Transcript M150 Data, Computing and Information

DATA, COMPUTING AND INFORMATI
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Haifaa Elayyan
M150 – UNIT 5
UNIT 5: FOUR THEMES
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Data transmission:
How the Internet works?
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Data storage:
Logical and physical storage.
Large collections of data:
Databases facilitate storage, access & data
protection
Metadata helps providing access to multimedia DB
Keeping data secure:
Privacy issues
Ownership of data
Risks on data
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Haifaa Elayyan
A data storage device is a device for recording
(storing) information (data). Recording can be done
using virtually any form of energy, spanning from
manual muscle power in handwriting, to acoustic
vibrations in phonographic recording, to
electromagnetic energy modulating magnetic tape
and optical discs.
A storage device may hold information, process
information, or both. A device that only holds
information is a recording medium. Devices that
process information (data storage equipment) may
either access a separate portable (removable)
recording medium or a permanent component to
store and retrieve information.
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UNIT 5: PERSISTENT DATA
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To facilitate subsequent retrieval, you store your
documents in some logical arrangement on a suitable
storage medium for holding persistent data such as
your computer’s hard disk.
The cabinet has several drawers and each drawer may
hold a large number of files. In turn, each file
contains a number of related documents. You label
each drawer of the filing cabinet with a name, and you
put a name tag on each file….
Now you have a strategy for retrieving your
documents quickly.
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UNIT 5: HIERARCHICAL OR NESTED
FOLDER STRUCTURE
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The lowest level of the hierarchy  documents
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This is called a hierarchical or nested folder structure, because each
folder may contain other folders
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An Explorer window has two panes. The right-hand pane is a folder
window (content can be displayed as icons or as a list)
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The left-hand pane does not show any
hard disks, folders and any icon which
share’).
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documents, but it does showF08
holds other items (e.g. ‘network
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UNIT 5: TREE STRUCTURE
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In computer science, a tree is a widely-used data structure that
emulates a hierarchical tree structure with a set of linked nodes. It is
an acyclic connected graph where each node has a set of zero or more
children nodes, and at most one parent node.
The elements at the very bottom of an
inverted tree (that is, those that have no
elements below them) are called leaves.
Inverted trees are the data structures used
to represent hierarchical file structures. In
this case, the leaves are files and the other
elements above the leaves are directories.
A binary tree is a special type of inverted
tree in which each element has only two
branches below it.
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UNIT 5: TREE STRUCTURE
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With a bit of imagination, you can think of the folder structure
loosely as a tree lying on its side. The desktop is the root of the tree,
and each folder is a branch. The leaves of the tree correspond to
documents.
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Any similar hierarchical arrangement of objects is frequently called a
tree structure or just a tree
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UNIT 5: PATH
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Haifaa Elayyan
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A path is the general form of a file or directory name, specifying a
unique location in a file system. A path points to a file system
location by following the directory tree hierarchy expressed in a
string of characters in which path components, separated by a
delimiting character, represent each directory. The delimiting
character is most commonly the slash ("/"), the backslash
character ("\"), or colon (":"), though some operating systems may
use a different delimiter. Paths are used extensively in computer
science to represent the directory/file relationships common in
modern operating systems, and are essential in the construction
of Uniform Resource Locators (URLs).
A path can be either absolute or relative. A full path or absolute
path is a path that points to the same location on one file system
regardless of the working directory or combined paths. It is
usually written in reference to a root directory.
A relative path is a path relative to the working directory of the
user or application, so the full absolute path may not need to be
given.
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UNIT 5: PATH
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There are two distinct folders called ‘mail’ (prev. figure)
but no confusion arises because each is in a different
place and has a different path leading to it.
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How many doc./folders should you keep in a single folder?
There is no limit !! Preferably do NOT exceed 20!
A path contains the names of all the folders that lead to
it from the root.
In effect a path allows you to identify unambiguously aAO
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folder or document and is often referred to as its fullLeb.
name or full path name (documents in the same folder F08
must have different names)
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UNIT 5: SEARCH/FIND FUNCTION
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Search function  find items you have ‘lost’
In Windows XP, the search will begin in ‘My Computer’ (or
elsewhere by using the drop-down list).
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Consider the Windows path name:
C:\Projects\M150\Assignments\TMA02.doc
‘C:’ is the root. ‘Projects’ is the name of a folder at the
top level of the hard disk which contains a folder called
‘M150’ which in turn contains an ‘Assignments’ folder.
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The document ‘TMA02.doc’ is in the ‘Assignments’ folder.
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Another place where you can see full path names is in a
‘Search Results’ window.
UNIT 5: DIRECTORIES
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The topmost directory in any file is called the
root directory. A directory that is below another
directory is called a subdirectory. A directory
above a subdirectory is called the parent
directory. Under DOS and Windows, the root
directory is a back slash (\).
Haifaa Elayyan
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An organizational unit, or container, used to
organize folders and files into a hierarchical
structure. Directories contain bookkeeping
information about files that are, figuratively
speaking, beneath them in the hierarchy. You
can think of a directory as a file cabinet that
contains folders that contain files. Many
graphical user interfaces use the term folder
instead of directory. Computer manuals often
describe directories and file structures in terms
of an inverted tree. The files and directories at
any level are contained in the directory above
them. To access a file, you may need to specify
the names of all the directories above it. You do
this by specifying a path.
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To read information from, or
write information into, a
directory, you must use an
operating system command. You
cannot directly edit directory
files. For example, the DIR
command in DOS reads a
directory file and displays its
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contents.
UNIT 5: DIRECTORIES
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Part of the directory for a given folder can be displayed
on screen in a number of ways (alphabetically , in order of
last modification date, size , type …) to aid human
identification of the contents.
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Each folder has a list, or directory, of the folders and
documents that it contains (A folder is sometimes loosely
referred to as a directory !!!)
The directory of a folder also lists the address or
physical location (more discussion to come!) on the disk of
each document and subfolder in that folder. This address
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is internal to the operating system and cannot be seen in
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user window.
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UNIT 5: STORAGE TECHNOLOGIES
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There are various technologies of data storage:
Storing data can be on the hard disk (typically is built into your
computer) or on removable storage media such as CDs, DVDs, Zip
disks, high-capacity tape cartridges…
There are various measures of storage size (Capacity):
PS: ‘kilo’ usually means 1,000 but in computing it denotes 1024
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UNIT 5: HARD DISK STORAGE
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The general principle of hard-disk storage:
coated with a magnetic material that can be magnetised
into a pattern representing a sequence of bits i.e the
surface consists of millions of tiny magnets, which can
each be magnetised in two possible directions
representing 1 or 0.
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Nowadays a typical PC comes with a 40/80 GB hard disk
A hard disk is 1 to 3 inches in diameter, and consists of
one or more circular plates (aluminum, ceramic or glass…),
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each having two surfaces.
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Data is recorded on each surface by magnetising a series
of concentric circles called tracks.
UNIT 5: HARD DISK STORAGE
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The disk surface is divided into a number of equal sized wedgeshaped regions called sectors. Within a sector each track holds
the same amount of data – usually 512 bytes.
This is the basic unit of data handled by the disk control
mechanism, and is called a block (each block of data is
guaranteed to be the same size)
Question: How can a sector hold the same amount of data on
each track when the outer tracks are large than the inner
tracks? (check answer on page 14)
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UNIT 5: HARD DISK STORAGE
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The actual reading from and writing to the disk surface is
performed by a read/write head, which is attached to an
arm that moves to and from the centre. The disk is kept
spinning continuously, so each sector is under the head at
some time.
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The head hovers close to the spinning surface, which
needs to be engineered carefully to avoid physical contact
between the head and the surface (disk crash).
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For each plate in a disk there are two read/write heads,
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one for each surface. In a read operation the head Leb.
detects a magnetised pattern. In a write operation, the
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UNIT 5: HARD DISK STORAGE
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The heads associated with all surfaces move in and out
together; at any one time they can read from the
corresponding tracks on both surfaces of every plate in the
disk. This set of tracks is called a cylinder (a disk may have
16000 cylinders i.e. each plate would have 16000 tracks on
it).
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Typical rotation speeds for hard disks are 5,000–15,000
revolutions per minute. A disk which rotates at 10,000
rev/min and has 60 sectors takes 0.1 millisecond (ms) for a
512-byte block of data to pass under the head.
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On average it takes half a rotation (3ms) for the desired
sector to reach the head – this is called the average
latency. You also need to add to this the seek time (time
taken for the head to move to the relevant track ~3–10ms)
UNIT 5: REMOVABLE STORAGE
MEDIA DEVICES
Read/Write floppy disks: capacity limited to 1.4MB.
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Zip drive: 100MB or 250MB capacity
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Memory card: A removable medium which is very popular
(works on a principle that will not be discussed here).
Optical discs:
CD (compact disc)
 DVD (digital versatile disc):
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Store documents using a different technology based on
the optical properties of the surface. The capacity of a
CD is 650MB and data is stored on only one side of it in a
single spiral groove which winds round the disc 22,188
times.
UNIT 5: OPTICAL DISCS
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The data is packed uniformly along the groove, so that
outer tracks of the groove hold more data than inner ones
( the disc spins more slowly when accessing data near the
centre)
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Conventional CDs are called CD-ROMs (Read-Only
Memory), and have bits of data stored as ‘pits’ in their
groove. Beams of laser light are used to burn the pits on
the disc. A CD drive works by shining a low-power laser
beam on the disc, which detects the presence or absence
of a pit (the pits do not reflect the light).
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DVDs (also called DVD-ROMs) pack the data more tightly,
using smaller pits and a narrower groove which increase
the capacity of a simple DVD to 4.7GB (Some DVDs use
both sides having one or two layers each, yielding a
theoretical maximum capacity of 19MB).
UNIT 5: HARD DISKS VS. OPTICAL
DISKS
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One important difference between CD/DVD-ROM discs
and magnetic disks is the ability to write to them (once a
pit has been burned, it cannot be erased).
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There are two kinds of CDs:
1. Recordable CDs, known as CD-R. Instead of burning pits on
the CD, the writing process dyes the relevant parts of the
groove. When read by a CD drive these dye spots are
indistinguishable from pits on a conventional CD. The process
is not reversible.
2.
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Rewritable CDs, known as CD-RW, use a different technology
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altogether (heat a point on the disk to one of two
temperatures corresponding to different states of the F08
material). This process is reversible.
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UNIT 5: LABELLING VOLUMES
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Identifying a CD or other removable medium is necessary.
It is important that each is given a label with a title. The
volume can be stored in a rack with many similar looking
ones but can be identified by its external label.
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A hard disk or Zip disk or CD is called a volume.
Besides its physical label a volume should also have an
electronic label, which, for consistency, should be the AO
same as the physical label. This electronic label is the U
name of the volume, and it will be displayed when you Leb.
search the contents of your computer.
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UNIT 5: SENSIBLE ORGANISATION
OF STORAGE
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In the case of a magnetic disk three numbers are required to identify
a block of data: cylinder number, surface number and sector number.
This set of three numbers is called the address of the block.
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Each volume contains a large number of documents, so there has to be
a means of locating the one you want.
To this end each volume has a volume table of contents or VTOC. The
VTOC is a table with one line for each document.
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UNIT 5: MOVING DOCUMENTS
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What actually happens when you move a document
‘M150notes.doc’ from a folder (say ‘Current’) to another
(say ‘Models’) on your hard disk?
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Of course, you expect that from now on it will no longer
be displayed in ‘Current’, but will appear when you inspect
‘Models’. What happens to the document itself, though?
The simple answer is nothing. Moving a document between
folders on a disk is really an illusion because the document
does not move at all!
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What really happens is something different. The
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document’s physical location remains unchanged, but the
directories change (as illustrated in page 21).
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UNIT 5: DELETING DOCUMENTS
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What actually happens when you delete a document?
The operating system does not obey your instruction, but,
instead moves the document to a special folder called
‘Recycle Bin’ or ‘Trash’ from which it can be retrieved.
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When your hard disk is becoming too full – you can decide
to ‘empty’ it. Once done, is there a hope for recovery?
When you moved documents to the recycle bin? They did
not go anywhere; they remained in the same physical
position on the disk. It was the directory entry for theAO
document that was removed, with a new directory entryU
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being created in the recycle bin.
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UNIT 5: DELETING DOCUMENTS
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What you perceive when you navigate through the folders
on your computer is not where the documents are located
physically, but where they are located logically. That is,
you are given a logical view of your documents which shows
their relationship to each other in a hierarchical (nested)
structure.
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The operating system hides from you where items are
located physically. The document does not need to be
moved when you empty the bin but marked for
deletion!!!!!!!!
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So the document may remain on your disk for a long time
without being overwritten. However, it is inaccessible
since its directory entries have disappeared.
Specialized software can recover  Privacy issue???
UNIT 5: OTHER STORAGE MEDIA
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Magnetic tape is a linear storage medium which is slow and
difficult to access. So why would you want to use it?
The main strengths of tape are its high capacity, its
reusability and its cheapness.
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A hologram is a three-dimensional image made with the aid
of a laser which helps storing much higher volumes of
data.
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A Biological storage media idea is to represent 0s and 1s
using two colour states of a suitable form of synthetic
DNA.
A number of such memory units would be attached to a
support substrate to form a memory cell.
COMPUTER NETWORKING
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Haifaa Elayyan
Computer networking is the engineering
discipline concerned with communication
between computer systems or devices.
Networking, routers, routing protocols, and
networking over the public Internet have their
specifications defined in documents called
RFCs.[1] Computer networking is sometimes
considered a sub-discipline of
telecommunications, computer science,
information technology and/or computer
engineering. Computer networks rely heavily
upon the theoretical and practical application
of these scientific and engineering disciplines
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COMPUTER NETWORKING
Haifaa Elayyan
A COMPUTER NETWORK IS ANY SET OF COMPUTERS OR
DEVICES CONNECTED TO EACH OTHER WITH THE ABILITY TO
EXCHANGE DATA.[2] EXAMPLES OF DIFFERENT NETWORKS
ARE:
LOCAL AREA NETWORK (LAN), WHICH IS USUALLY A SMALL
NETWORK CONSTRAINED TO A SMALL GEOGRAPHIC AREA.
WIDE AREA NETWORK (WAN) THAT IS USUALLY A LARGER
NETWORK THAT COVERS A LARGE GEOGRAPHIC AREA.
WIRELESS LANS AND WANS (WLAN & WWAN) ARE THE
WIRELESS EQUIVALENT OF THE LAN AND WAN.
ALL NETWORKS ARE INTERCONNECTED TO ALLOW
COMMUNICATION WITH A VARIETY OF DIFFERENT KINDS OF
MEDIA, INCLUDING TWISTED-PAIR COPPER WIRE CABLE,
COAXIAL CABLE, OPTICAL FIBER, AND VARIOUS WIRELESS
TECHNOLOGIES.[3] THE DEVICES CAN BE SEPARATED BY A
FEW METERS (E.G. VIA BLUETOOTH) OR NEARLY UNLIMITED
DISTANCES (E.G. VIA THE INTERCONNECTIONS OF THE
INTERNET[
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UNIT 5: COMPUTER NETWORKING
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Networks of computers have been around for more than
thirty years. In recent years, the internet has become a
part of society, like the telephone, radio and television.
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The web, which is based on the internet, has become the
platform on which all kinds of information are
disseminated. For example, it has generated e-commerce,
new educational system …
Such applications, all of which use the internet, are
creating novel forms of communication.
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Besides the internet, many other computer networks exist
(banks, Police, Travel agents and airline…).
UNIT 5: NETWORKING ISSUES
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A network of computers is linked together by
communication links. These links may be:
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dedicated cable links;
public telephone networks;
radio or microwaves links.
Networks do not have to be dispersed over a wide area;
their benefits are available locally. Any organisation using
more than one computer is likely to have a local area
network (LAN) to exploit the benefits of resource
sharing.
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Pocket-sized computers known as PDAs (personal digital
assistants) can communicate with each other and with
desktop computers using infra-red / Bluetooth signals.
They form a small local network.
UNIT 5: THE INTERNET
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The Internet is a worldwide collection of computer networks,
cooperating with each other to exchange data using a common
software standard. Through telephone wires and satellite links,
Internet users can share information in a variety of forms. The
size, scope and design of the Internet allows users to:
connect easily through ordinary personal computers and local
phone numbers;
exchange electronic mail (E-mail) with friends and colleagues
with accounts on the Internet;
post information for others to access, and update it frequently;
access multimedia information that includes sound,
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photographic images and even video; and
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access diverse perspectives from around the world.
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UNIT 5: THE INTERNET
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The internet has its roots in the American militaryfunded research community of the early 1970s. The first
applications to use the internet were based purely on
text.
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The internet comprises a huge collection of computers
(called hosts) with telecommunications links between
them.
The internet then began to be used for email and for file
transfer. Modern graphical tools for accessing the
internet (like Netscape Navigator) are much more recent
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dating back to the early 1990s.
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How big is the internet? ARPANET was a network of just
four computers. By 1996 that figure had grown to 15
million host computers, in 2002 it had multiplied ten times
UNIT 5: THE INTERNET
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The internet links together not just one type of computer
but any type of computer running any operating system.
By adopting the internet protocol each of these
computers can become an internet host.
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The telephone system (which uses analogue signals
consisting of a continuously varying voltage) was designed
for voice transmission.
As your computer communicates using digital signals
(consisting of discrete bit patterns), a modem (modulatordemodulator) which is a piece of equipment will be sitting
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between your computer and the telephone socket.
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The modem converts the data signals from the computer
into analogue signals, a modem at the other end will
convert the signal back into digital.
BROWSERS :
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Haifaa Elayyan
A web browser is a software application which
enables a user to display and interact with text,
images, videos, music, games and other
information typically located on a Web page at a
website on the World Wide Web or a local area
network. Text and images on a Web page can
contain hyperlinks to other Web pages at the
same or different website. Web browsers allow a
user to quickly and easily access information
provided on many Web pages at many websites
by traversing these links. Web browsers format
HTML information for display, so the appearance
of a Web page may differ between browsers.
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UNIT 5: BROWSING THE WEB
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In 1990, Tim Berners-Lee at CERN created the
forerunner of the web which today is a collection of
hypertext documents distributed worldwide and linked by
the internet.
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The value of the web is that trillions of pages of web
content are linked together via multiple hyperlinks.
As you have learned in previous units:
the software you use on your computer to access and view
documents on the web is called a web browser;
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 the basic unit of web content is the web page which is an HTML
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The browser accesses the page, held on a remote
computer (web server), and downloads it to your computer
(the client).
PROTOCOLS AND STANDARDS
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Haifaa Elayyan
Web browsers communicate with Web servers
primarily using HTTP (hypertext transfer
protocol) to fetch webpages. HTTP allows Web
browsers to submit information to Web servers as
well as fetch Web pages from them. The most
commonly used HTTP is HTTP/1.1, which is fully
defined in RFC 2616. HTTP/1.1 has its own
required standards that Internet Explorer does
not fully support, but most other currentgeneration Web browsers do.
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PROTOCOLS AND STANDARDS
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Pages are located by means of a URL (uniform
resource locator, RFC 1738 ), which is treated as
an address, beginning with http: for HTTP
access. Many browsers also support a variety of
other URL types and their corresponding
protocols, such as gopher: for Gopher (a
hierarchical hyperlinking protocol), ftp: for FTP
(file transfer protocol), rtsp: for RTSP (real-time
streaming protocol), and https: for HTTPS (an
SSL encrypted version of HTTP).
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PROTOCOLS AND STANDARDS
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Early Web browsers supported only a very simple version
of HTML. The rapid development of proprietary Web
browsers led to the development of non-standard dialects of
HTML, leading to problems with Web interoperability.
Modern Web browsers support a combination of standardsand defacto-based HTML and XHTML, which should
display in the same way across all browsers. No browser
fully supports HTML 4.01, XHTML 1.x or CSS 2.1 yet.
Currently many sites are designed using WYSIWYG
HTML generation programs such as Adobe Dreamweaver
or Microsoft FrontPage. Microsoft FrontPage often
generates non-standard HTML by default, hindering the
work of the W3C in developing standards, specifically with
XHTML and CSS (cascading style sheets, used for page
layout). Dreamweaver and other more modern Microsoft
HTML development tools such as Microsoft Expression
Web and Microsoft Visual Studio conform to the W3C
standards
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UNIT 5: INTERNET ADDRESSING
The address has several levels to it (like the posting system):
 At the highest level: domain (a collection of internet hosts).
Two types of top-level
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codes of three letters
codes with two-letter (normally countries)
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UNIT 5: INTERNET ADDRESSING
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Country code domains are usually subdivided
ac.uk (academic community),
 co.uk (commercial),
 gov.uk (national and local government)
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Many individual domain names are available within each
top-level domain. Within ac.uk there is open.ac.uk, the OU
domain (the central address for the OU on the internet)
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Once the domain name open.ac.uk has been approved by an
external agency, the OU is free to allocate subdomains
and host names within this naming scheme.
UNIT 5: INTERNET ADDRESSING
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How addressing down to the document level?
The address associated with a hyperlink is given in the
form of a URI (uniform resource indicator), which
specifies the service requested and the full address of
the document.
An example of a URI:
http://mcs.open.ac.uk/mcsexternal/courses/m150.htm
http:// identifies the protocol (HTTP)
 mcs.open.ac.uk’, specifies the server. The host address is in
two parts:
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1) ‘mcs’ , which identifies a particular computer
2) ‘open.ac.uk’, which identifies its domain.
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The rest of the address is the path within ‘mcs’ that
the required document.
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UNIT 5: NAMING HOSTS
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It is usually convenient to assign a name to each computer
on a network so that users can identify it easily. In a large
network it is common to use a systematic naming scheme.
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A typical naming system might use names like ‘anemone’ or
‘buttercup’. These hosts in the OU domain would then be
known to the internet as ‘anemone.open.ac.uk’ and
‘buttercup.open.ac.uk’
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UNIT 5: IP NUMBERS
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The naming scheme (convenient for humans) is not actually
used by the messages that travel across the internet.
Instead each host has a 4-byte number associated with it,
called its IP (internet protocol) number.


The IP number carried by a message ensures that it
reaches the correct destination. How does the message
discover the IP number of its destination host? Answer is
that special directories, called domain name servers, keep
this infor.
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The first thing that happens when a URI is executed is
that the host name is sent to a domain name server to be
resolved (new hosts are being added so domain name
servers need to be kept up to date).
UNIT 5: LOGICAL AND PHYSICAL NAMES

Suppose that your website on ‘orchid.open.ac.uk’ has
crashed. And you acquired an up to-date computer which
you name ‘peony.open.ac.uk’ and move your website there.
44
Problem: Now no one can find your web pages any more, as
they are no longer at the same address.


A good solution is to avoid reliance on named physical
machines. The way to do this is to identify the web server
to the internet not by the name of the physical computer it
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resides on, but by a logical name.
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This means that an index must be kept which associates
the logical name of a server with its current physical host.
UNIT 5: EMAIL




A mail application, or mail client, combines high speed
with a
permanent record. Email is asynchronous
45

Very popular use of computer networking: Email
Email combines immediacy and permanent record
Organisations can have their own internal email system
that is independent of the internet
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Email achieves its universality by using text messages
comprising ASCII-coded text only. Unlike other URIs, an
email address identifies a user: [email protected]
UNIT 5: EMAIL

46
The internet works using a standard protocol. You
despatch an email message and expect it to arrive. You do
not know which route it took or which countries it passed
through.


When data travels across the internet, it is broken up into
units of a standard size called packets. Each packet carries
the address information so that it will reach its intended
destination. The packets are re-assembled into a single
item on arrival.
Along with the actual data, an email also carries
transmission information in a number of lines, called
headers.
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UNIT 5: EMAIL
Email address of the recipient
 Date and time of despatch.
 Subject of the message
 ‘Reply-to’ field gives the reply
address of the sender

Headers whose names begin
with ‘X-’ are are used to
convey additional information.
The header X-Mailer reveals
that the message was
composed using version 4.01 of
the Pegasus
47

AO
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UNIT 5: SENDING ATTACHMENTS

48
Email transmission is restricted to text but how is it possible to attach
documents of any kind to an email message?
Ans: By encoding the attached file as a series of alphabetic characters
and appending them to the end of the message.
(An arbitrary attachment can be converted into ASCII code)


In order to enable the receiving mail client to decode the attachment,
the encoding scheme must conform to a standard. One of the internet
standard for encoding mail attachments is MIME (Multipurpose
Internet Mail Extensions)
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Content-type: text/plain indicates the nature of data and will usually
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be followed by ‘charset=iso-8859-1’ for English text
Leb.
F08
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UNIT 5: HOW DOES DATA TRAVEL?




Communication takes place in the form of serial transmission
49

The transmission medium could be a wire carrying electrical signals or
an optical fibre carrying light signals. Wireless connection could be
infra-red, radio or microwave link.
Protocols are needed to ensure that, on arrival, the receiving computer
interprets the stream of bits with its original meaning.
The receiver need to know when the transmission begins. A simple
protocol is to include start & stop bits with each byte
If something has gone wrong, the receiver sends a simple
the transmitter indicating a failed transmission.
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messageLeb.
to
F08
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UNIT 5: DATABASES

A database (db) is a collection of data stored in a computer
system according to a set of rules, and organised to
facilitate access involving complex searches and selection.
50


The primary emphasis of database applications is on
making the data persistent, and structuring it so as to
minimise redundancy, avoid inconsistency & maximise the
usefulness of the data for the purposes of access and
updating.
AO
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Leb.
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A query (request to db) is used to get specific information
from the db. The response to the query ideally extracts
from the database all the relevant information. So a
database is part of an information system.
UNIT 5: DATABASES
Table ???
Row ???
Field ???
51
Key
???
What is the engine size of Tom Cobbley’s car?
(Hint: use the two tables in combination)
AO
U
Leb.
F08
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
UNIT 5: Dconsist
ATABASES
Databases
of many tables holding vast amounts of
data, designed with great care in order to be able to
provide answers to (possibly complex) queries.
52

A typical industrial database system will consist of:





A collection of tables
Data (called metadata) which describes the tables (what each
column in a table means, and how many tables there are in a
database….)
Facilities for backing up the tables.
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Facilities for ensuring security (credit card details)
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A query facility.
F08
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


A data object (sound, image, video) may be contained
within a database as ‘an object in a box’. The box has a
name and the database can access the object using this
name.
53

UNIT database
5: OBJECT
Object
mayDATABASE
contain video, voice and music along
with more traditional forms of data.
An object stored in this way is called a BLOB (binary large
object). There are facilities for the storage and retrieval of
these data types but the weakness of this approach is that
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you can NOT query the content of a BLOB (the BLOB has
U
Leb.
NO structure to it).
Solution: Metadata
F08
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UNIT 5: METADATA


54

In order to describe anything other than the simplest of
data, it is necessary to provide some form of explanatory
data (i.e. metadata) about the data. Email headers are
good examples.
Web pages have a rudimentary form of metadata in the
form of keywords that can be used by search engines to
locate web pages of a particular topic.
Each item in the <HEAD> section of an HTML document
AO
U
is an example of metadata. It is not part of the contentLeb.
of
the document; rather, it says something about the content.
F08
M15
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UNIT 5: METADATA

An adequate collection of metadata (hooks or pointers) will
identify the various features of multimedia databases to
make it searchable.
55


Each still picture, audio clip and video will have a number
of associated items of metadata.
MPEG-7 provided a visual descriptors (colour, texture,
shape, position, motion and face recognition) and audio
descriptors (key, mood, tempo and tempo changes).
Example: This is a scene with a barking brown dog on the
left and a blue ball on the right, with the sound of passing
cars in the background.
AO
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Leb.
F08
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UNIT 5: PRIVACY


There is a lot of data about us in a semi-public domain
that we may not even be aware of (loyalty card at a
supermarket allow to track all your purchases!).
56

Privacy – keeping some things removed from general or
public knowledge – is central to our way of thinking.
Computer systems provide the facility for government,
local authority, tax authorities, your bank … to have
control over vast, and detailed, amounts of data about you.
AO
They can store it for indefinite periods and use it in a U
Leb.
variety of ways.
F08
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UNIT 5: DATA AND THE LAW

57
Data protection or data access legislation may vary from
jurisdiction to jurisdiction, and depends on the willingness
of the relevant authorities to enforce it, and of most
individuals and organisations to adhere to it.

Data protection laws in any jurisdiction are likely to have
some or all of the following characteristics:





a legal definition of data (limited to electronic forms or also
covers handwritten and typed data, photographs..);
a description of how data may be acquired lawfully;
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what uses the data may be put to;
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any time limits on storage;
who may lawfully access and use the data, and for what. F08
M15
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UNIT 5: COMPUTER ETHICS

Ethics is defined as a set of moral principles that should
guide our acts as a citizen.
58
1) Thou shalt not use a computer to harm other people.
2) Thou shalt not interfere with other people’s computer work.
3) Thou shalt not snoop around in other people’s computer files.
4) Thou shalt not use a computer to steal.
5) Thou shalt not use a computer to bear false witness.
6) Thou shalt not copy or use software for which you have not paid.
7) Thou shalt not use other people’s computer resources without
authorisation or proper compensation.
8) Thou shalt not appropriate other people’s intellectual output.
AO
9) Thou shalt think about the social consequences of the program U
you
Leb.
are writing or the system you are designing.
F08
10) Thou shalt always use a computer in ways that ensure
consideration and respect for your fellow humans.
M15
0Unit
UNIT 5: LINKING AND THEFT

Web pages often contain links to web pages developed by
other users (gateways).
59

Consider a site which sets itself up as an internet
newspaper and contains links to individual stories stored
at other online newspaper sites. What should the ethical
position be on this?
It could be regarded as an example of intellectual property theft.
AO
 It could be argued that the material has not been stolen because
U
the text has not been cut and pasted but simply linked to!!! Leb.

F08
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UNIT 5: SECURITY

60
Legislation to protect data, and in particular computerised
data, is desirable. However, the law itself is never
sufficient.
(You never leave your home open when you go out!!)


Once you link your computer to the internet, you need to
think about ways of making it less accessible to unwanted
visitors who, in computer jargon, are termed hackers.
One solution is to allow access using a password only. It
AO
may be desirable to secure a whole network of computers
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from unauthorised outside access by using a firewall (aLeb.
F08
software system which controls data traffic entering and
leaving the network).
M15
0Unit
UNIT 5: OWNERSHIP AND RIGHTS OVER DATA

The concept of data ownership is legally very unclear in
most countries. Who actually ‘owns’ medical records??
(Patients, Doctors, pharmacists or health departments)
61
Intellectual Property Rights (IPR): the right to gain
financially from the products one creates
 Moral rights: the right to say how one’s products can be
used
(the content of letters that you write, and even the
content of assignments that you prepare …)
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 Copyright laws: afford some sort of protection for
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intellectual property (databases are subject to
F08
copyright)

M15
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UNIT 5: WORM, VIRUSES AND TROJAN HORSES
 A worm is a program intended to subvert a whole
network of computers. It transfers copies of itself to
other machines on the network.


A virus is a program designed to cause specific damage
to your software by attaching itself to documents. Ex:
deleting important documents from your hard disk.
62

A Trojan horse is a code which looks legitimate but
attempts to do something quite different (modify
documents on your hard disk, collect passwords…).
Typically the name of the document will be misleading.AO
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F08
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Anti-virus softwares will protect your system but needs
regular updates because new viruses appear on a daily
basis.