1. Course Name: Computer Application Topic: Computer Networks
LECTURE
Course Name: Computer Application
Topic: Computer Networks

2. Computer Network
A computer network is a system in which multiple computers are connected to each other to share information and resources.
Sharing of resources: Primary goal of a computer network is to share resources.
Sharing information: Information on a single computer can be accessed by other computers in the network. Duplication of data file on separate PCs can be avoided.
Communication: When several PCs are connected to each other, messages can be sent and received
Safe : The data received is the same as the data sent
Secure : The data being transferred cannot be damaged either will fully or accidentally.
Reliable: Both the sender and the receiver knows the status of the data sent. Thus the sender knows whether the receiver got the correct data or not

3. Importance of Networking
Computer networks allow the user to access remote programs and remote databases either of the same organization or from other enterprises or public sources.
Computer networks provide communication possibilities faster than other facilities. Because of these optimal information and communication possibilities, computer networks may increase the organizational learning rate.
There are other reasons why any organization should have a computer network
cost reduction by sharing hardware and software resources
high reliability by having multiple sources of supply
cost reduction by downsizing to microcomputer-based networks instead of using mainframes
greater flexibility because of possibility to connect devices from various vendors

4. Data Communication
The data communication is the movement or transmission of data between two devices or computers.
OR, it is the transfer of data between two points either in analog or digital form via a communication medium.
Modes of Data Communications
Simplex mode:
In simplex mode, data can be transmitted in one direction as shown in the figure. The device using the simplex mode of transmission can either send or receive data, but it cannot do both. An example is the traditional television broadcast, in which the signal is sent from the transmitter to the TV. There is no return signal. In other words a TV cannot send a signal to the transmitter.

5. 2. Half-Duplex mode:
In Half duplex mode data can be transmitted back and forth between two stations. But at any point of time data can go in any one direction only. This arrangement resembles traffic on a one lane bridge. When traffic moves in one direction, traffic on the opposite direction is to wait and take their turn.
The common example is the walky-talky, wherein one waits for his turn while the other talks. In half-duplex mode, each station can both transmit and receive data, but not at the same time.

6. 3. Full duplex mode
In full duplex mode a device can simultaneously send or receive data. This arrangement resembles traffic on a two-way bridge, traffic moving on both directions simultaneously.
An example is two people on the telephone talking and listening simultaneously. Communication in full duplex mode is faster. Full duplex transmission is used in large computer systems. Products like “Microsoft Net Meeting’ supports such two way interaction

7. Metropolitan Area Network (MAN)
Types of Network
Local Area Networks (LAN)
Metropolitan Area Network (MAN)
Wide Area Network (WAN)

8. Even more useful is the ability to share information
Local Area Network (LAN)
A LAN (Local Area Network) is a system whereby individual PCs are connected together within a company or organization.
For instance if ten people are working together within an office, it makes sense for them all to be connected. In this way, the office can have a single printer and all ten people can print to it. In a similar way, other devices such as modems or scanners can be shared.
Even more useful is the ability to share information
when connected to a network.

9. Metropolitan Area Networks (MAN)
A Network that spans a geographical area covering a Metropolitan city is called Metropolitan Area Network (MAN)
Wide Area Networks (WAN)
A WAN is typically two or more LANs connected together across a wide geographical area. The individual LANs separated by large distances may be connected by dedicated links, fibre optic cables or satellite links
Wide area networks (WANs) are used to connect LANs together. Typically, WANs are used when the LANs that must be connected are separated by a large distance.

10. Network Topologies

11. Network Topologies
Network Topology refers to the physical layout and connectivity of computers in a network. Structure of a network and may be depicted physically or logically. Physical topology is the placement of the various components of a network, including device location and cable installation, while logical topology illustrates how data flows within a network, regardless of its physical design

12. Types of network Topology
1. Star Topology
In a star topology all the nodes (server, workstations, peripherals) on the network are connected directly to a centralized connectivity device called a hub, switch, or router. Each computer is connected with its own cable to a port on the hub. Data on a star network passes through the hub, switch, or router before continuing to its destination. The hub, switch, or router manages and controls all functions of the network. It also acts as a repeater for the data flow.
Advantages of a Star Topology
Centralized management. It helps in monitoring the network.
Easy to install and configure.
No disruptions to the network when connecting or removing devices.
Easy to detect faults and to remove parts.
Failure of one node or link doesn’t affect the rest of network.
Disadvantages of a Star Topology
Requires more cable length than a Bus topology.
If the hub, switch, or concentrator fails, nodes attached become disable.
More expensive than linear bus topologies because of the cost of the hubs.

13. 2. Ring Topology
In a ring every node is logically connected to two other preceding and succeeding nodes, forming a ring. Traffic flows through the entire ring until it reaches its destination.
Data packets travel in a single direction around the ring from one network device to the next. Each network device acts as a repeater, meaning it regenerates the signal the packets they receive and then send them on to the next computer in the ring.
Advantages of Ring Topology
Even when the load on the network increases, its performance is better than that of Bus topology.
There is no need for network server to control the connectivity between workstations.
Additional components do not affect the performance of network.
Each computer has equal access to resources.
Disadvantages of Ring Topology
Each packet of data must pass through all the computers between source and destination. This makes it slower than Star topology.
If one workstation or port goes down, the entire network gets affected.
Network is highly dependent on the wire which connects different components.
Ring topologies can be difficult to troubleshoot.
Adding or removing computers from this type of topology can disrupt the operation of the network.

14. Bus Topology
In the bus topology, each node is attached to a single common cable. This topology type is considered a passive topology because the computers on a bus just sit and listen. When they "hear" data on the wire that belongs to them, they accept that data. When they are ready to transmit, they make sure no one else on the bus is transmitting and then they send their packets of information on the network.
Advantages of a Bus Topology
Bus topology costs very less.
Easy to connect a computer or peripheral to a linear bus.
Requires less cable length than a other topologies.
It is easy to set-up and extend bus network.
Linear Bus network is mostly used in small networks.
Disadvantages of a Bus Topology
Entire network shuts down if there is a break in the main cable. Loose and detached connections may also affect the entire network.
There is a limit on central cable length and number of nodes that can be connected.
Proper termination is required to dump signals. Use of terminators is must.
It is difficult to detect and troubleshoot fault at individual station.
It is not suitable for networks with heavy traffic.

15. Mesh Topology
In a mesh network topology, each of the network node, computer and other devices, are interconnected with one another.
In a full mesh, every device in the network is connected to every other device. In reality, a partial mesh is commonly used in backbone environments to provide fault-tolerant connections between critical servers and network devices. This type of topology is very expensive as there are many redundant connections, thus it is not mostly used in computer networks. It is commonly used in wireless networks. Flooding or routing technique is used in mesh topology.
Advantages of Mesh topology
Data can be transmitted from different devices simultaneously. This topology can withstand high traffic.
Even if one of the components fails there is always an alternative link present. So data transfer doesn’t get affected.
Expansion and modification in topology can be done without disrupting other nodes.
Disadvantages of Mesh topology
There are high chances of redundancy in many of the network connections.
Overall cost of this network is too high as compared to other network topologies.
Set-up and maintenance of this topology is very difficult. Even administration of the network is challenging.

16. OSI MODEL

17. THE OSI MODEL
Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late 1970s.

18. LAYERED TASKS
We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office.

19. Tasks involved in sending a letter

20. Seven layers of the OSI model

21. Physical Layer:
The physical layer is responsible for movements of individual bits from one (node) to the next. The unit of communication at the physical layer is a bit. Provides physical interface for transmission of information. Defines rules by which bits are passed from one system to another on a physical communication medium.
Data Link Layer:
The data link layer is responsible for moving frames from one (node) to the next. The unit of communication at the data link layer is a frame. Data link layer attempts to provide reliable communication over the physical layer interface. Breaks the outgoing data into frames and reassemble the received frames.
Network Layer:
The network layer is responsible for the delivery of individual packets from the source host to the destination host. Implements routing of frames (packets) through the network. Defines the most optimum path the packet should take from the source to the destination.

22. Session Layer: Presentation Layer : Application Layer:
Transport Layer:
The transport layer is responsible for the delivery of a message from one process to another. Purpose of this layer is to provide a reliable mechanism for the exchange of data between two processes in different computers. Ensures that the data units are delivered error free. Ensures that data units are delivered in sequence.
Session Layer:
Session layer provides mechanism for controlling the dialogue between the two end systems. It defines how to start, control and end conversations (called sessions) between applications. This layer requests for a logical connection to be established on an end-user’s request.
Presentation Layer :
The presentation layer is responsible for translation, compression, and encryption. Presentation layer defines the format in which the data is to be exchanged between the two communicating entities. Also handles data compression and data encryption (cryptography).
Application Layer:
The application layer is responsible for providing services to the user. Application layer interacts with application programs and is the highest level of OSI model. Examples of application layer are applications such as file transfer, electronic mail, remote login etc.

25. Summary of the OSI Model