QBasic Tutorial 2

Q Basic Tutorial 2

QBasic Tutorial 1

Q Basic Tutorial 1

Three-Schema Architecture (3 view of DBMS)

 

Data Abstraction

Data abstraction is hiding the complex data structure in order to simplify the user’s interface of the system. It is done because many of the users interacting with the database system are not that much computer trained to understand the complex data structures of the database system.

To achieve data abstraction, we will discuss a Three-Schema architecture which abstracts the database at three levels discussed below:

 Three-Schema Architecture:

The main objective of this architecture is to have an effective separation between the user interface and the physical database. So, the user never has to be concerned regarding the internal storage of the database and it has a simplified interaction with the database system.

The three-schema architecture defines the view of data at three levels:

1. Physical level (internal level/view)
2. Logical level (conceptual level/view)
3. View level (external level/view)
   
   
   

1. Physical Level/ Internal Level

The physical or the internal level schema describes how the data is stored in the hardware. It also describes how the data can be accessed. The physical level shows the data abstraction at the lowest level and it has complex data structures. Only the database administrator operates at this level.

2. Logical Level/ Conceptual Level

It is a level above the physical level. Here, the data is stored in the form of the entity set, entities, their data types, the relationship among the entity sets, user operations performed to retrieve or modify the data and certain constraints on the data. Well adding constraints to the view of data adds the security. As users are restricted to access some particular parts of the database.

It is the developer and database administrator who operates at the logical or the conceptual level.

3. View Level/ User level/ External level 

It is the highest level of data abstraction and exhibits only a part of the whole database. It exhibits the data in which the user is interested. The view level can describe many views of the same data. Here, the user retrieves the information using different application from the database.

Schema in DBMS

The term "database schema" can refer to a visual representation of a database, a set of rules that govern a database, or to the entire set of objects belonging to a particular user. 

A database schema represents the logical configuration of all or part of a relational database. It can exist both as a visual representation and as a set of formulas known as integrity constraints that govern a database. These formulas are expressed in a data definition language, such as SQL. As part of a data dictionary, a database schema indicates how the entities that make up the database relate to one another, including tables, views, stored procedures, and more.

There are two main kinds of database schema:

1. A logical database schema conveys the logical constraints that apply to the stored data. It may define integrity constraints, views, and tables.
2. A physical database schema lays out how data is stored physically on a storage system in terms of files and indices.

 What is sub schema?

A sub schema is a subset of the schema and inherits the same property that a schema has. The plan (or scheme) for a view is often called sub schema. Sub schema refers to an application programmer's (user's) view of the data item types and record types, which he or she uses.

INTRODUCTION & CHARACTERISTICS OF COMPUTER

 

INTRODUCTION TO COMPUTER

Computer is an electronic device which takes input from user, processes it according to instructions and gives the desired output. It is basically a programmable machine. It works on binary digits, in short binary digit is known as bit. Computer was developed to produce accurate result at a very fast speed.

CHARACTERISTICS OF COMPUTERS

 

1. Speed: A computer is very fast device; it can perform a work in a few second. The amount of work that a human being can do in an entire year, a computer can do in a few minutes or in seconds.

While talking about the speed of a computer we do not talk in terms of seconds or even milliseconds ( but in terms of microseconds (, nanoseconds (, and even picoseconds (. A powerful computer is capable of performing several billion ( simple arithmetic operations per second.

2.  Power of Remembering: As a human being acquires new knowledge, his/her   brain selects what it feels to be important and forgets unimportant things. This is not the case with computers. A computer can store and recall any amount of information because of its secondary storage capacity. It can retain a piece of information as long as a user desires and the user can recall the information whenever required. Even after several years, a user can recall exactly the same information that he/she had stored in the computer several years ago. A computer forgets or looses certain information only when a user asks it to do so. Hence, it is entirely depend upon to the user to make a computer retain or forget some information.

3.   Accuracy: It provides a high degree of accuracy. It can accurately gives the result of division of any two numbers up to 10 decimal places.

4. Diligence: Computer, being a machine, does not suffer from the tiredness and lack of concentration. If millions of calculations to be performed then the computer will perform the last calculation with the same accuracy and speed as the first calculation. 

5.  Intelligence: It has no I.Q. It possesses no intelligence of its own.

 

Plotter

 A plotter is a special output device used to produce hard copies of large graphs and designs on paper, such as construction maps, engineering drawings, architectural plans and business charts. The plotter is either a peripheral component that you add to your computer system or a standalone device with its own internal processor.

A plotter is a computer hardware device much like a printer that is used for printing vector graphics. Instead of toner, plotters use a pen, pencil, marker, or another writing tool to draw multiple, continuous lines onto paper rather than a series of dots like a traditional printer. Though once widely used for computer-aided design, these devices have more or less been phased out by wide-format printers. 

Advantages of plotters

• Plotters can work on very large sheets of paper while maintaining high resolution.
• They can print on a wide variety of flat materials including plywood, aluminum, sheet steel, cardboard, and plastic.
• Plotters allow the same pattern to be drawn thousands of times without any image degradation.

Disadvantages of plotters

• Plotters are quite large compared to a traditional printer.
• Plotters are also much more expensive than a traditional printer

UNIX Commands List

 

File/Directory operation related Unix Commands

• cp – copy a file
• mv – move or rename files or directories
• tar – create and use archives of files
• gzip – compress a file
• ftp – file transfer program
• lpr – print out a file
• mkdir – make a directory
• rm – remove files or directories
• rmdir – remove a directory
• mount – attaches a file system to the file system hierarchy at the mount_point, which is the pathname of a directory.
• umount – unmounts a currently mounted file system.

Navigational type Unix Commands

• cd – change directory
• pwd – display the name of your current directory
• ls – list names of files in a directory

Disk, File and Folder Size/Usage

• du – Use this command to see the size/usage of the folder you are in. Example usage: du -sk *
• df – Report file system disk space usage. Example usage: df -k

Display file content

• cat – concatenate and display files.
• more – The more utility is a filter that displays the contents of a text file on the terminal, one screenful at a time.
• less – Less is a program similar to  more  (1), but  which  allows backward  movement  in the file as well as forward movement. Also, less does not have  to  read  the  entire  input  file before starting,so with large input files it starts up faster than text editors like vi

File Editing

• vi – The vi (visual) utility is a display-oriented text editor.
• nano – nano is a small, free and friendly editor.

Search

• find – find files of a specified name or type.
• grep – searches files for a specified string or expression.

Administration

• top – Top displays the top 10 processes on the system and periodically  updates this information. Raw cpu percentage is used to rank the processes.
• chmod – change the permissions of a file or a directory.
• ps – The ps command prints information  about  active  processes.
• kill – kill a process.

Information

• date – display the current date and time.
• cal – The cal utility writes a Gregorian calendar to standard output.
• diff – display differences between text files.

Help Related

• man – The man command  displays information  from  the reference manuals.
• help – The help utility retrieves information  to  further  explain errors  messages and warnings from SCCS commands.

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AMPLIFIER & REPEATER

AMPLIFIER

An amplifier is an electronic device that increases the voltage, current, or power of a signal. Amplifiers are used in wireless communications and broadcasting, and in audio equipment of all kinds. They can be categorized as either weak-signal amplifiers or power amplifiers.

REPEATER

In digital communication systems, a repeater is a device that receives a digital signal on an electromagnetic or optical transmission medium and regenerates the signal along the next leg of the mediu

Video Tutorial

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MODEM

Modem is short for MOdulator DEModulator. It’s an electronic device used to access the Internet that modulates carrier waves to encode information to be transmitted and also demodulates incoming carrier waves to decode the information they carry.

What is a modem?

A modem is a very important piece of network hardware that allows a computer to send and receive data through a telephone line or cable connection. In simple words, it’s the device that connects a computer to the Internet using telecommunication network.

The Importance of a Modem

Back in the old days, when landline phones were the primary tool to communicate over long distances, modems came in pretty handy to gain Internet connectivity using telephone lines. In fact, without modems, it would have been impossible for most users to connect to the Internet. While computer technology is purely digital, i.e., it relies on numbers to transmit and receive information, telephone technology, even to this day, is partly analog, meaning that it uses continuously varying electrical signals to transmit information.

Since your modem sends information through a telephone line by modulating digital signals, it also needs to have another kind of translator that helps it demodulate the analog signals it receives via the telephone line.

That’s why a modem is named as such, because it both modulates and demodulates signals.

ETHERNET

Ethernet is the technology that is most commonly used in wired local area networks (LANs). A LAN is a network of computers and other electronic devices that covers a small area such as a room, office, or building. It is used in contrast to a wide area network (WAN), which spans much larger geographical areas. Ethernet is a network protocol that controls how data is transmitted over a LAN. Technically it is referred to as the IEEE 802.3 protocol. The protocol has evolved and improved over time to transfer data at the speed of a gigabit per second. 

How Ethernet works

The Institute of Electrical and Electronics Engineers Inc. (IEEE) specifies in the family of standards called IEEE 802.3 that the Ethernet protocol touches both Layer 1 -- the physical layer -- and Layer 2 -- the data link layer -- on the OSI network protocol model. Ethernet defines two units of transmission: packet and frame. The frame includes not just the payload of data being transmitted, but also:

• the physical media access control (MAC) addresses of both the sender and receiver;
• VLAN tagging and quality of service information;
• Error correction information to detect transmission problems.

Advantages of using wired Ethernet network

• It is very reliable.
• Ethernet network makes use of firewalls for the security of the data.
• Data is transmitted and received at very high speed.
• It is very easy to use the wired network.

Disadvantages of using wired Ethernet network

• The wired Ethernet network is used only for short distances.
• The mobility is limited.
• Its maintenance is difficult.
• Ethernet cables, hubs, switches, routers increase the cost of installation.

Types of Ethernet network

The maximum data rate of the original Ethernet technology is 10 megabits per second (Mbps), but a second generation fast ethernet carries 100 Mbps, and the latest version called gigabit ethernet works at 1000 Mbps. Ethernet network can be classified into 3 types:

Fast Ethernet

This type of Ethernet can transfer data at a rate of 100 Mbps.  Fast Ethernet makes use of twisted pair cable or fiber optic cable for communication.
There are three types of fast Ethernet, which are as follows:
• 100BASE-TX
• 100BASE-FX
• 100BASE-T4

Gigabit Ethernet

This type of Ethernet network can transfer data at a rate of 1000 Mbps. Gigabit Ethernet also makes use of twisted pair cable or fiber optic cable. 48 bits used for addressing in Gigabit Ethernet. Nowadays gigabit Ethernet is very popular. The latest Gigabit Ethernet is a 10 Gigabit Ethernet, which can transfer data at a rate of 10 Gbps. Gigabit Ethernet was developed so that it can meet the needs of the user like faster communication network, faster transfer of data etc.

Collision and Broadcast Domain

Collision Domain

A collision domain is a network segment connected by a shared medium or through repeaters where simultaneous data transmissions collide with one another. The collision domain applies particularly in wireless networks, but also affected early versions of Ethernet. A network collision occurs when more than one device attempts to send a packet on a network segment at the same time. Members of a collision domain may be involved in collisions with one another. Devices outside the collision domain do not have collisions with those inside.

The following example illustrates collision domains.

As you can see, we have 6 collision domains.

Here, each port on a hub is in the same collision domain. Each port on a bridge, a switch or router is in a separate collision domain.

 Broadcast Domain 

A broadcast domain is a domain in which a broadcast is forwarded. A broadcast domain contains all devices that can reach each other at the data link layer (OSI layer 2) by using broadcast. All ports on a hub or a switch are by default in the same broadcast domain. All ports on a router are in the different broadcast domains and routers don’t forward broadcasts from one broadcast domain to another.

The following example clarifies the concept.

In the picture above we have three broadcast domains, since all ports on a hub or a switch are in the same broadcast domain, and all ports on a router are in a different broadcast domain.