Download Module Materials Computer Architecture and Systems Level 5 C20012

CERTIFICATE IN INFORMATION TECHNOLOGY
Module Materials
Computer Architecture
and Systems
Level 5 C20012
Specific Learning Outcomes
Unit 1 Introduction to Computer Systems
1.1
list the main components of a computer system
1.2
state the function of each of these components
1.3
distinguish between hardware and software
1.4
list common examples of the use of a computing system
1.5
list the main types of computer
1.6
list uses of each type of computer
1.7
know the main elements of an information system (a typical business system)
1.8
list two examples of information systems
1.9
explain how the computer can be used as a means of communication
1.10 outline the historical development of computers.
Unit 2 Computer Architecture
2.1
explain the role of the CPU in a computer
2.2
list the component parts in the CPU e.g. ALU, registers, decoder
2.3
explain the role of each component
2.4
explain the purpose of the instruction pointer
2.5
outline the steps involved in processing an instruction
2.6
explain the fetch – execute cycle.
2.7
describe the purpose of a memory cell
2.8
distinguish between random access memory (RAM) and read only memory (ROM)
2.9
list the different types of read only memory
2.10 distinguish between bit, byte, word, kilobyte and megabyte
2.11 distinguish between primary and secondary memory
2.12 explain the term cache memory
2.13 explain how cache memory can be used to improve the performance of the CPU
2.14 define a bus
2.15 distinguish between internal buses and external buses
2.16 list the different buses, which connect the CPU to the computer's main memory chips.
2.17 distinguish between different types of character recognition devices
2.18 list applications of character recognition devices
2.19 explain bar codes and list their uses
2.20 give examples of different types of user interface devices
2.21 explain the purpose of a magnetic tape
2.22 list advantages and disadvantages of using tapes
2.23 explain the purpose of a magnetic disk
2.24 explain the terms: track and sector
2.25 distinguish between hard disks and floppy disks
2.26 outline the basic disk structure
2.27 draw a diagram of a floppy disk, outlining its main components
2.28 draw a diagram of a hard disk, outlining its main components
2.29 explain the terms: access time, seek time and latency
2.30 list the advantages of hard disks over floppy disks
2.31 explain how direct memory access (DMA) improves the transfer of data from disks to
main memory
2.32 explain the purpose of optical disks
2.33 explain how data is stored on optical disks
2.34 explain how data compression works
2.35 explain what voice recognition software does
2.36 list the uses of voice recognition software
2.37 explain how voice recognition makes computer systems accessible to those with a
2.38
2.39
2.40
2.41
2.42
disability
list devices used to produce computer output
list the different classifications of printers
describe how images are displayed on a visual display unit
describe specialised types of input/output devices, e.g. scanners, digital cameras, etc.
describe how special purpose storage devices such as smart cards can be used.
Unit 3 Data Communications
3.1
define the term communications
3.2
list examples of how communications technology is used to-day
3.3
list the components that make up a communications system
3.4
describe the different types of transmission media used for communications channels
3.5
describe the ways in which the transmission media are connected
3.6
explain how data is transmitted
3.7
describe the communications equipment used in a communications system
3.8
list the functions performed by communications software
3.9
list the categories of network
3.10 describe the most common network layouts
3.11 explain the use of communications protocols
3.12 describe the Internet and how it works
3.13 list services provided by the Internet (email, ftp, etc.)
3.14 explain how to connect to the Internet and WWW.
Unit 4 Operating Systems
4.1
list the functions of an operating system
4.2
explain how an operating system makes the computer hardware usable
4.3
explain the different types of operating system architecture: single-user, multi-tasking,
multi-user, networks
4.4
name and describe the major operating systems in use to-day
4.5
list some services provided by an operating system to a user
4.6
use the operating system user interface
4.7
give examples of different user interfaces
4.8
list the advantages and disadvantages of different types of user interface.
Unit 5 System Applications
5.1
outline the main features of the Data Protection Act
5.2
discuss the role of computing systems in modern society
5.3
list the advantages of computing technology
5.4
identify examples of this technology in the local environment
5.5
list the advantages of electronic mail
Unit 1: Introduction to Computer Systems
Probably nothing has influenced our lives more in the past 50 years than the invention of the
electronic computer. Today computers are nearly omnipresent. They are in our homes, our cars,
our microwaves, our cellular phones, and even in our toys. Tiny computers called
microprocessors are responsible for controlling many of the common appliances that we use
every day from the automatic coffee maker to the hi-fi VCR. What exactly are computers and
how do they look like? Where do they come from? And what do we use them for?
1.1 What is a computer?
It would be possible to build a specific machine to carry out a specific task - but this would be
expensive and inconvenient. A general purpose machine can be given new instructions to
perform new tasks, offering a more flexible, economical solution. A computer can be defined as a
general purpose programmable digital machine which function is to run instructions. A computer
system comprises hardware and software. The hardware components of a computer system are
the electronic and mechanical parts while the software components of a computer system are the
intangible parts: the data and the computer programs.
The major hardware components of a computer system are the processor, main memory,
secondary memory, peripheral interfaces and buses.
• The processor or Central Processing Unit (CPU) is the most significant component of a
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computer: it is the heart and brain of the system and contains all the circuitry that the
computer needs to manipulate data and execute instructions.
The main memory is used to store and retrieve data and programs being accessed by the
processor. Two of the most commonly used types of memory are RAM (Random Access
Memory) and ROM (Read Only Memory). RAM stores the data and general-purpose
programs that the machine executes. RAM is temporary: its content can be changed at any
time and it is erased when power to the computer is turned off. ROM is permanent; it can
retain its content even when the machine is powered-off and is used to store programs for
the initial boot-up of the machine.
Secondary memory is a slower and cheaper form of memory and is used to store
programs and data that are not currently used. The processor does not access the
secondary memory directly. The content in it must first be copied into the main memory
for the CPU to process. Secondary memory devices include hard drives, floppy disks,
CDs and CDROMs etc.
Peripheral interfaces mediate between the outside world and the interior world of the
computer. They allow the computer to communicate to the user and to Input/Output
devices. Input devices allow entering information into a computer while output devices
display information that has been held of generated within a computer. Input devices
include keyboards, joysticks, mice, light pens, touch-sensitive screens, scanners, graphics
tablets, speech-recognition devices, and vision systems. Some examples of output devices
are: monitors, speakers, printers.
Buses are bundles of cables that carry information across the different units.
A typical desktop computer has its microprocessor, main memory, and other essential
components on a motherboard (fig 1.1). Other components such as external storage, controllers
for video display and sound, and peripheral devices may be attached to the motherboard as plugin cards or via cables, although in modern computers it is increasingly common to integrate some
of these peripherals into the motherboard itself.
Fig1.1: A computer motherboard showing the location of the CPU, memory and I/O card sockets.
1.2 Where do computers come from?
Modern computing can probably be traced back to the 'Harvard Mk I' and Colossus (both of
1943). Colossus was an electronic computer built in Britain at the end 1943 and designed to crack
the German coding system - Lorenz cipher. The 'Harvard Mk I' was a more general purpose
electro-mechanical programmable computer built at Harvard University with backing from IBM.
These computers were among the first of the 'first generation' computers.
First generation computers were normally based around wired circuits containing vacuum valves
and used punched cards as the main (non-volatile) storage medium. Another general purpose
computer of this era was 'ENIAC' (Electronic Numerical Integrator and Computer) which was
completed in 1946. It was typical of first generation computers, it weighed 30 tonnes contained
18,000 electronic valves and consumed around 25KW of electrical power. It was, however,
capable of an amazing 100,000 calculations a second.
The next major step in the history of computing was the invention of the transistor in 1947. This
replaced the inefficient valves with a much smaller and more reliable component. Transistorised
computers are normally referred to as 'Second Generation' and dominated the late 1950s and
early 1960s. Despite using transistors and printed circuits these computers were still bulky and
strictly the domain of Universities and governments.
The explosion in the use of computers began with 'Third Generation' computers. These relied
Jack St. Claire Kilby's invention - the integrated circuit or microchip; the first integrated circuit
was produced in September 1958 but computers using them didn't begin to appear until 1963.
While large 'mainframes' such as the I.B.M. 360 increased storage and processing capabilities
further, the integrated circuit allowed the development of Minicomputers that began to bring
computing into many smaller businesses. Large scale integration of circuits led to the
development of very small processing units, an early example of this is the processor used for
analyising flight data in the US Navy's F14A `TomCat' fighter jet. This processor was developed
by Steve Geller, Ray Holt and a team from AiResearch and American Microsystems.
On November 15th, 1971, Intel released the world's first commercial microprocessor, the 4004.
Fourth generation computers were developed, using a microprocessor to locate much of the
computer's processing abilities on a single (small) chip. Coupled with one of Intel's inventions the RAM chip (Kilobits of memory on a single chip) - the microprocessor allowed fourth
generation computers to be even smaller and faster than ever before. The 4004 was only capable
of 60,000 instructions per second, but later processors (such as the 8086 that all of Intel's
processors for the IBM PC and compatibles is based) brought ever increasing speed and power to
the computers. Supercomputers of the era were immensely powerful, like the Cray-1 which could
calculate 150 million floating point operations per second. The microprocessor allowed the
development of microcomputers, personal computers that were small and cheap enough to be
available to ordinary people. The first such personal computer was the MITS Altair 8800,
released at the end of 1974, but it was followed by computers such as the Apple I & II,
Commodore PET and eventually the original IBM PC in 1981.
Although processing power and storage capacities have increased beyond all recognition since
the 1970s the underlying technology of LSI (large scale integration) or VLSI (very large scale
integration) microchips has remained basically the same, so it is widely regarded that most of
today's computers still belong to the fourth generation.
1.3 Common uses for computers
Computers can be instructed to perform a variety of individual functions. A set of instructions
that tells a computer what to do is called a program. Today, more than 10,000 application
programs are available for use on personal computers. They include such popular programs as
word processing programs, spreadsheet programs, database programs, and communication
programs.
• Word processing programs are used to type, correct, rearrange, or delete text in letters,
memos, reports, and school assignments.
• Spreadsheet programs enable individuals to prepare tables easily. The users of such
programs establish rules for handling large groups of numbers. For example, using a
spreadsheet program, a person can enter some numbers into a table and the program will
calculate and fill in the rest of the table. When the user changes one number in the table,
the other numbers will change according to the rules established by that user.
Spreadsheets may be used for preparing budgets and financial plans, balancing a
chequebook, or keeping track of personal investments.
• Database programs allow a computer to store large amounts of data (information) in a
systematic way. Such data might include the name, address, telephone number, salary,
and starting date of every employee in a company. The computer could then be asked to
produce a list of all employees who receive a certain salary.
•
Communication programs connect a personal computer to other computers. People can
thereby exchange information with one another via their personal computers. In addition,
•
communication programs enable people to link their personal computers with databanks.
Databanks are huge collections of information stored in large centralized computers.
News, financial and travel information, and other data of interest to many users can be
obtained from a databank.
Other programs include recreational and educational programs for playing games,
composing and hearing music, and learning a variety of subjects. Programs have also
been written that turn household appliances on and off. Some people develop their own
programs to meet needs not covered by commercially prepared programs. Others buy
personal computers mainly to learn about computers and how to program them.
1.4 Main types of computers
The main types of computer include Personal Computer, Workstation, Mainframe and
Microcontroller.
Fig. 1.2: Examples of computer systems
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Personal Computers (PC) are usually designed to be used by one person at the time for
general data processing, multimedia and communication. Desktop PC are fixed and
usually required a constant supply of current from the main for power supply. Laptops
are portable PC take can operate from a rechargeable battery.
A workstation station is a high-performance computer designed for technical or scientific
applications. Workstations are commonly used as servers connected to a local area
network and running multi-user applications.
Mainframe computers are computers used mainly by large organisations for critical
applications, typically bulk data processing such as census, industry and consumer
statistics, Enterprise Resource Planning and financial transaction processing.
A computer wholly contained within a machine, e.g. mobile phone, is said to be
‘embedded.’ Examples of embedded computers are mobile phones, PDA, mp3 players,
etc… The market for embedded computers is larger than for ‘ordinary’ computers.
A Microcontroller is a small computer on a integrated circuit package, consisting of a
relatively of a CPU, memory, and peripheral interfacing. Microcontrollers are used in
automatically controlled products and devices, such as automobile engine control
systems, remote controls, office machines, appliances, power tools, and toys.
1.5 Computers in Information Systems and communication
Computer systems can be integrated in business and organisation to increase productivity,
effectiveness and competivity. Information’s Systems relate to the combination of hardware,
software, data, network infrastructure and trained personnel organized to facilitate planning,
control, coordination, and decision making. The main elements of an Information Systems in a
typical business are:
Major types of Information systems include structural databases and information management
software that can include the following:
• Transaction Processing Systems (TPS) -- collect, store, modifie and retrieve the data
transactions of an enterprise;
• Management Information Systems (MIS) -- collect, process, store and disseminate data in
the form of information needed to carry out the functions of management;
• Decision-Support Systems (DSS) -- help decision makers compile useful information
from raw data, documents, personal knowledge, and/or business models to identify and
solve problems and make decisions;
• Executive Support Systems (ESS): produce useful summarized reports and provide
analysis tools that predict a series of performance outcomes over time using the input
data. Collect information from all company levels and departments such as billing, cost
accounting, staffing, scheduling, etc… .
Computers can be used as a means of communication by being part of a network. Computer
networks allow multiple computers to be interconnected in a single building (e.g., the home) and
cable services permit a high-speed connection to the Internet. Unlike with conventional
telecommunications, the Internet is very affordable and it costs little to transmit information
across the world.
Conclusion
1. The main hardware components of a computer system are the processor, main memory,
secondary memory, peripheral interfaces and buses.
2. The processor manipulates data and executes instructions. Main memory is used to store
and retrieve data and programs being accessed by the processor. Secondary memory is
used to store programs and data that are not currently used. Peripheral interfaces mediate
between the outside world and the interior world of the computer. They are all connected
via buses that carry information.
3. The hardware components of a computer system are the electronic and mechanical parts
while the software components of a computer system are the intangible parts: the data and
the computer programs.
4. Common examples of the use of a computing system include such popular programs as
word processing programs, spreadsheet programs, database programs, and
communication programs.
5. The main types of computer include Personal Computer, Workstation, Mainframe,
Microprocessor and Microcontroller.
6. Personal Computers (PC) are usually designed to be used by one person at the time for
general data processing, multimedia and communication. Workstations are commonly
used as servers connected to a local area network and running multi-user applications.
Mainframes are used mainly by large organisations for critical applications.
Microcontrollers are used in automatically controlled products and devices.
7. The main elements of an information system hardware, software, data, networks and
people.
8. Examples of information systems include Transaction Processing Systems, Management
Information Systems, Decision-Support Systems, and Executive Support Systems.
9. A computer can be used as a means of communication by being interconnected with other
computers as part of a network.
10. The historical development of computers: The first generation computers were introduced
in 1943, based on vacuum valves and punched cards. The invention of the transistor in
1947 allows the design of a second generation of computers that dominated until the early
1960s. The introduction of integrated circuits accelerated the development of a third
generation in the 1960s. In 1971, Intel released the first commercial microprocessor used
to develop the fourth generation. Although processing power and storage capacities have
dramatically increased since the 1970s, it is regarded that most of today's computers
belong to the fourth generation.
Unit 2 Computer Architecture
A computer can be seen as a machine that manipulates coded data and responds to events
occurring in the external world. This is called the stored-program or Von Neumann machine
architecture. In this unit, our primary goal is to develop an understanding of this machine
architecture.
2.1 Central Processing Unit (CPU)
A simple representation of a computer consists of a processor connected to some memory and
some input and output (I/O) devices by a bus or data highway. This design principle separates
special functions into special purpose devices.
Fig. 2.1 Representation of the Von Neumann machine architecture
The Concept of Stored Program: The core of the Von Neumann architecture states that the
memory is used to store both data and programs. In order for a CPU to accomplish meaningful
work, it must have two inputs: instructions and data. Instructions tell the CPU what actions need
to be performed on the data. Instructions are represented with binary codes just like data. In fact,
the CPU makes no distinction about the whether it is storing instructions or data in memory. This
concept is called the stored-program concept.
Early computing devices were not known for their flexibility, as the program that each device
executed tended to be built into the control unit as a part of the machine. One approach used to
gain flexibility in early electronic computers was to design the control units so they could be
conveniently rewired. A breakthrough came with the realization that the program, just like data,
can be coded and stored in main memory. If the control unit is designed to extract the program
from memory, decode the instructions, and execute them, a computer's program can be changed
merely by changing the contents of the computer's memory instead of rewiring the control unit.
This stored-program concept has become the standard approach used today. To apply it, a
machine is designed to recognize certain bit patterns as representing certain instructions.
The Central Processing Unit (CPU) is where all the work is done. It contains the hardware
resources required to execute instructions. The CPU reads the sequence of commands that make
up a program one by one from memory and executes them.
The CPU functions by following a cycle of fetching an instruction, decoding it, and executing it.
This process is known as the fetch-decode-execute cycle. The cycle begins when an instruction is
transferred from memory to the CPU along the data bus. Then the unique bit patterns that make
up the machine-language are extracted and decoded. A decoder recognises which operation the
bit pattern represents and activates the correct circuitry to perform the operation. Once the
operation is performed, the cycle repeats after the CPU takes the time to put away results to offprocessor locations. Fig. 2.2 illustrates the CPU’s functional units that make possible the
instruction execution cycle. The CPU registers and units involved in the process are:
MAR The Memory Address Register holds the address of the next location in the store to be
accessed. The contents of the MAR point to the location of information in memory. For
example, if the MAR contains the value $4000, the CPU is going to access address
number $4000 in the memory.
PC
The Program Counter contains the address of the next instruction to be executed.
MBR The Memory Buffer Register holds the data just read from the main store, or data to be
written into the main store. All information that flows into or out of memory passes
through the MBR.
IR
The Instruction Register holds the most recently read instruction from the main store.
ALU The Arithmetic and Logic Unit calculates a function of one or two inputs. The actual
function performed by the ALU is determined by the bit pattern of the instruction in the
IR.
CU
The Control Unit interprets the instruction in the IR. That is, the CU is responsible for
converting the bit pattern of an instruction into the sequence of actions necessary to
execute the instruction.
D0
Data Register D0 can be used as an accumulator to hold temporary data in the CPU
during arithmetic and logical operations.
CCR The Condition Code Register is used to implement conditional branches.
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Fetching an instruction begins with the contents of the Program Counter being moved to
the Memory Address Register ([MAR] <- [PC]). The MAR is now pointing at the next
instruction to be executed. Once the PC has done its job, it is automatically incremented
to point the next instruction in sequence. After the increment, the PC points at the next
instruction while the current instruction is being executed.
The next step is to read the contents of the memory location pointed at by the MAR. The
data read from memory is first deposited in a temporary holding register, the Memory
Buffer Register ([MBR]<-[MAR]).
In the final step of the fletch cycle the contents of the MBR are copied the Instruction
Register ([IR]<-[MBR]). The IR holds the instruction while it is decoded by the Control
Unit. One field of the IR contains the operation code (op-code) that tells the CPU what
operation is to be carried out. A second field, called the operand field, contains the
address of the data to be used by the instruction.
The fletch phase is followed by an execution phase in which the Control Unit generates
all signals necessary to execute the instruction. The CU controls all parts of the CPU
including all programmable registers and the Arithmetic and Logic Unit.
A path between the CCR and the CU is used by the CU to decide whether to continue
with the next instruction in series, or to jump to the address field specified by the branch
field of a conditional instruction.
Fig. 2.2: Structure of a typical CPU
2.2 MEMORY
Computer storage is classified into primary (main) memory and secondary (peripheral) storage.
Primary storage is usually RAM (~10ns access time) while secondary storage is usually hard disk
drives (~10ms access time). Secondary storage is a lot cheaper than primary and secondary
storage is persistent when RAM is volatile. This section will give an overview of the memory
hierarchy from cache to secondary storage.
2.2.1 Why defining a Memory Hierarchy?
A memory hierarchy is required because no single technology provides all the necessary
requirements of ideal memory; that is, low-cost, fast access time, non-volatile. By combining
various memory technologies, it is possible to create a system with a memory system that has,
collectively, the required requirements. Fig. 1 illustrates the nature of a memory hierarchy.
Fig. 2.3 Memory hierarchy
2.2.2 RAM and ROMs
The RAM (random access memory) is the place in a computer where the OS, application
programs, and data in current use is kept so that they can be quickly reached by the processor. The
RAM is volatile.
The ROM (Read-Only Memory) is computer memory containing data that normally can only be
read, not written to. The ROM contains the programs that allow a computer to be "booted up"; it is
a non-volatile type of memory.
Programmable read-only memory (PROM) is a form of digital memory used to store programs
permanently. The key difference from a strict ROM is that the programming is applied after the
device is constructed. They are frequently seen in video game consoles, or such products as
electronic dictionaries, where PROMs for different languages can be substituted.
FLASH memory is a semiconductor read-mostly memory that uses EPROM (Electronically
Programmable ROM) technology to store data. Data can be read electronically and written
electronically into data cells. It has a fast read access time but a relatively slow write time.
FLASH memory can be used to store data (e.g. BIOS). FLASH memory is now the basis of digital
cameras and MP3 memory systems. FLASH memory is important because an entire branch of
computing (personal, mobile, pervasive) depends on it.
EEPROM (electrically erasable programmable read-only memory) is user-modifiable read-only
memory (ROM) that can be erased and reprogrammed (written to) repeatedly through the
application of higher than normal electrical voltage. Unlike EPROM chips, EEPROMs do not
need to be removed from the computer to be modified.
Memory cells are the building blocks of computer data storage. The bit is the smallest unit of
digital information; however data is usually represented as groups of multiples bits to form
nybbles, bytes, words, longwords, kilobytes, megabyte, gigabytes and terabytes:
• 1nybble = 4 bits
• 1byte= 8 bits = 2 nybbles
• 1word=2 bytes
• 1longword=2 words = 4 bytes
• 1quadword=4words= 8bytes
• 1kilobyte (KB) = 1024 bytes
• 1 megabyte (MB) = 1024 KB
• 1 gigabyte (GB) = 1024 MB
• 1 terabyte (TB) = 1024 GB
Symbols: k = 103=1000, K =210= 1024, b = bit, B = byte
2.2.3 The system bus
All units of a computer system are connected via a bus that carries information: it is a bundle of
cables, like a highway where information travels. The bus has a limited speed which determines
its ability to carry information.
• An internal bus links units within the computer: CPU, main memory and peripheral
interfaces.
• An external bus connects external Input/Output devices to the peripheral interfaces.
Three types of information can travel via a bus: data, address and control signals. Then some
computer systems have three separate buses for each type of signal.
2.2.4 Cache memory
There is a high time cost in getting information from the memory to the processor via the bus,
which is why programmers try to keep as much information in the CPU as much as possible. The
processor is a special purpose device dedicated in executing instructions, not storing them. But
what you can do is to have the processor store a tiny piece of a program, perhaps the piece of a
program that is frequently used, that you can easily access. This can dramatically speed up
program execution. That is called the cache.
Cache memory is a special high-speed storage mechanism. A memory, sometimes called a cache
store or RAM cache, is a portion of memory made of high-speed static RAM (SRAM) instead of
the slower and cheaper dynamic RAM (DRAM) used for main memory. Memory caching is
effective because most programs access the same data or instructions over and over. By keeping
as much of this information as possible in SRAM, the computer avoids accessing the slower
DRAM.
SRAM
Transistors
per bit
6
DRAM 1
Access
time
1X
Persistant ? Sensitive ? Cost
Yes
No
100x
10X
No
Yes
1X
Applications
cache memories
Main memories,
buffers
frame
Disk caching works under the same principle as memory caching, but instead of using high-speed
SRAM, a disk cache uses conventional main memory. The most recently accessed data from the
disk is stored in a memory buffer. When a program needs to access data from the disk, it first
checks the disk cache to see if the data is there. Disk caching can dramatically improve the
performance of applications, because accessing a byte of data in RAM can be thousands of times
faster than accessing a byte on a hard disk.
2.3 COMPUTER PERIPHERALS
So far we've examined the internal structure and operation of the computer's central processing
unit. However, if a computer is to be of any real value to people it must have some way of
communicating with them. After all, there's no point in creating a supercomputer the size of a
paperback that can solve the ultimate question of life, the universe, and everything, if it can't tell
us the answer. In this chapter we look at the ways and means by which information gets into and
out of a computer and the devices or peripherals that are connected to the computer (e.g. the
printer and the display).
2.3.1 Structure and operation of magnetic storage devices
We will look at the operation of three magnetic storage devices: hard disks, floppy disks and
tapes.
Disk drives are flat, circular, rigid sheet of aluminium coated with a thin layer of magnetic
material. Hard disk drives contain multiple rotating platters in which a head (one per disk
surface) records data along a concentric track. The track is divided into basic units called sectors
(a sector is the smallest unit of data that can be read from or written to a disk).
The read write head hovers above the surface (but not touching it) at a tiny fraction of an inch
(the head to surface distance is the order of a wavelength of light).
Spindle
Read/write heads
Track
Boom
Platters
Fig. 2.4 typical hard disk drive layout
There are three stages in reading/writing to a disk drive:
• Seek time - head movement to the correct track: ~6ms;
• Rotational latency - wait for the desired sector to arrive under the head: ~4ms;
• Data transfer - relatively small.
Therefore, reading from physically adjacent blocks is relatively fast.The performance of disks has
increased dramatically with 1 TB now available. However, data access times have improved
little. Disk typically rotate at 7,200 rpm and the fastest disks rotate at 15,000 rpm.
The floppy disk is a removable secondary storage
medium that can be transported from one system to
another. The original floppy disk, created in 1971 by
IBM, was made of plastic coated with a magnetic
material enclosed in an eight inch square protective
envelope.
In operation, the drive grabs the floppy's centre and
spins it inside its housing. The read/write head contacts
the surface through an opening
in the plastic shell or envelope. Floppies rotate at 300
RPM, which is from 10 to 30 times slower than a hard
disk. They are also at rest until a data transfer is
requested. Following are the three types developed,
from newest to oldest, and their raw, uncompressed
storage capacity:
Housing
3.5" rigid
3.5" rigid
5.25"
flexible
8" flexible
Max. Storage
Capacity
1.44MB
2.88MB
1.2MB
Creator
Sony
IBM
Shugart
500KB
IBM
Floppy disks have long access times and much lower capacity than hard disks. Woefully
undersized for today's use, the floppy disk is no longer Fig. 2.5 structure of a floppy disk
standard equipment on computers.
Tape-based secondary systems were popular in the 1970s
and 1980s and were largely associated with mainframe
computers. Today, tape is mainly found in small cartridgebased systems where it provides backup and archival
storage.
Tapes provide an easy media method for preserving
multiple full-system backups. In general, tape drives are
used where high capacity and high reliability are
paramount. They can be expensive initially but are
extremely inexpensive when you factor in the low cost of
the media over time. However, tape-base backup systems
present non-negligible drawbacks:
• Creating a tape backup copy of files or of a drive
requires the use of a special backup program in most
cases.
• Retrieving data from most tape backup drives requires
that the data files be restored to the hard disk.
• Tape backups store and retrieve data sequentially. The
last file backed up can't be accessed until the rest of
the tape is read.
Fig. 2.6 examples of removable tapes
The increase in the capacity and decrease in the cost of hard drives together with the growth of
optical storage systems make tape much less attractive than it once was.
2.3.2 Structure and operation of optical
disks
An optical memory disk is a rigid plastic disk
whose surface is covered with a long spiral
track. The track is laid down on a substrate
inside the disk and is covered with a
transparent plastic protective layer.
Like a magnetic disk, information is stored
along a track in binary form. Unlike the
magnetic disk, the track in an optical is in a
continuous spiral. The spiral on an optical disk
begins at the innermost track and spirals
outwards.
An optical disk stores information by means of
reflecting or non-reflective metallic dots along
the track. A beam of light produced by a
semiconductor laser is focused on the surface
of the track and a photosensitive transistor or
diode detects the light reflected back from the
surface.
Fig.2.7: structure & operation of a CD-ROM
The functional problems of optical storage are reliability, detecting the presence of tiny dents
called pits on the surface of the disk, optically tracking the reflective elements, and
encoding/decoding the data.
Compact Disk (CD) are Digital Versatile Disks (DVD) are the most common forms of optical
stage disks and can be found in multiple declinations (single/double side, single/double layers,
read-only memory, rewritable, etc…). The CD has a capacity of about 600 Mbytes. The DVD is a
replacement for the CD which uses virtually identical technology. However, modern
manufacturing processes have allowed the system to be scaled down to fit more bits on the disk
(typically 4.7 GBytes). Sophisticated focusing mechanisms have allowed two planes of data (dual
layer) within a DVD which has doubled the capacity to over 9 GBytes.
Hard drive vs DVD: The capacity of hard drives has increased more rapidly than that of optical
storage. The standard DVD has a capacity of 4.7 Gbytes which is 1% of that of a large hard drive.
However, the DVD provides very low cost removable storage and can be used for backup and
archival purposes. The access time of DVDs is mot lower than that of the hard disk because the
DVD rotates more slowly (it is not in a sealed enclosure). Moreover, the DVD’s track is not
concentric and is a spiral. Locating a specific point on a DVD is therefore slower than locating a
sector on a disk. The head assembly in a DVD is far heavier than that in a hard disk and that
means that the head cannot step rapidly from track to track. This reduces the access time of the
DVD. At the moment, two new optical storage mechanisms are becoming available (both are
modified DVDs). Blu-ray has a capacity of 50 GByte/disk and high density DVD, HD-DVD, has
a capacity of 15-45 GBytes. Unfortunately, these mechanisms are mutually incompatible. Such
large storage mechanisms are required for both high definition video and backing up the
increasingly low-cost large magnetic disk drives.
2.3.3 Direct Memory Access (DMA)
Consider the way in which a block of data is written to a disk drive. It's often impractical to use
programmed data transfers for disk I/O because programmed transfers are too slow. The output
strategy most frequently used is direct memory access (DMA), in which the data is transferred
from the computer's memory to a peripheral, or vice versa, without passing through the CPU's
registers. The CPU simply tells the DMA hardware to move a block of data and the DMA
hardware gets on with the task, allowing the CPU to continue its main function of information
processing. This strategy (i.e. DMA) requires special hardware to implement it.
Fig. 2.8 illustrates the DMA mechanism. An interface chip called a DMA controller (DMAC) is
responsible for moving the data between the memory and the peripheral.
Fig. 2.8: description of DMA mechanism
The DMAC must provide addresses for the source or destination of data in memory, and signal to
the peripheral that data is needed or is ready. Furthermore, the DMAC must collect the
computer's internal data and address buses for the duration of a data transfer. Data transfer by
DMA must be done while avoiding a conflict with the CPU for the possession of the buses. The
bus switches connect the CPU, DMA controller, and peripheral to the data bus. In normal
operation the CPU accesses the buses. The DMA controller requests the CPU to give up the bus
and turns off the CPU‘s bus switch and turns on the DMA controller bus switch.
2.3.4 Data Compression
Data compression consists of recombining the bits and bytes that make up your data into a
smaller, more compact form. The actual information doesn't change (in case of loseless
compression), but the internal representation of it does. Without data compression, it would be
virtually impossible to implement technologies such as streaming media, large database
applications, or any other application that handles lots and lots of information. There is simply
too much data to handle easily.
Data compression is a coding/decoding process. The coding side of it uses a particular algorithm
to rearrange the internal representation of your data. Generally, an application handles that
transparently and in many cases you don't need to know how or why it works. When it's time to
actually use the information it has to be decoded, using the reverse process of the method used in
the coding process.
An example of this is the popular WinZip program, which compresses data files. To add files to a
WinZip archive, you use the available menu option to add a file. When you want a copy of a file
within an archive, you choose to extract it. In both cases, a mouse click is all that's required of
you. But behind the scenes, the WinZip program is using a particular algorithm to compress and
uncompress the information in your files.
2.3.5 Voice recognition software
Speech recognition (also known as automatic speech recognition or computer speech recognition)
converts spoken words to machine-readable input (for example, to key presses, using the binary
code for a string of character codes). The term "voice recognition" is sometimes incorrectly used
to refer to speech recognition, when actually referring to speaker recognition, which attempts to
identify the person speaking, as opposed to what is being said. Confusingly, journalists and
manufacturers of devices that use speech recognition for control commonly use the term Voice
Recognition when they mean Speech Recognition.
Speech recognition applications include voice dialing (e.g., "Call home"), call routing (e.g., "I
would like to make a collect call"), domotic appliance control and content-based spoken audio
search (e.g., find a podcast where particular words were spoken), simple data entry (e.g., entering
a credit card number), preparation of structured documents (e.g., a radiology report), speech-totext processing (e.g., word processors or emails), and in aircraft cockpits (usually termed Direct
Voice Input).
People with disabilities are another part of the population that benefit from using speech
recognition programs. It is especially useful for people who have difficulty with or are unable to
use their hands, from mild repetitive stress injuries to involved disabilities that require alternative
input for support with accessing the computer. In fact, people who used the keyboard a lot and
developed RSI became an urgent early market for speech recognition. Speech recognition is used
in deaf telephony, such as spinvox voice-to-text voicemail, relay services, and captioned
telephone. Individuals with learning disabilities who have problems with thought to paper
communication (essentially they think of an idea but it is processed incorrectly causing it to end
up differently on paper) can benefit from the software as it helps to overlap that weakness.
2.3.6 Character recognition devices
Optical character recognition (OCR) is a process of capturing an image of a document and then
extracting the text from that image. In recent years, it has become possible to scan in paper copies
of documents so as to form computerized images of such documents, analyze images in the text
areas of the documents so as to recognize individual characters in the text data, and form
computer readable files of character codes corresponding to the recognized characters. With the
advent of optical imaging capabilities and optical character recognition (OCR) software, data
placed on a form can be digitized by such instruments as a scanner or fax machine and the
digitized data can be interpreted as text by the OCR software. A number of optical character
recognition (OCR) systems are known. Typically, such systems comprise apparatus for scanning
a page of printed text and performing a character recognition process on a bit-mapped image of
the text, which is a pixel-by-pixel representation of the overall image in a binary form. The
recognition system reads characters of a character code line by framing and recognizing the
characters within the image data. During the recognition process, the document is analyzed for
several key factors such as layout, fonts, text and graphics. The document is then converted into
an electronic format that can be edited with application software. The output image is then
supplied to a computer or other processing device, which performs an OCR algorithm on the
scanned image. The document can be of many different languages, forms and features. The
purpose of the OCR algorithm is to produce an electronic document comprising a collection of
recognized words that are capable of being edited. In general, electronic reading machines using
computer-based optical character recognition (OCR) comprise personal computers outfitted with
computer scanners, optical character recognition software, and computerized text-to-voice
hardware or software.
An optical scanner is a device that can read text or illustrations printed on paper and translate
the information into a form the computer can use. A scanner works by digitising an image which
consists of dividing it into a grid of boxes and representing each box with either a zero or a one,
depending on whether the box is filled in. For colour and grey scaling, the same principle applies,
but each box is then represented by up to 24 bits. The resulting matrix of bits, called a bit map,
can then be stored in a file, displayed on a screen, and manipulated by programs.
Optical scanners do not distinguish text from illustrations; they represent all images as bit maps.
Therefore, it is not possible to directly edit text that has been scanned. To edit text read by an
optical scanner, one needs an optical character recognition (OCR) system to translate the image
into ASCII characters. Most optical scanners sold today come with OCR packages.
A range of scanners are available: flat bed scanners or hand held scanners. The latter scanners
are commonly used in the retail trade (shops and supermarkets) or libraries to read barcodes.
A barcode is an optical machine-readable representation of data. Originally, bar codes
represented data in the widths (lines) and the spacings of parallel lines and may be referred to as
linear or 1D (1 dimensional) barcodes or symbologies. But they also come in patterns of squares,
dots, hexagons and other geometric patterns within images termed 2D (2 dimensional) matrix
codes or symbologies. In spite of there being no bars, 2D systems are generally referred to as
barcodes as well. The first use of barcodes was to automate grocery checkout systems, a task
where they have become almost universal today. Their use has spread to many other roles as
well, tasks that are generically referred to as Auto ID Data Capture (AIDC).
Magnetic-Ink Character Recognition, MICR, is a type of font capable of recognition using
magnetically charged ink. Computers equipped with the right hardware and software can print
and/or read the character printed in such ink.
MICR font is commonly used to print checks, deposit slips, mortgage coupons, etc. There are
various types of MICR fonts, the MICR E-13B font is used in the Canada, Panama, Puerto Rico,
UK, and the United States. The MICR CRC-7 was created according to the ISO standards and is
a font used in France, Mexico, Spain, and most other Spanish speaking countries.
2.3.7 User interface devices
The user
interface (also
known
as Human
Computer
Interface or Man-Machine
Interface (MMI)) is the aggregate of means by which users interact with the system—a
particular machine, device, computer program or other complex tool. The user interface provides
means of input, allowing the users to manipulate a system, and Output, allowing the system to
indicate the effects of the users' manipulation. Currently the following types of user interface are
the most common:
• Graphical user interfaces (GUI) accept input via devices such as computer keyboard and
mouse and provide articulated graphical output on the computer monitor. There are at
least two different principles widely used in GUI design: Object-oriented user interfaces
(OOUIs) and application oriented interfaces[verification needed].
• Web-based user interfaces or web user interfaces (WUI) accept input and provide output
by generating web pages which are transmitted via the Internet and viewed by the user
using a web browser program. Newer implementations utilize Java, AJAX, Adobe Flex,
Microsoft .NET, or similar technologies to provide real-time control in a separate
program, eliminating the need to refresh a traditional HTML based web browser.
Administrative web interfaces for web-servers, servers and networked computers are
often called Control panels.
• Command line interfaces, where the user provides the input by typing a command string
with the computer keyboard and the system provides output by printing text on the
computer monitor. Used for system administration tasks etc.
•
•
•
•
•
•
•
•
Tactile interfaces supplement or replace other forms of output with haptic feedback
methods. Used in computerized simulators etc.
Touch interfaces are graphical user interfaces using a touchscreen display as a combined
input and output device. Used in many types of point of sale, industrial processes and
machines, self-service machines etc.
Attentive user interfaces manage the user attention deciding when to interrupt the user,
the kind of warnings, and the level of detail of the messages presented to the user.
Crossing-based interfaces are graphical user interfaces in which the primary task consists
in crossing boundaries instead of pointing.
Gesture interfaces are graphical user interfaces which accept input in a form of hand
gestures, or mouse gestures sketched with a computer mouse or a stylus.
Multi-screen interfaces, employ multiple displays to provide a more flexible interaction.
This is often employed in computer game interaction in both the commercial arcades and
more recently the handheld markets.
Noncommand user interfaces, which observe the user to infer his / her needs and
intentions, without requiring that he / she formulate explicit commands.
Voice user interfaces, which accept input and provide output by generating voice
prompts. The user input is made by pressing keys or buttons, or responding verbally to the
interface.
2.3.8 Common output devices
An output device is any piece of computer hardware equipment used to communicate the results
of data processing carried out by an information processing system (such as a computer) to the
outside world. The most common outputs are visual display units, speakers and printers.
A visual display unit (also called VDU, monitor, or screen) offers a two-dimensional visual
presentation of information.
A speaker can be used for various sounds mean to alert the user, as well as music and spoken
word.
Printers produce a permanent hard copy of the information on paper. The most common
technologies rely on inkjet or laser.
Inkjet printers use a print head that squirts a tiny drop of ink onto the paper. The drop is ejected
from the print head either by heating the ink and momentarily boiling it, or by flexing a quartz
crystal to send a shockwave through the ink to eject a drop. A print head has multiple nozzles.
Laser printers use a drum coated with a photoconductive material. First a high voltage charge is
placed on the drum. An image is projected on the drum by a laser (or other means) which causes
the charge to leak away in light areas. The drum is then covered with a toner (a fine powder)
which sticks to the charged (dark) areas. The drum is then rotated against the paper and the toner
transferred to the paper where it is set (fused) by heating.
Both ink jet and laser printers are fairly reliable. However, the inkjet printer is normally used for
domestic and small office applications with small print runs. Inkjet printers operate a line at a
time and scan the paper by moving the print head along a line. This means that inkjet printers
are ultimately slower than laser printers that print an image at a time. Inkjet printers are
remarkably cheap. This is partially because they are mass produced and partially because the
complexity of the printer is in the print head which is thrown away when the ink is used. This
approach means that the life of the most critical part of the printer need be measured only in
terms of hundreds of sheets of paper rather than tens of thousands. Moreover, one problem of
inkjet printing is closing œ the nozzles become clogged by dried ink and fail. By replacing the
print head when the ink is used, no maintenance is required.
2.3.9 Example of special purpose storage devices: smart cards
A smart card, chip card, or integrated circuit card (ICC), is in any pocket-sized card with
embedded integrated circuits which can process data. This implies that it can receive input which
is processed — by way of the ICC applications — and delivered as an output. There are two
broad categories of ICCs. Memory cards contain only non-volatile memory storage components,
and perhaps some specific security logic. Microprocessor cards contain volatile memory and
microprocessor components. The card is made of plastic, generally PVC, but sometimes ABS.
The card may embed a hologram to avoid counterfeiting. Using smartcards also is a form of
strong security authentication for single sign-on within large companies and organizations.
Unit 3: Data Communications
3.1 Network communication
The primary purpose of any network is to provide a method to communicate information. From
the very earliest primitive humans to the most advanced scientists of today, sharing information
with others is crucial for human advancement. All communication begins with a message, or
information, that must be sent from one individual or device to another. The methods used to
send, receive and interpret messages change over time as technology advances. All
communication methods have three elements in common. The first of these elements is the
message source, or sender. Message sources are people, or electronic devices, that need to
communicate a message to other individuals or devices. The second element of communication is
the destination, or receiver, of the message. The destination receives the message and interprets
it. A third element, called a channel, provides the pathway over which the message can travel
from source to destination.
3.2 Components of a communications system
There are many components that can be part of a network, for example personal computers,
servers, networking devices, and cabling. These components can be grouped into four main
categories:
• Hosts
• Shared peripherals
• Networking devices
• Networking media
The network components that people are most familiar with are hosts and shared peripherals.
Hosts are devices that send and receive messages directly across the network.
Shared peripherals are not directly connected to the network, but instead are connected to hosts.
The host is then responsible for sharing the peripheral across the network. Hosts have computer
software configured to enable people on the network to use the attached peripheral devices.
The network devices, as well as networking media, are used to interconnect hosts.
Some devices can play more than one role, depending on how they are connected. For example, a
printer directly connected to a host (local printer) is a peripheral. A printer directly connected to a
network device and participates directly in network communications is a host.
All computers connected to a network that participate directly in network communication are
classified as hosts. Hosts can send and receive messages on the network. In modern networks,
computer hosts can act as a client, a server, or both. The software installed on the computer
determines which role the computer plays.
Servers are hosts that have software installed that enable them to provide information, like email
or web pages, to other hosts on the network. Each service requires separate server software. For
example, a host requires web server software in order to provide web services to the network.
Clients are computer hosts that have software installed that enable them to request and display
the information obtained from the server. An example of client software is a web browser, like
Internet Explorer.
A computer with server software can provide services simultaneously to one or many clients.
Additionally, a single computer can run multiple types of server software. In a home or small
business, it may be necessary for one computer to act as a file server, a web server, and an email
server.
A single computer can also run multiple types of client software. There must be client software
for every service required. With multiple clients installed, a host can connect to multiple servers
at the same time. For example, a user can check email and view a web page while instant
messaging and listening to Internet radio.
The role of computers in a network
3.3 Types of transmission media used for communications channels
In order for communication to occur a source, destination, and some sort of channel must be
present. A channel, or medium, provides a path over which the information is sent. In the
networked world, the medium is usually some sort of physical cable. It may also be
electromagnetic radiation, in the case of wireless networking. The connection between the source
and destination may either be direct or indirect, and may span multiple media types.
Many different types of cables exist to interconnect the various devices in a NOC or local
network. There are two kinds of physical cable. Metal cables, usually copper, have electrical
impulses applied to them to convey information. Fiber optic cables, made of glass or plastic, use
flashes of light to convey information.
Twisted Pair: Modern Ethernet technology generally uses a type of copper cable known as
twisted pair (TP) to interconnect devices. Because Ethernet is the foundation for most local
networks, TP is the most commonly encountered type of network cabling.
Coaxial cable is usually constructed of either copper or aluminum, and is used by cable television
companies to provide service. It is also used for connecting the various components which make
up satellite communication systems.
Fiber optic cables are made of glass or plastic. They have a very high bandwidth, which enables
them to carry very large amounts of data. Fiber is used in backbone networks, large enterprise
environments and large data centers. It is also used extensively by telephone companies.
In addition to the wired network, various technologies exist that allow the transmission of
information between hosts without cables. These are known as wireless technologies.
Wireless technologies use electromagnetic waves to carry information between devices. An
electromagnetic wave is the same medium that carries radio signals through the air. The
electromagnetic spectrum includes such things as radio and television broadcast bands, visible
light, x-rays and gamma-rays. Each of these has a specific range of wavelengths and associated
energies as shown in the diagram. Some types of electromagnetic waves are not suitable for
carrying data. Other parts of the spectrum are regulated by governments and licensed to various
organizations for specific applications. Certain areas of the spectrum have been set aside to allow
public use without the restriction of having to apply for special permits. The most common
wavelengths used for public wireless communications include the Infrared and part of the Radio
Frequency (RF) band.
Infrared (IR) is relatively low energy and cannot penetrate through walls or other obstacles.
However, it is commonly used to connect and move data between devices such as Personal
Digital Assistants (PDAs) and PCs. A specialized communication port known as an Infrared
Direct Access (IrDA) port uses IR to exchange information between devices. IR only allows a
one-to-one type of connection. IR is also used for remote control devices, wireless mice, and
wireless keyboards. It is generally used for short-range, line-of-sight, communications. However,
it is possible to reflect the IR signal off objects to extend the range. For greater ranges, higher
frequencies of electromagnetic waves are required.
Radio Frequency (RF) waves can penetrate through walls and other obstacles, allowing a much
greater range than IR. Certain areas of the RF bands have been set aside for use by unlicensed
devices such as wireless LANs, cordless phones and computer peripherals. This includes the 900
MHz, 2.4 GHz, and the 5 GHz frequency ranges. These ranges are known as the Industrial
Scientific and Medical (ISM) bands and can be used with very few restrictions.
Bluetooth is a technology that makes use of the 2.4 GHz band. It is limited to low-speed, shortrange communications, but has the advantage of communicating with many devices at the same
time. This one-to-many communications has made Bluetooth technology the preferred method
over IR for connecting computer peripherals such as mice, keyboards and printers.
Other technologies that make use of the 2.4 GHz and 5 GHz bands are the modern wireless LAN
technologies that conform to the various IEEE 802.11 standards. They are unlike Bluetooth
technology in that they transmit at a much higher power level, which gives them a greater range.
3.4 Network categories
There are many types of networks that provide us with different kinds of services. In the course
of a day, a person might make a phone call, watch a television show, listen to the radio, look up
something on the Internet, or even play a video game with someone in another country. All of
these activities depend on robust, reliable networks. Networks provide the ability to connect
people and equipment no matter where they are in the world. People use networks without ever
thinking about how they work or what it would be like if the networks did not exist.
Communication technology in the 1990s, and before, required separate, dedicated networks for
voice, video and computer data communications. Each of these networks required a different type
of device in order to access the network. Telephones, televisions, and computers used specific
technologies and different dedicated network structures, to communicate. But what if people
want to access all of these network services at the same time, possibly using a single device?
New technologies create a new kind of network that delivers more than a single type of service.
Unlike dedicated networks, these new converged networks are capable of delivering voice, video
and data services over the same communication channel or network structure.
New products are coming to market that take advantage of the capabilities of converged
information networks. People can now watch live video broadcasts on their computers, make a
telephone call over the Internet, or search the Internet using a television. Converged networks
make this possible.
Networks come in all sizes. They can range from simple networks consisting of two computers,
to networks connecting millions of devices. Networks installed in small offices, or homes and
home offices, are referred to as SOHO networks. SOHO networks enable sharing of resources,
such as printers, documents, pictures and music between a few local computers.
In business, large networks can be used to advertise and sell products, order supplies, and
communicate with customers. Communication over a network is usually more efficient and less
expensive than traditional forms of communication, such as regular mail or long distance phone
calls. Networks allow for rapid communication such as email and instant messaging, and provide
consolidation, storage, and access to information on network servers.
Business and SOHO networks usually provide a shared connection to the Internet. The Internet is
considered a "network of networks" because it is literally made up of thousands of networks that
are connected to each other.
3.5 Network topology
In a simple network consisting of a few computers, it is easy to visualize how all of the various
components connect. As networks grow, it is more difficult to keep track of the location of each
component, and how each is connected to the network. Wired networks require lots of cabling
and network devices to provide connectivity for all network hosts.
When networks are installed, a physical topology map is created to record where each host is
located and how it is connected to the network. The physical topology map also shows where the
wiring is installed and the locations of the networking devices that connect the hosts. Icons are
used to represent the actual physical devices within the topology map. It is very important to
maintain and update physical topology maps to aid future installation and troubleshooting efforts.
In addition to the physical topology map, it is sometimes necessary to also have a logical view of
the network topology. A logical topology map groups hosts by how they use the network, no
matter where they are physically located. Host names, addresses, group information and
applications can be recorded on the logical topology map.
The graphics illustrate the difference between logical and physical topology maps.
3.6 Communication protocols
Protocols are specific to the characteristics of the source, channel and destination of the message.
The rules used to communicate over one medium, like a telephone call, are not necessarily the
same as communication using another medium, such as a letter.
Protocols define the details of how the message is transmitted, and delivered. This includes issues
of: Message format, message size, timing, encapsulation, encoding and standard message pattern.
Many of the concepts and rules that make human communication reliable and understandable
also apply to computer communication. Computers, just like humans, use rules, or protocols, in
order to communicate. Protocols are especially important on a local network. In a wired
environment, a local network is defined as an area where all hosts must "speak the same
language" or in computer terms "share a common protocol". If everyone in the same room spoke
a different language they would not be able to communicate. Likewise, if devices in a local
network did not use the same protocols they would not be able to communicate.
The most common set of protocols used on local wired networks is Ethernet. The Ethernet
protocol defines many aspects of communication over the local network, including: message
format, message size, timing, encoding, and message patterns.
IP traffic is managed based on the characteristics and devices associated with each of the three
layers: Access, Distribution and Core. The IP address is used to determine if traffic should
remain local or be moved up through the layers of the hierarchical network.
The Access Layer provides a connection point for end user devices to the network and allows
multiple hosts to connect to other hosts through a network device, usually a hub or switch.
Typically, all devices within a single Access Layer will have the same network portion of the IP
address. If a message is destined for a local host, based on the network portion of the IP address,
the message remains local. If it is destined for a different network, it is passed up to the
Distribution Layer. Hubs and switches provide the connection to the Distribution Layer devices,
usually a router.
The Distribution Layer provides a connection point for separate networks and controls the flow
of information between the networks. It typically contains more powerful switches than the
Access Layer as well as routers for routing between networks. Distribution Layer devices control
the type and amount of traffic that flows from the Access Layer to the Core Layer.
The Core Layer is a high-speed backbone layer with redundant (backup) connections. It is
responsible for transporting large amounts of data between multiple end networks. Core Layer
devices typically include very powerful, high-speed switches and routers. The main goal of the
Core Layer is to transport data quickly.
3.7 The Internet
Every day millions of people exchange information through the Internet - but what exactly is the
Internet? The Internet is a worldwide collection of computer networks, cooperating with each
other to exchange information using common standards. Through telephone wires, fiber optic
cables, wireless transmissions and satellite links, Internet users can exchange information in a
variety of forms.
The Internet is a network of networks that connects users in every country in the world. There are
currently over one billion Internet users worldwide.
Up to now the networks we have discussed have been controlled by one individual or
organization. The Internet is a conglomerate of networks and is owned by no one individual or
group. There are, however, several major International organizations that help manage the
Internet so that everyone uses the same rules.
Any home, business or organization that wants to connect to the Internet must use an Internet
Service Provider (ISP). An ISP is a company that provides the connections and support to access
the Internet. It can also provide additional services such as Email and web hosting.
ISPs are essential to gaining access to the Internet. No one gets on the Internet without a host
computer, and no one gets on the Internet without going through an ISP.
ISPs range in size from small to very large and differ in terms of the area they service. ISPs may
provide limited services to a small geographical area or can have a wide variety of services and
support entire countries with millions of customers. ISPs also differ in the types of connection
technologies and speeds they offer. Examples of well known ISPs include AOL, EarthLink, and
Roadrunner.
Individual computers and local networks connect to the ISP at a Point of Presence (POP). A POP
is the connection point between the ISP's network and the particular geographical region that the
POP is servicing.
An ISP may have many POPs depending on its size and the area it services. Within an ISP, a
network of high-speed routers and switches move data between the various POPs. Multiple links
interconnect the POPs to provide alternate routes for the data should one link fail or become
overloaded with traffic and congested.
ISPs connect to other ISPs in order to send information beyond the boundaries of their own
network. The Internet is made up of very high-speed data links that interconnect ISP POPs and
ISPs to each other. These interconnections are part of the very large, high capacity network
known as the Internet Backbone.
Connecting to the ISP at the POP provides users with access to the ISP's services and the
Internet.
ISPs provide a variety of ways to connect to the Internet, depending on location and desired
connection speed. In a major city there are typically more choices for ISPs and more connection
options than in a rural area. For example, cable Internet access is only available in certain
metropolitan areas where cable TV service is available. Remote areas may only have access via
dial-up or satellite. Each Internet access technology uses a network access device, such as a
modem, in order to connect to the ISP. It may be built in to your computer or may be provided by
the ISP.
The simplest arrangement is a modem that provides a direct connection between a computer and
the ISP. However, if multiple computers connect through a single ISP connection, you will need
additional networking devices. This includes a switch to connect multiple hosts on a local
network, and a router to move packets from your local network to the ISP network. A home
networking device, such as an integrated router, can provide these functions, as well as wireless
capability, in a single package.
The choice of Internet access technologies depends on availability, cost, access device used,
media used and the speed of the connection. Most of the technologies shown are used for both
home and small business. Leased lines are typically used for business and large organizations, but
can be used to provide high speed connectivity in areas where cable or DSL are not available.
Conclusion
The term Local Area Network (LAN) refers to a local network, or a group of interconnected local
networks that are under the same administrative control. In the early days of networking, LANs
were defined as small networks that existed in a single physical location. While LANs can be a
single local network installed in a home or small office, the definition of LAN has evolved to
include interconnected local networks consisting of many hundreds of hosts, installed in multiple
buildings and locations.
The important thing to remember is that all of the local networks within a LAN are under one
administrative control. Other common characteristics of LANs are that they typically use
Ethernet or wireless protocols, and they support high data rates.
The term Intranet is often used to refer to a private LAN that belongs to an organization, and is
designed to be accessible only by the organization's members, employees, or others with
authorization.
Unit 4
Operating Systems
4.1 Functions of an operating system
System components and peripherals, by themselves, are nothing more than a collection of
electronics and mechanical parts. To get these parts to work together to perform a specific task, a
special type of computer program, known as an operating system (OS), is required.
Suppose that a user wants to write a report and print it out on an attached printer. A word
processing application is required to accomplish this task. Information is entered from the
keyboard, displayed on the monitor, saved on the disk drive and then finally sent to the printer.
In order for the word processing program to accomplish all of this, it must work with the OS,
which controls input and output functions. In addition, the entered data is manipulated inside of
the computer, stored in RAM and processed by the CPU. This internal manipulation and
processing is also controlled by the OS. All computerized devices, such as servers, desktops,
laptops or handhelds, require an OS in order to function.
4.2 An operating system makes the computer hardware usable
The OS acts like a translator between user applications and the hardware. A user interacts with
the computer system through an application, such as a word processor, spreadsheet, computer
game or instant messaging program. Application programs are designed for a specific purpose,
such as word processing, and know nothing of the underlying electronics. For example, the
application is not concerned with how information is entered into the application from the
keyboard. The operating system is responsible for the communication between the application
and the hardware.
When a computer is powered on, it loads the OS, normally from a disk drive, into RAM. The
portion of the OS code that interacts directly with the computer hardware is known as the kernel.
The portion that interfaces with the applications and user, is known as the shell. The user can
interact with the shell using either the command line interface (CLI) or graphical user interface
(GUI).
When using the CLI, the user interacts directly with the system in a text-based environment by
entering commands on the keyboard at a command prompt. The system executes the command,
often providing textual output. The GUI interface allows the user to interact with the system in an
environment that uses graphical images, multimedia, and text. Actions are performed by
interacting with the images on screen. GUI is more user friendly and requires less knowledge
than CLI of the command structure to utilize the system. For this reason, many individuals rely
on the GUI environments. Most operating systems offer both GUI and CLI.
Operating systems have complete control of local hardware resources. They are designed to work
with one user at a time. They enable the user to multitask. The operating system keeps track of
which resources are used by which application. In order to work with resources that are not
directly connected to the computer system, a special piece of software must be added that allows
a device to send and receive data from the network. This software, known as a redirector, may
either be an integral part of the OS or may need to be installed separately as a network client.
When installed, the operating system becomes a network operating system (NOS). A NOS offers
complex scheduling and user management software that allow a device to share resources
between many users and treat networked resources as though they are directly connected.
4.3 Types of operating system architecture
As computers have progressed and developed so have the types of operating systems. Below is a
basic list of the different types of operating systems and a few examples of operating systems that
fall into each of the categories. Many computer operating systems will fall into more than one of
the below categories.
GUI - Short for Graphical User Interface, a GUI Operating System contains graphics and icons
and is commonly navigated by using a computer mouse. See our GUI dictionary definition for a
complete definition. Below are some examples of GUI Operating Systems. E.g.: System 7.x,
Windows 98 and Windows CE.
Multi-user - A multi-user operating system allows for multiple users to use the same computer at
the same time and/or different times. See our multi-user dictionary definition for a complete
definition for a complete definition. Below are some examples of multi-user operating systems.
E.g.: Linux, Unix and Windows 2000.
Multiprocessing - An operating system capable of supporting and utilizing more than one
computer processor. Below are some examples of multiprocessing operating systems. E.g.:
Linux, Unix and Windows 2000.
Multitasking - An operating system that is capable of allowing multiple software processes to
run at the same time. Below are some examples of multitasking operating systems. E.g.: Unix
and Windows 2000.
Multithreading - Operating systems that allow different parts of a software program to run
concurrently. Operating systems that would fall into this category are: Linux, Unix and Windows
2000.
4.4 Major operating systems in use today
There are many different operating systems available. The major groupings are listed here with
some examples.
Microsoft Windows: XP, Vista, 2003 Server
UNIX-Based: IBM AIX, Hewlett Packard HPUX, and Sun Solaris
BSD - Free BSD
Linux-Based (Many varieties)
Macintosh OS X
Non-Unix Proprietary: IBM OS/400, z/OS
While most of these operating systems require the user to purchase and agree to a commercial
license, there are several operating systems released under a different type of licensing scheme
known as the GNU Public License (GPL).
Commercial licenses usually deny end-users the ability to modify the program in any way.
Windows XP, Mac OS X and UNIX are all examples of commercial OS software.
In contrast, the GPL allows end-users to modify and enhance the code, if they desire, to better
suit their environment. Some common operating systems, which are released under the GPL,
include Linux and BSD.
Operating systems require a certain amount of hardware resources. These resources are specified
by the manufacturer and include such things as:
Amount of RAM
Hard disk space required
Processor type and speed
Video resolution
Manufacturers often specify both a minimum and recommended level of hardware resources.
System performance at the minimum specified hardware configuration is usually poor and only
sufficient to support the OS and no other functionality. The recommended configuration is
usually the better option and is more likely to support standard additional applications and
resources.
To take advantage of all of the features provided by an operating system, additional hardware
resources such as sound cards, NICs, modems, microphones, and speakers are generally required.
Many of the OS developers test various hardware devices and certify that they are compatible
with the operating system. Always confirm that the hardware has been certified to work with the
operating system before purchasing and installing it.
Choosing an appropriate OS requires many factors to be considered before deciding which one to
use in a given environment.
The first step in selecting an OS is to ensure that the OS being considered fully supports the
requirements of the end user. Does the OS support the applications that will be run? Is the
security and functionality sufficient for the needs of the users?
Next, conduct research to make sure that sufficient hardware resources are available to support
the OS. This includes such basic items as memory, processors, and disk space, as well as
peripheral devices such as scanners, sound cards, NICs and removable storage.
Another consideration is the level of human resources needed to support the OS. In a business
environment, a company may limit support to one or two operating systems and discourage, or
even disallow, the installation of any other OS. In the home environment, the ready availability of
technical support for an OS may be a determining factor.
When considering implementing an OS, it is the total cost of ownership (TCO) of the OS that
must be considered in the decision making process. This not only includes the costs of obtaining
and installing the OS, but also all costs associated with supporting it.
Another factor that may come into play in the decision making process is the availability of the
operating system. Some countries and/or businesses have made decisions to support a specific
type of OS or may have restrictions barring individuals from obtaining certain types of
technologies. In this type of environment, it may not be possible to consider a particular OS
regardless of its suitability to the task.
The process for selecting an operating system must take all of these factors into account.
Unit 5
System Applications
Technology has played a massive role in solving social ills. Every technology has been created
for a purpose. Technological innovation has certainly had a major impact on our modern day
society. One of the technologies that have been used worldwide are computers. The computer's
era has
provided society with a variety of facilities. The computer era began after the industrial
revolution, when people started thinking about their problems critically. Society has taken
advantage of the features provided by computer technologies. This has made the computer
revolution have a greater impact on modern society than the industrial revolution; without
computer technology nothing would be possible today.
The Industrial Revolution took place during the 18th and 19th centuries. Countries such as Britain
and France were involved in machine-based manufacturing. Heavy use of machines was made in
order to produce certain products. Innovations were made such as textiles, steam power and iron
founding. The Industrial revolution transformed the agricultural environment to an industrial
environment. Other innovations included glass making, roads, railways, etc. The Industrial
Revolution has thus had enormous impact on society. There was a major shift from agriculture to
industry. However, the computer revolution didn't lag behind in many fields.
5.1 Principles of Data Protection Act
Personal data should:
Only be obtained and processed lawfully and fairly
Only be held for specified, lawful and registered purposes
Only be used for registered purposes and only disclosed to registered recipients
Shall be adequate, relevant and not excessive for their registered purposes
Shall be accurate and, where necessary, kept up to date
Shall not be kept for longer than is necessary for their stated purpose
Individuals shall have the right of access to any data held about them that is capable of being
processed automatically
Appropriate security measures shall be taken against unauthorised access to, or alteration,
disclosure or destruction of, or accidental loss of, personal data
Governmental exemptions
Police
Secret service
MI5
MI6
GCHQ
Ministry of Defence
Conditional exemptions
Only for the production of documents (implicitly meant to be word processing)
Only for the calculation of payments in accounting, payroll and pensions
Only for the distribution of information or articles to the data subjects (e.g. lists of their
names and
addresses)
Unconditional exemption for management of personal, family or household affairs.
5.2 Advantages of electronic mail
Electronic mail or e-mail, the electronic transmission of messages, letters, and documents. In its
broadest sense electronic mail includes point-to-point services such as telegraph and facsimile
(fax) systems. It is commonly thought of, however, in terms of computer-based message systems
where the electronic text file that is received can be edited, replied to, excerpted, or even pasted
into another electronic document that can be used or manipulated by a word processor , desktop
publishing system, or other computer program .
Advantages of electronic mail include convenience, cost effectiveness, improvement of
communication skills, records of all correspondence, student participation, and rapidity of
response time by professor
5.3 Computing technology at the workplace
For Western European society, debates about the changes computers have brought to the
workplace present many conflicting images. Critics insist on an information age marked by job
losses and dehumanized work whilst optimistics praise the creation of a post-industrial economy
of skilled workers. Advantages of computer and automated technology in the workplace include:
• Employment: technological innovation is associated with further employment prospects
and not an overall reduction in the employment numbers. e.g.: the number of employees
engaging in research and development, information processing and servicing increases
whilst the number of workers engaging production and office work decreases.
• Productivity: gains are obtained in terms of administrative effectiveness. e.g. the use of
electronic mail, calendars and filling all make for swifter and more informed decisions.
• Skills: many information workers are IT-competent and poised to negotiate higher wages
and receive more status from what has been traditionally regarded as low-status work.
e.g.: secretaries can assess the various technologies on the market, be in charge of
recommending purchases and they need high degree of computer literacy.
• Social: the office become mobile since processing can be done by distance from portable
terminal. e.g.: with developments such as email and teleconferencing home working can
be implemented.
5.4 Computing technology in business
The use of information technology to monitor a business’s performance can also enable the
business to highlight areas where they are not making the most use of their resources. The use of
information technologies can also increase the businesses income through advertising in the
various available forums. Advances in information technology over the last thirty years have lead
to the television for example being more widely used today than thirty years ago (e.g. the
introduction of transistor based televisions reduced costs while increasing reliability).
An e-commerce approach would provide a world wild market for products advertising and sales.
According to analysts from the online consulting firm Strategic eCommerce Solution (SeS),
benefits of e-commerce for a new starting up business include the expansion of geographical
reach and costumer base as well as an increased visibility. Besides, using Internet as a market
platform could help providing “customers valuable information about the business”, “building
customer loyalty” and “collecting customer data”. In addition, marketing and advertising
expenses can be greatly reduced and sales are likely to grow since the business is continuously
open.
According to the business analyst Evan I Schwartz, the essential principles for growing the new
businesses on the World Wide Web include:
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•
•
•
•
•
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Prioritize the quality of the experience over the quantity of people visiting the web site;
Valorize results instead of exposure on the web;
Compensate costumers for disclosing data about themselves;
Be aware that consumers will shop online only of information-rich products;
Consider that self-service provides for the highest level of customer comfort;
"Value-Based Currencies" Enable You to Create Your Own Monetary System;
Trusted Brand Names Matter Even More on the Web
Even the Smallest Business Can Compete in the Web's Global "Marketspace"
Agility Rules - Web Sites Must Continually Adapt to the Market
5.5 Computing technology for News & Media organisations
How has the development of information technology influenced the way news and media
organisations deliver content?
Due to the nature of news and media organisations, the information technologies have particular
relevance to them. As noted earlier "Information technology is the technology used to store,
manipulate, distribute or create information". News and media organisations are intimately
acquainted with each of these elements of information technology. However this report will focus
on the distribution and creation of information.
Developments such as the Internet and satellite television have created new medium and
audiences through which and to which these organisations can disseminate their information.
Given the situation thirty years ago the developments that we have seen have enabled the news
and media organisations access to more people, they have a wider audience. The audience
however now has a wider, global choice. News reports can be received which highlight many
different sides of an international conflict for example than was possible before. The relative
cheapness of being able to publish information on the Internet for example means that virtually
anyone can publish information accessible anywhere in the world.
5.6 Computing technology in educational organisations
How has the development of information technology influenced the way educational resources
are provided?
As information technology has developed over the last thirty years, educational establishments
have been influenced in various ways. The most obvious example has been the introduction of
information technology related courses. These courses are introduced to try to satisfy the demand
that society has for qualified people to develop these information technologies.
The developments that have occurred in information technology have also had other influences
on educational establishments. As was discussed earlier about news and media organisations,
educational organisations also have a goal to distribute information from a source (lecturer,
books, on-line resources etc.) to the student. The processes by which educational establishments
distribute information have become increasingly diverse, and the effectiveness of the process has
also improved.
The distribution of information is not the only concern of educational establishments. For
example one of the aims of Universities is to create information. This "creation" is done by
research. Information technologies have enabled researchers to access a wider source of
information than previously available through such technologies as the Internet (the original
ArpaNet being set up primarily to assist research). The Internet and other related technologies
such as electronic mail, also enable collaborative projects to be undertaken between
geographically distant groups.
Collective Activities and Interests
One of the aims of the report is to analyse the influence of "information technology" on the
collective activities and interests of a broad range of people. The phrase "collective activities and
institutions" has a wide scope. This report divides these into two main areas:
Conclusion
It has been shown that the developments in information technology have had an impact on
general society perception of information. Without going into specific detail about individual
situations, it has been shown that that impact has been fourfold:
Storage, manipulation, distribution and creation are four areas dealing with information are the
four areas in which societies perception of information has changed. As communication and
information technologies have been developed, the various elements that makeup society,
whether they be individuals or organisations, expect to be able to use information in ways that
were not possible thirty years ago. Society expects to be able to store more than was previously
conceived. Society expects to be able to manipulate the information they have for their benefit, to
increase understanding and discover new relationships. Society expects to be able to distribute
information quickly, efficiently and cheaply. Society now expects the creation of new
information to be facilitated by these new technologies.