QUANTUM CRYPTOGRAPHY

The sender transmits the changing key sequence over a secure fiber-optic link as a stream  of polarized photons (indivisible particles of light). Because the polarization reflects the amount of electromagnetic radiation allowed to radiate at an angle to a light beam's direction, it can be considered to be a measure of the angular dependence of the light. Should an eavesdropper tap into the secure fiber-optic line, he would disrupt this stream of polarized photons by the very act of observing them and the tampering could be

Instantly detected

QC is quite different because it encodes a single bit of information onto a single photon of light. The laws of quantum physics protect this information because:

Heisenberg’s Uncertainty Principle prevents anyone directly measuring

the bit value without introducing errors that can be detected.

A single photon is indivisible which means that an eavesdropper cannot

split the quantum signal to make measurements covertly.

The quantum ‘no-cloning’ theorem means that it is not possible to

receive a single photon and copy it.

A potential snag is that in real systems, not all the photons will be received, due to inherent losses in the transmission medium. A practical QC protocol needs to incorporate some method of determining which photons have been correctly received and also of detecting any attempt by an eavesdropper to sit in the middle of the channel and act as a relay.

The first provably secure protocol for QC that resolved these problems is known as BB84, and named after its inventors Bennett and Brassard in 1984. Each bit of information is encoded as the polarization state of a single photon of light.

As you all now Computer is now a need for every human being. All the old ways of doing a work is now changed into a digital format in the form of a computer, due to this the need for learning computer arises with a rapid speed. Today I like to share the three fundamental of computer with you. These are as under:

1. The first basic thing you must now is that - What is a computer?

Though there is various definition made on computer but the perfect one according to me is a "Computer can be defined as a high speed Electronic Data Processing Machine"

Let as now want the definition says...

· Electronic Machine - It is a machine, which consists of various Electronic circuits.

· Data Processing Machine – Now how it is a data processing machine because it accept data as its input. Process it by certain calculations & Manipulation & give you the desired result in the form of output.

2. The second thing is that when you buy a computer you don’t know – What can computer do for you?

Some of the uses computer do are as follows:

· It can play game with you.

· It can help to teach student.

· It can help to solve difficult problem that would take a person hours or years to solve.

· It can help to maintain accounts, payroll, inventory, mailing list etc.

· It can help to predict your future.

· It can be used in large application like railways/airlines booking.

· It can be used in space technology, it figures out and controls the take off and touch down of planes and rockets.

· It can be used for every possible application Remember that the computer does not know anything.

Computer only works on per instruction given to it. Computer can not think like a man. You have to give some of the input to get some output.

3. Now the last thing is that what is Hardware and Software?

People are confused what these things do in the computer, what are they?? The answer is as below:

· The hardware can be generally is physical devices like Monitor, Disk drive and Chips are called Hardware.

· Hardware definition can be modified by me and presented as the physical component of the machine which can be seen, touch (tangible) is known as the Hardware.

· Now I will show a diagram in which the computer structure is represented in a demo form as under:

· Now about software:

Software is a program which makes Hardware useful. You should remember that hardware is useless without the appropriate Software.

I have come with a definition of software as under:

“The set of instruction generally referred to as program is the Software”.

The storage medium in a magnetic-disk system consists of one or more disks mounted on a common spindle. A thin magnetic film is deposited on each disk, usually on both sides. The disks are placed in a rotary drive so that the magnetized surfaces move in close proximity to read/write heads. The disk rotates at a uniform speed. The following factors determine the performance of a hard disk.

Access Time: Disk access time is the interval between the time a computer makes a request for transfer of data from a disk system to the primary storage and the time this operation is completed. To access information stored on a disk, the disk address of the desired data has to be specified. The disk address is specified in terms of the surface number track/cylinder number and sector number. Information is always written from the beginning of a sector and can be read only from the track beginning. Hence disk access time depends on the following three parameters.

1. Seek time - As soon as a read/write command is received by the disk unit, the read/write heads are first positioned on to the specific track number by moving the access arms assembly in the proper direction. The time required to position the read/write head over the desired track is called the seek time. The seek time varies depending on the position of the access arms assembly when a read/write command is received. If the access arms assembly is positioned on the outer most track and track to be reached is the inner most one the seek time will be maximum. It will be zero if the access arms assembly already happens to be on the desired track. The average seek time is thus specified for most systems. It is of the order of 10 to 100 milliseconds.

We also know that some disk systems have multiple read/write heads on each access arm. This is done to reduce the seek time. For example: a disk system may have two sets of read/write heads for each surface one for reading/writing on the inside tracks and another for the outside tracks. This will reduce the average seek time by half because each read/write head needs to cover and move across only half of the total number of tracks.

2. Latency- Once the heads are positioned on the desired track the head on the specified surface is activated. Since the disk is continuously rotating this head should wait for the desired data to come under it. This rotational waiting time i.e., the time required to spin the desired sector under the head is called the latency. The latency also known as the rotational delay time is also a variable and depends on the distance of the desired sector from the initial position of the head on the specified track. It also depends on the rotational speed of the disk which may be anywhere from 300 rpm to 7200 rpm. An average latency time is thus normally specified which is of the order of 5 to 80 milliseconds. Note that the average latency of a disk system is equal to half the time taken by the disk to rotate once. Hence the average latency of a disk system whose rotational speed is 3600 rpm will be 0.5/3600 minute=8.3 milliseconds.

3. Transfer rate -Transfer rate refers to the rate at which data are read from or written to the disk. Once the read/write head is positioned over the desired sector the data are read, written at a speed determined by the rotational speed of the disk. If the rotational speed of a disk is 3600 rpm and the disk has 125 sectors/track and 512 bytes/sector the amount of data transferred in one full revolution of the disk will be 125*512=64000 bytes=64k bytes(approximately). Hence the transfer rate of the disk system will be 64,000 * 3600/60 bytes/second = 38,40,000 bytes/second = 3.8 megabytes/second. Notice that the transfer rate of a disk system depends on the density of the stored data and the rotational speed of the disk.

Since the data transfer time is negligible as compared to seek time and latency the average access time for a disk system is the sum of its average seek time and average latency. The average access time for different types of disk systems varies over a wide range and may be anywhere from 10 to 600 milliseconds.

Since the access time for a piece of data stored on a disk depends on the physical location of the data it is more correct to refer to a disk system as direct access storage device instead of random access storage device. Random access refers to a storage device in which the access the access time is independent of the physical location of the data. For example primary storage is a random access storage. However this distinction is not always observed strictly and hence disk systems are often referred to as random access storage devices.

Switch is a small device that joins multiple computers together of a low-level network protocol layer. Technically, Switch operates at layer two, that is, data link layer of the OSI Model.

Switches looks nearly identical to Hubs, but Switchitch generally contains more intelligence than hub. Unlike hubs, Switches are capable of inspecting the data packets as they are received, determining the source and destination address of the Packet and forwarding that Packet appropriatly.

By delivering messages only to the connected device that it was intended for, that is, signals are transmitted only to the port where they need to go, rather than to all ports unlike hun.

Typrs of Switch :

There are therr basic types of Switch :

1> Cut-Through Switch.

2> Store-and -forward Switch.

3> Fragment free Switch.

Cut-Through Switch :

A Cut-Through Switch forward the Packet as soon as it is received. No error checking is performed on the Packet, so Packet is moved through very quickly. This typw of Switch is relatively inexpensive and minimizes the delay incurred during the processing Packets by the Switch.

Store-and -forward Switch :

In Store-and -forward configuration, the Switch waits to receive the entire Packet before beginning to forward it. It performs basic error checking. This type of Switch is a shared-memory Switch, which has a comman memory buffer that stores the incomming data from all the ports.

Fragment free Switch :

Fragment free Switch works by reading only the port of the packet that enables it to identify fragments of a transmission.