PROGRAMMING CONCEPT

PROGRAMMING CONCEPT

The earliest computers

It is an astonishing fact that as we enter the 21st century, computers has been around for less than 60 years. Many of those who worked on the earliest computer are still alive to tell us of their experiences. If you ever get a chance to visit Bletchley Park, don't miss it - it is the location of the breaking of the Enigma code (used by the Germans to code all their most secret messages), with the aid of the first programmable computer, named Colossus, built for this purpose in 1943. A fascinating exhibition the fie story and Colossus has been reconstructed there.

Generations of programming language

Machine language - the first generation

Programming languages are often characterised by 'generation'- The first generation of computer language, known as machine code, executes directly without translation. Machine code is the actual pattern of 0s and 1s used in a computer's memory. The programming of Colossus and other early computers was laboriously done with toggle switches representing a pattern of binary codes for each instruction.
Machine language, however it is entered into a computer, is time-consuming, laborious and error-prone. Few programmers code in it today. A language suited to fie particular application would be used instead.

Assembly code - the second generation

In the 1950s when computers were first used commercially, machine code gave way to assembly code, which allowed programmers to use mnemonics (abbreviations that represent the instructions in a more memorable way) and denary numbers (i-e.0-9) instead of 0s and 1s. Thus ADD or ADX might be an instruction to add two numbers, SIJB or SBX an instruction to subtract.
Programs written in assembly languages have to be translated into machine code before they can be executed, using a program called an assembler-There is more or less a one-to-one correspondence between each assembly code statement and its equivalent machine code statement, which means that programs can be written in the most efficient way possible, occupying as little space as possible and executing as fast as possible. For this reason assembly code is still used for applications where timing or storage space is critical. Assembly languages are called low level languages because they are close to machine code and the detail of the computer architecture.
Since different types of computer have different instruction sets, which depend on how the machine carries out the instructions, both machine code and assembly code are machine-dependent - each type of computer will have its own assembly language.

lmperative high level languages - the third generation

As computer use increased dramatically in the 1950s, the need grew to make it easier and faster to write error-free programs. Computer manufacturers and user groups started to develop so-called high-level languages such as Algol (standing for AlGOrithmic Language) and Fortran (standing for FORmula TRANslation).In the 1950s most of the people actually writing programs were scientists, mathematicians and engineers and so both these languages were created to be used in mathematical applications.
COBOL (Common Business Oriented Language) was invented by the redoubtable Admiral Grace Hopper in 1960 specifically for writing commercial and business, rather than scientific, programs.-Whilst serving in the US Navy in 1947, Grace Hopper was investigating why one of the earliest computers was not working, and discovered a small dead moth in the machine. After removing it (and taping it in her logbook) the machine worked fine, and from then on computer errors were known as 'bugs'.
Other third generation languages followed: BASIC was created in the 1960s as a language for students to learn programming. Early versions of the language however did not contain the facilities to write well-structured programs that were easy to maintain and debug, although the language has since developed. In I97l Nicklaus Wirth designed Pascal (named after the seventeenth century French mathematician) to teach structured programming to students.
High level languages are so-called because they are independent of the architecture of any particular computer; one statement written in a high level language is translated into several machine code instructions before it can be executed. The term imperative high level language refers to languages such as Pascal, BASIC, COBOL and FORTRAN - in contrast to object-oriented and declarative languages, which you will learn about in the second year of this course.

Fourth-Generation programming language

A fourth-generation programming language (4GL) is a programming language closer to human languages than typical high-level programming languages. Such languages arose after the introduction of modern, block-structured third-generation programming languages, which improved the process of software development. Most 4GLs are used to access databases. For example, a typical 4GL command is
SQL > SELECT * FROM tblStudent WHERE gender='Male'
Why use assembly code?
Assembly language, although it is laborious to write and hard to debug, is still used in some circumstances, for example:

• when there is a need for the program to execute as fast as possible;
• when the program must occupy as little space as possible;

Parts of an operating system, and device drivers that control the operation of devices such as a printer, mouse or CD-ROM may be written in assembly code. Programs in embedded systems like satellite decoders, encryption and decryption software, and routines that are called frequently from high-level programs may also be written in assembly code.

Types of program translator

There are three types of program used for translating the code that a programmer writes into a form (i.e. machine code) that the computer can execute. These are:

• assembler
• compiler
• interpreter.

Assembler

An assembler is a program that translates an assembly code program into machine code ready for the computer to execute it. Since each type of computer has its own assembly language, it also has its own assembler. The assembler itself could be written in assembly code or in a high level language such as C, which has special facilities useful for this type of programming.

Compiler

A compiler is a program that translates a high level language program into machine code. The Turbo Pascal compiler, for example, translates a program written in Turbo Pascal on a PC into object code, which can be run on a PC. The code written by the programmer is known as the source code, and the compiled code is known as the object code.
A compiler is a complex program which takes the source code and scans through it several times, each time performing different checks and building up tables of information needed to produce the fina1 object code. When you write a short program, this process appears to happen almost instantaneously, but a long program of several thousand lines can take several minutes to compile.
Compiler: Translates the whole high level language source code into object code, which can then be executed without the presence of a compiler.

Interpreter

An interpreter also translates high-level source code. However the crucial difference between a compiler and an interpreter is that an interpreter translates one line at a time and then executes it; no object code is produced, and so the program has to be interpreted each time it is to be run. If the program performs a section of code 10,000 times, then that section of code is translated into machine code 10,000 times as each line is interpreted and then executed.
Interpreter: Analyses the source code statement by statement as execution proceeds decoding each statement and calling routines to carry out each instruction.

Relative advantages of compilers and interpreters

A compiler has many advantages over an interpreter:

• the object code can be saved on disk and run whenever required without the need to recompile. However, if an error is discovered in the program, the whole program has to be recompiled.
• the object code executes faster than interpreted code.
• the object code produced by a compiler can be distributed or executed without having to have the compiler present.
• the object code is more secure, as it cannot be read without a great deal of 'reverse engineering'.

An interpreter has some advantages over a compiler:

• it is useful for program development as there is no need for lengthy recompilation each time an error is discovered.
• it is easier to partially test and debug programs.

Typically, a programmer might use an interpreter during program development. A program that is tested and ready for distribution would then be compiled and the saved object code would be distributed.

NETWORK PROTOCOL:

NETWORK PROTOCOL:

Internet Protocol Suite

Protocols are the sets of rules by which communication over a network is achieved. Protocols are responsible for enabling and controlling network communication: They set the rules for the representation of data, the signals used in communications (e.g., signals regarding how the connection will be established or how information is exchanged), the detection of errors, and the authentication of computing devices on the network.
In order for different computers to communicate with each other they must both speak the same language. The rules for these languages are called Network Protocols. A protocol describes how a packet of information is organized and the rules it follows when traveling across the network.

Types of Protocol

1. TCP/IP (Transmission Control Protocol/Internet Protocol)

TCP/IP is the basic communication language or protocol of the Internet. It can also be used as a communications protocol in a private network (either an intranet or an extranet). When you are set up with direct access to the Internet, your computer is provided with a copy of the TCP/IP program just as every other computer that you may send messages to or get information from also has a copy of TCP/IP.
TCP/IP is a two-layer program. The higher layer, TCP, manages the assembling of a message or file into smaller packets that are transmitted over the Internet and received by a TCP layer that reassembles the packets into the original message. The lower layer, IP, handles the address part of each packet so that it gets to the right destination. Each gateway computer on the network checks this address to see where to forward the message. Even though some packets from the same message are routed differently than others, they'll be reassembled at the destination.
TCP/IP uses the client/server model of communication in which a computer user (a client) requests and is provided a service (such as sending a Web page) by another computer (a server) in the network. TCP/IP communication is primarily point-to-point, meaning each communication is from one point (or host computer) in the network to another point or host computer.

2. HTTP(Hypertext Transfer Protocol)

HTTP is the set of rules for transferring files (text, graphic images, sound, video, and other multimedia files) on the World Wide Web. As soon as a Web user opens their Web browser, the user is indirectly making use of HTTP. HTTP is an application protocol that runs on top of the TCP/IP suite of protocols (the foundation protocols for the Internet).
HTTP concepts include the idea that files can contain references to other files whose selection will extract additional transfer requests. HTTP daemon, a program that is designed to wait for HTTP requests and handle them when they arrive. Your Web browser is an HTTP client, sending requests to server machines. When the browser user enters file requests by either "opening" a Web file (typing in a Uniform Resource Locator or URL) or clicking on a hypertext link, the browser builds an HTTP request and sends it to the Internet Protocol address (IP address) indicated by the URL. The HTTP daemon in the destination server machine receives the request and sends back the requested file or files associated with the request.

3. FTP(File Transfer Protocol)

A FTP better known as File Transfer Protocol basically is applied to transfer data from one particular computer to another above a world-wide network.
Usually, FTP is a primarily applied protocol for exchanging data over a particular network which normally supports TCP/IP protocol. Generally there are two types of computer occupied in an FTP data transfer they are the server and the client. The FTP server, administrating server software, depends on the network for network connection request from supplementary computers. The consumer computer, which is administrating the FTP client software, starts a connection to the server. As soon as the connection is started, the client can perform numerous file manipulations like uploading them, downloading them, giving them new names, etc. Generally any software company or programming individual can create an FTP server because the protocol is open standard.

4. SMTP (Simple Mail Transfer Protocol)

SMTP is a delivery protocol only. It cannot pull messages from a remote server on demand. Other protocols, such as the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP) are specifically designed for retrieving messages and managing mail boxes. However, SMTP has a feature to initiate mail queue processing on a remote server so that the requesting system may receive any messages destined for it. POP and IMAP are preferred protocols when a user's personal computer is only intermittently powered up, or Internet connectivity is only transient and hosts cannot receive message during off-line periods.

5. TELNET

Telnet is a user command and an underlying TCP/IP protocol for accessing remote computers. Through Telnet, an administrator or another user can access someone else's computer remotely. On the Web, HTTP and FTP protocols allow you to request specific files from remote computers, but not to actually be logged on as a user of that computer. With Telnet, you log on as a regular user with whatever privileges you may have been granted to the specific application and data on that computer.
The result of this request would be an invitation to log on with a userid and a prompt for a password. If accepted, you would be logged on like any user who used this computer every day.Telnet is most likely to be used by program developers and anyone who has a need to use specific applications or data located at a particular host computer.

E-MAIL

E-MAIL

E-mail (electronic mail) is the exchange of computer-stored messages by telecommunication. E-mail messages are usually encoded in ASCII text. However, you can also send non-text files, such as graphic images and sound files, as attachments sent in binary streams. E-mail was one of the first uses of the Internet and is still the most popular use. A large percentage of the total traffic over the Internet is e-mail. E-mail can also be exchanged between online service provider users and in networks other than the Internet, both public and private.

E-mail can be distributed to lists of people as well as to individuals. A shared distribution list can be managed by using an e-mail reflector. Some mailing lists allow you to subscribe by sending a request to the mailing list administrator. A mailing list that is administered automatically is called a list server.

E-mail is one of the protocols included with the Transport Control Protocol/Internet Protocol (TCP/IP) suite of protocols. A popular protocol for sending e-mail is Simple Mail Transfer Protocol and a popular protocol for receiving it is POP3. Both Netscape and Microsoft include an e-mail utility with their Web browsers.

Advantages of Email

The benefits of e-mail are huge in number.

• Easy to use: E-mail frees us from the tedious task of managing data of daily use. It helps us to manage our contacts, send mails quickly, maintain our mail history, store the required information, etc.
• Speed: The e-mail is delivered instantly, anywhere across the globe. No other service matches the e-mail in terms of speed.
• Easy to prioritize: Since the mails have subject lines, it is easy to prioritize them and ignore unwanted mails.
• Reliable and secure: Constant efforts are being taken to improve the security in electronic mails. Thus making it one of the secured ways of communication.
• Informal and conversational: The language used in e-mails is generally simple and thus makes the communication informal. Sending and receiving e-mails takes less time, so it can be used as a tool for interaction.
• Easier for reference: When one needs to reply to a mail, there is a provision in the mailing system to attach the previous mails as references. This refreshes the recipient's knowledge, on what he is reading.
• Automated e-mails: It is possible to send automated e-mails using special programs like the autoresponders. The autoresponders reply back to the sender with generalized pre-written text messages.
• Environment friendly: Postal mails use paper as a medium to send letters. Electronic mail thus, saves a lot of trees from being axed. It also saves fuel needed in transportation.
• Use of graphics: Colorful greeting cards and interesting pictures can be sent through e-mails. This adds value to the e-mail service.
• Advertising tool: Many individuals and companies are using e-mails to advertise their products, services, etc.

Disadvantages of Email

The e-mails, though beneficial in our day-to-day life, has got its own drawbacks that are off late coming to the fore.

• Viruses: These are computer programs having the potential to harm a computer system. These programs copy themselves and further infect the computer. The recipient needs to scan the mails, as viruses are transmitted through them and have the potential to harm computer systems.
• Spam: E-mails when used to send unsolicited messages and unwanted advertisements create nuisance and is termed as Spam. Checking and deleting these unwanted mails can unnecessarily consume a lot of time, and it has become necessary to block or filter the unwanted e-mails by means of spam filters. Spamming includes, sending hoax e-mails. E-mail spoofing is another common practice, used for spamming. Spoofing involves deceiving the recipient by altering the e-mail headers or the addresses from which the mail is sent.
• Hacking: The act of breaking into computer security is termed as hacking. After the e-mail is sent and before it is received by the desired recipient, it "bounces" between servers located in different parts of the world. Hence, the e-mail can be hacked by a professional hacker.
• Misinterpretation: One has to be careful while posting any kind of content through an e-mail. If typed in a hurry, the matter could be misinterpreted.
• Lengthy mails: If the mail is too long and not properly presented the reader may lose interest in reading it.
• Not suitable for business: Since the content posted via e-mails is considered informal, there is a chance of business documents going unnoticed. Thus, urgent transactions and especially those requiring signatures are not managed through e-mails.
• Crowded inbox: Over a period of time, the e-mail inbox may get crowded with mails. It becomes difficult for the user to manage such a huge chunk of mails.

SEARCH ENGINES

SEARCH ENGINES

A web search engine is designed to search for information on the World Wide Web and FTP servers. The search results are generally presented in a list of results and are often called hits. The information may consist of web pages, images, information and other types of files. Some search engines also mine data available in databases or open directories. Unlike web directories, which are maintained by human editors, search engines operate algorithmically or are a mixture of algorithmic and human input.

List of Search Engines

1. Google (http://www.google.com)

Voted four times Most Outstanding Search Engine by Search Engine Watch readers, Google has a well-deserved reputation as the top choice for those searching the web. The crawler-based service provides both comprehensive coverage of the web along with great relevancy. It's highly recommended as a first stop in your hunt for whatever you are looking for.

2. Yahoo (http://www.yahoo.com)

Launched in 1994, Yahoo is the web's oldest "directory," a place where human editors organize web sites into categories. However, in October 2002, Yahoo made a giant shift to crawler-based listings for its main results. These came from Google until February 2004. Now, Yahoo uses its own search technology

3. Ask (http://www.ask.com)

Ask Jeeves initially gained fame in 1998 and 1999 as being the "natural language" search engine that let you search by asking questions and responded with what seemed to be the right answer to everything.

4. AltaVista (http://www.altavista.com)

AltaVista opened in December 1995 and for several years was the "Google" of its day, in terms of providing relevant results and having a loyal group of users that loved the service. Sadly, an attempt to turn AltaVista into a portal site in 1998 saw the company lose track of the importance of search. Today, AltaVista is once again focused on search. Results come from Yahoo, and tabs above the search box let you go beyond web search to find images, MP3/Audio, Video, human category listings and news results.

5. Live Search (http://www.live.com/)

Live Search (formerly Windows Live Search) is the name of Microsoft's web search engine, successor to MSN Search, designed to compete with the industry leaders Google and Yahoo. The search engine offers some innovative features, such as the ability to view additional search results on the same web page (instead of needing to click through to subsequent search result pages) and the ability to adjust the amount of information displayed for each search-result (i.e. just the title, a short summary, or a longer summary). It also allows the user to save searches and see them updated automatically on Live.com. Now a days this name is changed int BING.COM

FEW TERM OF INTERNET

FEW TERM OF INTERNET

IP Address ( Internet Protocol Address)

An IP Address is a usually numerical label assigned to each device (e.g., computer, printer) participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: network interface identification and location addressing.
In order for you to receive snail mail at home the sending party must have your correct mailing address (IP address) in your town (network) or you do not receive letter. The same is true for all equipment on the internet. Without this specific address, information cannot be received.

Domain Name System (DNS):

DNS implements a distributed database to store this name and address information for all public hosts on the Internet. DNS allows the IP address to be translated to words. It is much easier for us to remember a word than a series of numbers. The same is true for email addresses.
For example, it is much easier for you to remember a web address name such as mystudyroom.com.np  than it is to remember 192.168.1.1 or in the case of email it is much easier to remember email@gmail.com than email@192.168.1.1

Client-Server Architecture

A network architecture in which each computer or process on the network is either a client or a server. Servers are powerful computers or processes dedicated to managing disk drives (file servers), printers (print servers), or network traffic (network servers). Clients are PCs or workstations on which users run applications. Clients rely on servers for resources, such as files, devices, and even processing power.
The Internet revolves around the client-server architecture. Your computer runs software called the client and it interacts with software known as the server located at a remote computer. The client is uses a browser such as Internet Explorer or Mozilla. User type web address to open a site like mystudyroom.com.np, user's request is sent to the website's server and then users are able to view the website. This is how client and server work together.

World Wide Web (WWW)

WWW is collection of internet resources (such as FTP, telnet, Usenet), hyperlinked text, audio, and video files, and remote sites that can be accessed and searched by browsers based on standards such as HTTP and TCP/IP. Also called the web as an easier way to access information scattered across the internet.
The WWW is a spiderweb-like interconnection of millions of pieces of information located on computers around the world. Web documents use hypertext, which incorporates text and graphical links to other documents and files on Internet-connected computers.

Static and Dynamic Pages

Web pages can be either static or dynamic. "Static" means unchanged or constant, while "dynamic" means changing or lively. Therefore, static Web pages contain the same prebuilt content each time the page is loaded, while the content of dynamic Web pages can be generated on-the-fly.
Standard HTML pages are static Web pages. They contain HTML code, which defines the structure and content of the Web page. Each time an HTML page is loaded, it looks the same. The only way the content of an HTML page will change is if the Web developer updates and publishes the file.
Other types of Web pages, such as PHP, ASP, and JSP pages are dynamic Web pages. These pages contain "server-side" code, which allows the server to generate unique content each time the page is loaded. For example, the server may display the current time and date on the Web page. It may also output a unique response based on a Web form the user filled out. Many dynamic pages use server-side code to access database information, which enables the page's content to be generated from information stored in the database. Websites that generate Web pages from database information are often called database-driven websites.
You can often tell if a page is static or dynamic simply by looking at the page's file extension in the URL, located in the address field of the Web browser. If it is ".htm" or ".html," the page is probably static. If the extension is ".php," ".asp," or ".jsp," the page is most likely dynamic. While not all dynamic Web pages contain dynamic content, most have at least some content that is generated on-the-fly.

Advantages of static websites

• Quick to develop website
• Cheap to develop and host website

Disadvantages of static websites

• Requires web development expertise to update site
• Site not as useful for the user
• Content can get stagnant

Advantages of dynamic websites

• Much more functional website
• Much easier to update
• New content brings people back to the site and helps in the search engines
• Can work as a system to allow staff or users to collaborate

Disadvantages of dynamic websites

• Slower / more expensive to develop
• Hosting costs a little more

TYPE OF TRANSMISSION

TYPE OF TRANSMISSION

Analog and Digital Transmission

Analog Transmission

Analog transmission uses signals that are exact replicas of a sound wave or picture being transmitted. Signals of varying frequency or amplitude are added to carrier waves with a given frequency of electromagnetic current to produce a continuous electric wave. The term "analog signal" came about because the variations in the carrier waves are similar, or analogous, to that of the voice itself.
For example, in analog transmission, say a telephone system, an electric current or the reproduction of patterned sound waves are transmitted through a wire and into the telephone receiver. Once this is completed, they are then converted back into sound waves.

Digital Transmission

In digital transmission the signals are converted into a binary code, which consists of two elements-positive and non-positive. Morse code and the "on and off" flashing of a light are basic examples. Positive is expressed as the number 1, while non-positive is expressed as the number 0. Numbers that are expressed as a string of 0s and 1s are called binary numbers. Every digit in a binary number is referred to as a bit .
As an example of digital transmission, in a type of digital telephone system, coded light signals produced by a rapidly flashing laser travels through optical fibers (thin strands of glass) and are then decoded by the receiver. When transmitting a telephone conversation, the light flashes on and off about 450 million times per second. This high rate enables two optical fibers to carry about 15,000 conversations simultaneously.

TOPOLOGY

TOPOLOGY

Bus Topology

Ring Topology

Star Topology

Tree Topology

Mesh Topology

What is a Topology?

The physical topology of a network refers to the configuration of cables, computers, and other peripherals. Physical topology should not be confused with logical topology which is the method used to pass information between workstations.

Main Types of Physical Topologies

The following sections discuss the physical topologies used in networks and other related topics.

• Bus Topology
• Ring Topology
• Star Topology
• Tree Topology
• Mesh Topology

1. Bus Topology

A linear bus topology consists of a main cable in middle where nodes(file server, workstations, and peripherals) are connected. Terminator is connected at each end. T-Connector is used in the cable to add new nodes in the network.
 Advantages

• Easy to connect a computer or peripheral to a linear bus.
• Requires less cable length than a star topology.

Disadvantages of a Linear Bus Topology

• Entire network shuts down if there is a break in the main cable.
• Terminators are required at both ends of the backbone cable.
• Difficult to identify the problem if the entire network shuts down.
• Not meant to be used as a stand-alone solution in a large building.

2. Ring Topology

A ring topology is a network topology or circuit arrangement in which each network device is attached along the same signal path to two other devices, forming a path in the shape of a ring. Each device in the network that is also referred to as node handles every message that flows through the ring. Each node in the ring has a unique address. Since in a ring topology there is only one pathway between any two nodes, ring networks are generally disrupted by the failure of a single link.
Ring topologies are used on token ring networks. Each device processes and retransmits the signal, so it is capable of supporting many devices in a somewhat slow but very orderly fashion. A token, or small data packet, is continuously passed around the network. When a device needs to transmit, it reserves the token for the next trip around, then attaches its data packet to it. The receiving device sends back the packet with an acknowledgment of receipt, then the sending device puts the token back out on the network.
Advantages

• An orderly network where every device has access to the token and the opportunity to transmit
• Under heavy network load performs better than a start topology.
• To manage the connectivity between the computers it doesn't need network server.

Disadvantages

• One malfunctioning workstation can throw away the entire network.
• Moves, adds and changes of devices can affect the entire network.

3. Star Topology

A star topology is designed with each node (file server, workstations, and peripherals) connected directly to a central network hub or switch.
Data on a star network passes through the hub or switch before continuing to its destination. The hub manages and controls all functions of the network. It also acts as a repeater for the data flow. This configuration is common with twisted pair cable; however, it can also be used with coaxial cable or fiber optic cable.
Advantages of a Star Topology

• Easy to install and wire.
• No disruptions to the network when connecting or removing devices.
• Easy to detect faults and to remove parts.

Disadvantages of a Star Topology

• Requires more cable length than a linear topology.
• If the hub, switch, or concentrator fails, nodes attached are disabled.
• More expensive than linear bus topologies because of the cost of the hubs, etc.

4. Tree Topology or Expanded Star

A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable .  Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs.
Advantages of a Tree Topology

• Point-to-point wiring for individual segments.
• Supported by several hardware and software venders.

Disadvantages of a Tree Topology

• Overall length of each segment is limited by the type of cabling used.
• If the backbone line breaks, the entire segment goes down.
• More difficult to configure and wire than other topologies.

5. Mesh Topology

Mesh Network is a network where all the nodes are connected to each other and is a complete network. In a Mesh Network every node is connected to other nodes on the network through hops. Some are connected through single hops and some may be connected with more than one hope.
While the data is traveling on the Mesh Network it is automatically configured to reach the destination by taking the shortest route which means the least number of hops. Data travels by hopping from one node to another and then reaches the destination node in a Mesh Topology Network. An example of a Mesh Network is the Internet.

6. Hybrid Topology

Hybrid topology is the combination of two or more topologies together. It has the advantage of more than one topologies as well as disadvantages, too.

COMMUNICATION MEDIUM

COMMUNICATION MEDIUM

Twisted Pair

Coaxial Cable

Fiber Optics

Microwave and Satellite Transimission

• Guided Media

• Unguided media

Guided Media

Guided Transmission Media uses a "cabling" system that guides the data signals along a specific path. The data signals are bound by the "cabling" system. Guided Media is also known as Bound Media. Cabling is meant in a generic sense in the previous sentences and is not meant to be interpreted as copper wire cabling only. Some of the guided media used in networking is as following:

• Twisted Pair
• Coaxial Cable
• Fiber Optics

1. Twisted Pair

It is one types of copper cables, it consists of two insulated copper wires that are twisted together in helical form. There are two types of twisted pairs, Unshielded Twisted Pair (UTP) cable and Shielded Twisted Pairs (STP). Both of types are used Ethernet LANs, they consists of four pairs of color coded wires that have been twisted together and then encased in a flexible plastic sheath.
The color codes identify the individual pairs and wires in the pairs and aid in cable termination. The purpose of twisting the wire is to reduce electrical interference from similar pairs closed by.
Twisted pairs can run approximately 100 meter without amplification, but for longer distances, repeaters are needed. Twisted pairs can be used for either analogue or digital transmission. The bandwidth depends on the thickness of wire and the distance traveled. Twisted pair can carry 10Mbps, 100Mbps or 1000MBps, this variation depends on the category of cable (Cables are placed into categories according to their ability to carry higher bandwidth rates, For example, Category 5 (Cat5), Category 6 (Cat6).

2. Coaxial Cable

It is one type of copper cables. It consists of a stiff copper conductor as a core surrounded by a layer of flexible insulation, the insulator is incased by cylindrical conductor often as a closely woven braided mesh. The outer conductor is covered in a protective plastic sheath.
The constriction and shielding of coaxial cable give it a good combination of high bandwidth and excellent noise immunity. The bandwidth possible depends on the cable length. For 1-Km cables, a data rate of 1 to 2 Gbps is feasible. Longer cables can also be used but at lower data rate or with periodic amplifiers.
At one time, coaxial cable was the most widely used network cabling. There were a couple of reasons for coaxial wide usage. Coaxial was relatively inexpensive, and it was light, flexible, and easy to work with. It was so popular that it became a safe, easily supported installation. Coaxial cables are widely used in telephone system but have now largely replaced by fiber optics on long-land routers. However coaxial cables are still widely used for TV cables and some LAN. In Nepal SUBISU Network company is using this cable to supply TV channels and Internet.

3. Fiber Optics

Fiber-optic cabling uses either glass or plastic fibers to guide light impulses from source to destination. The bits are encoded on the fiber as light impulses. Optical fiber cabling is capable of very large raw data bandwidth rates. With current optical fiber technology, the achievable bandwidth is certainly in excess of 50,000 Gbps (50 Tbps).
An optical transmission system has three components the light source, the transmission medium and the detector. The pulse of light indicates "1" bit and the absence of light indicates a"0" bit. The transmission medium is ultrathin fiber of glass. The light source attached to one end, the light moves over the fiber optics, the detector will generate an electrical signal when the light falls on it.
By attaching a light source to one end of an optical fiber and a detector to the other, we have a unidirectional data transmission system that accept an electrical signal, convert and transmits it by light pulses, and then reconverts the output to an electrical signal at the receiving end.

Unguided Media

Unguided Transmission Media consists of a means for the data signals to travel but nothing to guide them along a specific path. The data signals are not bound to a cabling media and as such are often called Unbound Media. Some of the unguided media used in networking is as following:

• Microwave System
• Communication Satellites

1. Microwave Transmission

Above the 100 MHz, the waves travel in straight lines and can therefore be narrowly focused. Concentrating all energy into a small beam using a parabolic antenna gives a much higher signal to noise ratio, but the transmitting and receiving antennas must be accurately aligned with each other. In addition, this directionality allows multiple transmitters lined up in a raw to communicate with multiple receivers in a raw without interference. Before fiber optics, for decades these microwaves formed the heart of long distance telephone transmission systems.
Since the microwaves travel in a straight line, if the towers are far apart, the earth will get in the way. Consequently, repeaters are needed periodically. The distance between repeaters goes up very rough, with the tower height. For 100 M tower, repeater can be spaced 80 Km a part. Unlike radio waves at lower frequencies, microwave does not pass through building wall. In addition, even through the beam may be well focused at the transmitter; there is still some divergence in space.

2. Communication Satellite

Communication satellites have some interesting properties that make them attractive for many applications. A Communication satellite can be thought to be as big microwave repeater in the sky. It contains several "Transponders", each of which listens to some portion of the spectrum, amplify the incoming signal and then rebroadcast it at another frequency to avoid interference with the incoming signal. The down word beams can be broad covering a substantial fraction of the earth surface.

Satellite versus Fiber

• The bandwidth of Fiber is more than the bandwidth of satellite.
• For mobile communication, fiber optics link are of no use to them, but Satellite links are potentially are.
• The Satellite has a broadcast nature.
• With Satellite, communication is available in a hostile terrain or poorly developed terrestrial infrastructure.

TYPES OF NETWORK

TYPES OF NETWORK

Local Area Network (LAN)

Metropolitan Area Network

Wide Area Network (WAN)

LAN (Local Area Network):

A group of computers and other devices dispersed over a relatively limited area and connected by a communications link that enables any device to interact with any other on the network. LANs commonly include microcomputers and shared resources such as laser printers and large hard disks. The devices on a LAN are known as nodes, and the nodes are connected by cables through which messages are transmitted. LAN have data rate in the 4 to 16 Mbps range and can reach 100 Mbps.

Advantages of LAN

• The reliability of network is high because the failure of one computer in the network does not effect the functioning for the other computer.
• Addition of a new computer to network is easy.
• High rate of data transmission is possible.
• Other computers can share peripheral devices like magnetic disk and printer.

Disadvantage of LAN

• Of the communication fail, the entire network system breaks down.
• User authority and user identification is poor.

MAN (Metropolitan Area Network):

Metropolitan Area Network. A high-speed network that can carry voice, data, and images at up to 200 Mbps or faster over distances of up to 75 km. Based on the network architecture, the transmission speed can be higher for shorter distances. A MAN, which can include one or more LANs as well as telecommunications equipment such as microwave and satellite relay stations, is smaller than a WAN but generally operates at a higher speed.

WAN (Wide Area Network):

A communications network that connects geographically separated areas. In the other word the term WAN (Wide area network) is used to describe a computer network spanning a regional, national and global area. It provides long-distance transmission of data, voice, image, and video information over a large geographical area that may comprise a country, a continent or even the world wide.