Definition of Computer Science:

 The study of the design and operation of computer hardware and software.

A branch of science that deals with the theory of computation or the design of computers.

The computer science discipline concerned with developing large applications.

Software Engineering: A branch of computer science that deals with the design,

implementation, and maintenance of complex computer programs.

Software engineering (SE) is concerned with developing and maintaining software systems that behave reliably and efficiently, are affordable to develop and maintain, and satisfy all the requirements that customers have defined for them.

Software engineering covers not only the technical aspects of building software systems, but also management issues, such as directing programming teams, scheduling, and budgeting.

Definition of Information Technology: The study or use of systems (esp. computers and

telecommunications) for storing, retrieving, and sending and manipulating information.

Information Technology is a term that covers all forms of technology used to create, store, transmit, interpret and manipulate information in its various formats. (e.g. business data, voice conversations, pictures and multimedia presentation etc.)

Processing and distribution of data using people, hardware, software, computer networking and telecommunications.

Difference Between Computer Science & Information Technology:

Computer science is the systematic engineering process of software or hardware, while

Information technology is referred as the use of such products to share the information fast and efficiently to make the computers more easy and useful to the users.

COMPUTER NETWORK:A collection of computers and other devices that are connected together by communication channels for sharing information and resources is called computer network.

The resources may include printers, scanners, and hard disks etc.

Computer networks support applications such as access to the World Wide Web, shared use of application and storage servers, printers, and fax machines, and use of email and instant messaging applications.

Uses / Advantages of Computer Networks:

The main benefits or uses of computer network are:

Communication Sharing Resources Sharing Software Data Sharing

1- Communication:Using a network, different people can communicate with each other all over the world. People can communicate at very low cost via e-mail, chatting, telephone, video telephone, video conferencing, groupware, and SMS services etc.2- Sharing Resources:In a computer network, resources such as, printers, scanners, fax machines and modems can be shared among different users. Suppose several personals computers and a laser printer are connected to a network. Each users can access the printer.

3- Sharing Software:In a computer network, usually application programs and other software are stored on the central computer. Users connected to a network can access these programs or software.4- Data Sharing:In a network environment, any authorized user can access data stored on other computers on the network. For example, on the Internet, a large number of Internet users can access same database.

COMPUTER NETWORK TOPOLOGIES:

Network topology means, “the ways in which wires (or other media) can be run to link those computers together”. Network topology defines the structure of the network.

The four basic and common topologies are:

Bus Star Ring Mesh (Hybrid)

Bus Topology:The bus topology is often used when a network installation is small, simple or temporary.

“A bus topology uses a single backbone cable that is terminated at both ends and all the hosts (computers) are connected directly to this backbone”.

Advantages of the Bus:The bus is simple, reliable in very small networks, easy to use, and easy to understand.The bus requires the least amount of cable to connect the computers together and is therefore less expensive than other cabling arrangements.It is easy to extend a bus. Two cables can be joined into one longer cable with a BNC barrel connector, making a longer cable and allowing more computers to join the network. Disadvantages of the Bus:Heavy network traffic can slow a bus considerably.Each barrel connector weakens the electrical signal, and too many more prevent the signal from being correctly received all along the bus.It is difficult to troubleshoot a bus. A cable break or malfunctioning computer anywhere between two computers can cause them not to be able to communicate with each other. A cable break or loose connector will also cause reflections and bring down the whole network, causing all network activity to stop.

Star Topology:

In a star topology, “all the cables run from the computers to a central location/point, where they are all connected by a device called a hub”. 

Advantages of the Star:It is easy to modify and add new computers to a star network without disturbing the rest of the network. You simply run a new line from the computer to the central location and plug it into the hub. When the capacity of the central hub is exceeded, you can replace it with one that has a larger number of ports to plug lines into.The center of a star network is a good place to diagnose network faults.Single computer failures do not necessarily bring down the whole network. The hub can detect a network fault and isolate the offending computer or network cable and allow the rest of the network to continue operating.

Disadvantages of the Star: If the central hub fails, the whole network fails to operate.

Many star networks require a device at the central point to rebroadcast or switch network traffic.

It costs more to cable a star network because all network cables must be pulled to one central point, requiring more cable than other networking topologies.

Ring Topology: A ring topology connects one node/host to the next

and the last node/host to the first. This creates a physical ring of cable.

Advantages of the Ring:In ring, every computer is given equal access to the token, no one computer can monopolize the network.

Disadvantages of the Ring:Failure of one computer on the ring can affect the whole network.It is difficult to troubleshoot a ring network.Adding or removing computers disturbs the network.

Mesh Topology: A mesh topology is implemented to provide as much

protection as possible from interruption of service. “Each host has its own connections to all other

hosts; the mesh topology is distinguished by having redundant links between devices”.

Advantages of Mesh:A mesh topology is fault tolerance. A mesh topology guarantees communication channel capacity.Disadvantages of Mesh:The difficulty of installation and reconfiguration, as well as the cost of maintenance redundant links.

NETWORK TYPES (BY AREA): One way to categorize the different types of computer

network designs is by their scope or area.

The networking industry refers to nearly every type of design as some kind of area network.

LAN and WAN are the original categories of area networks, while the others have gradually emerged over many years of technology evolution.

WLAN, MAN, PAN, SAN, HAN etc. are other types of network by area.

Note: These network types are a separate concept from network topologies such as bus, ring, star and mesh.

LAN- LOCAL AREA NETWORK LAN is the Local Area Network that allows the computer to be

connected with the network within the specified and limited distance/boundaries up to hundred’s meters ( or within a small geographic area e.g. within an office/home building).

In addition to operating in a limited space, LANs are also typically owned, controlled, and managed by a single person or organization.

The LAN connection can be used by the schools, universities, colleges and on a small scale level too. Hundreds or thousands of computers may be connected through LAN.

As the connection is used on a smaller level of organization so it had been found out that the speed of the connection is quite faster.

The local area network connection can be of different types but the major networking technologies among LAN are the token- ring, the Ethernet and FDDI (Fiber Distributed Data Interface).

The speed of communications between any two devices on an Ethernet LAN can be 2 to 1000 millions bits per second (Mbps).

A LAN can be one of two kinds: wired or wireless.

WAN – WIDE AREA NETWORK As the term implies, a WAN spans a large physical

distance up to kilometers.

A WAN is a geographically-dispersed collection of LANs.

A WAN differs from a LAN in several important ways. Most WANs (like the Internet) are not owned by any one organization but rather exist under collective or distributed ownership and management.

The Internet is the largest WAN, spanning on the Earth.

WANs tend to use technology like ATM (Asynchronous Transfer Mode), Frame Relay and X.25 for connectivity over the longer distances.

NETWORK TECHNOLOGY/ARCHITECTURE:

Network technologies are designed to solve specific problems of the computer on network.

As networking technology improves and new uses for computers place different demands on networks.

The number of technologies grows and networks are became more complex and sophisticated.

The Ethernet, Token Ring and FDDI are popular LAN technologies, while ATM, Frame Relay and X.25 are WAN technologies, that are mostly used.

NIC- Network Interface Card A network interface card (NIC) is a printed circuit board that provides

network communication capabilities to and from a personal computer, also called a LAN card/adapter.

Network adapters perform all the functions required to communicate on the network. They convert data from the form stored in the computer to the form transmitted or received on the cable and provide a physical connection to the network.

NICs are peripheral cards that plug into the motherboard of your computer and into a network, with the help of a network cable.

Some computers have built in Ethernet network interfaces. Network cards/adapters (e.g. Ethernet, Token Ring, Fiber-optic Ethernet and

Microwave network interfaces etc.) are used in networks, depends upon type of network you are using.

A network adapter/card must match both the bus of the computer, it is placed in and the network to which it will be attached.

Most modern computers have both an ISA and a PCI bus. For Ethernet and Token Ring, ISA is sufficiently fast, but for all higher-speed networks you should only use the PCI bus.

In general, use the fastest bus in your computer for your network connection to ensure that the bus does not become a bottleneck.

ETHERNET: In 1972, Robert Metcalfe and David Boffs at Xerox implemented the network

architecture with a cabling and signaling scheme.

In 1975, they introduced the first Ethernet product. This original Ethernet could connect over 100 computers at just under 3 Mbps over a distance of up to one kilometer.

Ethernet is the most widely-installed local area network (LAN) technology, specified in a standard IEEE 802.3 (IEEE Stands for Institute of Electrical and Electronics Engineers).

An Ethernet LAN typically uses coaxial cable, optical fiber and special grades of twisted pair wires.

Ethernet is also used in wireless LANs.

The most commonly installed Ethernet systems are called 10BASE-T and provide transmission speeds up to 10 Mbps.

Devices are connected to the cable and compete for access using a Carrier Sense Multiple Access with Collision Detection (CSMA/CD ) protocol, this means that only one workstation can use network at a time. Using the rules of this model, the workstations must contend for the opportunity to transmit across the network. For this reason, Ethernet is referred to as a contention-based system.

TOKEN RING: A Token Ring network is Local Area Network (LAN) in which all

computers are connected in a ring or star topology and a bit- or token-passing scheme is used in order to prevent the collision of data between two computers that want to send messages at the same time. The token scheme can also be used with bus topology LANs.

The Token Ring protocol is the second most widely-used protocol on local area networks after Ethernet.

The IEEE 802.5 Token Ring technology provides for data transfer rates of either 4 or 16 megabits per second.

Token Ring comes in standard 4 and 16 Mbps and high-speed Token Ring at 100Mbps (IEEE 802.5t) and 1Gbps (IEEE 802.5v). Token Ring does come with a higher price tag because token ring hardware is more complex and more expensive to manufacture.

The Fiber Distributed-Data Interface (FDDI) also uses a Token Ring protocol.

FDDI: FDDI (Fiber Distributed Data Interface) is a set of ANSI (American

National Standards Institute) and ISO (International Organization for Standardization ) technology for data transmission on fiber optic lines in a local area network (LAN).

Although, FDDI was also later specified to use copper cable, in which case it may be called CDDI (Copper Distributed Data Interface), standardized as TP-PMD (Twisted-Pair Physical Medium-Dependent), also referred to as TP-DDI (Twisted-Pair Distributed Data Interface).

The FDDI protocol is based on the token ring protocol.

The FDDI network contains two rings, one as a secondary backup in case the primary ring fails. The primary ring offers up to 100 Mbit/s capacity. When a network has no requirement for the secondary ring to do backup, it can also carry data, extending capacity to 200 Mbit/s.

FDDI is frequently used on the backbone for a wide area network (WAN).

MEDIUM, MEDIA AND N/W MEDIA A medium is a third-party or element through which a

message is communicated.

Media is a Plural of medium.

A network media is a physical channel used for transmission in the network. Wire, fiber and air are the three media.

In computer networks, media refers to the cables linking hosts/nodes/Pc’s together. There are many different types of cabling transmission media, the most popular are: twisted-pair cable (normal electrical wire), coaxial cable (the type of cable used for cable television), fiber optic cable (cables made out of glass).

TWISTED PAIR CABLES: Twisted-pair cable is a type of cabling that is used for

telephone communications and most modern Ethernet networks.

A pair of wires forms a circuit that can transmit data. The pairs are twisted to provide protection against crosstalk, the noise generated by adjacent pairs.

Twisted pair cables were invented by Alexander Graham Bell in 1881.

Today, most of the millions of kilometers of twisted pairs in the world are outdoor landlines, owned by telephone companies, used for voice service, and only handled or even seen by telephone workers.

Types of Twisted Pair: The two basic types of twisted-pair cable are:

Unshielded Twisted Pair (UTP) Shielded Twisted Pair (STP)

UTP CABLE: UTP cable is a medium that is composed of pairs of wires. UTP cable has group of 2,4,6,8 copper wires with colorful plastic coating.

UTP cable is used in a variety of networks. Each of the individual copper wires in UTP cable is covered by an

insulating material. In addition, the wires in each pair are twisted around each other.

STP CABLE: STP cable combines the techniques of shielding, cancellation, and

wire twisting. Each pair of wires is wrapped in a metallic foil. The four pairs of wires then are wrapped in an overall metallic braid

or foil. As specified for use in Ethernet network installations, STP reduces

electrical noise both within the cable (pair-to-pair coupling, or crosstalk) and from outside the cable (EMI).

STP prevents interference better than UTP, it is more expensive and difficult to install. Because of its cost and difficulty in installation, STP is rarely used in Ethernet networks.

Types of UTP/STP:Following are the commonly used types of UTP and STP cable:

Cat1: used for telephone communications. Not suitable for transmitting data.Cat2: Capable of transmitting data at speeds up to 4 megabits per second (Mbps).Cat3: Used in 10BASE-T networks. Can transmit data at speeds up to 10 Mbps.Cat 4: Used in Token Ring networks. Can transmit data at speeds up to 16 Mbps.Cat5: Can transmit data at speeds up to 100 Mbps.Category 5e: Used in networks running at speeds up to 1000 Mbps (1 gigabit per second [Gbps]).Cat6: Typically, Category 6 cables are currently the fastest standard for UTP.

Coaxial Cable Coaxial cable is one of the most common types of transmission media

used to create networks. Coaxial cable has a core of solid copper wire surrounded by a

layer of plastic. An outer layer of metal mesh or foil surrounds the plastic layer. The entire cable is then covered in a protective plastic coating.

Coaxial cable must be linked using devices called British Naval Connectors (BNC). A BNC connector that links a computer or other device to a coaxial cable is called a T connector.

Most coaxial networks transmit information at speeds of up to 10 megabits per second (Mbps), although thicker coaxial cable can transmit information must faster.

Coaxial is inexpensive than fiber-optics but more expensive than Twisted Pair.

Fiber-optic cable:

Fiber-Optic cable is made of glass or plastic. Fiber-optic cable uses light signals to transfer information through a network.

Information is encoded onto a beam of light as a series of ON-OFF flashes that represents 1 and 0 bits.

Fiber-optic cable transmits light signals through a core made of glass or plastic. The core is surrounded with plastic to protect it from damage and signal loss. The cable is then covered with a plastic coating.

The light is guided down the center of the fiber called CORE that is surrounded by an optical material called CLADDING, that trap the light in the CORE using an optical technique called TOTAL INTERNAL REFLECTION. The fiber is coated with a protective plastic covering/coating, also called the PRIMARY BUFFER COATING, which protects it from moisture and other damage.

Features/advantages: Interference: Fiber-optic cable does not emit electrical

signals and is not susceptible to interference from other devices, such as photocopiers. Due to these factors, fiber-optic cable is ideal for companies concerned about security.

Bandwidth: Fiber-optic cable can transfer information at speeds of over 100 Mbps. Some types of fiber-optic cables are capable of transferring information at well over 2 Gbps, but equipment that can utilize such a high rate of speed is not readily available.

Signal Degradation: With many types of transmission media, the farther a signal travels over the medium, the weaker the signal becomes. Signals transmitted through fiber-optic cable are not greatly affected by attenuation. It is possible to transmit signals for many miles without any detectable degradation in the signal.

Cost: Fiber-optic cable and related equipment are very expensive to purchase and install. Fiber-optic cable is usually used as the main cable, or the backbone, of a network because of the high cost.

Some Networking Equipment's: Hub:All networks (except those using coaxial cable) require a central location to bring media segments together. These central locations are called hubs.The easiest way to understand this concept is to think of the necessity of connecting multiple cables. If you just connected the media segments together by soldering them, the signals would interfere with each other and create problems. “Hubs concentrate connections, it organizes the cables and relays signal to the other media segments”. In other words, they take a group of hosts and allow the network to see them as a single unit.

Switch:Switches add more intelligence to data transfer management. Switches can determine whether data should remain on a LAN or not, and they can transfer the data to only those connections that needs that data.

Repeater: 

All transmission media attenuate (weaken) the signals (analog or digital) that travel through them. Attenuation therefore limits the distance of any medium can carry data. Adding a device that amplifies (regenerates) the signal the signal can allow it to travel farther, increasing the size of network, hence a repeater is a network device used to amplifies (regenerates) a signal.