Storage devices and media, Exercises of Technology

Download Storage devices and media and more Exercises Technology in PDF only on Docsity! In this chapter you will learn about:  back-up storage  why it is necessary to back up data and files  the types of access used by the backing stores  the types of internal and external backing storage devices:  magnetic  optical  solid state. 3.1 Backing up data The first two sections in this chapter consider the need for backing up data and the different ways of storing and accessing data. Section 3.3 then discusses many forms of backing storage and compares the advantages and disadvantages of each type. The comparative performance and main uses for each type of store are also discussed in some depth. What is backing up of data? Backing up refers to the copying of files and data to a different medium (disk, tape, flash drive, etc.) in case of a problem with the main storage device. Backing up files and data on a regular basis is seen as good computing practice and many computer systems can be set to back up files automatically on a regular basis. The backups are often stored in a different place to the main storage. This is in case of fire or some other situation which could lead to irretrievable loss of key data and files. Why back up data? There are various reasons why backups are made. Some of the more common reasons are considered below:  Data could be lost due to failure of the original storage device. This could be due to hardware failure (e.g. head crash on a hard drive unit), problems caused by files being over-written accidentally (or otherwise) or possible corruption of files (e.g. caused by power surges).  Hackers could be responsible for the corruption or even loss of data. This may not be their intention (they may only want to gain access to the information for other purposes, e.g. to find personal information such as bank account details). However, the very act of hacking into files could cause problems such as corruption or data loss.  Backups are also made in case the files need to be used elsewhere. The original files are then protected against possible corruption or loss. However, backups do not necessarily guard against the effect of a virus. The virus could attach itself to the files which could mean that the backups were also affected. If the computer was ‘cleaned’ of the virus and then the backup files were re-loaded 28 Storage devices and media CHAPTER 3 C03_IGCSE_ICT_9780340983829.QXD:983829_IGCSE_BP_02 16/12/09 12:51 Page 28 there would remain the risk that the same virus could infect the computer system again. The best protection is not to get a virus in the first place (discussed in Chapter 6). 3.2 Types of access The way data is stored and read by different backing storage devices varies considerably. This section briefly describes the two main methods of accessing data. Serial access With this system, to access data it is necessary to start at the beginning and then access each piece of data in turn until the required information is found. It is primarily used on magnetic tape systems and is a very slow form of access. It is used in applications where speed of access or where the order in which the data is accessed is not important, for example in utility billing, clearing bank cheques or producing pay slips. When a magnetic tape needs updating, an additional tape is required so that the old information can be merged with the new data (itself often on another tape, but the new data could be stored in various ways) to produce the updated tape (see Figure 3.1). Direct access This method is used with magnetic disks and with optical media (such as CDs and DVDs). The computer uses a key field to calculate where data has been stored. It is then able to access the data directly from the calculated position. Consequently, access is much faster than with serial access. It is used in applications where access speed is vital (e.g. in real-time process control systems such as controlling a chemical plant or online systems such as booking air tickets or automatic stock control). When updating media which uses direct access, the new data is written to the next available location and its position is calculated using the built-in algorithm. 3.3 Backing storage media Dating back to the development of the personal computer, all computer systems have come equipped with some form of backing storage. When a user types data into a computer, the information is stored temporarily on the RAM – however, this information would be lost as soon as the computer was turned off. Backing storage devices ensure that data is stored permanently and can be used at a later date. This section will be considering various types of backing storage and the media used. Backing storage devices are either internal or external (i.e. plug-in devices) to the computer, and are one of three types:  magnetic  optical  solid state. Storage devices and media 29 New data to be merged with old data B Old data stored on magnetic tape A New tape produced by merging A and B Figure 3.1 Updating the data on a magnetic tape C03_IGCSE_ICT_9780340983829.QXD:983829_IGCSE_BP_02 7/1/10 11:37 Page 29 Disadvantages  Access time is very slow.  When updating, another tape is needed (i.e. original tape + tape with the changes produces an updated tape). Optical storage media Optical storage devices, such as CD and DVD, all use optical (i.e. light) read/write methods, unlike tapes and floppy/hard drive disks which are magnetic media. A laser beam is used to write to and read from the optical media. The CDs and DVDs are manufactured either from a single polycarbonate disk or from two polycarbonate disks bonded together. A very thin layer of metal or organic dye is used as the recording media. The big advantage of these storage media is that they are portable and can store large data files (e.g. films, music or multimedia files) which would be too large for a floppy disk. CD-ROM and DVD-ROM CD-ROMs and DVD-ROMs are read only memory (ROM), which means they cannot be written over and can only be read. The data is stored as a series of pits (equivalent to a binary value of 1) and lands (equivalent to the binary value of 0) in the metallic optical layer. The pits are formed by a laser beam etching the surface at the manufacturing stage. Only a single track exists which spirals out from the centre of the disk. The pits and lands are read by a low-powered laser beam which follows the data stream and reads from the centre outwards in a spiral. The light reflects differently off a pit than it does off a land and this is interpreted as 1s and 0s (i.e. data) – hence the term digital media. Uses  CD-ROMs are used by manufacturers to store music files and software, computer games and reference software (such as an encyclopedia).  DVD-ROMs have much larger storage capacity than CD-ROMs and are used to store films. They are now increasingly used to store computer data and ever-more sophisticated computer and arcade games. Advantages  They hold far more data than floppy disks, so one CD/DVD could replace several floppy disks in some applications.  They are less expensive than hard disk drive systems. Disadvantages  The data transfer rate and data access time are slower than for hard disks. CD-R and DVD-R The letter ‘R’ here means the disk is recordable once only and then it becomes a CD-ROM or DVD-ROM. These use a thin layer of an organic dye as the recording media; DVDs also use an additional silver alloy or gold reflector. A laser beam produces heated spots and unheated spots. On reading the disk, a laser beam is Information and Communication Technology 32 C03_IGCSE_ICT_9780340983829.QXD:983829_IGCSE_BP_02 16/12/09 12:51 Page 32 capable of distinguishing between the two types of spots and effectively reads the data stream from the centre outwards in a spiral action. This data is then interpreted as 1s and 0s. Uses  They are used for home recordings of music (CD-Rs) and films (DVD-Rs).  They are used to store data to be kept for later use or to be transferred to another computer.  They are used in applications where it is necessary to prevent the deletion or over-writing of important data). Advantages  CD-Rs and DVD-Rs are cheaper than RW disks.  Once burned (and finalised), they are like ROM disks. Disadvantages  They can only be recorded once, so if an error occurs then the disk has to be thrown away.  Not all CD/DVD players can read CD-R/DVD-R. CD-RW and DVD-RW The ‘RW’ means that these disks are a re-writable media and can be written over several times. Unlike CD-R/DVD-R, they don’t become ROMs. The recording layer uses a special phase-changing metal alloy. The alloy can switch between crystalline and amorphous (non-crystalline) phases, thus changing its reflectivity to light, depending on the laser beam power. Spots are produced which can be read by a laser and then interpreted as 1s and 0s. The system allows data to be written, erased and re-written many times. Uses  CD-RWs and DVD-RWs are used to record radio and television programmes, but can be recorded over time and time again.  They are used in closed circuit television (CCTV) systems. Advantages  CD-RWs and DVD-RWs can be re-used many times.  They can use different file formats each time they are used.  The RW format is not as wasteful as the R format since files or data can be added at a later stage. Disadvantages  CD-RWs and DVD-RWs can be relatively expensive media.  It is possible to accidentally overwrite data. DVD-RAM DVD-RAM is a recent addition to the optical media group. Unlike other CD and DVD formats, DVD-RAMs have several discrete concentric tracks rather than a single spiral track. This gives them the advantage that writing and reading can occur at the same time. This makes it possible to watch an already recorded television Storage devices and media 33 C03_IGCSE_ICT_9780340983829.QXD:983829_IGCSE_BP_02 16/12/09 12:51 Page 33 programme at the same time as a different programme is being recorded. DVD-RAMs can be written to many times. Figure 3.2 compares the single spiral track found on normal CDs and DVDs with the discrete single tracks found on a DVD-RAM. The recording layer is made from a similar phase-changing material to that used in RW technology. When writing, a laser heats the phase- changing alloy on the disk to about 500–700ºC, changing the reflective properties from shiny to dull (i.e. pits). If the disk needs to be erased, a laser heats the surface to about 200ºC to return the disk to its original shiny state. A low power laser is used to read the written marks on the surface. The shiny and dull (pits) marks represent data to a computer where they are interpreted. Uses  DVD-RAMs are used in recording devices such as satellite receivers to allow simultaneous recording and playback.  They are used in camcorders to store films. Advantages  DVD-RAMs have a long life – minimum life is estimated to be 30 years.  It is possible to do a re-write operation over 100,000 times, compared with the RW format which only allows about 1,000 re-writes.  Writing on DVD-RAMs is very reliable, as they have in-built verification software to ensure the accuracy of the data.  Access is very fast if the files are fairly small.  There is no need to finalise the disk.  They have a very large capacity (about 10 Gbyte if double-sided format is used).  They offer the ability to read data at the same time as data is being written. Disadvantages  DVD-RAMs are not as compatible as R or RW format, as many systems will not recognise their format.  They are relatively expensive, costing about 4 times as much as a DVD-RW disk. Blu-ray disks Blu-ray disks have the largest capacity of all the optical media available and go up to 100 Gbyte (at the present time). The laser beam used is at the blue/violet end of the spectrum, rather than red which is the colour of the lasers used in other optical media. Consequently, the light used has a shorter wavelength, allowing more data to be stored/read on the disk. Uses  Blu-ray disks are used in home video consoles.  They are used for storing and playing back films: 1 high-definition film of two hours duration uses 25 Gbyte of memory.  PCs can use this technology for data storage or backing up hard drives.  Camcorders can use this media (in cartridge form) to store film footage. Information and Communication Technology 34 a b Figure 3.2 a Spiral tracks on a normal CD or DVD b Discrete tracks on a DVD-RAM C03_IGCSE_ICT_9780340983829.QXD:983829_IGCSE_BP_02 16/12/09 12:51 Page 34