REFERENCES . .

 > relation between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to refer to the second object. The second object – the one to which the first object refers – is called the referent of the first object.

The term reference is used in many spheres of human knowledge, adopting shades of meaning particular to the contexts in which it is used.

References can take on many forms, including: a thought, a sensory perception that is audible (onomatopoeia), visual (text), olfactory, or tactile, emotional state, relationship with other,^[1]spacetime coordinate, symbolic or alpha-numeric, a physical object or an energy projection; but, other concrete and abstract contexts exist as methods of defining references within the scope of the various fields that require an origin, point of departure, or an original form. This includes methods that intentionally hide the reference from some observers, as in cryptography.

5 Generations of Computer

First Generation (1940-1956) Vacuum Tubes 

The first computers used vacuum tubes for circuitry and magnetic drums for memory, and were often enormous, taking up entire rooms. They were very expensive to operate and in addition to using a great deal of electricity, generated a lot of heat, which was often the cause of malfunctions.

First generation computers relied on machine language, the lowest-level programming language understood by computers, to perform operations, and they could only solve one problem at a time. Input was based on punched cards and paper tape, and output was displayed on printouts. 

Second Generation (1956-1963) Transistors 

Transistors replaced vacuum tubes and ushered in the second generation of computers. The transistor was invented in 1947 but did not see widespread use in computers until the late 1950s. The transistor was far superior to the vacuum tube, allowing computers to become smaller, faster, cheaper, more energy-efficient and more reliable than their first-generation predecessors. Though the transistor still generated a great deal of heat that subjected the computer to damage, it was a vast improvement over the vacuum tube. Second-generation computers still relied on punched cards for input and printouts for output. 

Third Generation (1964-1971) Integrated Circuits  

The development of the integrated circuit was the hallmark of the third generation of computers. Transistors were miniaturized and placed on silicon chips, called semiconductors, which drastically increased the speed and efficiency of computers.

Instead of punched cards and printouts, users interacted with third generation computers through keyboards and monitorsand interfaced with an operating system, which allowed the device to run many different applications at one time with a central program that monitored the memory. Computers for the first time became accessible to a mass audience because they were smaller and cheaper than their predecessors. 

Fourth Generation (1971-Present) Microprocessors   

The microprocessor brought the fourth generation of computers, as thousands of integrated circuits were built onto a single silicon chip. What in the first generation filled an entire room could now fit in the palm of the hand. The Intel 4004 chip, developed in 1971, located all the components of the computer—from the central processing unit and memory to input/output controls—on a single chip.

In 1981 IBM introduced its first computer for the home user, and in 1984 Apple introduced the Macintosh. Microprocessors also moved out of the realm of desktop computers and into many areas of life as more and more everyday products began to use microprocessors.

As these small computers became more powerful, they could be linked together to form networks, which eventually led to the development of the Internet. Fourth generation computers also saw the development of GUIs, the mouse and handhelddevices. 

Fifth Generation (Present and Beyond) Artificial Intelligence 

Fifth generation computing devices, based on artificial intelligence, are still in development, though there are some applications, such as voice recognition, that are being used today. The use of parallel processing and superconductors is helping to make artificial intelligence a reality. Quantum computation and molecular and nanotechnology will radically change the face of computers in years to come. The goal of fifth-generation computing is to develop devices that respond to natural language input and are capable of learning and self-organization.

WORLD WIDE WEB IS ?

The World Wide Web (abbreviated as WWW or W3commonly known as the web), is a system of interlinked hypertext documents accessed via the Internet. With a web browser, one can view web pages that may contain text, images, videos, and other multimedia, and navigate between them via hyperlinks. 

 

WHAT IS INTERNET ??

The Internet is a global network connecting millions of computers. More than 100 countries are linked into exchanges of data, news and opinions. According to Internet World Stats, as of December 31, 2011 there was an estimated 2,267,233,742 Internet users worldwide. This represents 32.7% of the world's population.

Unlike online services, which are centrally controlled, the Internet is decentralized by design. Each Internet computer, called a host, is independent. Its operators can choose which Internet services to use and which local services to make available to the global Internet community. Remarkably, this anarchy by design works exceedingly well. There are a variety of ways to access the Internet. Most online services offer access to some Internet services. It is also possible to gain access through a commercial Internet Service Provider (ISP).  

  

WHO OWNS THE INTERNET ? 

No one actually owns the Internet, and no single person or organization controls the Internet in its entirety. The Internet is more of a concept than an actual tangible entity, and it relies on a physical infrastructure that connectsnetworks to other networks. 

IS WEB AND INTERNET IS SAME ? 

The Internet is not synonymous with World Wide Web. The Internet is a massive network of networks, a networking infrastructure. It connects millions of computers together globally, forming a network in which any computer can communicate with any other computer as long as they are both connected to the Internet. The World Wide Web, or simply Web, is a way of accessing information over the medium of the Internet. It is an information-sharing model that is built on top of the Internet.

COMMUNICATION DEVICES

Technology changes so quickly that new communication devices are being invented and improved all the time. Computers and mobile phones are smaller and are able to do more things. Can you imagine what Marconi would think of mobile phones today? Communication makes our modern world a small place because it allows us to talk easily with people all over the globe. In Chapter 3, you can discover how the internet and email have had a major impact on the way we communicate. This chapter looks at some of the other modern devices that we use to send and receive information and stay in touch with each other.  

SATELLITES 

There are more than 100 communication satellites that are orbiting (travelling around) the Earth. They are called  geostationary satellites, which means that they travel at the same speed as the Earth. The satellites receive signals from transmitter dishes and send the signals to other stations on Earth. This is how information is sent between countries around the world. See image 2

Some portable telephones are linked to satellites so that the person using the phone can make a call from anywhere in the world. Satellite phones are an important link for people who are living or working in remote areas where mobile phones do not work. 

CLEVER MOBILE PHONES  
Computer chips mean that mobile phones can be used to make and receive phone calls, send text messages, take photos and movies, tell the time, and listen to the radio. 

SMS  
SMS stands for Short Messaging Service. This is a feature of mobile phones that lets you type messages by using the letters on your keypad. Once you have finishing writing your message, you can send it to other mobile phones. It is useful for people who want to send short messages and do not want to make a telephone call. 
















Video phones and video conferencing

In the 1990s, videophones were created by attaching a video screen and camera to a telephone. This allowed people to have a conversation and see a picture of the other person on the screen. Videophones were used by some companies but were not a common feature on many phones.

These days, high-speed internet connections send information more quickly. This makes it easier to hold a videoconference. This happens when people talk to each other over the phone and see each other on a screen. This is a very useful communication tool for people who live in outback areas because it lets them see and talk to people who might be thousands of kilometres away.

Music

The way that we listen to music has changed a lot since the times of vinyl records and cassettes. You can buy music on a compact disc or you can find it on the internet. Music can be downloaded from the internet onto most home computers. Special music files, called MP3 files, can then be transferred onto an MP3 player or a compact disc. You then listen to your favourite songs. 

TYPE OF TRANSMISSION MEDIA.

TRANSMISSION MEDIA : 

The means through which data is transformed from one place to another is called transmission or communication media. There are two categories of transmission media used in computer communications.    

• BOUNDED/GUIDED MEDIA 
• UNBOUNDED/UNGUIDED MEDIA  

1. BOUNDED MEDIA:

Bounded media are the physical links through which signals are confined to narrow path. These are also called guide media. Bounded media are made up o a external conductor (Usually Copper) bounded by jacket material. Bounded media are great for LABS because they offer high speed, good security and low cast. However, some time they cannot be used due distance communication. Three common types of bounded media are used of the data transmission. These are

• Coaxial Cable
• Twisted Pairs Cable
• Fiber Optics Cable

COAXIAL CABLE:

Coaxial cable is very common & widely used commutation media. For example TV wire is usually coaxial.

Coaxial cable gets its name because it contains two conductors that are parallel to each other. The center conductor in the cable is usually copper. The copper can be either a solid wire or stranded martial.

Outside this central Conductor is a non-conductive material. It is usually white, plastic material used to separate the inner Conductor form the outer Conductor. The other Conductor is a fine mesh made from Copper. It is used to help shield the cable form EMI.

Outside the copper mesh is the final protective cover. (as shown in Fig)

The actual data travels through the center conductor in the cable. EMI interference is caught by outer copper mesh. There are different types of coaxial cable vary by gauge & impedance.

Gauge is the measure of the cable thickness. It is measured by the Radio grade measurement, or RG number. The high the RG number, the thinner the central conductor core, the lower the number the thicker the core. 

Twisted Pair Cable

The most popular network cabling is Twisted pair. It is light weight, easy to install, inexpensive and support many different types of network. It also supports the speed of 100 mps.Twisted pair cabling is made of pairs of solid or stranded copper twisted along each other. The twists are done to reduce vulnerably to EMI and cross talk. The number of pairs in the cable depends on the type. The copper core is usually 22-AWG or 24-AWG, as measured on the American wire gauge standard. There are two types of twisted pairs cabling

1. Unshielded twisted pair (UTP)

2. Shielded twisted pair (STP)

Fiber Optics

Fiber optic cable uses electrical signals to transmit data. It uses light. In fiber optic cable light only moves in one direction for two way communication to take place a second connection must be made between the two devices. It is actually two stands of cable. Each stand is responsible for one direction of communication. A laser at one device sends pulse of light through this cable to other device. These pulses translated into “1’s” and “0’s” at the other end.

In the center of fiber cable is a glass stand or core. The light from the laser moves through this glass to the other device around the internal core is a reflective material known as CLADDING. No light escapes the glass core because of this reflective cladding.  

2.Unbound transmission media  

Extend beyond the limiting confines of cabling. They provide an excellent Communication Networks alternative for WANS. The lack of physical restrictions provides larger bandwidth as well as wide area capabilities. Unbound media typically operate atvery high frequencies.  The three types of unbound transmission media are:  

1.Radio wave 

2.Micro wave

 3.Infrared.  

Radiowaves Transmission

Although Radio waves are prevalent and well understood, we are just beginning to realize their enormous potential as a networking medium. Radio waves can operate on a single or multiple frequency bands. 

Microwave Transmission

Microwaves have been used in data communications for a long time. They have a higher frequency than radio waves and therefore can handle larger amounts of data. 

Infra Red.

Infrared offers a great unbound photonic solution. Like fiber-optic cabling, infrared communications use light, so they are not bound by the limitations of electricity.