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unit 3 cyber security 19.4.22.pptx

  1. Information storage and communication Unit 3
  2. Information storage • Computer data or information storage, often called storage or memory, refers to computer components, devices, and recording media that retain digital data used for computing for some interval of time. • Computer data storage provides one of the core functions of the modern computer, that of information retention. • Different types of information is stored in different ways, depending on what the information is, how much storage space it requires, and how quickly it needs to be accessed. • This information is stored in its “short term” memory and its “long term” memory.
  3. • Your system memory (or RAM) holds information that you are working with right now. This is the computer’s “short- term memory”, and is designed to be able to feed information to the processor at high speed so the processor is not slowed down too much while waiting for it. • However, this short-term memory disappears when the computer is turned off. This is why you must always save a file that you are working on before turning off the machine. • Longer-term storage is provided by your hard disk drive, floppy drive and other devices, where information is stored permanently in the form of files, ready for you to retrieve when you need it. When you want to use your spreadsheet program, for example, the computer loads the instructions that are stored on the hard disk that tell the computer how to run it, from long-term storage (your hard disk) into short-term memory.
  4. • Every computer comes with a certain amount of Physical memory, usually referred to as main memory or RAM. • You can think of main memory as an array of boxes, each of which can hold a single byte of information. A computer that has 1 megabyte of memory, therefore, can hold about 1 million bytes (or characters) of information. • There are several different types of memory: • RAM (random-access memory): This is the same as main memory. When used by itself, the term RAMrefers to read and write memory; that is, you can both write data into RAM and read data from RAM. • Thisis incontrast to ROM which permits you only to read data. Most RAM i volatile,which means that it requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, whatever data was in RAM is lost.
  5. • ROM (read-only memory):Computers almost always contain a small amount of read-only memory that holds instructions for starting up the computer. Unlike RAM, ROM cannot be written to. • PROM (programmable read-only memory): A PROM is a memory chip on which you can store a program. But once the PROM has been use ,you cannot wipe it clean and use it to store something else. Like ROMs, PROMs are non- volatile. • EPROM (erasable programmable read-only memory): An EPROM is a special type of PROM that can be erased by exposing it to ultraviolet light. • EEPROM (electrically erasable programmable read-only memory):An EEPROM is a special type of PROM that can be erased by exposing it to an electrical charge. •
  6. PURPOSE OF STORAGE • A digital computer represents data using the binary numeral system. Text, numbers, pictures, audio, and nearly any other form of information can be converted into a string of bits, or binary digits, each of which has a value of 1 or 0. • The most common unit of storage is the byte, equal to 8 bits. A piece of information can be handled by any computer whose storage space is large enough to accommodate the binary representation of the piece of information, or simply data. • For example, using eight million bits, or about one megabyte, a typical computer could store a small novel. • Without a significant amount of memory, a computer would merely be able to perform fixed operations and immediately output the result. It would have to be reconfigured to change its behavior. • This is acceptable for devices such as desk calculators or simple digital signal processors. Von Neumann machines differ in that they have a memory in which they store their operating instructions and data.
  7. • Such computers are more versatile in that they do not need to have their hardware reconfigured for each new program, but can simply be reprogrammed with new in- memory instructions; they also tend to be simpler to design, in that a relatively simple processor may keep state between successive computations to build up complex procedural results. Most modern computers are von Neumann machines. • In practice, almost all computers use a variety of memory types, organized in a storage hierarchy around the CPU, as a tradeoff between performance and cost. • Generally, the lower a storage is in the hierarchy, the lesser its bandwidth and the greater its access latency is from the CPU. • This traditional division of storage to primary, secondary, tertiary and off-line storage is also guided by cost per bit.
  8. TYPES OF STORAGE DEVICES • Storage Devices are the data storage devices that are used in the computers to store the data. • The computer has many types of data storage devices. Some of them can be classified as the removable data Storage Devices and the others as the non removable data Storage Devices. • The data Storage Devices come in many sizes and shape so altogether different. The storage devices are one of the most important components of the computer system. • The memory is of two types; one is the primary memory and the other one is the secondary memory.
  9. • The primary memory is the volatile memory and the secondary memory is the non volatile memory. • The volatile memory is the kind of the memory that is erasable and the non-volatile memory is the one where in the contents cannot be erased. • The secondary memory is used to store the data permanently in the computer. The secondary storage devices are usually as follows: hard disk drives — this is the most common type of storage device that is usedinalmostallthecomputersystems. • The other one include the floppy disk drives,the CDROM,and the DVD ROM. The flash memory, the USB data card etc.
  10. • Thestoragedevicesareusedto recordthedataoverany storagesurface.Thememoriesmay alsobeof different types depending upon the architecture and the design like the optical data storage memory, magnetic media storage and the mechanical storage media etc and also the flash memory devices etc. • • The storage devices are actually defined as the peripheral unit which holds the data like the tape, disk, or flash memory card etc. The most of the drives that are used for the purpose of data storage are fragile and the data can be easily corrupted in them. The data storage devices are the ones that are also usedforthebackup andthearchivingofthedata.Thedatastoragedeviceswereatatimeinthe pa • stused to be too costly and expensive. But these days the data storage devices are becoming cheap day by day. Hence the data storage devices price is falling. So, we are in a position to get a storage device for a comparatively cheaper price than the earlier drive. The technology is improving a lot and now the memory storage capacity has gone up TB. •
  11. • The data in the storage devices can be in the form of the files, databases, digital video and the audio etc.The storagedevices thatarecalled asthe nonvolatile canstorethe datapermanently untilotherwise erased purposely. This is in the case of the hard disk drives or the floppy disk drives. • • The otherkinds ofthe storagemedia likefor examplethe CDand theDVD caneven haveagain two types of the storage; the first one is that in which the data once written cannot be erased. It is stored permanently over it. While the second type of the CD’s or the DVD’s are called as the rewritable; where inthedatathatisoncewrittencanbeerasedcompletely andthesamestoragedevicecanbeusedagainfor storing the different data. – FILE ORGANIZATION • Studying the file organization is an important aspect of computer science as all the information is stored on the hard disk organized in some formats which are collectively known as files. • Given that a file consists, generally speaking, of a collection of records, a key element in file management is the way in which the records themselves are organized inside the file, since this heavily affects system performances as far as record finding and access is concerned.
  12. FILE ORGANIZATION • Note that by “organization” we refer here to the logicalarrangement of the records in the file (their ordering or, more generally, the presence of “closeness” relations between them based on their content), and not instead to the physical layout of the file as stored on a storage media. File organization is the methodology which is applied to structured computer files. • Here we will look at two components of file organization: • The way the internal file structure is arranged and • The external file as it Is presented to the O/S or program that calls it. Here we will also examine the concept of file extensions. • We will examine various ways that files can be stored and organized. Files are presented to the application as a stream of bytes and then an EOF (end of file) condition. • A program that uses a file needs to know the structure of the file and needs to interpret its contents. •
  13. Internal File Structure Methods and Design Paradigm • It is a high-level design decision to specify a system of file organization for a computer software program or a computer system designed for a particular purpose. • Performance is high on the list of priorities for this design process, depending on how the file is being used. • The design of the file organization usually depends mainly on the system environment. • For instance, factors such as whether the file is going to be used for transaction-oriented processes like OLTP or Data Warehousing, or whether the file is shared among various processes like those found in a typical distributed system or standalone. • It must also be asked whether the file is on a network and used by a number of users and whether it may be accessed internally or remotely and how often it is accessed.
  14. important considerations might be: • Rapid access to a record or a number of records which are related to each other. • The Adding, modification, or deletion of records. • Efficiency of storage and retrieval of records. • Redundancy, being the method of ensuring data integrity. • A file should be organized in such a way that the records are always available for processing with no delay. • This should be done in line with the activity and volatility of the information.
  15. Types of File Organization • Organizing a file depends on what kind of file it happens to be a file in the simplest form can be a text file (in other words a file which is composed of ASCII (American Standard Code for Information Interchange) text. • Files can also be created as binary or executable types (containing elements other than plain text.) Also, files are keyed with attributes which help determine their use by the host operating system. Techniques of File Organization The three techniques of file organization are: • Heap (unordered) • Sorted • Sequential (SAM) • Line Sequential (LSAM) • Indexed Sequential(ISAM) • Hashed or Direct • In addition to the three techniques, there are four methods of organizing files. They are sequential, line sequential, indexed- sequential, inverted list and direct or hashed access organization.
  16. Sequential Organization • • A sequential file contains records organized in the order they were entered. • The order of the records is fixed. • The records are stored and sorted in physical, contiguous blocks within each block the records are in sequence. • Records in these files can only be read or written sequentially. Once stored in the file, the record cannot be made shorter, or longer or deleted. • However, the record can be updated if the length does not change. • New records will always appear at the end of the file. • If the order of the records in a file is not important, sequential organization will suffice, no matter how many records you may have. • Sequential output is also useful for report printing or sequential reads which some programs prefer to do.
  17. Line-Sequential Organization • Line-sequential files are like sequential files, except that the records can contain only characters as data. • Line-sequential files are maintained by the native byte stream files of the operating system. In the COBOL environment, line- sequential files that are created with WRITE statements with the ADVANCING phrase can be directed to a printer as well as to a disk.
  18. Indexed-Sequential Organization • Key searches are improved by this system too. • The single-level indexing structure is the simplest one where a file, whose records are pairs, contains a key pointer. • This pointer is the position in the data file of the record with the given key. • A subset of the records, which are evenly spaced along the data file, is indexed, in order to mark intervals of data records. • This is how a key search is performed: the search key is compared with the index keys to find the highest index key coming in front of the search key, while a linear search is performed from the record that the index key points to, until the search key is matched or until the record pointed to by the next index entry is reached. • Regardless of double file access (index + data) required by this sort of search, the access time reduction is significant compared with sequential file searches.
  19. Direct or Hashed Access • With direct or hashed access a portion of disk space is reserved and a “hashing” algorithm computes the record address. • So there is additional space required for this kind of file in the store. • Records are placed randomly throughout the file. • Records are accessed by addresses that specify their disc location. • Also, this type of file organization requires a disk storage rather than tape. • It has an excellent search retrieval performance, but care must be taken to maintain the indexes.
  20. External File Structure and File Extensions Microsoft Windows and MS-DOS File Systems • The external structure of a file depends on whether it is being created on a FAT or NTFS partition. • FAT(File Allocation Table) • NTFS(New Technology File System) • VFAT(Virtual File Allocation Table) • The maximum filename length on a NTFS partition is 256 characters, • 11 characters on FAT (8 character name+”.”+3 character extension.) • NTFS filenames keep their case, whereas FAT filenames have no concept of case • Also, there is the new VFAT which permits 256 character filenames.
  21. UNIX and Apple Macintosh File Systems • The concept of directories and files is fundamental to the UNIX operating system. • On Microsoft Windows-based operating systems, directories are depicted as folders and moving about is accomplished by clicking on the different icons. • In UNIX, the directories are arranged as a hierarchy with the root directory being at the top of the tree. • The root directory is always depicted as /. Within the / directory, there are subdirectories • C:/system/bin/cplus.exe • Files can be written to any directory depending on the permissions. • Files can be readable, writable and/or executable.
  22. DATA COMMUNICATION—AN OVERVIEW • Data Communication defined as the transfer of information between two points, • either via an analogue (sine wave) electrical signal • or digital (binary) signal via electrical pulses • or optically via light pulses • A simple scenario would be two personal computers in the same building, but 50 feet away from each other. • By hooking up a cable between the two personal computers, we now have Data Communications. • The extent of Data Communications builds from this point on, since there are many factors such as • distance, topology, protocol, signalling, security, etc. that determine how Data Communications will take place.
  23. Analog Signals Vs Digital Signal • Analog signal is any continuous signal for which the time varying feature (variable) of the signal is a representation of some other time varying quantity. • Analog is usually thought of in an electrical context; however, mechanical, pneumatic, hydraulic, and other systems may also convey analog signals. • Digital signals consist of patterns of bits of information. • Modern digital computers store and process all kinds of information as binary patterns. • All the pictures, text, Sound and video stored in this computer are held and manipulated as patterns of binary values. • The main advantage of digital signals over analog signals is that the precise signal level of the digital signal . •
  24. Signals • Signals can be in analog or digital form. • Analog signals can have an infinite number of values in a range; digital signals can have only a limited number of values. • The difference between analog and digital is similar to the difference between continuous- time and discrete-time. • Analog corresponds to a continuous set of possible function values, • while digital corresponds to a discrete set of possible function values. • A common example of a digital signal is a binary sequence, where the values of the function can only be one or zero
  25. Periodic vs Aperiodic • Periodic signals repeat with some period T, while aperiodic, or non periodic, signals do not. We can define a periodic function through the following mathematical expression, wheretcan be any number and T is a positive constant: f(t)=f(T+t) (1) • If a continuous time signal does not have a definite pattern and does not repeat at regular intervals of time is known as continuous time aperiodic signal or nonperiodic signal. • In other words, a signal x(t) for which no value of time t satisfies the condition of periodicity, is known as aperiodic or non-periodic signal. .
  26. Periodic Aperiodic
  27. • Data Rate limits . how fast we can send data, in bits per second over a channel. Data rate depends on three factors: • The bandwidth available • The levels of signals we can use • The quality of the channel(level of the noise) Noiseless Channel: Nyquist Bit Rate For a noiseless channel, The Nyquist bit rate formula defines the theoretical maximum bit rate Bit Rate=2*Bandwidth*Log2L Here bandwidth is the bandwidth of the channel , L is the number of signal levels used to represent data and Bit Rate is bits per second.
  28. Noisy Channel: Shannon Capacity • In reality, we cannot have a noiseless channel: the channel is always noisy. In 1944 Claude Shannon introduced a formula, called the Shannon Capacity to determine the theoretical highest data rate for a noisy channel. • Capacity=Bandwidth* Log2(1+SNR) • Bandwidth is the Bandwidth of the channel, • SNR is the signal-to- noise ratio and • capacity is the capacity of the channel in bits per second. • SNR is the ratio of the power of the signal to power of the noise.
  29. Basic Data Communication Model • Communication is the conveyance of a message from one entity, called the source to the destination. • A simple example of such a communication system is conversation; people commonly exchange verbal messages, with the channel consisting of waves of compressed air molecules at frequencies which are audible to the human ear. • The only way that a message source can be certain that the destination properly received the message is by some kind of acknowledgment response from the destination. • Conversing people might say "I understand" in response to a statement made by their peer. This acknowledged form of dialogue is the basis of reliable communications. • The conveyance of a message could be direct between the corresponding entities or it could be indirect, with one or more intermediaries participating in the message transport.
  30. • Communication can be from a source to a single destination, known as point-to-point. • To multiple destinations, known as point-to-multipoint or multicast. • A special case of multicast is the conveyance of a message from a source to every possible destination, which is referred to as broadcast. • The broadcast can be local or global in scope. • Depending on the definitions of source, destination and channel, the communication could be Asynchronous or Synchronous . • Communication is called asynchronous if the sender and receiver do not need to synchronies before each transmission • In synchronous communication, there is a minimal assumed timing relationship between the source and destination. •
  31. The Communications Channel • A communications channel is a pathway over which information can be conveyed. • A communication channel can be simplex, in which only one party can transmit. • Full-duplex, in which both correspondents can transmit and receive simultaneously, • half-duplex, in which the correspondents alternate between transmitting and receiving states .
  32. • Communication between two entities can be considered either in- band or out-of-band, depending on context. • In-band communication is communication which occurs via the primary channel between the communicating entities. • Out-of-band communication occurs via an alternative channel, which is not considered to be the primary channel between the entities.
  33. • The message source is the transmitter, and the destination is the receiver. • A channel whose direction of transmission is unchanging is referred to as a simplex channel. • For example, a radio station is a simplex channel because it always transmits the signal to its listeners and never allows them to transmit back. • A half-duplex channel is a single physical channel in which the direction may be reversed. • Messages may flow in two directions, but never at the same time, in a half-duplex system. • In a telephone call, one party speaks while the other listens. After a pause, the other party speaks and the first party listens. • Speaking simultaneously results in garbled sound that cannot be understood.
  34. • A full-duplex channel allows simultaneous message exchange in both directions. • It really consists of two simplex channels, a forward channel and a reverse channel, linking the same points. • The transmission rate of the reverse channel may be slower if it is used only for flow control of the forward channel.
  35. Channel Characteristics • bandwidth -how much information can be conveyed across the channel in a unit of time, commonly expressed in bits per second or bps. • A higher bandwidth is usually a good thing in a channel because it allows more information to be conveyed per unit of time. • High bandwidths mean that more users can share the channel, depending on their means of accessing it. • High bandwidths also allow more demanding applications (such as graphics) to be supported for each user of the channel.
  36. • Quality - how reliably can the information be correctly conveyed across the channel, commonly in terms of bit error rate ( BER) and whether the channel is dedicated (to a single source) or shared (by multiple sources). • A low quality channel is prone to distorting the messages it conveys; • a high quality channel preserves the integrity of the messages it conveys. • Depending on the quality of the channel in use between communicating entities, the probability of the destination correctly receiving the message from the source might be either very high or very low. • If the message is received incorrectly it needs to be retransmitted.
  37. Serial Communications • Bit-serial transmission conveys a message one bit at a time through a channel. • Each bit represents a part of the message. • The individual bits are then reassembled at the destination to compose the message. • In general, one channel will pass only one bit at a time. • Thus, bit-serial transmission is necessary in data communications if only a single channel is available. • Bit-serial transmission is normally just called serial transmission and is the chosen communications method in many computer peripherals
  38. • Byte-serial transmission conveys eight bits at a time through eight parallel channels. • The raw transfer rate is eight times faster than in bit-serial transmission, eight channels are needed, and the cost may be as much as eight times higher to transmit the message. • When distances are short, it may nonetheless be both feasible and economic to use parallel channels in return for high data rates. • The baud rate refers to the signalling rate at which data is sent through a channel and is measured in electrical transitions per second. • The data rate of a channel is often specified by its bit rate . • However, an equivalent measure channel capacity is bandwidth. • In general, the maximum data rate a channel can support is directly proportional to the channel’s bandwidth and inversely proportional to the channel’s noise level.
  39. Parallel Communications • Parallel communication implies more than one wire in addition to a ground connection. • An 8-bit parallel channel transmits eight bits (or a byte) simultaneously. • A serial channel would transmit those bits one at a time. • If both operated at the same clock speed, the parallel channel would be eight times faster. • A parallel channel will generally have additional control signals such as a clock, to indicate that the data is valid, and possibly other signals for handshaking and directional control of data transmission. •
  40. communications protocol • A communications protocol defines the order and meaning of bits in a serial transmission. • It specify a procedure for exchanging messages. • A protocol will define how many data bits compose a message unit, the framing and formatting bits, any error-detecting bits that may be added and other information that governs control of the communications hardware. • Channel efficiency is determined by the protocol design rather than by digital hardware considerations. • Reliability-protocols provide greater immunity to noise by adding error-detecting and -correcting codes. • Baud Rate : The baud rate of a data communications system is the number of symbols per second transferred
  41. MODULATIONTECHNIQUES • Modulation techniques are methods used to encode digital information in an analog world. • The 3 basic modulation techniques are: • AM (amplitude modulation) • FM (frequency modulation) • PM (phase modulation) • All 3 modulation techniques employ a carrier signal. • A carrier signal is a single frequency that is used to carry the data. • For digital, the DATA is either a1 or 0. When we modulate the carrier, we are changing its characteristics to correspond to either a 1 or 0.
  42. AM - Amplitude Modulation • Amplitude Modulation modifies the amplitude of the carrier to represent 1s or 0s. • In the below, example, a 1 is represented by the presence of the carrier for a predefined period of 3 cycles of carrier. • Absence or no carrier indicates a 0. Advantages • Simple to design. Disadvantages • Noise spikes on transmission medium interfere with the carrier signal. • Loss of connection is read as 0s.
  43. AM - Amplitude Modulation
  44. Frequency Modulation (FM) • Frequency Modulation modifies the frequency of the carrier to represent the 1s or 0s. • In the example below, a 0 is represented by the original carrier frequency and a 1 by a much higher frequency -the cycles are spaced closer together. • Advantages • Immunity to noise on transmission medium. • Always a signal present. Loss of signal easily detected • Disadvantages • Requires 2 frequencies • Detection circuit needs to recognize both frequencies when signal is lost.
  45. Frequency Modulation (FM)
  46. Phase Modulation (PM) Phase Modulation modifies the phase of the carrier to represent a 1 or 0. • The carrier phase is switched at every occurrence of a 1 bit but remains unaffected for a 0 bit. • The phase of the signal is measured relative to the phase of the preceding bit. • The bits are timed to coincide with a specific number of carrier cycles (3 in this example = 1 bit). • Advantages • Only 1 frequency used • Easy to detect loss of carrier • Disadvantages • Complex circuitry required to generate and detect phase changes.
  47. Phase Modulation (PM)
  48. REVIEW QUESTIONS • • What is the need for storage of information? • What considerations are there while choosing a storage device for information? • What are short & long term storage devices? • Studying the file organization is an important aspect of computer science. Explain why? • A file should be organized in such a way that the records are always available for processing with no delay. What are the schemes for doing so? • What are the techniques for searching records in a file? • Write short notes on: • Sequential (SAM) • Line Sequential (LSAM) • IndexedSequential(ISAM) • What is data communication? • Define signals. Distinguish between periodic and a periodic signals. • What are the factors on which data rate depends on? • Describe Nyquist bit rate formula for noisy channel. 12.Describe Shannon Capacity formula for noisy channel. 13.Distinguish between serial & parallel data communication. 14.What are communication protocols and there relevance? 15.What are signal modulation techniques? Describe the following • AM (amplitude modulation) • FM (frequency modulation) • PM (phase modulation)