Overview of storage technology and data representation (Notes)
The storage technology refers to data storage systems that enable a computer to store and retrieve data and programs.
The basic functions of a computer are to
- Accept input data
- Process data
- Store data
- Produce output
Data storage system
Each data storage system has two main components:
a) Storage medium
Is a medium that holds data. Examples are hard disks (external or internal), multimedia card or memory card, Zip disk, tape cartridge, floppy disk, CD, DVD, and so on.
b) Storage device
Is the mechanical apparatus that records and retrieves data from a storage medium. Examples are hard disk drives, card readers, tape drives, CD drives, DVD drives, flash drives, solid state drives, floppy drives and Zip drives.
Computers can be configured with a variety of storage devices e.g. Hard disk drive, CD drive or DVD drive, flash drive or solid state card reader, floppy disk drive, or Zip drive.
Storing and retrieving data
- The process of recording or storing data is often referred to as “writing data” or “saving a file”.
- The storage device writes the data on the storage medium to save it for later use.
- The process of retrieving data is often referred to as “reading data”, “loading data”, or “opening a file.”
Data is copied from a storage device into Random Access Memory (RAM) of a computer, where it waits to be processed.
RAM is a temporary holding area of working file (e.g. word processing document) for the operating system and application program instructions hence it is important to the storage process. In other word, RAM is the connection between computer’s storage devices and its storage media.
RAM is very volatile (data can be lost easily) and doesn’t store data permanently. Thus, after data is processed in RAM, it is usually copied to a storage medium for more permanent safekeeping.
Please don’t confuse between RAM and hard disk storage. Note their differences below:
Data presentation
Letters, numbers, musical notes, and pictures do not pass from the keyboard or other input device through the circuitry of a computer and then jump onto the screen or printer.
So how is it that a computer can work with documents, photos, videos, and sound recording? The answer to that question is what data representation and digital electronics are all about.
Data representation makes it possible to convert letters, sound, and images into electrical signals.
Digital electronics makes it possible for a computer to manipulate simple “on” and “off” signals, which are represented by 1s and 0s, to perform complex tasks.
Data representation is based on the binary number system, which uses two numbers, 1 and 0, to represent all data.
When data is stored, these 1s and 0s must be converted into a signal or mark that is fairly permanent, but which can be changed when necessary.
The data is not literally written as “1” or “0.” Instead, the 1s and 0s must be transformed to change the surface of a storage medium. Exactly how this transformation happens depends on the storage technology. For example, floppy disks store data in a way that is different from the way CD-ROMs store data.
Three common types of storage technologies are
a) Magnetic storage
Stores data by magnetizing microscopic particles on the disk or tape surface.
Disk consists of circular platter constructed of nonmagnetic material, called the substrate, coated with a magnetizeable material. Substrate was traditionally aluminium; now is glass. Examples of magnetic storage are hard disk, floppy disk, Zip disk, and tape; refer figure below.
Magnetic Storage Read write Mechanisms
Data recording and retrieval (Read or write operations) are done via a stationary conducting coil named as the head on top of the platter which rotates beneath it.
Current flowing through a coil produces a magnetic field
A magnetic field moving relative to a coil produces current in the coil and pulses are sent to the head
Magnetic patterns are recorded on the surface below.
When the surface of the disk passes under the head, current of the same polarity is generated as the one already recorded as result read the data.
Read and write heads may be combined or separated and sometimes are measured in revolutions per minutes (rpm). The faster a drive spins, the more rapidly it can position the read write head over specific data. i.e. 7,200 rpm is faster than a 5,400 rpm drive.
Heads read information while copying it from disk to RAM and write information when copying it from RAM to disk.
Internal hard disk is a magnetic storage device with one or more thin platters that store information sealed inside the disk drive.
External hard disks are magnetic storage media which are portable storage units that can be connected to computer as necessary.
Durability of magnetic media
Magnetic media is not very durable as a result data stored on it can be altered by magnetic field, dust, mould, smoke particles, heat, and mechanical problems with a storage device.
Thus, a magnet should never be placed on or near a magnetic media like floppy disks because it will alter the magnetic particles on the disk, destroying the data.
Magnetic media gradually lose their magnetic charge, which results in lost data.
Some experts estimate that the reliable life span of data stored on magnetic media is about three years.
b) Optical Storage
Stores data as microscopic light and dark spots on the disk surface whereas the dark spots are called pits and the lighter, nonpitted surface areas of the disc are called lands.
Disk’s surface is made of polycarbonate resin whereby computer data are stored as a series of microscopic pits using a highly focused laser.
Polycarbonate resin is coated with highly reflective material such as aluminium and Acrylic coating added for protection.
Both CDs (“Compact Discs”) and DVDs (“Digital Versatile Disc”) use optical storage technologies.
Optical storage media use one of three technologies Read only (ROM), Recordable (R), Rewritable (RW).
Thus, the suffix used to help recognizing the type of technology used to created as CD-ROM specifies “CD read-only” technology, CD-R specifies “CD recordable” technology, CD-RW specifies “CD rewritable” technology.
DVDs have two types of recordable and two types of rewritable formats.
i. The recordable formats are designated as DVD-R and DVD+R
ii. The rewritable formats are designated as DVD-RW and DVD+RW.
Three technologies used by optical storage media are
1. Read only memory (ROM) technology
Involves data that is stamped on disc surface when it was manufactured hence data cannot be added, changed, or deleted
e.g. Commercial software, music, and movies. Examples are CD-DA (Compact disc digital audio), DVD-Video (digital versatile disc video), CD-ROM and DVD-ROM
2. Recordable (R) technology-
The laser creates dark spots in the dye of medium that are read as pits.
The change in dye is permanent, so data cannot be changed once it has been recorded and it can be done in multiple sessions. Provides a permanent record of large volumes of user data. Examples are CD-R, DVD-R or DVD+R.
3. Rewritable (RW) technology
Uses “phase change” technology where there are two different reflections in two different phase states
Laser used to change state
> Amorphous state – molecules have random orientation – poor reflections
> Crystalline state – smooth surface – good reflections to alter a crystal structure on the disc surface.
The crystal structure can be changed from light to dark and back again many times as a result data can be recorded and erased multiple times. Examples are CD-RW, DVD+RW or DVD-RW.
Optical Storage Read write Mechanisms
- CD drives and DVD drives are storage devices that use optical storage (laser) technology to read data on computer or audio CDs or DVDs respectively.
- CD and DVD drives contain a spindle that rotates the disc over a laser lens. The reflecting laser directs a beam of light toward the underside of the disc.
- When the beam strikes a pit, no light is reflected. When the laser strikes a reflective surface, light bounces back to the read head.
- Thus, dark “pits” and light “lands” on the disc surface reflect the light differently whereby the reflective intensity determines value.
- The patterns of light and dark between pits and lands are interpreted as the 1s and 0s that represent data as result data recording.
- As a drive reads the disc, these differences are translated into the 0s and 1s that represent data
- Disk contains a single spiral track that starts at the centre ends at the edge. Note a constant linear velocity disk rotates slower towards the edge
- Data recorded on optical media is generally considered to be less susceptible to environmental damage than data recorded on magnetic media.
The useful life of a CD-ROM disc is estimated to exceed 200 years.
c) Solid State Storage
Stores data in a non volatile, erasable, low-power chip and its medium stores data in a microscopic grid of cells.
The chip’s circuitry is arranged as a grid, and each cell in the grid contains two transistors that act as gates.
When the gates are open, current can flow and the cell has a value that represents a ”1” bit. When the gates are closed, the cell has a value that represents a “0” bit.
Very little power is required to open or close the gates, which makes solid state storage ideal for battery-operated devices, such as digital camera.
Once the data is stored, it is non-volatile-chips retains the data without the need for an external power source.
Some solid state storage requires a device called a card reader to transfer data to or from a computer, others plug directly into a computer’s system unit.
When connected, file stored in a solid state storage medium can be opened, edited, deleted and run as those stored in hard disk.
A Universal Serial Bus (USB) flash drive
Is a portable storage device featuring a built-in connector that plugs directly into a computer’s USB port.
CompactFlash (CF) cards
Are about the size of a matchbook and provide high storage capacities and access speeds.
They use a card reader to transfer data from the card to computer whereas a built in controller tells the card reader what to do.
With their high storage capacities and access speeds, CompactFlash cards are ideal for use on high-end digital cameras that require megabytes of storage for each photo.
Multimedia cards (MMC)
0ffer solid state storage in a package about the size of a postage stamp. Initially used in mobile phones and pagers, use of MMC has spread to digital cameras and MP3 players. Like CFC, MMC include a built-in controller, so MMC readers are electronically simple and very expensive.
Secure Digital (SD) cards
Are based on MMC technology, but they feature significantly faster data transfer rates and include cryptographic security protection for copyrighted data and music.
They use a card reader to copy data from the card to a computer and it’s popular for digital images and MP3 storage.
Smart Media cards
0riginally called ‘solid state floppy disk cards’ because they look much like a miniature floppy disk.
They do not include a built in controller, which means that the Smart Media card reader manages the read or write process.
These cards are the least durable of the solid state storage media and should be handled with care.
USB Flash drive, Compact Flash Card, Smart media cards, Multimedia cards, Secure digital cards, Card readers and computer's USB port,
Comparing storage technologies
There are four criteria to determine the best storage technology systems namely:
i. Versatility
Is the ability of a device and its media to work with more than one storage technologies. Some storage device can access data from only one type of medium. More versatile devices can access data from several different media.
e.g. A floppy disk drive can access only floppy disks but a DVD drive can access DVDs, DVD movies, audio CDs, data CDs, and CD-Rs.
ii. Durability
Determines the ability of the device or media to last so that data will be accessible.
Most storage technologies are susceptible to damage from mishandling or other environmental factors, such as heat and moisture. Some technologies are less susceptible than others.
e.g. Optical and solid state technologies tend to be less susceptible than magnetic technologies to damage that could cause data loss.
iii. Speed
Is the time that storage technology takes to retrieve or access the data (efficiency). It involves two factors namely:
Access time which is the average time taken by a computer to locate data on the storage device, measured in milliseconds. Lower number indicate faster access times.
Random access (direct access) is the ability of a device to jump directly to the requested data. Thus, the random access devices have the fastest access times. Floppy disk, hard disk, solid state, CD, and DVD drives are random-access devices.
Sequential access is the ability to read from the beginning to the end and hence it is slower. e.g. Tape drive.
Data transfer rate is the amount of data that a storage device can move from medium to the computer per second. Higher numbers indicate faster transfer rates.
iv. Capacity
Is the maximum amount of data that can be stored on a storage medium, measured in kilobytes (KB), megabytes (MB), terabytes (TB), Petabyte (PB) or Exabyte (EB) each technology can store. The amount of data that a disk stores-its capacity-depends on its density.
Disk density refers to the closeness of the data on the disk surface. The higher the disk density, the more data it can store. Higher capacity is almost always preferred.
Examples of storage capacities are 40-200 GB for fixed hard disk, 160 GB (average) for External hard drive and removable hard disk. Also 700MB for CD and CD-RW, 4.7GB for writable DVD, 32MB - 8GB USB flash drive, 32MB - 4GB for memory card and 30 GB for magnetic tape.
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