TYPES OF INTERNET CONNECTIONS

 DIAL UP CONNECTION: Dial-up Internet access is a form of Internet access that uses the facilities of the public switched telephone network (PSTN) to establish a connection to an Internet service provider (ISP) by dialing a telephone number on a conventional telephone line.
This type of connection is called dial-up because the initiating device, usually an Internet user's personal computer, will literally pick up the telephone line signal and dial up the telephone number of a receiving device, usually an Internet-connected modem.
The condition of the telephone line has a lot to do with the maximum speed of the dial up connection. Under optimal circumstances, a 56kbps throughput can be maintained. Real world speeds typically vary between 18kbps and 48kbps.

Broadband Connections:

The term broadband commonly refers to high-speed Internet access that is always on and faster than the traditional dial-up access. Broadband includes several high-speed transmission technologies such as:

• Digital Subscriber Line (DSL)
• Cable Modem
• Fiber
• Wireless
• Satellite
• Broadband over Powerlines (BPL)

The broadband technology you choose will depend on a number of factors. These may include whether you are located in an urban or rural area, how broadband Internet access is packaged with other services (such as voice telephone and home entertainment), price, and availability.

 

Digital Subscriber Line (DSL)

DSL is a wireline transmission technology that transmits data faster over traditional copper telephone lines already installed to homes and businesses. DSL-based broadband provides transmission speeds ranging from several hundred Kbps to millions of bits per second (Mbps). The availability and speed of your DSL service may depend on the distance from your home or business to the closest telephone company facility.

The following are types of DSL transmission technologies:

• Asymmetrical Digital Subscriber Line (ADSL) – Used primarily by residential customers, such as Internet surfers, who receive a lot of data but do not send much. ADSL typically provides faster speed in the downstream direction than the upstream direction. ADSL allows faster downstream data transmission over the same line used to provide voice service, without disrupting regular telephone calls on that line.
• Symmetrical Digital Subscriber Line (SDSL) – Used typically by businesses for services such as video conferencing, which need significant bandwidth both upstream and downstream.

Faster forms of DSL typically available to businesses include:

• High data rate Digital Subscriber Line (HDSL); and
• Very High data rate Digital Subscriber Line (VDSL).

Cable Modem

Cable modem service enables cable operators to provide broadband using the same coaxial cables that deliver pictures and sound to your TV set.

Most cable modems are external devices that have two connections: one to the cable wall outlet, the other to a computer. They provide transmission speeds of 1.5 Mbps or more.

Subscribers can access their cable modem service by simply turning on their computers, without dialing-up an ISP. You can still watch cable TV while using it. Transmission speeds vary depending on the type of cable modem, cable network, and traffic load. Speeds are comparable to DSL.

Fiber

• Fiber optic technology converts electrical signals carrying data to light and sends the light through transparent glass fibers about the diameter of a human hair. Fiber transmits data at speeds far exceeding current DSL or cable modem speeds, typically by tens or even hundreds of Mbps.
• The actual speed you experience will vary depending on a variety of factors, such as how close to your computer the service provider brings the fiber and how the service provider configures the service, including the amount of bandwidth used. The same fiber providing your broadband can also simultaneously deliver voice (VoIP) and video services, including video-on-demand.
• Telecommunications providers sometimes offer fiber broadband in limited areas and have announced plans to expand their fiber networks and offer bundled voice, Internet access, and video services.
• Variations of the technology run the fiber all the way to the customer’s home or business, to the curb outside, or to a location somewhere between the provider’s facilities and the customer.

Wireless

• Wireless broadband connects a home or business to the Internet using a radio link between the customer’s location and the service provider’s facility. Wireless broadband can be mobile or fixed.
• Wireless technologies using longer-range directional equipment provide broadband service in remote or sparsely populated areas where DSL or cable modem service would be costly to provide. Speeds are generally comparable to DSL and cable modem. An external antenna is usually required.
• Wireless broadband Internet access services offered over fixed networks allow consumers to access the Internet from a fixed point while stationary and often require a direct line-of-sight between the wireless transmitter and receiver. These services have been offered using both licensed spectrum and unlicensed devices. For example, thousands of small Wireless Internet Services Providers (WISPs) provide such wireless broadband at speeds of around one Mbps using unlicensed devices, often in rural areas not served by cable or wireline broadband networks.
• Wireless Local Area Networks (WLANs) provide wireless broadband access over shorter distances and are often used to extend the reach of a "last-mile" wireline or fixed wireless broadband connection within a home, building, or campus environment. Wi-Fi networks use unlicensed devices and can be designed for private access within a home or business, or be used for public Internet access at "hot spots" such as restaurants, coffee shops, hotels, airports, convention centers, and city parks.
• Mobile wireless broadband services are also becoming available from mobile telephone service providers and others. These services are generally appropriate for highly-mobile customers and require a special PC card with a built in antenna that plugs into a user’s laptop computer. Generally, they provide lower speeds, in the range of several hundred Kbps.

Satellite

Just as satellites orbiting the earth provide necessary links for telephone and television service, they can also provide links for broadband. Satellite broadband is another form of wireless broadband, and is also useful for serving remote or sparsely populated areas.

Downstream and upstream speeds for satellite broadband depend on several factors, including the provider and service package purchased, the consumer’s line of sight to the orbiting satellite, and the weather. Typically a consumer can expect to receive (download) at a speed of about 500 Kbps and send (upload) at a speed of about 80 Kbps. These speeds may be slower than DSL and cable modem, but they are about 10 times faster than the download speed with dial-up Internet access. Service can be disrupted in extreme weather conditions.

Broadband over Powerline (BPL)

BPL is the delivery of broadband over the existing low- and medium-voltage electric power distribution network. BPL speeds are comparable to DSL and cable modem speeds. BPL can be provided to homes using existing electrical connections and outlets. BPL is an emerging technology that is available in very limited areas. It has significant potential because power lines are installed virtually everywhere, alleviating the need to build new broadband facilities for every customer.

 

VSAT (Very Small Aperture Terminal)

 VSAT stands for “Very Small Aperture Terminal” and refers to receive/transmit terminals installed at dispersed sites connecting to a central hub via satellite using small diameter antenna dishes (.75 to 3.8 meter).
VSAT technology represents a cost effective solution for users seeking an independent communications network connecting a large number of geographically dispersed sites. VSAT networks offer value-added satellite-based services capable of supporting the Internet, LAN, voice/fax communications, video, security, and provide powerful, dependable private and public network communications solutions.
Generally, these systems operate in the Ku-band and C-band frequencies, and soon Ka-band. Ku-band based networks are used primarily in Europe and North America and utilize the smaller sizes of VSAT antennas. C-band, used extensively in Asia, Africa and Latin America, require larger antenna sizes. These are quite common in Cyber Cafes throughout the rest of the world.

VSAT networks can be configured to receive only or transmit and receive. Examples of uses we commonly see for receive only are:

• Stock market & other news broadcasting
• Training or continuing education from a distance
• Distribute financial trends & analyses
• Introduce new products at geographically dispersed locations
• Update market related data, news, and catalog prices
• Distribute video or TV programs (Directv and DISH)
• Distribute music in stores & public areas
• Relay advertising to electronic signs in retail stores.
Examples of uses we see for receive/transmit are:
• Interactive computer transactions
• Internet
• Distance Learning Video Teleconferencing
• Database inquiries
• Bank transactions, ATM
• Reservation systems
• Distributed remote process control and telemetry
• VoIP communications
• Airport flight and weather data
• Emergency services
• Electronic fund transfer at Point-of-Sale
• E-mail
• Medical data transfer
• Sales monitoring & stock control
• Surveillance and monitoring.
VSAT networks come in various shapes and sizes ranging from point-to-point, point-to-multipoint, and customized private hubs for thousands of sites. Mesh systems have traditionally been somewhat smaller in size than star systems—5 to 30 sites is a good rule of thumb.

Fundamentals Notes

 Generation of Computers, etc

Fundamentals Notes

Differences between Virtual Circuits and Datagram Networks

 

Differences between Virtual Circuits and Datagram Networks

 

Virtual Circuits	Datagram Networks
Virtual circuits are connection-oriented, which means that there is a reservation of resources like buffers, bandwidth, etc. for the time during which the newly setup VC is going to be used by a data transfer session.	It is connectionless service. There is no need for reservation of resources as there is no dedicated path for a connection session.
A virtual circuit network uses a fixed path for a particular session, after which it breaks the connection and another path has to be set up for the next the next session.	A Datagram based network is a true packet switched network. There is no fixed path for transmitting data.
All the packets follow the same path and hence a global header is required only for the first packet of connection and other packets will not require it.	Every packet is free to choose any path, and hence all the packets must be associated with a header containing information about the source and the upper layer data.
Packets reach in order to the destination as data follows the same path.	Data packets reach the destination in random order, which means they need not reach in the order in which they were sent out.
Virtual Circuits are highly reliable.	Datagram networks are not as reliable as Virtual Circuits.
Implementation of virtual circuits is costly as each time a new connection has to be set up with reservation of resources and extra information handling at routers.	But it is always easy and cost-efficient to implement datagram networks as there is no need of reserving resources and making a dedicated path each time an application has to communicate.

Operating System Notes PART 1 PDF

OS NOTES PART 4 

 

OS NOTES PART 3 

 

OS NOTES PART 2  

 

OS NOTES PART 1

 

OS NOTES NEXT

Computer Fundamentals PDF 3

 

Computer Fundamentals PDF Notes 2

 

Computer Fundamentals PDF Notes

 

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.