The OSI Model: Understanding the 7 Layers of Networking

The OSI (Open Systems Interconnection) model was developed by the International Organization for Standardization (ISO) in 1978 and adopted by all major computer and telecommunication companies since in 1984.

The OSI model is a reference model it is not practically implemented.

The OSI model full form is Open Systems Interconnection define how to transfer data between two systems having different hardware and software across the globe. For example, if two computer having different operating system so, how they communicate with each other, to solve this issue and for successful communication 7 layer OSI model will help.

The OSI mode is a 7 layer architecture, where every layer having his own specific functionality. Here each or every layers interact and help to its lower and upper layer.

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7 Layers of OSI Model

OSI model 7 layers is divided into 3 parts. The upper three layer (Application layer, Presentation layer and Session layer) is called software layers, the lower three layer (Network layer, Datalink layer and Physical layer ) is called hardware layers and the middle layer (Transport layer) is called the heart of the OSI model.

Here’s a detailed explanation of each layer:

Application Layer

The Application Layer (also known as End User layer or desk top layer) is the top most layer of OSI model. It is closest to the end user. It interacts with software applications to implement a communicating component and provides services such as email, file transfer, and network management.

Functions:

1. It act as an interface between system and the user.
2. It provide interface through which we ca send data easily.
3. it Facilitates various protocols and services (e.g., HTTP, FTP, SMTP, Telnet, DNS).

Examples: HTTP (Hyper Text Transfer Protocol), FTP (File Transfer Protocol), SMTP (Simple Mail Transfer Protocol), DNS (Domain Name System).

Presentation Layer

The presentation layer is also known as Translation layer. On the sender’s end, the presentation layer translates the data from a user-dependent format to the common binary format. On the receiver’s end, their presentation layer translates it to another, receiver-dependent format.

After translation, presentation layer compress data bit by the process known as data compression and then Encryption and decryption are carried out by the presentation layer to secure data over the computer network. It ensures that only the communicating devices can understand pertinent data.

Functions of the presentation layer

3 main function perform by presentation layer

1. Data translation and encoding (e.g., ASCII, EBCDIC, Unicode).
2. Data compression and decompression.
3. Data encryption and decryption.

Translation : On the sender’s end, the presentation layer translates the data from a user-dependent format to the common binary format. On the receiver’s end, their presentation layer translates it to another, receiver-dependent format.

Data Compression: Data compression is the process of encoding, restructuring or otherwise modifying data to reduce its size. Compression is often used to maximize the use of bandwidth across a network. Data compression id done at sender side and decompression is perform at receiver end.

Encryption/Decryption : Encryption and decryption are carried out by the presentation layer to secure data over the network. It ensures that only the communicating devices can understand pertinent data.

                                                                       OR

Encryption is the process by which a readable message is converted to an unreadable form to prevent unauthorized parties from reading it. Decryption is the process of converting an encrypted message back to its original (readable) format. The original message is called the plaintext message.

 Protocol Use:  JPEG, MPEG, GIF

Session layer

The session layer establish, controls and maintains connections between two systems to share data. It establishes, maintains, and ends sessions across all channels. In case of a network error, it checks the authenticity and provides recovery options for active sessions.

Functions of the session layer

1. Session establishment, maintenance, and termination: This layer allows to establish, use, and terminate a connection between the two processes.
2. Dialog control: Session layer acts as a dialog controller that creates a dialog between two processes or we can say that it allows the communication between two processes which can be either be in half-duplex or full-duplex.
3. 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. This process is known as Synchronization and recovery.

Protocol use: SQL (Structured Query Language) sessions, NetBIOS (Network Basic Input/Output System) sessions.

Transport layer

The transport layer layer is the heart of OSI model. It provides services to the application layer and takes services from the network layer. It receives the data from the session layer (upper layer) and converts them into small-small units known as segments.

This layer provide end-to-end communication and ensures complete data transfer. It manages error detection, data flow control, and retransmission of lost data packets.

Functions of Transport layer

1.Segmentation : Transport layer receives the data from the upper layer (session layer), it divides the data unit into multiple segments, and each segment is assigned with a sequence number that uniquely identifies each segment. When the message has arrived at the destination, then the transport layer reassembles the message based on their sequence numbers.

2.Flow Control: The transport layer also responsible for flow control end to end. Flow control define amount of data to be send.

3.Error Control: The transport layer is also responsible for Error control. Error control is performed end-to-end rather than across the single link. The sender transport layer ensures that message reach at the destination without any error. For error control transport layer use ARQ Algorithm (Automatic Repeat Request ) and check Sum.

4.Connection-oriented and connection less transmission: A connectionless service treats each segment as an individual packet, and they all travel in different routes to reach the destination. A connection-oriented service makes a connection with the transport layer at the destination machine before delivering the packets. In connection-oriented service, all the packets travel in the single route.

Protocols Use: TCP (Transmission Control Protocol), UDP (User Datagram Protocol).

Network Layer

The Network Layer is responsible for determining the best physical path for data to travel from the source to the destination. It manages packet routing, addressing, and forwarding.

Functions of Network layer

1. Logical Addressing: To identify each and every device in a networks uniquely, the network layer defines an addressing scheme known as logical address or IP addressing. The sender & receiver’s IP addresses are placed in the header by the network layer. Such an IP address distinguishes each and every device uniquely.
2. Path Determination and Routing: Out of so many paths in inter-network network layer find the best path to reach the destination.
3. Packetizing: A Network Layer receives the segments from the upper layer (transport layer) and add source and destination IP address to each segment so this segment converts into packets. This process is known as Packetizing. It is achieved by internet protocol (IP).

Protocol Use: IP (Internet Protocol), ICMP (Internet Control Message Protocol), and routers.

Data link Layer (DLL)

The data link layer is responsible for the node-to-node delivery of the data. 

The Data Link Layer ensures reliable data transfer between two physically connected devices. It manages error detection and correction, frame synchronization, and flow control. When a packet arrives in a network, it is the responsibility of the DLL to transmit it to the Host using its MAC address.

Functions of Data Link Layer – OSI layer

1. Framing: Data Packet coming from upper layer (network layer), data link layer add physical address ( known as MAC address ) of source and destination in header and a tailer in each packet to make a frame.
2. Physical Addressing: DLL (data link layer) uses MAC (Media Access Control) addresses also known as physical address to ensure that data is sent to the correct destination within the same network segment. MAC addresses is a physical addresses that uniquely identify devices on a local network, enabling proper routing of frames to their intended recipients.
3. Error Detection and Correction: This layer includes mechanisms to detect and correct errors that may occur during data transmission. A method of detecting errors in transmitted messages by using a checksum. These techniques ensure data integrity by allowing the receiver to detect errors and request retransmission if necessary.
4. Flow Control: Flow control manage the rate of data transmission between two devices to prevent a fast sender from overwhelming a slow receiver. The technique use for flow control is Stop-and-Wait Protocol and Sliding Window Protocol.
5. Access Control: These mechanisms ensure that only one device transmits data at a time, reducing the chances of collision and ensuring efficient use of the network medium. Techniques use for access control is Carrier Sense Multiple Access with Collision Detection (CSMA/CD) and Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).

Protocol use: Ethernet (MAC addresses), PPP (Point-to-Point Protocol), and switches.

Physical Layer

This is the lowest layer of the OSI model. This layer is responsible for the physical connection between devices via physical medium. It handles the transmission of raw binary data over a physical medium, such as cables, radio frequencies, or optical fibers.

The function of the physical layer

• Line configuration: It defines how two or more devices are connected to a link.

•Representation of Bits: It encodes the bit stream into electrical and optical signals. 

•Establishment of Physical Connections:

•Transmission media: Wired or wireless

•Data Rate: How many bits send per second?

•Synchronization: The sender and receiver must be synchronized.

•Transmission mode: It describes the direction of the data flow.

•Topology: the physical and logical arrangement of nodes and connections in a network.

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