The lowest layer of the OSI model is the physical layer. He is responsible for the physical connection between devices. The physical layer is responsible for the transfer of individual bits from one node to another.
Physical layer functions:
Data rate control.
Defines the network topology (bus, star, etc.).
Defines the data transmission mode (simplex, half duplex and full duplex).
2. Data Link Layer
Key words: frame, MAC address, switch
The Data Link layer is responsible for delivering a message from node to node.
To deliver packets, the physical address of the device on the network is used - MAC address.
Data Link layer functions:
Framing allows the sender to transmit a set of bits that are meaningful to the receiver. This can be achieved by attaching special bit patterns to the start and end of the frame.
Physical addressing is performed by containing in the header of each frame the physical address (MAC address) of the sender and recipient.
An error control mechanism that detects and retransmits corrupted or lost frames.
3. Network Layer
Key words: packet, IP address, router
A segment of the network layer is called a packet.
The network layer is implemented by network devices such as routers.
The network layer works to transfer data from one host to another located on different networks. It also takes care of packet routing, i.e. about choosing the shortest path for transmitting a packet from among the available routes. For packet routing, the source and destination IP addresses are placed in the header of each packet.
Network layer functions:
Routing determines which route is suitable from source to destination using network layer protocols.
Logical addressing identifies each device on the internetwork with an IP address. This address makes each device unique and versatile.
4. Transport Layer
Key words: segment, TCP, UDP, port
The transport layer is called "the heart" of the OSI model.
The data in the transport layer are called segments.
The transport layer is controlled by the operating system. It is part of the OS and interacts with the application layer.
The transport layer receives data from the upper layers, performs segmentation, and controls flow and errors to ensure correct data transfer. It also adds the source and destination port number to its header and forwards the shared data to the network layer.
The port number is usually specified by default, or manually.
The functions of the transport layer are :
Segmentation: This layer accepts the message from the (session) layer, and breaks the message into smaller units. Each of the segments produced has a header associated with it. The transport layer at the destination station reassembles the message.
Service Point Addressing: In order to deliver the message to the correct process, the transport layer header includes a type of address called service point address or port address. Thus by specifying this address, the transport layer makes sure that the message is delivered to the correct process.
5. Session Layer
Key words: data, session, authentication
This layer is responsible for establishing a connection, maintaining sessions, authenticating, and also providing security.
Session layer functions:
Session establishment, maintenance, and termination: The layer allows two processes to establish, use, and terminate a connection.
Dialogue controller: The session layer allows two systems to initiate communication with each other in half-duplex or full-duplex mode.
At the Presentation layer, data is retrieved from the application layer and processed according to the required format for transmission over the network.
Presentation layer functions:
Translation: for example, from ASCII to EBCDIC.
Encryption / Decryption: Data encryption translates data into another form or code. The encrypted data is known as cipher text, and the decrypted data is known as plain text. The key value is used for both encryption and decryption of data.
Compression: Reduces the number of bits that need to be transmitted over the network.
The application layer produces data that needs to be transmitted over the network. This layer also serves as the interface between the user and services to access the network.
Example: Application - What's Up, Browsers, Skype Messenger, etc.
Application layers functions are:
The application layer provides the end user with software such as web browsers, email clients, and others.
It provides protocols that allow software to send and receive information and present meaningful data to users.
The application layer uses HTTP/ FTP/ POP/ SMTP/ DNS protocols
This layer provides various e-mail services
This application offers distributed database sources and access to global information about various objects and services.
Benefits of using the OSI model:
It will help you standardize your router, switch, motherboard, and other hardware.
Question 1. Explain the OSI reference model with its layered architecture. (Marks: 4)
Answer. Provides a way to understand how internetwork operates(1)
7 layers(1): Application Layer, Presentation Layer, Session Layer, Transport Layer, Network Layer, Data link Layer, Physical Layer(1)
Takes the task of internetworking and divides that up into what is referred to as a vertical stack (1) of 7 Layers
Question 2. Which layer of the Open System Interconnection (OSI) model is also known as the network interface layer? (Marks: 1)
Answer. Data link (1)
Question 3. Identify the layer according to the description: "This layer establishes, manages and terminates connections between applications. It sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end." (Marks: 1)
