Minerals found in earth crust:

Most minerals are made up of a cation (a positively charged ion) or several cations and an anion (a negatively charged ion (e.g., S2–)) or an anion complex (e.g., SO42–). For example, in the mineral hematite (Fe2O3), the cation is Fe3+ (iron) and the anion is O2– (oxygen). We group minerals into classes on the basis of their predominant anion or anion group. These include oxides, sulphides, carbonates, silicates, and others. Silicates are by far the predominant group in terms of their abundance within the crust and mantle.  Some examples of minerals from the different mineral groups are given in Table

Group	Examples
Oxides	Hematite (iron oxide Fe2O3), corundum (aluminum oxide Al2O3), water ice (H2O)
Sulphides	Galena (lead sulphide PbS), pyrite (iron sulphide FeS2), chalcopyrite (copper-iron sulphide CuFeS2)
Sulphates	Gypsum (calcium sulphate CaSO4·H2O), barite (barium sulphate BaSO4) (Note that sulphates are different from sulphides. Sulphates have the SO4–2 ion while sulphides have the S–2 ion)
Halides	Fluorite (calcium flouride CaF2), halite (sodium chloride NaCl) (Halide minerals have halogen elements as their anion — the minerals in the second last column on the right side of the periodic table, including F, Cl, Br, etc. — see Appendix 1.)
Carbonates	Calcite (calcium carbonate CaCO3), dolomite (calcium-magnesium carbonate (Ca,Mg)CO3)
Phosphates	Apatite (Ca5(PO4)3(OH)), Turquoise (CuAl6(PO4)4(OH)8·5H2O)
Silicates	Quartz (SiO2), feldspar (sodium-aluminum silicate NaAlSi3O8), olivine (iron or magnesium silicate (Mg,Fe)2SiO4)   (Note that in quartz the anion is oxygen, and while it could be argued, therefore, that quartz is an oxide, it is always classed with the silicates.)
Native minerals	Gold (Au), diamond (C), graphite (C), sulphur (S), copper (Cu)

Table:  The main mineral groups and some examples of minerals in each group

Oxide minerals have oxygen (O2–) as their anion, but they exclude those with oxygen complexes such as carbonate (CO32–), sulphate (SO42–), and silicate (SiO44–). The most important oxides are the iron oxides hematite and magnetite (Fe2O3 and Fe3O4, respectively). Both of these are important ores of iron. Corundum (Al2O3) is an abrasive, but can also be a gemstone in its ruby and sapphire varieties. If the oxygen is also combined with hydrogen to form the hydroxyl anion (OH–) the mineral is known as a hydroxide. Some important hydroxides are limonite and bauxite, which are ores of iron and aluminium respectively. Frozen water (H2O) is a mineral (an oxide), but liquid water is not because it doesn’t have a regular lattice.

Sulphides are minerals with the S–2 anion, and they include galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS2), and molybdenite (MoS2), which are the most important ores of lead, zinc, copper, and molybdenum respectively. Some other sulphide minerals are pyrite (FeS2), bornite (Cu5FeS4), stibnite (Sb2S3), and arsenopyrite (FeAsS).

Sulphates are minerals with the SO4–2 anion, and these include anhydrite (CaSO4) and its cousin gypsum (CaSO4.2H2O) and the sulphates of barium and strontium: barite (BaSO4) and celestite (SrSO4). In all of these minerals, the cation has a +2 charge, which balances the –2 charge on the sulphate ion.

The halides are so named because the anions include the halogen elements chlorine, fluorine, bromine, etc. Examples are halite (NaCl), cryolite (Na3AlF6), and fluorite (CaF2).

The carbonates include minerals in which the anion is the CO3–2 complex. The carbonate combines with +2 cations to form minerals such as calcite (CaCO3), magnesite (MgCO3), dolomite ((Ca,Mg)CO3), and siderite (FeCO3). The copper minerals malachite and azurite are also carbonates.

In phosphate minerals, the anion is the PO4–3 complex. An important phosphate mineral is apatite (Ca5(PO4)3(OH)), which is what your teeth are made of.

The silicate minerals include the elements silicon and oxygen in varying proportions ranging from Si : O2 to Si : O4.

Native minerals are single-element minerals, such as gold, copper, sulphur, and graphite.

Silicate Minerals

The vast majority of the minerals that make up the rocks of Earth’s crust are silicate minerals. These include minerals such as quartz, feldspar, mica, amphibole, pyroxene, olivine, and a great variety of clay minerals. The building block of all of these minerals is the silica tetrahedron, a combination of four oxygen atoms and one silicon atom. These are arranged such that planes drawn through the oxygen atoms form a tetrahedron (Figure 2.6). Since the silicon ion has a charge of +4 and each of the four oxygen ions has a charge of –2, the silica tetrahedron has a net charge of –4.

In silicate minerals, these tetrahedra are arranged and linked together in a variety of ways, from single units to complex frameworks (Figure 2.9). The simplest silicate structure, that of the mineral olivine, is composed of isolated tetrahedra bonded to iron and/or magnesium ions. In olivine, the –4 charge of each silica tetrahedron is balanced by two divalent(i.e., +2) iron or magnesium cations. Olivine can be either Mg2SiO4 or Fe2SiO4, or some combination of the two (Mg,Fe)2SiO4. The divalent cations of magnesium and iron are quite close in radius (0.73 versus 0.62 angstroms[1]). Because of this size similarity, and because they are both divalent cations (both have a charge of +2), iron and magnesium can readily substitute for each other in olivine and in many other minerals.

T

As already noted, the +2 ions of iron and magnesium are similar in size (although not quite the same). This allows them to substitute for each other in some silicate minerals. In fact, the common ions in silicate minerals have a wide range of sizes, as shown in Figure 2.11. All of the ions shown are cations, except for oxygen. Note that iron can exist as both a +2 ion (if it loses two electrons during ionization) or a +3 ion (if it loses three). Fe2+ is known as ferrous iron. Fe3+ is known as ferric iron. Ionic radii are critical to the composition of silicate minerals, so we’ll be referring to this diagram again.

Figure 2.11 The ionic radii (effective sizes) in angstroms, of some of the common ions in silicate minerals

The structure of the single-chain silicate pyroxene is shown on Figures 2.12 and 2.13. In pyroxene, silica tetrahedra are linked together in a single chain, where one oxygen ion from each tetrahedron is shared with the adjacent tetrahedron, hence there are fewer oxygens in the structure. The result is that the oxygen-to-silicon ratio is lower than in olivine (3:1 instead of 4:1), and the net charge per silicon atom is less (–2 instead of –4), since fewer cations are necessary to balance that charge. Pyroxene compositions are of the type MgSiO3, FeSiO3, and CaSiO3, or some combination of these. Pyroxene can also be written as (Mg,Fe,Ca)SiO3, where the elements in the brackets can be present in any proportion. In other words, pyroxene has one cation for each silica tetrahedron (e.g., MgSiO3) while olivine has two (e.g., Mg2SiO4). Because each silicon ion is +4 and each oxygen ion is –2, the three oxygens (–6) and the one silicon (+4) give a net charge of –2 for the single chain of silica tetrahedra. In pyroxene, the one divalent cation (2+) per tetrahedron balances that –2 charge. In olivine, it takes two divalent cations to balance the –4 charge of an isolated tetrahedron.

The structure of pyroxene is more “permissive” than that of olivine — meaning that cations with a wider range of ionic radii can fit into it. That’s why pyroxenes can have iron (radius 0.63 Å) or magnesium (radius 0.72 Å) or calcium (radius 1.00 Å) cations.

Figure 2.12 A depiction of the structure of pyroxene. The tetrahedral chains continue to left and right and each is interspersed with a series of divalent cations. If these are Mg ions, then the formula is MgSiO3.

Figure 2.13 A single silica tetrahedron (left) with  four oxygen ions per silicon ion (SiO4). Part of a single chain of tetrahedra (right), where the oxygen atoms at the adjoining corners are shared between two tetrahedra (arrows). For a very long chain the resulting ratio of silicon to oxygen is 1 to 3 (SiO3).

Exercises

Exercise 2.4 Oxygen Deprivation

The diagram below represents a single chain in a silicate mineral. Count the number of tetrahedra versus the number of oxygen ions (yellow spheres). Each tetrahedron has one silicon ion so this should give the ratio of Si to O in single-chain silicates (e.g., pyroxene).

The diagram below represents a double chain in a silicate mineral. Again, count the number of tetrahedra versus the number of oxygen ions. This should give you the ratio of Si to O in double-chain silicates (e.g., amphibole).

In amphibole structures, the silica tetrahedra are linked in a double chain that has an oxygen-to-silicon ratio lower than that of pyroxene, and hence still fewer cations are necessary to balance the charge. Amphibole is even more permissive than pyroxene and its compositions can be very complex. Hornblende, for example, can include sodium, potassium, calcium, magnesium, iron, aluminum, silicon, oxygen, fluorine, and the hydroxyl ion (OH–).

In mica structures, the silica tetrahedra are arranged in continuous sheets, where each tetrahedron shares three oxygen anions with adjacent tetrahedra. There is even more sharing of oxygens between adjacent tetrahedra and hence fewer charge-balancing cations are needed for sheet silicate minerals. Bonding between sheets is relatively weak, and this accounts for the well-developed one-directional cleavage (Figure 2.14). Biotite mica can have iron and/or magnesium in it and that makes it a ferromagnesian silicate mineral (like olivine, pyroxene, and amphibole). Chlorite is another similar mineral that commonly includes magnesium. In muscovite mica, the only cations present are aluminum and potassium; hence it is a non-ferromagnesian silicate mineral.

Figure 2.14 Biotite mica (left) and muscovite mica (right). Both are sheet silicates and split easily into thin layers along planes parallel to the sheets. Biotite is dark like the other iron- and/or magnesium-bearing silicates (e.g., olivine, pyroxene, and amphibole), while muscovite is light coloured. (Each sample is about 3 cm across.)

Apart from muscovite, biotite, and chlorite, there are many other sheet silicates (or phyllosilicates), which usually exist as clay-sized fragments (i.e., less than 0.004 mm). These include the clay minerals kaolinite, illite, and smectite, and although they are difficult to study because of their very small size, they are extremely important components of rocks and especially of soils.

All of the sheet silicate minerals also have water in their structure.

Silica tetrahedra are bonded in three-dimensional frameworks in both the feldspars and quartz. These are non-ferromagnesian minerals — they don’t contain any iron or magnesium. In addition to silica tetrahedra, feldspars include the cations aluminum, potassium, sodium, and calcium in various combinations. Quartz contains only silica tetrahedra.

The three main feldspar minerals are potassium feldspar, (a.k.a. K-feldspar or K-spar) and two types of plagioclase feldspar: albite (sodium only) and anorthite (calcium only). As is the case for iron and magnesium in olivine, there is a continuous range of compositions (solid solution series) between albite and anorthite in plagioclase. This is because the calcium and sodium ions are almost identical in size (1.00 Å versus 0.99 Å). Any intermediate compositions between CaAl2Si3O8 and NaAlSi3O8 can exist (Figure 2.15). This is a little bit surprising because, although they are very similar in size, calcium and sodium ions don’t have the same charge (Ca2+ versus Na+). This problem is accounted for by corresponding substitution of Al3+ for Si4+. Therefore, albite is NaAlSi3O8 (one Al and three Si) while anorthite is CaAl2Si2O8 (two Al and two Si), and plagioclase feldspars of intermediate composition have intermediate proportions of Al and Si. This is called a “coupled-substitution.”

The intermediate-composition plagioclase feldspars are oligoclase (10% to 30% Ca), andesine (30% to 50% Ca), labradorite (50% to 70% Ca), and bytownite (70% to 90% Ca). K-feldspar (KAlSi3O8) has a slightly different structure than that of plagioclase, owing to the larger size of the potassium ion (1.37 Å) and because of this large size, potassium and sodium do not readily substitute for each other, except at high temperatures. These high-temperature feldspars are likely to be found only in volcanic rocks because intrusive igneous rocks cool slowly enough to low temperatures for the feldspars to change into one of the lower-temperature forms.

In quartz (SiO2), the silica tetrahedra are bonded in a “perfect” three-dimensional framework. Each tetrahedron is bonded to four other tetrahedra (with an oxygen shared at every corner of each tetrahedron), and as a result, the ratio of silicon to oxygen is 1:2. Since the one silicon cation has a +4 charge and the two oxygen anions each have a –2 charge, the charge is balanced. There is no need for aluminum or any of the other cations such as sodium or potassium. The hardness and lack of cleavage in quartz result from the strong covalent/ionic bonds characteristic of the silica tetrahedron.

Exercises

Exercise 2.5 Ferromagnesian Silicates?

Silicate minerals are classified as being either ferromagnesian or non-ferromagnesian depending on whether or not they have iron (Fe) and/or magnesium (Mg) in their formula. A number of minerals and their formulas are listed below. For each one, indicate whether or not it is a ferromagnesian silicate.

Mineral	Formula	Ferromagnesian Silicate?
olivine	(Mg,Fe)2SiO4
pyrite	FeS2
plagioclase	CaAl2Si2O8
pyroxene	MgSiO3
hematite	Fe2O3
orthoclase	KAlSi3O8
quartz	SiO2

Mineral	Formula*	Ferromagnesian Silicate?
amphibole	Fe7Si8O22(OH)2
muscovite	K2Al4 Si6Al2O20(OH)4
magnetite	Fe3O4
biotite	K2Fe4Al2Si6Al4O20(OH)4
dolomite	(Ca,Mg)CO3
garnet	Fe2Al2Si3O12
serpentine	Mg3Si2O5(OH)4

                                    Quantum Theory

Planck’s radiation law

By the end of the 19th century, physicists almost universally accepted the wave theory of light. However, though the ideas of classical physics explain interference and diffraction phenomena relating to the propagation of light, they do not account for the absorption and emission of light. All bodies radiate electromagnetic energy as heat; in fact, a body emits radiation at all wavelengths. The energy radiated at different wavelengths is a maximum at a wavelength that depends on the temperature of the body; the hotter the body, the shorter the wavelength for maximum radiation. Attempts to calculate the energy distribution for the radiation from a blackbody using classical ideas were unsuccessful. (A blackbody is a hypothetical ideal body or surface that absorbs and reemits all radiant energy falling on it.) One formula, proposed by Wilhelm Wien of Germany, did not agree with observations at long wavelengths, and another, proposed by Lord Rayleigh (John William Strutt) of England, disagreed with those at short wavelengths.

In 1900 the German theoretical physicist Max Planck made a bold suggestion. He assumed that the radiation energy is emitted, not continuously, but rather in discrete packets called quanta. The energy E of the quantum is related to the frequency ν by E = hν. The quantity h, now known as Planck’s constant, is a universal constant with the approximate value of 6.62607 × 10−34 joule∙second. Planck showed that the calculated energy spectrum then agreed with observation over the entire wavelength range.

Einstein and the photoelectric effect

In 1905 Einstein extended Planck’s hypothesis to explain thephotoelectric effect, which is the emission of electrons by a metal surface when it is irradiated by light or more-energetic photons. The kinetic energy of the emitted electrons depends on the frequency ν of the radiation, not on its intensity; for a given metal, there is a threshold frequency ν0 below which no electrons are emitted. Furthermore, emission takes place as soon as the light shines on the surface; there is no detectable delay. Einstein showed that these results can be explained by two assumptions: (1) that light is composed of corpuscles or photons, the energy of which is given by Planck’s relationship, and (2) that an atom in the metal can absorb either a whole photon or nothing. Part of the energy of the absorbed photon frees an electron, which requires a fixed energy W, known as the work function of the metal; the rest is converted into the kinetic energy meu2/2 of the emitted electron (me is the mass of the electron and u is its velocity). Thus, the energy relation is 

If ν is less than ν0, where hν0 = W, no electrons are emitted. Not all the experimental results mentioned above were known in 1905, but all Einstein’s predictions have been verified since.

Bohr’s theory of the atom

BRITANNICA LISTS & QUIZZES

A major contribution to the subject was made by Niels Bohr of Denmark, who applied the quantum hypothesis to atomic spectra in 1913. The spectra of light emitted by gaseous atoms had been studied extensively since the mid-19th century. It was found that radiation from gaseous atoms at low pressure consists of a set of discrete wavelengths. This is quite unlike the radiation from a solid, which is distributed over a continuous range of wavelengths. The set of discrete wavelengths from gaseous atoms is known as a line spectrum, because the radiation (light) emitted consists of a series of sharp lines. The wavelengths of the lines are characteristic of the element and may form extremely complex patterns. The simplest spectra are those of atomic hydrogen and the alkali atoms (e.g., lithium, sodium, and potassium). For hydrogen, the wavelengths λ are given by the empirical formula 

where m and n are positive integers with n > m and R∞, known as theRydberg constant, has the value 1.097373157 × 107 per metre. For a given value of m, the lines for varying n form a series. The lines for m = 1, the Lyman series, lie in the ultraviolet part of the spectrum; those for m = 2, the Balmer series, lie in the visible spectrum; and those for m = 3, the Paschen series, lie in the infrared.

Bohr started with a model suggested by the New Zealand-born British physicist Ernest Rutherford. The model was based on the experiments ofHans Geiger and Ernest Marsden, who in 1909 bombarded gold atoms with massive, fast-moving alpha particles; when some of these particles were deflected backward, Rutherford concluded that the atom has a massive, charged nucleus. In Rutherford’s model, the atom resembles a miniature solar system with the nucleus acting as the Sun and the electrons as the circulating planets. Bohr made three assumptions. First, he postulated that, in contrast to classical mechanics, where an infinite number of orbits is possible, an electron can be in only one of a discrete set of orbits, which he termed stationary states. Second, he postulated that the only orbits allowed are those for which the angular momentum of the electron is a whole number n times ℏ (ℏ = h/2π). Third, Bohr assumed that Newton’s laws of motion, so successful in calculating the paths of the planets around the Sun, also applied to electrons orbiting the nucleus. The force on the electron (the analogue of the gravitational force between the Sun and a planet) is the electrostatic attraction between the positively charged nucleus and the negatively charged electron. With these simple assumptions, he showed that the energy of the orbit has the form

where E0 is a constant that may be expressed by a combination of the known constants e, me, and ℏ. While in a stationary state, the atom does not give off energy as light; however, when an electron makes a transition from a state with energy En to one with lower energy Em, a quantum of energy is radiated with frequency ν, given by the equation

 Inserting the expression for En into this equation and using the relation λν = c, where c is the speed of light, Bohr derived the formula for the wavelengths of the lines in the hydrogen spectrum, with the correct value of the Rydberg constant.

Bohr’s theory was a brilliant step forward. Its two most important features have survived in present-day quantum mechanics. They are (1) the existence of stationary, nonradiating states and (2) the relationship of radiation frequency to the energy difference between the initial and final states in atransition. Prior to Bohr, physicists had thought that the radiation frequency would be the same as the electron’s frequency of rotation in an orbit.

SCIENCE AND TECHNOLOGY

Religion

From the beginning of time, religion has been considered as the panacea for all ills, and mankind, despite moment of doubt, has always leaned on religious faith for solace.

Religion has been defined as a system of faith and worship in practice within a group of people living in a community. Belief in god or Supreme Being is the basic premise of all religions and even those who worship Nature bow down to a super power who make all the marvels of nature possible. Thousands of believers have pursued with single mindedness, the path of devotion for future salvation.

By religious behavior human seeks to adopt to cope with, or understand    dimensions of life beyond their explanations or control. These manifestations differed according to place and time. The simple folk never really doubted the existence of god.

Unfortunately although a number of people believe in him vaguely, each set of believers had its own version of God. They had their own theories of what he looked like and what he said to them. The most effective way to identify religious faith in society had always been through the ritualistic expressions. When divisions began to appear in these ritualistic expressions, conflict between various practitioners of faith resulted.

Religion has played a phenomenal role in shaping our history from the ancient to the modern times. When the age of reasoning succeeded the age of faith, God was temporarily buried and people wondered if human reason was so powerful did men need God?

Critical thinkers in 19th and 20th centuries began to say that religion is the opium of the people. According to Karl marks, religion suppressed social change. Darwin’s theory blew up basic religious tenets. Karl Marks saw close links between the ruling class and the heads of religions and was eager to blow up both. God was dead, announced Nietzsche.

Traditional function of religion seems to have been one of providing a system of meaningful interaction by defining taboos or reinforcing rules without which society could disintegrate. Young people today are perplexed looking for a fresh concept of faith which will give them freedom, side by side with stability.

E. M. Forster asserts that tolerance, good temper, and sympathy- they are what matter really, and if the human race is not to collapse, they must come to the front before long. The function of our universities is to produce in the students the quality of compassion for the suffering humanity and the quality which enables the individuals to treat one another in a truly democratic spirit.

Economic growth

Scientific discoveries and the consequential technological changes have completely revolutionized the life style and living standards of people.

Since the advent of industrial revolution, different periods have been marked by advances in different clusters of inventions. The first wave of invention that lasted for 60 years beginning 1785 was marked by progress in water, power, textiles and iron. The second wave lasted for 55 years between 1845 and 1900 and this was propelled by inventions in rail and steel. The third wave beginning 1900 and going up to the end of the first half of the century was marked by inventions in electricity, chemicals and internal combustion engine. The fourth wave was powered by oil, electronics, aviation and mass production. India is in the midst of fifth wave dominated by semiconductors, fiber optics, genetics and software.

Strides in the 20th century

The century opened on a bright note—with the electric powered lamp. Science then advanced at supersonic speed.

The automobile rolled out, the airplane took off, and man in a great leap conquered space. Information technology made possible a global village and artificial intelligence opened new windows to cyberspace. Man played God to create and to destroy. He split the atom to destroy his brothers and cloned beings to create a brave new world.

Information Technology

Information technology (IT), which comprises electronic computer technology and telecommunication technology, has in a few decades changed our society. Behind this development lies an advanced scientific and technical development originating from fundamental scientific inventions.

Information technology has been in the process of bringing about openness, networking, democratic functioning and social transformation. Technology is changing societies across the globe in terms of work, education thought process and overall work and life style. It brings transparency, responsibility, accountability and better social justice.

The rapid development of electronic computer technology started with the invention of the integrated circuit (IC) around 1960 and the microprocessor in 1970s; when the number of components on a chip became sufficiently large to allow the creation of a complete microcomputer.

Chip development has been marked to be equally dynamic and powerful development in telecommunication technology. Just as the IC has been and is a prime mover for electronic computer technology, ultra-rapid transistors and semiconductor lasers based on hetero structures of semiconductors are playing a decisive part in telecommunication.

The invention of the transistor just before1947 is usually taken to mark the start of the development of modern semiconductor technology (Nobel prize in physics 1956 to William B Shockley, John Bardeen and Walter H Brattain). With the transistor there came a component that was considerably smaller, more reliable and less energy consuming than the radio valve, which thus lost its importance.

In the beginning of the 1950s there were ideas and thoughts about manufacturing transistors resisters and condensers in a composite semiconductor block, an IC. The IC is more a technical invention than a discovery. However it is evident that it embraces many physical issues. One example is the question of how aluminum and gold, which are part of an IC, differ regarding their adhesion to silicon. Another question is how to produce dense layers that are only a few atoms thick.

M. J. Kelly director of research of Bell Laboratories had the fore-sight to recognize that reliable, expanded telephone communication required electronic, rather than electro-mechanical switching and better amplifiers. He formed a solid-state research group consisting of theoretical and experimental physicists.

The Transistor was born on 16-12-1947.

In 1951, three years after the discovery of amplifier in solid, transistors were produced commercially. Silicon transistor was produced in 1954 by Texas. By 1961, Texas and others commercially produced ICs in USA.

1. discrete transistors       

1. small scale integration        <100 components

1. medium scale integration    100 to 1000 components

1. large scale integration        1000 to 10000 components

1. very large scale integration    >10000 components           

Junction field effect transistor (JFET) was produced by Terzner in France in 1958.Metal oxide semiconductor field effect transistor (MOSFET) by Bell laboratories in 1960. Operation amplifier (a709) ICs were produced in 1964.

ICs have made the marriage of communication and computation possible-the digital signal processing.

The first microprocessor the Intel 4004 was launched in 1971. It contained 2300 transistors and ran at 0.1 MHz. in the early eighties, at the dawn of the PC era, the clock speed of a PC’s processor was 5 MHz. Fifty years on, we are surrounded by millions of transistors – in radios, television, telephone and computers.

THE PERSIONAL COMPUTER (PC)

California—the first Silicon Valley in USA; started in 1970-the experimental PC. The 1975 saw an Altair 8800 PC. But fully developed PC of Intel hardware and Microsoft software- emerged in 1981 only. Within two decades from 1981 to 2001, one billion PCs were sold all over the world.

INTERNET BACKGROUND

Advanced Research Project Agency (ARPA) was launched in USA around 1969 to set up a pocket switched network consisting of a subnet and host computers.  By 1974 ARPA invented a model of protocols known as TCP/IP for data communication over internetwoks. The TCP/IP model and protocols were specifically designed to handle communication over internetworks. By 1983 ARPANET was stable and successful. By 1984 NSF decided to build a backbone network. By 1990 internet was born in USA 3000 networks and 200,000 computers.  In 1992 internet society was formed. By 1995 there was exponential growth of internet services throughout the world.

TCP/IP reference model and TCP/IP protocol stack makes universal service possible and can be compared to telephone system.

INTERNET

Internet is a network of connections through which information from one point can be transmitted to another; in a way it is quite similar to the network of roads which facilitate movement of vehicles from one place to another. In road transport, a highway is a rather wide road unencumbered by obstacles so that a vehicle can move on them at very high speeds. Information super highways are similar connections that permit communication of digital information at very high speeds.

The rules of communication are often referred to as protocols. When a message is sent through internet, it is not transmitted through a dedicated line, as is the case with telephone. Instead the message is broken up into pieces (pockets) using a transmission protocol and the internet protocol assigned to each pocket, its distinctive identification, which includes the address of the sender as well as the receiver. The message is then re-assembled at the received end.

The transmission control protocol (TCP) breaks up the information sent on the internet, each containing 1-150 bytes. It numbers each of the units, puts each into a pocket and thus helps to send it over the network. Internet protocol (IP) governs the way these pockets are addressed and routed along the internet. Thus the various pockets that comprise a message may travel a different route and take a different time to arrive at its destination. Some may even get damaged on the way. At the recipient’s end TCP extracts the data from each pocket, checks for its accuracy, and reassembles them into their original order. If it finds that any data are lost or damaged, it requests the sender computer to transmit them again. Thus these protocols (TCP/IP) really make communication through the internet possible.

A machine (PC) is on the internet if it runs the TCP/IP protocol stack, has an IP address, and has the ability to send IP pockets to all the other machines on the internet. Internet had four main applications: E mail, news, remote login and file transfer.

Internet as a global information system

Transmission control protocol (TCP) and the internet protocol (IP) – these protocols are usually lumped together as TCP/IP and are embedded in the software for operating systems.

Servers

Servers are computers dedicated to the purpose of providing information to the internet. They run specialized software for each type of internet application. These include e-mail, discussion groups, long distance computing and file transfers.

Routers

Routers are computers that form part of the communication net and that route or direct the data along the best available paths into the networks.

The network architecture is referred to as TCP/IP. The data are transmitted in pockets. Many separate functions are to e performed in pocket transmission; such as pocket addressing, routing, and coping with pocket congestion.

Internet protocol (IP)

In this layer, the pockets of information are passed along the internet from router to router and to the host stations. No exact path is laid out before hand and the IP layers in the routers must provide the destination address for the next leg of the journey so to speak. This destination address is part of the IP header attached to the pocket. The source address is also included as part of the IP header. The problems of lost pockets or pockets arriving out of sequence are not a concern of the IP layer.

Transmission control protocol (TCP)

With TCP, information is passed back and forth between transport layers, which control the information flow. This includes such information as the correct sequencing of the pockets, replacement of lost pockets and adjusting the transmission rate of pockets to prevent congestion. The TCP layer is termed connection oriented, because sender and receiver must be in communication with each other to implement the protocol.

All TCP connections are full duplex and point to point. Every byte on a TCP connection has its own 32-bit sequence number. Sending and receiving TCP entities exchange data in the form of segments. The TCP protocol has to address the following:

1. the TCP segment header
2. TCP connection management
3. TCP transmission policy
4. TCP congestion control
5. TCP time management.

TCP link

A virtual communication link exists between corresponding layers in the network. The send and receive layers have buffer memories. The receive buffer holds incoming data while they are being processed. The send buffer holds data until they are ready for transmission. It also holds copies of data already sent until it receives an acknowledgement that the original has been received correctly.

The receive window is the amount of receive buffer space available at any given time. This changes as the received data are processed and removed from the buffer. The receive layer sends an acknowledgement signal to the send TCP layer when it has cleared data from its buffer, and the acknowledgement also provides an update on the current  size of the received window, and so on.

IP Address

Every host and router on the internet has as IP address, which encodes its network number and host number. The combination is unique: no two machines have the same IP address. All IP addresses are 32 bit long and are used in the source address and destination address fields of IP pockets. The 32 bit numbers are usually written in dotted decimal notation. Example, the hexadecimal address C0290614 is written as 192.41.6.20. The lowest IP address is 0.0.0.0 and the highest is 255.255.255.255.

Subnet

Subnet means the set of all routers and communication lines in a network. Each router has a table listing some number of network IP addresses and some number of host IP addresses. The first kind tells how to get to distant networks. The second kind tells how to get to local hosts. When an IP pocket arrives, its destination address is looked up in the routing table. If the pocket is for a distant network, it is forwarded to the next router on the interface given in the table. If it is a local host, it is sent directly to the destination. If the network is not present, the pocket is forwarded to a default router with more extensive tables. Subnet reduces router table space by creating a three level hierarchy.

Necessity of Modem

Attenuation and propagation speed are frequency dependent. Square waves in digital data have a wide spectrum and thus are subject to strong attenuation and delay distortion. These effects make base band (DC) signaling unsuitable except at slow speed and over short distances. To get around the problems associated with DC signaling, especially on telephone lines, AC signaling is used. A continuous tone in the 1000 to 2000 Hz range is introduced. Its amplitude, frequency, or phase can be modulated to transmit information.

Internet services

Telephone companies and others have begun to offer networking services to any organization that wishes to subscribe. The subnet is owned by the network operator, providing communication service for the customers’ terminals.