do not need to be licensed or pass any standardized (or governmentally regulated) certification tests in order to call themselves "programmers" or even "software engineers." Because the discipline covers many areas, which may or may not include critical applications, it is debatable whether licensing is required for the profession as a whole. In most cases, the discipline is self-governed by the entities which require the programming, and sometimes very strict environments are defined (e.g. United States Air Force use of AdaCore and security clearance). However, representing oneself as a "Professional Software Engineer" without a license from an accredited institution is illegal in many parts of the world.
Another ongoing debate is the extent to which the programming language used in writing computer programs affects the form that the final program takes. This debate is analogous to that surrounding the Sapir–Whorf hypothesis^[2] in linguistics and cognitive science, which postulates that a particular spoken language's nature influences the habitual thought of its speakers. Different language patterns yield different patterns of thought. This idea challenges the possibility of representing the world perfectly with language, because it acknowledges that the mechanisms of any language condition the thoughts of its speaker community.
source: http://en.wikipedia.org/wiki/Computer_programming

 

What is a Network?

What is a Network?

 

A network is basically a set of two or more articles that are linked so the computers can share resources, such as printers, software, and internet connections. Networked computers can also share files without having to transfer data using a disk or data key. And users of networked computers can also communicate electronically without use of the internet.
Computers within a network can be linked several ways: though cables, telephone lines, radio waves, satellites, or infrared beams. There are also three basic types of networks: Local Area Networks (LANs), Metropolitan Area Networks (MANs), and Wide Area Networks (WANs).
 

Local Area Networks

A Local Area Network (LAN) is basically a smaller network that's confined to a relatively small geographic area. LAN computers are rarely more than a mile apart. Examples of common LANs are networked computers within a writing lab, school, or building.
Within a LAN network, one computer is the file server. This means that it stores all software that controls the network, and it also stores the software that can be shared among computers in the network. The file server is the heart of the LAN.
The computers attached to the file server are called workstations. Workstations can be less powerful than the file server because they don't have to store as many files and applications as the file server, and they are not always on and working to keep the network up and running. However, workstations may also have additional software stored on their hard drives. Most LANS are connected using cables.
 

Metropolitan Area Networks

A Metropolitan Area Network (MAN) connect 2 or more LANs together but does not span outside the boundaries of a city, town, or metropolitan area. Within this type of network is also the Campus Area Network (CAN), which is generally smaller than a MAN, connecting LANs within a limited functional area, like a college campus, military base, or industrial complex.
 

Wide Area Networks

Wide Area Networks connect larger geographic areas. Often, smaller LANs are interconnected to form a large WAN. For instance, an office LAN in Los Angeles may be connected to office LANs for the same company in New York, Toronto, Paris, and London to form a WAN spanning the whole company. The individual offices are no longer part of individual LANs, they are instead part of a worldwide WAN.
The connection of this type of network is complicated. WANs are normally connected using multiplexers connect local and metropolitan networks to global communications networks like the Internet.
source: http://www.spamlaws.com/what-is-network.html

Communication media

Communication media

 

Computer networks can be classified according to the hardware and associated software technology that is used to interconnect the individual devices in the network, such as electrical cable (HomePNA, power line communication, G.hn), optical fiber, and radio waves (wireless LAN). In the OSI model, these are located at levels 1 and 2.
A well-known family of communication media is collectively known as Ethernet. It is defined by IEEE 802 and utilizes various standards and media that enable communication between devices. Wireless LAN technology is designed to connect devices without wiring. These devices use radio waves or infrared signals as a transmission medium.

Wired technologies

The order of the following wired technologies is, roughly, from slowest to fastest transmission speed.

• Twisted pair wire is the most widely used medium for telecommunication. Twisted-pair cabling consist of copper wires that are twisted into pairs. Ordinary telephone wires consist of two insulated copper wires twisted into pairs. Computer networking cabling (wired Ethernet as defined by IEEE 802.3) consists of 4 pairs of copper cabling that can be utilized for both voice and data transmission. The use of two wires twisted together helps to reduce crosstalk and electromagnetic induction. The transmission speed ranges from 2 million bits per second to 10 billion bits per second. Twisted pair cabling comes in two forms: unshielded twisted pair (UTP) and shielded twisted-pair (STP). Each form comes in several category ratings, designed for use in various scenarios.

• Coaxial cable is widely used for cable television systems, office buildings, and other work-sites for local area networks. The cables consist of copper or aluminum wire surrounded by an insulating layer (typically a flexible material with a high dielectric constant), which itself is surrounded by a conductive layer. The insulation helps minimize interference and distortion. Transmission speed ranges from 200 million bits per second to more than 500 million bits per second.

• 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.

• An optical fiber is a glass fiber. It uses pulses of light to transmit data. Some advantages of optical fibers over metal wires are less transmission loss, immunity from electromagnetic radiation, and very fast transmission speed, up to trillions of bits per second. One can use different colors of lights to increase the number of messages being sent over a fiber optic cable.

Wireless technologies

• Terrestrial microwave – Terrestrial microwave communication uses Earth-based transmitters and receivers resembling satellite dishes. Terrestrial microwaves are in the low-gigahertz range, which limits all communications to line-of-sight. Relay stations are spaced approximately 48 km (30 mi) apart.

• Communications satellites – The satellites communicate via microwave radio waves, which are not deflected by the Earth's atmosphere. The satellites are stationed in space, typically in geosynchronous orbit 35,400 km (22,000 mi) 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. The systems divide the region covered into multiple geographic areas. Each area has a low-power transmitter or radio relay antenna device to relay calls from one area to the next area.

• Radio and spread spectrum technologies – Wireless local area network use a high-frequency radio technology similar to digital cellular and a low-frequency radio technology. Wireless LANs use spread spectrum technology to enable communication between multiple devices in a limited area. IEEE 802.11 defines a common flavor of open-standards wireless radio-wave technology.

• Infrared communication can transmit signals for small distances, typically no more than 10 meters. In most cases, line-of-sight propagation is used, which limits the physical positioning of communicating devices.

• A global area network (GAN) is a network used for supporting mobile across an arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge in mobile communications is handing off user communications from one local coverage area to the next. In IEEE Project 802, this involves a succession of terrestrial wireless LANs.^[4]

Exotic technologies

There have been various attempts at transporting data over exotic media:

• IP over Avian Carriers was a humorous April fool's Request for Comments, issued as RFC 1149. It was implemented in real life in 2001.^[5]

• Extending the Internet to interplanetary dimensions via radio waves.^[6]

Both cases have a large round-trip delay time, which prevents useful communication.
source :http://en.wikipedia.org/wiki/Computer_network

Computer network

A computer network, or simply a network, is a collection of computers and other hardware interconnected by communication channels that allow sharing of resources and information.^[1] Where at least one process in one device is able to send/receive data to/from at least one process residing in a remote device, then the two devices are said to be in a network. A network is a group of devices connected to each other. Networks may be classified into a wide variety of characteristics, such as the medium used to transport the data, communications protocol used, scale, topology, benefit, and organizational scope.
Communications protocols define the rules and data formats for exchanging information in a computer network, and provide the basis for network programming. Well-known communications protocols include two Ethernet, a hardware and link layer standard that is ubiquitous in local area networks, and the Internet protocol suite, which defines a set of protocols for internetworking, i.e. for data communication between multiple networks, as well as host-to-host data transfer, and application-specific data transmission formats.
Computer networking is sometimes considered a sub-discipline of electrical engineering, telecommunications, computer science, information technology or computer engineering, since it relies upon the theoretical and practical application of these disciplin

source: http://en.wikipedia.org/wiki/Computer_network

What are the components of a CPU?

 The CPU is comprised of three main parts:

ALU (Arithmetic Logic Unit): Does the actual logical comparisons that need to be processed.

Control Unit: Can execute or store the results coming out of the ALU.

Registers: Stores the data that is to be executed next.

How to use your computer

How to use your computer

The Mouse

The Mouse is the device you will use for most of your interactions with your PC. It is primarily used to move the pointer arrow around the screen and to select and activate options.
The mouse buttons are used to interact with whatever is on the screen where the pointer is located. The left mouse button is the one you will use most often. Clicking this tells the PC to select an item and is called 'left-clicking'. To activate an item and use it, you will often need to click the button twice or 'double-click'.
If you click and hold the left button and move the mouse, this has the effect of 'dragging' a selected object about the screen until you release the button.
Left-clicking on text in a document will place a flashing cursor at that point, which allows you to begin typing there.
The right mouse button is usually used to bring up a small menu window that gives options specific to the selected object. To use a particular option you just left-click it.
Common right-click menu options for an object include: Open (activate, same as if you double-click), Cut (remove to be placed elsewhere), Copy (make a copy of this object) , Create Shortcut (create a shortcut link elsewhere to give quick access to this object), Delete (remove permanently), Rename (give a new name), and Properties (find out information).
You may have a third smaller button in the center of your mouse that can be rolled forwards and backwards. This mouse-wheel can be used to move up and down through documents.

The Keyboard

Keyboards can come in a number of different layouts. QWERTY keyboards are so-called because the top line of letters, underneath the number line, begins with those six letters.
There are a number of other useful keys too. The Enter key confirms a selection, and pressing the Shift or Caps Lock keys activates uppercase letters when you type, or special characters on number keys.
There are a number of function keys beginning with F1, F2 and so on, and these are used to access different options in different programs. Other keys such as Home, End and Page Up and Page Down allow you to quickly move about within a document.
The arrow keys move the text typing cursor up, down, left or right.
You may have a number keypad too. This acts as an alternative to the number keys above the main letter keys. Pressing the Num Lock button will switch between using the keypad as number keys, or arrow keys.
There are certain key combinations that can be used to perform particular tasks. See Keyboard Shortcuts for more information.

The Desktop

The Desktop is the main background on your screen when you are working on your PC. It consists of a background picture or 'wallpaper', any program shortcuts you have created, and the Taskbar.
The Taskbar runs along the bottom of the screen (or the side, or top - you can click and drag it wherever you want to place it) and shows the date and time as well as giving you access to the Start menu. You can also put program shortcuts on the taskbar so that one click will activate a particular program.
Left-clicking on the Start menu brings up the All Programs list of shortcuts to programs you have on your computer. These are often arranged in 'folders', so that you can find all the programs made by one company under a folder in their name.
By holding the mouse pointer over a folder you will see a new list appear of whatever programs are within that folder. Some programs will have several related entries - so you might see a README (a document that has instructions for using the program), or an option to Uninstall (remove the program from your PC).
You can access your Desktop settings by right-clicking anywhere on the background wallpaper and then clicking on Properties.

Windows

The operating system that runs all your programs and allows you to easily interact with your PC is called Windows for a good reason. It allows you to work with several different items and programs at once, all in their own 'window'. A window is like a smaller screen contained within a box on your main screen.
When you are working with a window it is brought to the 'front' of the screen, overlapping the Desktop and any other items behind it, so that you can focus on whatever is in the window.

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In this way, you can have several windows open at the same time containing different programs that can be opened, closed, and re-arranged across the screen. This means you can do multiple tasks at the same time, such as playing your favorite music while writing an e-mail for example.

Files And Folders

Every item stored on your PC, whether it is a document, picture, song, game or whatever, is a file. A file can only be activated by programs that understand and are able to use it.
For example, you cannot use a music program to open a letter document. Each program on your computer understands what type of file it can or cannot use by the fact that every file has a filetype given to it.
This takes the form of a few extra letters or numbers added on to its name after a full stop. So for example, 'myletter.doc' is a file named 'myletter' with the filetype '.doc', meaning it will require a program that understands doc (document) files, such as Microsoft Word.
There are many different filetypes. Read Common Filetypes for a list of the more common.
Any one single program or application can actually consist of many different files, all working together to achieve the same task.
This means modern computers can have millions of files spread across different locations. Folders help organize these files, so that for each particular program there can be one folder that contains all the files it needs in one place.
A folder can even contain other folders inside it called 'sub-folders'. For example, to organize your holiday pictures together you might have a folder called 'Holidays 2006' and then other sub-folders within that for each particular holiday destination, for example 'France 2006'.
You can create your own folders. You can also create shortcuts to make it easier to find particular files. Read Organize Your Files/Folders/Shortcuts to learn more.
You can take a look at an example of a folder now. On your Desktop you should see a shortcut icon called My Documents. Double-clicking this will open a new window showing the contents of the My Documents folder.

My Documents

My Documents is a folder on your computer that is automatically created for you, so that you can store all your own files in one place away from all the program and system folders, making them easier to find.
Double-click the My Documents shortcut on the Desktop and you will see a list of folders, within which are your files. To see what is inside a folder, double-click on it and the current window will change to show the contents.
You may see some other folders which have been automatically created for you to help you store certain types of files, such as My Music, My Pictures and My Videos. However, you do not have to use these, and you can store any of your files in any folder you want to. You don't even have to use My Documents, though it is usually easier if you do.

My Computer

Close any open windows by clicking the X button in the top-right. On your Desktop, double-click the My Computer shortcut and you will see a list of locations where files are stored on your PC's disk drives.
If you double-click the Local Disk (your Hard Disk, usually C: ) you can see all the folders that are stored on your Hard Disk - this will be the part of your PC where all your files are usually kept, unless you have extra disk drives.
The Program Files and Windows folders here are especially important, and should usually not be changed as they contain the files necessary for programs and Windows to run.
All files take up 'disk space' and your computer only has so much room to store them all. If you want to see how much of your Hard Disk you have used up so far, and how much space you have left, double-click My Computer again and this time, instead of double-clicking the Local Disk, right-click it and select Properties.
If you find you are running out of disk space, you should try to clean up some unnecessary files. Read Clean Out And Clean Up Files for more information.
If you decide you need a lot more space than you have, it is possible to buy extra Hard Drives and connect them to your computer.
If you right-click the My Computer icon on the Desktop rather than double-clicking it, and then click on Properties, you can see detailed information about your PC including your hardware devices and performance settings.

The Control Panel

The Control Panel is a collection of all the important options you will need when you want to change settings on your computer. You can find control icons for display, sound, Internet, hardware, programs, security and system settings. To access the Control Panel, click the Start menu and then click Control Panel.
Changing settings in the Control Panel can make big changes to your PC's setup. For example, the Appearance and Themes or Display options in the Control Panel can affect the quality of your screen and the size of text. Read Change Display Settings And Text Size for more information.
Click on your Start menu and then Help and Support for more help with using your PC and particular Windows features.
source: http://www.easycomputertips.com/article-howtouseyourpc-thebasics.html