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AgendaUnit 5: Sections 3 - 6

About the Quiz

Unit 6: Section 2: Hardware, software and computer systemsSection 3: The resources of a computer and how they are managed

T. Jumana Abu Shmais – AOU - Riyadh

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Computer Networking• The Internet is an international network of computer

networks.

• In recent years, the Internet has become a part of society, like the telephone, radio and television.

• The Web, which is based on the internet, has become the platform on which all kinds of information are disseminated. For example, it has generated e-commerce, new educational system …

• Besides the Internet, many other computer networks exist (banks, police, travel agents and airlines…).

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Networking Issues • A network of computers is linked together by

communication links. These links may be:– dedicated cable links;– public telephone networks;– radio or microwaves links.

• Any organization using more than one computer is likely to have a Local Area Network (LAN) to exploit the benefits of resource sharing (printers, Internet access…etc.).

• Pocket-sized computers known as PDAs (Personal Digital Assistants) can communicate with each other and with desktop computers using infra-red/Bluetooth signals. They form a small local network.

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The Internet • The Internet has its roots in the American military-funded

research community (ARPA) of the early 1970s. • The first applications to use the Internet were based purely

on text (command-line interface). Modern graphical browsers appeared only in the early 1990s.

• The Internet comprises a huge collection of computers (called hosts) with telecommunications links between them.

• The Internet can connect any type of computer running any operating system. By adopting the Internet protocol each of these computers can become an Internet host; it becomes part of the Internet.

• How big is the internet? ARPANET was a network of just four computers. Now the Internet connect hundreds of millions of computers.

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The Internet • Most users connect to the Internet through the public

telephone network.• The telephone system (which uses analogue signals

consisting of a continuously varying voltage) was designed for voice transmission.

• Your computer communicates using digital signals (consisting of discrete bit patterns).

• So, we need to convert between the two signals. This is the job of a modem (modulator-demodulator).

• An external modem is attached to the computer as a peripheral. An internal one is a card installed inside the computer’s case.

• The modem converts the data signals from the computer into analogue signals, a modem at the other end will convert the signal back into digital.

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Digital/Analog Communications• In most countries the main telephone system is no longer

analogue but digital. However this applies only to the links between telephone exchanges. The copper wires that join individual phones to the ‘box’ in the road, called the local loop, still carry analogue signals.

• Two factors conspire to prevent the telephone network from becoming 100% digital:– The need to provide new cables to every single house; – The expense of replacing millions of handsets.

• A typical modem can download data to your computer at 56,000 bits per second – sometimes termed 56kbps modem.

• In recent years telephone companies have begun to offer ADSL (Asymmetric Digital Subscriber Lines). This is a technology which allows data to be transmitted digitally at high speed.

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Browsing the Web• In 1990, Tim Berners-Lee at CERN created the forerunner

of the Web which today is a collection of hypertext documents distributed worldwide and linked by the Internet.

• The software you use on your computer to access and view documents on the Web is called a Web browser.

• The basic unit of Web content is the Web page which is an HTML document.

• The browser accesses the page, held on a remote computer (Web server), and downloads it to your computer (the client).

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The factors affecting the speed at which a page downloads (using a dial-up modem):

• The amount of data in the page.• The speed of your modem. • The quality of the phone line. If the line is noisy

the speed will go down to increase the accuracy of the transmission.

• The speed of your computer. • The amount of traffic on the Internet. When

traffic is high, certain parts of the Internet become congested.

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Internet Addressing • Internet addresses have several levels.• At the highest level, there’s the top-level domain (domain

means a collection of Internet hosts). • Two types of top-level domains:

1. Codes of three letters or more: group users by category

2. Codes of two letters: are normally country-specific

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Internet Addressing • Country code domains are usually subdivided

– ac.uk (academic community), – co.uk (commercial),– gov.uk (national and local government)

• Many individual domain names are available within each top-level domain. Within ac.uk there is open.ac.uk, the OU domain.

• Once the domain name open.ac.uk has been approved by an external agency, the OU is free to allocate sub-domains and host names within this naming scheme.

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Internet Addressing (to the document level)

• The address associated with a hyperlink is given in the form of a URI (Uniform Resource Indicator), which specifies the service requested and the full address of the document.

• An example of a URI:http://mcs.open.ac.uk/mcsexternal/courses/m150.htm– http:// identifies the protocol (HTTP) – mcs specifies the server (host) – open.ac.uk identifies the domain in which mcs is

found.– uk is the top-level domain.– The rest of the address is the path within ‘mcs’ that

leads to the required document (m150.htm).

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Naming Hosts & IP Addresses• It is usually convenient to assign a name to each

computer on a network so that users can identify it easily. • The naming scheme (convenient for humans) is not

actually used by the messages that travel across the Internet. Instead each host has a 4-byte number associated with it, called its IP (Internet Protocol) number.

• The first thing that happens when a URI is executed is that the host name is sent to a Domain Name Server (like a directory) to be resolved (translated to an IP).

• New hosts are being added so DNSs need to be kept up to date. A DNS propagates updates to neighboring DNSs.

• If a DNS is asked for a host name which does not appear in its directory, it broadcasts a message across the Internet to other DNSs until the IP number is located.

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Logical and Physical Names• Suppose that your website on ‘orchid.open.ac.uk’ has

crashed. So you moved your website to another computer: ‘peony.open.ac.uk’.

• Problem: Now no one can find your web pages any more, as they are no longer at the same address.

• A good solution is to avoid reliance on named physical machines. The way to do this is to identify the Web server to the Internet by a logical name. So, although the physical name of the server might change, the logical name will remain the same.

• This means that an index must be kept which associates the logical name of a server with its current physical host.A DNS will ensure that.

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Email • Email is an asynchronous way of communication.

• A mail application, or mail client, combines high speed with a permanent record.

• Organizations can have their own internal email system that is independent of the Internet.

• Unlike other URIs, an email address identifies a user, e.g. Odysseus@homer.ac.gr

– The part before @ is the username

– What’s after the @ is the domain name

• When data travels across the Internet, it is broken up into units of a standard size called packets. Each packet carries the address information so that it will reach its intended destination. The packets are re-assembled into a single item on arrival.

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Email Headers• Along with the actual data, an email

also carries transmission information in a number of lines, called headers.

• Examples: date and time of dispatch, source and destination e-mails, subject…etc.

• Headers whose names begin with ‘X-’ are used to convey additional information. The header X-Mailer reveals that the message was composed using version 4.01 of the Pegasus.

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Sending Attachments• Q: Email transmission is restricted to text, but how is it

possible to attach documents of any kind to an email message?

A: By encoding the attached file as a series of alphabetic characters and appending them to the end of the message.

• In order to enable the receiving mail client to decode the attachment, the encoding scheme must conform to a standard. One of the Internet standards for encoding mail attachments is MIME (Multipurpose Internet Mail Extensions).

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How Does Data Travel?• Communication between computers takes place in

the form of serial transmission (a single channel carries a stream of bits in sequence).

• Protocols are needed to ensure that, on arrival, the receiving computer interprets the stream of bits with its original meaning.

• Communication which can take place in both directions simultaneously is called duplex or two-way communication (e.g. telephone).

• Communication that takes place in one direction is called one-way communication (e.g. radio, TV).

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Databases• A database (db) is a collection of data stored in a

computer system according to a set of rules, and organized to facilitate access involving complex searches and selection.

• The primary emphasis of database applications is on making the data persistent, and structuring it so as to minimize redundancy, avoid inconsistency & maximize the usefulness of the data for the purposes of access and updating.

• A query (request to db) is used to get specific information from the db. The response to the query ideally extracts from the database all the relevant information.

• A database is part of an information system.

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Databases• In databases data is stored in tables.• Each row (record) represents an item in the table.• Each column (field) represents a property of an item.• Each row in a table must be identified uniquely by the

values in one or more columns. Such column(s) is called a key (primary key).

• When the values of two columns in two different tables are equal, we say that the tables are related. The resulting database is called a relational database.

• The relations between tables enable us to use combinations of tables in queries.

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DatabasesWhat is the engine size of Tom Cobbley’s car?

Makes of cars

Details of cars

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Databases• A typical database system will consist of:

– A collection of tables – Data (called metadata) which describes the

tables (what each column in a table means, and how many tables there are in a database….)

– Facilities for backing up the tables. – Facilities for ensuring security – A query facility.

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Object Database • Object database may contain video, voice and music along

with more traditional forms of data. • A data object (sound, image, video) may be contained

within a database as ‘an object in a box’. The box has a name and the database can access the object using this name.

• An object stored in this way is called a BLOB (Binary Large OBject). The weakness of this approach is that you cannot query the content of a BLOB (the BLOB has no structure to it).

• Solution: to make BLOB fields searchable, their content can be described using metadata.

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Metadata• Metadata is explanatory, descriptive data about

the data. • Email headers are good examples.

• Each item in the <HEAD> section of an HTML document is an example of metadata. It is not part of the content of the document; rather, it says something about the content.

• HTML has a tag, <META>, used to include metadata and keywords that can be used by search engines.

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Metadata – MPEG-7• MPEG-7 is a new MPEG standard (MPEG stands

for ‘Moving Picture Experts Group’, a committee of ISO).

• MPEG-7 provides visual descriptors (color, texture, shape, position, motion and face recognition) and audio descriptors (key, mood, tempo and tempo changes).

• Using MPEG-7, a metadata description of a scene could be re-assembled into English as: This is a scene with a barking brown dog on the left and a blue ball on the right, with the sound of passing cars in the background.

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Privacy

• Privacy: keeping some things removed from general or public knowledge.

• Computer systems provide the facility for government, local authority, tax authorities, your bank … to have control over vast, and detailed, amounts of data about you. They can store it for indefinite periods and use it in a variety of ways.

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Data and the Law • Data protection laws or data access legislations in any

jurisdiction are likely to have some or all of the following characteristics:– a legal definition of data (limited to electronic forms or

also covers handwritten and typed data, photographs..);– a description of how data may be acquired lawfully;– what uses the data may be put to;– any time limits on storage;– who may lawfully access and use the data, and for

what. • They aim to enable people to maintain a

reasonable level of privacy.

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Computer Ethics• Ethics is defined as a set of moral principles that should guide our acts

as a citizen.

1) Thou shalt not use a computer to harm other people.

2) Thou shalt not interfere with other people’s computer work.

3) Thou shalt not snoop around in other people’s computer files.

4) Thou shalt not use a computer to steal.

5) Thou shalt not use a computer to bear false witness.

6) Thou shalt not copy or use software for which you have not paid.

7) Thou shalt not use other people’s computer resources without authorization or proper compensation.

8) Thou shalt not appropriate (take without permission) other people’s intellectual output.

9) Thou shalt think about the social consequences of the program you are writing or the system you are designing.

10) Thou shalt always use a computer in ways that ensure consideration and respect for your fellow humans.

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Linking and Theft • Web pages often contain links to web pages developed by

other users (e.g. gateways).

• Consider a site which sets itself up as an internet newspaper and contains links to individual stories stored at other online newspaper sites. What should the ethical position be on this?

– It could be regarded as an example of intellectual property theft.

– It could be argued that the material has not been stolen because the text has not been cut and pasted but simply linked to!!!

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Security • Legislation to protect data, and in particular

computerized data, is desirable. However, the law itself is never sufficient. We have to take precautions.

• Once you link your computer to the internet, you need to think about ways of making it less accessible to unwanted visitors who, in computer jargon, are termed hackers.

• It may be desirable to secure a whole network of computers from unauthorized outside access by using a firewall (a software system which controls data traffic entering and leaving the network).

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Ownership and Rights Over Data• The concept of data ownership is legally very unclear in

most countries. Who actually ‘owns’ medical records?? (patients, doctors, pharmacists or health departments)

• Intellectual Property Rights (IPR): the right to gain financially from the products one creates.

• Moral rights: the right to say how one’s products can be used. (the content of letters that you write, and even the content of assignments that you prepare …)

• Copyright laws: afford some sort of protection for intellectual property (databases are subject to copyright)

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Worms, Viruses and Trojan Horses• A worm is a program intended to subvert a whole

network of computers. It transfers copies of itself to other machines on the network.

• A virus is a program or piece of code designed to cause specific damage to your software by attaching itself to documents. E.g. deleting important documents from your hard disk.

• A Trojan horse is a program which looks legitimate but attempts to do something quite different (modify documents on your hard disk, collect passwords…). Typically the name of the document will be misleading.

• Anti-virus software will protect your system but needs regular updates because new viruses appear on a daily basis.

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Agenda

Unit 5: Sections 3 - 6

About the Quiz

Unit 6: Section 2: Hardware, software and computer systemsSection 3: The resources of a computer and how they are managed

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Agenda

Unit 5: Sections 3 - 6

About the Quiz

Unit 6: Section 2: Hardware, software and computer systemsSection 3: The resources of a computer and how they are managed

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Hardware, Software and Computer Systems

• Hardware: consists of the physical tangible parts of computer that

can be touched.

• Software: consists of sets of instructions that tell a computer how

to perform a particular task. It’s intangible.

• Computer hardware and software components work together in

order to execute computer programs.

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Firmware• Firmware is a sequence of instructions

(software) etched (fixed, carved…etc.) into the Read-Only Memory (ROM) of the computer, usually to perform some system function.

• Because these instructions are on a chip they form a permanent part of the computer and could be viewed as a combination between hardware and software.

• Firmware can be thought of as "semi-permanent" since it remains the same unless it is updated.

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Hardware vs. Software• Although hardware and software are different in

nature, there are related.

• A solution to a computer problem can be implemented using either hardware or software.

• The decision to implement some functions in hardware and others in software is based on factors such as speed, cost and reliability.

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The Computer as a System• A computer is a device that accepts data and

manipulates it by carrying out a sequence of instructions (a program), so as to produce some output. A computer also has the means of storing the input, the output and the program.

• It accepts input, processes that input, and

produces output.

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CPU – the Heart of the ComputerThe Central Processing Unit: (CPU also called processor) is the part of the computer that executes program instructions. The processor of a PC is usually on a single microchip, (or microprocessor) which can contain several tens of millions of circuits.

It consists of the following 3 parts:1. The Arithmetic/Logic Unit (ALU) is where all calculations and

logical operations (manipulations of binary data) are performed. – Arithmetic operations involve addition, subtraction,

multiplication, and division. – Logic operations basically consist of comparing or combining

two values:• All ALUs can compare >, <, and =.• Some ALUs can also compare >=, <=, and <>.• Comparisons can be combined using the logical operations

NOT, AND and OR. Truth tables are used to formally define these logical operations.

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CPU – the Heart of the Computer

2. The Control Unit (CU): The control unit is responsible for interpreting the instructions in a computer program and then sending appropriate control signals to the components that will carry them out. As soon as one instruction has been executed, the control unit moves on to deal with the next.

The control unit is in two-way communication with all the other devices in the system.

3. The ALU and CU registers: small numbers of high-speed memory locations which are used to hold single pieces of data or single instructions immediately before and after processing.

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CPU – the Heart of the Computer

• The CPU together with the way in which memory, internal communications and input/output are organized define an architecture (first described by John Von Neumann).

• Characteristic of the Von Neumann architecture is the fact that the program instructions are stored in the main memory along with the data.

• The data flow is coordinated by the control unit, through the sending and receiving of control signals.

• Processors come in ‘families’, such as the Pentium 4, Celeron, AMD Athlon XP. Each family of processors has what is termed an instruction set. This is the collection of basic instructions, called machine language instructions, which a particular processor understands.

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Main Memory• Usually referred to as RAM.• Random access memory (RAM): Data and programs must be loaded from external storage (such as

disk) into RAM before it can be used by the processor. RAM is volatile (lost when the computer is switched off).

• Random access means that each memory location takes the same amount of time to access, regardless of its address.

• Main memory can be envisaged as a large collection of sequentially ordered pigeon-holes, each of which can hold the same number of bits, equivalent to the word size of the computer.

• The word size refers to the number of bits that the CPU can manipulate at one time. It is determined by the size of the registers in the CPU and the number of bits that can be moved round the computer as a single unit. (‘32-bit’ or ‘64-bit’)

• The capacity of the memory varies in different computers and is measured in bytes. Each byte represents one character in memory.

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Peripheral Devices and Secondary Memory• A peripheral, or peripheral device, is any computer device that is not

part of the essential computer (the CPU and main memory). – input and output (I/O) devices – storage devices.

• PCs also have a number of external ports – connection points on the side of the computer’s case with associated control circuitry that allows I/O devices to communicate with the processor. A special purpose port can be added by plugging an expansion card (a small circuit board and port combined) into one of the expansion slots.

• Secondary memory or external storage is nonvolatile (permanent) and has more capacity than main memory.

• There are two approaches to external storage: sequential access (magnetic tape) and direct access (disks).

• The most widely used external storage media are floppy disks, hard disks, optical disks and magnetic tapes.

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The Operating System• An operating system is a complex piece of software

that acts as an interface between the user (or an application program) and the computer hardware, and manages the computer’s internal resources.

• Mainframes and minicomputers have powerful OS, including multiprocessing and multitasking, where many users can run different programs at the same time. Microcomputer OS focus on a single user.

• The most important operating systems are: DOS, Microsoft Windows, Unix, Linux and MAC.

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The Operating SystemThe Operating system has the following functions: Provision of user interface Management of the memory Coordination and control of peripheral devices. Scheduling access for the processor Interfacing between hardware and application

programs. Providing basic utilities (disk formatting, file

management… etc.).

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Read-Only Memory (ROM) and Bootstrapping

• Knowing that RAM is volatile, how can the computer run the operating system when it’s started?

• The answer is that there is another type of memory, called Read-Only Memory (ROM) installed in the computer. The data in ROM is built into the memory chips during manufacture and is permanent. It cannot be overwritten and will not disappear when power is lost to the computer.

• An important function of ROM is to store a program, called a boot program, which is automatically executed when the computer is first switched on. This small program will typically run a test of main memory and see what peripherals are connected to the system, before loading larger programs, such as the operating system.

• The process of using a short program to load a larger program is called bootstrapping, which comes from the idea of someone pulling themselves up by their own bootstraps.

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Agenda

Unit 5: Sections 3 - 6

About the Quiz

Unit 6: Section 2: Hardware, software and computer systemsSection 3: The resources of a computer and how they are managed

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Any

Questions