Important questions and answers of Computer Network

1. Explain the different layers of OSI model

Ans: The OSI (Open Systems Interconnection) model, developed by the International Organization for Standardization (ISO), serves as a blueprint for understanding how data moves from one device to another in a network.

The OSI model is a conceptual model that describes how data is transferred over a network. It is a layered model consisting of seven layers, each layer providing a specific functionality to enable communication between computers. The OSI model provides a systematic approach to understanding and designing communication systems. It helps ensure that different systems can communicate with each other and the communication is reliable.

The OSI model is a conceptual framework used to describe the functions of a communication system. It consists of seven layers, each with a specific set of responsibilities.

The seven layers of the OSI model are:

1. Physical Layer:

This is the lowest layer of the OSI model and deals with the physical transmission of data over the network medium. The physical layer is responsible for transmitting and receiving raw bit streams over a communication channel. This layer defines the electrical, mechanical, and physical characteristics of the communication medium, such as cables, connections and wireless frequencies.

The main function of the physical layer is to move data in the form of electromagnetic signals across a transmission medium.

Functions of Physical layer:

Line Configuration: It defines how two or more devices can be connected physically.

Data transmission: Defines whether the transmission is simplex, half duplex or full duplex.

Topology: It defines the way how network devices are arranged.

Signals: Determines the type of signals used for transmitting information.

2. Data Link Layer:

The data link layer is responsible for the node-to-node delivery of data over a physical network. Its main function is to provide a reliable and error free transmission of data between two devices in the network. When a packet arrives at the DLL, it is responsible for ensuring that the packet is delivered to the correct device on the network.

This layer is responsible for framing, error detection and correction of data and it also manages the flow of data between devices.

Functions of the DLL:

Framing: The DLL takes the packets from the network layer and encapsulates them into frames. It sends each frame bit by bit on the hardware.

Header: It contains the source and the destination addresses of the frame.

Trailer: It contains the error detection and error correction bits.

Physical addressing: The DLL adds a unique Media Access Control (MAC) address to each frame, which identifies the source and

destination devices on the network. The MAC address is used by the network hardware to deliver the frame to the correct destination.

Flow control: It is a speed matching mechanism. Flow control coordinates the amount of data that can be sent before receiving an acknowledgement. It prevents data loss and buffer overflow.

Access Control: Provides a mechanism for multiple devices to access the shared physical medium without causing data collisions.

Error Control: Detecting and correcting errors that may occur during transmission., using techniques such as cyclic redundancy check (CRC) and acknowledgement.

3. Network Layer:

The network layer is responsible for the delivery of data between nodes on different networks. It provides routing and address services to move data from the source to the destination network.

The network layer works for the transmission of data from one host to the other located in different networks. It also takes care of packet routing i.e., selection of the shortest path to transmit the packet, from the number of routes available. The sender and receiver’s IP address are placed in the header by the network layer. This layer defines the addressing scheme and the protocols used to route data between networks. The IP (Internet Protocols) is an example of a protocol used at this layer.

Functions:

Routing: The network layer protocols determines which route is suitable from source to destination. This function is known as routing.

Logical Addressing: To identify each device on the internet uniquely, the network layer defines an addressing scheme. The sender and receiver’s IP addresses are placed in the header by the network layer.

4. Transport Layer:

The transport layer is responsible for end-to-end communication between the sender and receiver. It ensures that data is delivered reliably and in the correct order through mechanisms like segmentation, flow control, and error correction. Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are the two main protocols at this layer.

Functions of Transport Layer:

Segmentation and reassembly: divide data into smaller segment for efficient transmission.

Connection Management: establish, maintain and terminate connections between applications.

Reliable data delivery: ensure error-free transmission through error detection and correction.

Flow control:

Congestion control:

5. Session Layer:

The session layer establishes, manages, and terminates connections between applications. It allows multiple applications to communicate over the network simultaneously and provides services like session establishment, maintenance, and synchronization.

Functions of Session Layer:

Synchronization: session layer adds some checkpoints when transmitting the data in a sequence. If some error occurs in the middle of the transmission of data, then the transmission will take place again from the checkpoint.

Dialog control: regulates the flow of data between applications to ensure orderly communication.

6. Presentation Layer:

The presentation layer is responsible for data translation, compression, encryption, and decryption. It translates the data from the application layer into a standardized format that can be understood by different systems. It also provides encryption and decryption services to protect data during transmission. The presentation layer is mainly concerned with the syntax and semantics of the information exchanged between two systems. It acts as a data translator for a network. It ensures that data sent by the application layer is properly formatted and readable by the receiving application.

Functions of Presentation Layer:

Data translations: It translates data between different character sets such as ASCII and Unicode.

Data formatting and syntax checking: it ensures that data is formatted properly and follows the correct syntax.

Data encryption and decryption: it encrypts data to ensure secure transmission, and decrypts it at the receiving end.

Data compression: it reduces the number of bits to be transmitted.

7. Application Layer:

The application layer is the closest layer to the end-user and provides network services directly to user applications. The application layer is responsible for providing services to end users. It is responsible for providing user interface and communication service that enables the applications to interact with the network. Application layer defines the protocols and interfaces used by application programs to access the network. Examples include HTTP, FTP, SMTP, and DNS.

Functions of the Applications Layer:

FTAM: File transfer, access and management provides service to transfer files between hosts and to manage the files on remote system.

Mail Service: sending and storing email messages.

Directory services: provides a way for users to locate and access resources on a network.

2. Define the terms broadcasting, multicasting and unicasting.

Ans: Casting means the transmission of data over a network. Broadcasting, multicasting and unicasting are three different methods of sending data over a network.

Broadcasting, multicasting and unicasting are terms used in computer network to describe different ways of sending data from one device to another. Broadcasting sends data to all devices on a network, multicasting sends data to a specific group of devices and unicasting sends data from one device to another device on a network. Understanding these terms is important in network design and troubleshooting to ensure that data is transmitted effectively and efficiently.

Broadcasting:  

Broadcasting is a method of sending data to all devices on a network. The sender transmits the data to a specific network address. The data is then sent to all devices connected to that network. In broadcasting, a single message is sent to every device on the network, regardless of whether the device is intended to receive the message or not. Broadcasting is typically used for tasks like network discovery or to send an alert message to all devices on the network.

Types:

Limited broadcast: data is sent from one host to all hosts residing in the same network.

Direct broadcast: transmits data from the source host to all the receivers host that exist in some other network.

Multicasting:

Multicasting is a method of sending data to a group of devices on a network. The sender transmits the data to a specific multicast address, which is a group of addresses that multiple devices can join. The data is then sent only to the devices that have joined the multicast group.

In multicasting, a single message is sent to a specific group of devices that have joined a multicast group. Devices that are not part of the multicast group do not receive the message. Multicasting is often used for tasks like video conferencing, online gaming or streaming media to a group of users.

Unicasting:

Unicasting is a method of sending data from one device to another device on a network. This is useful for sending messages to a specific device on the network, such as for requesting data or sending a command. In this method, the sender transmits the data to a specific destination IP address, and only the device with that address receives and processes the data. It is commonly used for one-to-one communication such as email or file transfer.

Unicasting is the most common method of transmitting data on networks, and it is used for tasks like sending emails or browsing the web.

3. What is the drawback of message switching? How is it overcome in packet switching?

Ans: Message switching and packet switching are two different methods of transmitting data over network.

Message switching is acommunication technique where an entire message is sent from one node to another through a series of intermediate nodes. In message switching, the entire message is transmitted from the source to the destination in one piece. The message is stored and forwarded from one node to another until it reaches its destination.

The main drawback of message switching is that it is inefficient in terms of network bandwidth utilization, as the entire message must be stored and forwarded at each intermediate node, leading to a slow delivery time for message. It requires a lot of memory to store each message, and network can become congested when multiple messages are being transmitted at the same time.

These limitations have led to the development of packet switching, which is a more efficient and faster technique used in modern computer networks. Packet switching overcomes the drawbacks of message switching by breaking data into smaller units called packets. Each packet contains a portion of the data along with addressing information that enables it to be routed to its destination independently of other packets. This allows for mor efficient use of network resources, as packets can be transmitted in parallel along different routes and reassembled at the destination.

Packet switching also enables faster communication with reduced latency, as packets can be transmitted immediately as they are ready without the need to wait for the entire message to be assembled. These benefits have made packet switching the preferred technique used in modern computer networks.

Packet switching can also improve network reliability as packets can be retransmitted if there is a problem with transmission. Additionally, packet switching allows for different types of traffic such as voice, video and data to be transmitted simultaneously over the same network which is not possible with message switching.

4. Suppose you want to transmit the data 100111001 and the generator polynomial is x⁶ + x⁵ + 1. What bit is actually sent?

Ans: To send the data 100111001 using a polynomial code with generator polynomial x⁶ + x⁵ + 1, we need to perform polynomial division using the data bits and the generator polynomial.

We represent the generator polynomial and the message polynomial as binary bit string.

X⁶ +x⁵ +1 isrepresented as 1100001

1.x⁶ +1.x⁵ +0.x⁴ +0.x³ +0.x² +0.x¹ + 1.x⁰

Divisor: 1100001

Append 6 zero bits to the message polynomial: 100111001000000

Divide the message polynomial by the generator polynomial

 

 

Or

The remainder of the division, 010100 is the CRC code that will be appended to the message polynomial.

Append the CRC to the message:

100111001010100

Therefore, the bit string which is actually sent to transmit the data 100111001 using a polynomial code generator polynomial x⁶ + x⁵ +1 is 100111001010100