OSI LAYERS AND FUNCTIONS - [Part 1] - INTERVIEW QUESTIONS

Question - 1 What is OSI (Open Systems Interconnection) reference model ?

• OSI (Open Systems Interconnection) reference model was developed by the (ISO) International Organization for Standardization. 
• Basically Open System Interconnection model determine how the data and network information are communicated from an application on one computer through the network media to an application on another computer. 
• The OSI reference model divides this process into layers and each layer has different role and functions. OSI has seven layers.

Layer 1 - Physical Layer
Layer 2 - Data Link Layer
Layer 3 - Network Layer
Layer 4 - Transport Layer
Layer 5 - Session Layer
Layer 6 - Presentation Layer
Layer 7 - Application Layer

Question - 2 What is the need of OSI model ?

Before sometime when OSI did not come in the real world, then only same manufacturer PC could communicate with each other. Two different manufacturer could not communicate with each other. So OSI came into the market to break this barrier 

Question - 3 What are the advantages of OSI model ?

• OSI divides the network communication process into smaller and simpler components, which are known as Layers, and each layer has different role and functions.
• It allows multiple-vendor development through standardization of network components.
• It allows to communicate various types of network hardware and software.
• Changes in one layer do not affect other layers, so it does not hamper development.
• OSI support both connection-oriented services and connection-less services.

Question - 4 Explain the function of each OSI layer.

Application Layer

• Provide a user interface and allow to access network resources.
• Application Layer is the first layer of the OSI Model and this layer is responsible to provide interface among users and the software application.
• It plays a role of intermediates between actual software application and the users.
• We can say that, application layer plays a front-end role, where user actually communicate or interact, other remaining layers play a back-end role.
• File transfer, emails etc. are the example of application layer where users interact.

Presentation Layer

• It is responsible to present data to the Application layer 
• Presentation Layer is responsible for data translation, conversion and code formatting like 

1. Data compression/Decompression
2. Data Encryption/Decryption
3. Graphic handling
4. Character/string conversion

• A data-transfer technique -  Data will be in standard format before transmission. After that computers will receive generically formatted data and then convert the data back into its native format for actual reading. 
• This layer ensures that data transferred from the Application layer of one system can be read by the Application layer of another system.
• So presentation layer is responsible for integrating all data formats into a standard format for efficient and effective communication. Best example is  EBCDIC (Extended Binary Coded Decimal Interchange Code) to ASCII (American Standard Code for Information Interchange) conversion. 

Session Layer

• Session Layer provides appropriate sessions between users and software, actually where users interacts.
• It is responsible for sessions establishment, session management, session termination and synchronization. Example : Online payment session and IRCTC login session
• It is also provides dialog control between devices, or nodes.

 Transport Layer

• The Transport layer is responsible to reassembles segments into a data stream.
• Transport layer provide end to end data transport services.
• It can establish a logical connection between the sending host and destination host on an inter-network.
• It is also responsible to hide sensitive details of data from networks and to providing data transmission in a transparent mode.
• Transport layer can be connection-less or connection oriented. Because TCP and UDP both work at the Transport layer. Where TCP is a connection oriented means provide reliable communication and UDP is connection-less means provide unreliable communication.
• Because Transport layer can be connection oriented, so it also will be use acknowledgments, sequencing, windowing, error control and flow control.

Network Layer

• Network layer is responsible to manages device addressing and it tracks the location of devices on the network.
• Network Layer is responsible for establishing data communication channel between multiple networks or nodes or devices or hosts.
• It determines the best path to move data, which means that the Network layer must act as a transport traffic between devices that are not locally attached.
• Network-Layer is used when a connection needs to establish between two or more different networks. If two or more nodes or devices are connected locally, then there is no any requirement of network layer.
• Router is a layer 3 devices, which is specified at the Network layer and provide the routing services within an inter-network.

Data Link Layer

• Data Link layer is responsible for physical transmission of the data and for successful data synchronization.
• It handles error notification, network topology, and flow control. 
• Data Link layer ensure that messages are delivered to the proper device on a LAN using physical address(MAC) 
• It translate messages from the Network layer into bits for the Physical layer to transmit.
• It reassembles the packet into data frames. Frames are containing the source address field, destination address field, packet, ether field and FCS.
• Data Link Layer is divided into two fields -Logical Link Control and Media Access Control.

Physical Layer

• Physical layers sends bits and receives bits. Bits is represented in only format of 1 or 0.
• It specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link between end systems.
• It identify the interface between the data terminal equipment (DTE) and the data communication equipment (DCE).

Question- 5 What is data encapsulation ?

Question - 6 Explain the data link layer services Data Encryption, Decryption, Compression and Decompression

• Data Encryption - It is process of translating the data into secret code or translating the data from plain text to cipher text.  
• Data Decryption - It is process of translating the secret code into normal readable data or translating the data from cipher text to plain text. 
• Data Compression - This process use to reduce the number of bits 
• Data Decompression - This process use to increase the number of bits mean convert back the data into their own original size.

Question - 7 What is difference between plain text and cipher text ?

• Plain Text -  It is normal user readable data. 
• Cipher Text -  It is encrypted secret code and not readable by user.

Question - 8 What are the Simplex, Half Duplex and Full Duplex ?

• Simplex - It is a communication channel that sends information in one direction only 
• Half Duplex - In a half duplex system, there are still two clearly defined path or channels and each party can communicate with each other but not simultaneously, the communication will be in one direction at a time. Oki-Toki is very good example of half duplex 
• Full Duplex - In a full duplex system both parties can communicate with each other simultaneously. Telephone communication is very good example of full duplex communication.

Question - 9 Explain TCP and UDP.

TCP (Transmission Control Protocol) - 

• TCP provide connection oriented reliable communication.
• There are Acknowledgement, Sequencing, Windowing and Flow control take place.
• TCP creates a lot of overhead.

UDP (User Datagram Protocol) - 

• UDP provide connection-less unreliable communication.
• There are no Acknowledgement, Sequencing, Windowing and Flow control take place. UDP does not sequence the segment and does not care in which order the segments arrive at the destination.
• There are low overhead.

Question - 10 Explain DCE and DTE.

DCE (Data Communication Equipment) -

• End of the WAN provider side of the communication facility.
• Responsible for providing clocking signal.
• The DCE is usually located at the service provider

DTE (Data Terminal Equipment) -

• End of the user's device on the WAN link.
• The DTE is the attached device.
• The services available to the DTE are most often accessed via a modem or channel service unit/data service unit (CSU/DSU).

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Anubhav Upadhyay

Hello and welcome to networktopic Blog. My name is Anubhav. I am a Senior Network Egineer. I have created this blog specially to serve interview questions and answer on Network Routing and Switching, I will try my best to serve correct and updated networking knowledge for you as per my corporate experience.

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