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Computer Science Clay
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14-06-2009, 01:46 AM



Perpendicular recording is forecast to be capable of delivering more than triple the storage density of traditional longitudinal recording. There was some interest in using the system in floppy disks in the 1980s, but the technology was never reliable. Today there is renewed interest in using it for hard drives, which are rapidly reaching the limits of longitudinal recording. Current hard disk technology with longitudinal recording has an estimated limit of 100 to 200 gigabit per square inch due to the superparamagnetic effect, though this estimate is constantly changing. Perpendicular recording is predicted to allow information densities of up to around 1 Tbit/sq. inch (1000 Gbit/sq. inch).
1.1 Introduction
Over the last 15 years the storage capacities of hard disk drives has risen by a factor of over 1000. Part of that increase is due to the number of platters and heads inside a drive. Yet a bigger part has come through greater tolerances and better manufacturing techniques. Weâ„¢ve moved from 5.25 inch drives to 2.5 inch drives today, and lately weâ„¢ve even reduced the number of platter and heads by increasing the areal density. [The areal density is the amount of data per square inch of media]. Yet the way forward to greater capacities and higher performance drives is changing the way that data is written to the surface of the disk. Current technology relies on Longitudinal Recording (LR) of data. This means that the positive and negative charges of each piece of data are written parallel to the disk surface. More importantly, they are also in the same plane, as opposed to being multi-layered like some optical technologies. The amount of data that we can store on a disk surface, in this way, is approaching its limit. As we seek to reduce the size of the charged particles we increase the risk of distortion. As a result increased areal density, the holy grail of disk manufacturing in recent years, is becoming ever hard to meet. Increased areal density has been the key to reducing the costs of drives, by removing the number of platters and heads. It has also allowed the disk format to reduce ensuring that high capacity hard disks have moved from the desktop and server market into the notebook, personal video recorder (pvr), mp3 player and other consumer technologies. As we keep pushing the areal density of storage higher, we need to find ways to overcome a serious problem - the limit. As we squeeze more and more data onto the surface of the media, the energy used to maintain the data has to be reduced to prevent corruption. Eventually the energy used to maintain each bit of data becomes so small that it is indistinguishable from the background energy created by the drive. At this point the entire recording field collapses in on itself. One way to get round the problem is to increase the coercivity (the magnetic field required to write data) of the disk. Material scientists believe that we are fast approaching the limit of what we can achieve in both the disk head and the magnetic media. To regain these advances in areal density disk manufacturers have been investing heavily in a new technology called Perpendicular Recording (PR). Where LR works in a single plane parallel to the disk surface, PR works at right angles to the disk surface piling the bits on top of each other. The concept is not new. In architecture we have long built large skyscrapers to house more people. Disk technology is finally catching up with that.
1.2 Why
As the volume of business and personal data continues to grow apace, technologists remain focused on increasing the amount of information that can be stored on computer disc drives. By increasing the areal density, or the amount of information that can be placed within a given area on a disc drive, technologists have increased the areal density of disc drives by more than 50 million- times since their introduction in 1957. A key byproduct of this rising density has been a reduction in hard drive cost since fewer platters, heads, and mechanical parts are necessary. Historically, areal density has increased more than 100 percent, or doubled, every year, though that rate has dropped to about 40 percent recently because of growing challenges in increasing areal densities of longitudinal recording “ a technology that has been used for nearly 50 years to record information on disc drives.
1.3 Increasing Areal Density
To increase areal densities in longitudinal recording and boost overall storage capacity, the data bits must be shrunk and packed more closely together. However, if the bit becomes too small, the magnetic energy holding the bit in place may also become so small that thermal energy can cause it to demagnetize, a phenomenon known as superparamagnetism. To avoid superparamagnetism, disc media manufacturers have been increasing the coercivity (the field required to write a bit) of the media. However, the fields that can be applied are limited by the magnetic materials making up the write head. In perpendicular recording, the magnetization of the disc, instead of lying in the discâ„¢s plane as it does in longitudinal recording, stands on end, perpendicular to the plane of the disc. The bits are then represented as regions of upward or downward directed magnetization. (In longitudinal recording, the bit magnetization lies in the plane of the disc and flips between pointing in the same and opposite directions of the head movement.) The media is deposited on a soft magnetic under-layer that functions as part of the write field return path and effectively produces an image of the recording head that doubles the recording field, enabling higher recording density than with longitudinal recording . Imagine having a 10-gigabyte hard drive in your cell phone or a terabyte of space on your laptop. Perpendicular recording, a new way of writing data to a hard disk, creates the possibility for these kinds of capacities, just as we’re approaching the physical limits of traditional recording methods. Hard drives store information by changing the polarization of microscopic magnetic bits aligned end-to-end on a surface called a platter. But you can pack in only so many bits before they interfere with one another, randomly switching orientation and turning your data into noise. Perpendicular recording stands the bits on-end so that more fit into the same space. It sounds simple, but it took Toshiba 20 years to reengineer the drive's innards to accommodate vertical bits. The entire hard-drive industry will go perpendicular within a few years, but Toshiba was the first to market with a 1.8-inch single-platter 40-gigabyte drive (previous 40-gig drives this size used two platters, burning more battery life) in its Gigabit MP3 player.
1.4 Recording technology
The main challenge in designing magnetic information storage media is to retain the magnetization of the medium despite thermal fluctuations caused by the superparamagnetic limit. If the thermal energy is too high, there may be enough energy to reverse the magnetization in a region of the medium, destroying the data stored there. The energy required to reverse the magnetization of a magnetic region is proportional to the size of the magnetic region and the magnetic coercivity of the material. The larger the magnetic region is and the higher the magnetic coercivity of the material, the more stable the medium is. There is a minimum size for a magnetic region at a given temperature and coercivity. If it is any smaller it is likely to be spontaneously de-magnetized by local thermal fluctuations. Perpendicular recording uses higher coercivity material because the head's write field penetrates the medium more efficiently in the perpendicular geometry.
The popular explanation for the advantage of perpendicular recording is that it achieves higher storage densities by aligning the poles of the magnetic elements, which represent bits, perpendicularly to the surface of the disk platter, as shown in the illustration. In this not-quite-accurate explanation, aligning the bits in this manner takes less platter than what would have been required had they been placed longitudinally so they can be placed closer together on the platter, thus increasing the number of magnetic elements that can be stored in a given area. The true picture is a bit more complex, having to do with the use of a magnetically "stronger" (higher coercivity) material as the storage medium. This is possible because in a perpendicular arrangement the magnetic flux is guided through a magnetically soft (and relatively thick) underlayer underneath the hard magnetic media films (considerably complicating and thickening the total disk structure). This magnetically soft underlayer can be effectively considered a part of the write head, making the write head more efficient, thus making it possible to produce a stronger write field gradient with essentially the same head materials as for longitudinal heads, and therefore allowing for the use of the higher coercivity magnetic storage medium. A higher coercivity medium is inherently thermally more stable, as stability is proportional to the product of bit (or magnetic grain) volume times the uniaxial anisotropy constant K u , which in turn is higher for a material with a higher magnetic coercivity. To achieve higher levels of areal density (usually expressed as the amount of data that can be placed in a square inch on a disk), drive manufacturers place bits perpendicular to the disk platter, rather than laying them flat as in longitudinal recording. In longitudinal recording the bits can be further shrunk, but the smaller they are the more susceptible they are to erasure from thermal energy
1.4.1 Superparamagnetism
Superparamagnetism is a form of magnetism. A superparamagnetic material is composed of small ferromagnetic clusters (e.g. crystallites), but where the clusters are so small that they can randomly flip direction under thermal fluctuations. As a result, the material as a whole is not magnetized except in an externally applied magnetic field (in that respect, it is like paramagnetism). Description
Specifically, superparamagnetism is a phenomenon in which magnetic materials may exhibit a behavior similar to paramagnetism at temperatures below the Curie or the Néel temperature. This is a small length-scale phenomenon, where the energy required to change the direction of the magnetic moment of a particle is comparable to the ambient thermal energy. At this point, the rate at which the particles will randomly reverse direction becomes significant. Normally, coupling forces in ferromagnetic materials cause the magnetic moments of neighboring atoms to align, resulting in very large internal magnetic fields. This is what distinguishes ferromagnetic materials from paramagnetic materials. At temperatures above the Curie temperature (or the Neel temperature for antiferromagnetic materials), the thermal energy is sufficient to overcome the coupling forces, causing the atomic magnetic moments to fluctuate randomly. Because there is no longer any magnetic order, the internal magnetic field no longer exists and the material exhibits paramagnetic behavior. If the material is non- homogeneous, one can observe a mixture of ferromagnetic and paramagnetic clusters of atoms at the same temperature, the super paramagnetic stage. The idea superparamagnetism is used in Super Paramagnetic Clustering algorithm (SPC) as well as in its extension global SPC. Superparamagnetism occurs when the material is composed of very small crystallites (1“10 nm). In this case even when the temperature is below the Curie or Neel temperature (and hence the thermal energy is not sufficient to overcome the coupling forces between neighboring atoms), the thermal energy is sufficient to change the direction of magnetization of the entire crystallite. The resulting fluctuations in the direction of magnetization cause the magnetic field to average to zero. Thus the material behaves in a manner similar to paramagnetism, except that instead of each individual atom being independently influenced by an external magnetic field, the magnetic moment of the entire crystallite tends to align with the magnetic field. The energy required to change the direction of magnetization of a crystallite is called the crystalline anisotropy energy and depends both on the material properties and the crystallite size. As the crystallite size decreases, so does the crystalline anisotropy energy, resulting in a decrease in the temperature at which the material becomes superparamagnetic. The rate at which particles will lose their direction is governed by the Néel- Arrhenius equation. In particular, it is a function of the exponential of the grain volume. Néel-Arrhenius equation
The Néel-Arrhenius equation (closely related to the standard Arrhenius equation) states:
t = t
exp(E / (k
t is the average length of time that it takes for a ferromagnetic cluster (often, a
crystallite) to randomly flip directions as a result of thermal fluctuations,
is a length of time, characteristic of the material, called the attempt time or
attempt period (its reciprocal is called the attempt frequency),
E is the magnetic anisotropy energy, which can be thought of as the energy
barrier associated with the magnetization moving from its initial "easy axis"
direction, through a "hard axis", ending at another easy axis,
is the Boltzmann constant,
T is the temperature.
In other words, when an external magnetic field is applied for a long time and then removed, the clusters will not randomize their direction immediately, but rather it will take some length of time to do so. This length of time can be anywhere from fractions of a second to years or much longer. Larger clusters tend to have larger anisotropy energy (the energy is approximately proportional to volume), and consistent with the Néel-Arrhenius equation, they tend to hold their magnetization for much longer. A superparamagnetic system can be measured with AC susceptibility measurements, where an applied magnetic field varies in time, and the magnetic response of the system is measured. A super paramagnetic system will show characteristic frequency dependence: When the frequency is much higher than 1/t, there will be a different magnetic response than when the frequency is much lower than 1/t, since in the latter case, but not the former, the ferromagnetic clusters will have time to respond to the field by flipping their magnetization. The precise dependence can be calculated from the Néel-Arrhenius equation, assuming that the neighboring clusters behave independently of one another (if clusters interact, their behavior becomes more complicated Effect on hard drives
Superparamagnetism sets a limit on the storage density of hard disk drives due to the minimum size of particles that can be used. This limit is known as the super paramagnetic limit. Current hard disk technology with longitudinal recording has an estimated limit of 100 to 200 Gbit/in², though this estimate is constantly changing. [1] One suggested technique to further extend recording densities on hard disks is to use perpendicular recording rather than the conventional longitudinal recording. This changes the geometry of the disk and alters the strength of the super paramagnetic effect. Perpendicular recording is predicted to allow information densities of up to around 1 Tbit/in² (1024 Gbit/in²). --reference is on the perpendicular recording. Another technique in development is the use of HAMR drives, which use materials that are stable at much smaller sizes. But, they require heating before the magnetic orientation of a bit can be changed. Seagate has demonstrated a recording areal density with perpendicular recording of 245 Gbpsi (Gigabits per square inch) with a data rate of 480 Mbits per second “ more than double the 110 Gbpsi used in today™s highest areal density disc drives “ and 500 Gbpsi, which will increase the capacity of today™s drives 5-fold, is possible with the new technology. At 500 Gbpsi, a 3.5-inch disc drive could store two terabytes of information; a 2.5- inch drive in a laptop could hold 500GB and a 1-inch drive, such as those in MP3 players, could store as much as 50GB of data
2 Longitudinal Recording Technologies
2.1 Recording technology
To understand how perpendicular recording has changed hard disk storage and increased storage capacities, you first need to understand conventional, longitudinal recording. As indicated by the name, longitudinal recording is a method of recording data to a hard disk drive (HDD) in such a way that the data bits are aligned horizontally in relation to the drive's spinning platter, which is parallel to the surface of the disk. In conventional longitudinal recording, the magnets representing the bits are lined up end-to-end along circular tracks in the plane of the disk. If you consider the highest- density bit pattern of alternating ones and zeros, then the adjacent magnets end up head-to-head (north-pole to north pole) and tail-to-tail (south-pole to south-pole). In this scenario, they want to repel each other, making them unstable against thermal fluctuations. In perpendicular recording, the tiny magnets are standing up and down. Adjacent alternating bits stand with North Pole next to South Pole; thus, they want to attract each other and are more stable and can be packed more closely. This geometry is the key to making the bits smaller without superparamagnetism causing them to lose their memory. Longitudinal recording has been the standard method of recording for more than 50 years ” the first commercial hard drive was introduced in 1956. Over the years we have seen many technological changes to longitudinal recording, which have resulted in higher-capacity drives. We've moved from 5.25 inch drives to 2.5 inch drives, the number of platters and heads have been reduced all the while increasing areal density (which is the amount of data per square inch of media). With all of these changes however, the need to physically change the way data was written to the drive was also being considered to reach higher storage capacities. Storage capacity with longitudinal recording was largely increased by decreasing the size of the magnetic grains that make-up data bits. As the magnetic grains became smaller, more data could then be stored on the disk. Unfortunately, magnetic grains have their limits. By continuing to shrink them, the point where data integrity would be compromised was on the horizon. This effect is called the super paramagnetic effect.
2.2 Longitudinal recording devices
In 1984 IBM introduced the 3480 cartridge drive and format as a replacement for the ageing 3420 series of nine-track drives. For large, off-line mainframe storage, 3480 became the de-facto standard. The 3480 is a longitudinal recording device that writes 18 tracks on a 1/2" cartridge. The initial storage capacity was 200Mb per cartridge with a transfer rate of up to 3Mb per second. The table below describes the main longitudinal recording devices:
Table 1 : Longitudinal Recording Technology
Storage Capacity (GB) Transfer Rate (MB/s) Price Band (GBP) 3480 0.2 3 5-10K 3490E 0.8 3/6 5-10K DLT-2000/3000 10/15 1.25/1.5 <5K DLT-4000/7000 20/35 1.5/5 <5K DLT-8000 40 6 <5K SuperDLT 110 11 <5K Magstar MP 3570 5 2.2 5-10k 20 20 Magstar 3590-B 10 9 10-20K Fujitsu M8100 10 13.5 10-20K Magstar 3590-E 20 14 20-30K IBM 3580 (Ultrium) 100 15 20-30K STK-9940 20 10 10-20K
As the above table denotes, the basic 3480 cartridge has evolved. IBM's first enhancement was the addition of a compression option Improved Data Recording Capability (IDRC). The 3490E uses 36 tracks and extended length tape to increase capacity to 800 Mb. However IBM continued research and development into a device named New Tape Product (NTP). This was later to be marketed under the name of Magstar (or 3590-Tape Series). The Magstar drive was formally announced in April 1995 and became readily available in 1996. With an initial price of £35,000 (half that of other competing high capacity drives), the Magstar was popular and has been in great demand ever since. The Magstar still maintained the 3480 form factor, but now the 3590-E has a native capacity of 20GB with a data transfer rate of 14Mb per second. Magstar uses a bi-directional longitudinal serpentine recording technique, and a second generation magneto-resistive head that reads and writes 16 track groups with 8 tracks per group, providing a total of 128 tracks. Increased reliability and integrity is also offered with improved error correction code and resident diagnostics, IBM claiming up to a 100-fold increase in data integrity over 3490E. Magstar also offers a 5 metres per second search mechanism to provide rapid access to stored data. The 3590-E drive mechanism offers a superior 16MB data buffer and tape speed of 3.14 m/s. Both StorageTek (a company formed from ex-IBM Storage specialists) and Fujitsu also offer IBM-3590 compatible tape subsystems. IBM also offer an ‘investment protection’ scheme enabling the upgrade from 3590-B to 3590-E for £10,000. The Magstar cartridges are still of the 3480/90 form factor, hence can co-exist in a traditional 3480 automated library system with few modifications. The cartridge is loaded with 1100 feet of 1/2" wide back coated metal particle media. A colour-coded leader block has been re-designed to prevent accidental threading and running in a 3480/90 drive. The latest release from IBM - The IBM 3580 (Ultrium) is the results of the Linear Tape Open (LTO) consortium. The LTO-open programme is a joint initiative involving IBM, Hewlett Packard, Seagate Corporation and 16 other licensed leading tape and drive manufacturers. Co-opetition is encouraged - co-operatively creating a new business opportunity then competing for market share. LTO utilizes multi- channel serpentine recording technology. The Ultrium specification is 384 tracks across (split into 4 bands of 96 tracks). The ½ inch LTO-Ultrium specification is 100GB native capacity at 15MB per second data transfer rate. Also of note is the DLT drive (Digital Linear Tape), another 128-track serpentine longitudinal recording device. Initially this drive was aimed at the server and enterprise backup market, offering storage capacities up to 40 GB with data transfer rates of 6 Mb per second. The price of a drive in the region of £1,500 to £4,500. The cartridges are1/2", based on Digital's CompacTape III design (similar to a TK cartridge).
DLT has proved to be popular with an estimated 1 million drives in operation worldwide, and an estimated 89% of the mid-range market segment. The DLT 8000 uses DLT IV tapes and read DLT II and DLT III backward compatible The drive utilizes variable speed recording technology set automatically according to the hosts bus speed. The latest release the SuperDLT drive is based on Quantum-developed Laser Guided Magnetic Recording. The SuperDLT 220 operates at a native capacity of 110GB with a transfer rate of 11MB/sec. The drive is backward-read compatible and able to read DLT-IV tapes but has no record capability on DLT-IV tapes. With SuperDLT cartridges costing in excess of £85.
3 Comparing Longitudinal and
3.1 Introduction
The majority of hard disk drives found on the market today use a recording technology called longitudinal, where bits are stored side by side on the magnetic surface. This recording technique has been used since the first hard disk drives. A new recording technology, called perpendicular, is being used on newer hard disk drives, allowing a higher recording density. In this tutorial we will explain you everything you need to know about perpendicular recording and how data is stored on the hard disk drive magnetic surface. Data is read and write on magnetic disks thanks to the electromagnetism physics phenomena. In 1820 a physicist called Hans Christian Oersted observed while he was preparing a lab class for his Physics students that an electrical current flowing in a wire moved the needle of a compass located near this wire. When the electrical current was shut down, the compass needle went back showing the location of Earthâ„¢s magnetic north pole. With that, he came to the conclusion that all conductors (wires) create a magnetic field around them when an electrical current is flowing. When the direction (polarity) of this electrical current is reversed, so is the polarity of the magnetic field. In 1831 another physicist called Michael Faraday found out that the inverse was also true, i.e. if a magnetic field strong enough was created near a wire, electrical current was to be produced (inducted) in this wire. If the direction of this magnetic field was reversed, the direction of the electrical current was reversed too.
To understand how data are read and written on hard disk drives and other magnetic devices, keep in mind these two electromagnetic properties:
? All conductors created magnetic fields around them when there is an electrical current flowing.
? A strong magnetic field can generate (induct) electrical current on a wire.
This is all you need to know in order to understand how data is read and written on a hard disk drive. An upside down U-shaped conductive material with a coil on it makes the hard disk drive read/write head. On the process of writing data to the hard disk drive, an electrical current is applied to the coil, creating a magnetic field around the read/write read. This field magnetizes the platter surface right below the head, aligning the magnetic particles to the left or to the right, depending on the polarity of the electrical current that was applied. Keep in mind that reversing the electrical current polarity will also reverse the polarity of the magnetic field. A stored bit is a sequence of magnetized particles.
On the process of reading data from the hard disk drive, when the head passes on a magnetized area either a positive or a negative current will be inducted, allowing the drive control circuit to read the stored bits.
3.2 Perpendicular vs. Longitudinal
Each hard disk drive platter is made of aluminum or glass and on it a magnetic material layer is applied, usually iron oxide mixed with other elements. We saw on previous that the read/write head magnetizes the magnetic particles found on the disk surface according to the applied current. We also saw that a sequence of magnetized particles represent a data bit
Under longitudinal recording technology, found in virtually all hard disk drives found on the market today, the magnetic particles are horizontally aligned, i.e. they are found side by side on the disk surface, as you can see on Figure During several years the most common method engineers used to increase storage capacity (storage density) was to decrease the size of the magnetic particles on the hard disk surface. The smaller the particles, the more data can be stored on the hard disk drive. Shrinking the magnetic particles, however, leads to a problem called superparamagnetism, which compromises data integrity. Superparamagnetism occurs when the particles are so small that temperature variations can reverse the magnetic fields from the particles, what would corrupt stored data. Superparamagnetism prevents manufactures from building hard disk drives with higher capacities. On perpendicular recording technology magnetic particles are vertically aligned, as you can see on Figure
3.3 Conclusion
With perpendicular recording technology more data can be stored on the hard disk and less superparamagnetism problems occur. With perpendicular recording technology we will see a increase in hard disk drive storage capacities in a short period of time and we will reach the 1 terabyte barrier very soon. Just to give you an idea, Seagate launched recently their new Barracuda 7200.10 hard disk drive family, which is based on perpendicular recording, featuring models up to 750 GB Portable devices will also benefit from this technology, as more bits will fit in a smaller physical space. We have to wait to see how the performance of hard disk drives based on perpendicular recording is compared to standard longitudinal recording ones.
4 Conclusions
With perpendicular recording, manufacturers have been able to lift the ceiling and squeeze more bits into hard-disk real estate. In perpendicular recording, the small magnetic regions are packed vertically, so each region uses less of a disk's surface area. Thus, more bits can be packed in per square inch. This technique has been known at least since the 1970s, when Shu-ichi Iwasaki developed perpendicular recording at the Tohoku Institute of Technology in Japan. The problem was that some other aspects of the technological mix required to read and write perpendicular magnetic regions caused a loss of reliability. The read and write head for perpendicular recording requires a "soft" magnetic under layer beneath the magnetic film where the digital ones and zeroes are recorded. This under layer is said to be magnetically soft because it is easily magnetized in one direction or another. That property allows it to work with a perpendicular read/write head to magnetize regions vertically in the magnetic film above. New techniques have allowed manufacturers to make the soft under layer thinner than was previously possible and less easily affected by stray magnetic fields, which could corrupt the information in the magnetic film above. Now that these problems have been overcome, recording data perpendicularly has become feasible. Manufacturers such as Hitachi Global Storage Technologies Inc., Seagate Technology LLC, Toshiba Corp. and Maxtor Corp. are moving toward or have already brought perpendicular drives to market. One additional benefit of perpendicular recording is that it may be possible to shrink the regions that need to be magnetized to record digital ones and zeroes. Perpendiculars read/write heads have a stronger magnetic field than those that are used for longitudinal recording. Therefore, it is possible to use magnetic films that have a higher magnetic coercivity, which means it's harder to change the orientation of a magnetic grain. The heat energy required to flip a grain has to be higher, and the superparamagnetic effect for these materials is limiting at smaller sizes. Reliable magnetic regions may, therefore, not require as many as 100 grains. Research is ongoing in other methods to shrink the size of magnetic regions. Patterning magnetic film may be the next advance, in which lasers are used to mill the magnetic film and isolate magnetic grains in such a way that very few are required in order to reliably record digital ones and zeroes. Research papers project and implimentation bit densities on patterned hard disks in the range of 300 gigabits per square inch

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