What is OSI Reference Model?

OSI reference model describes building blocks of functions for program-to-program communications between similar or dissimilar hosts. The model was the primary standard model for network communications. A conceptual framework has been developed by the International Organization of Standardization (ISO) in which these standards will fit. The OSI model is a seven-layered architecture where each layer is assigned a specific task.

  1. Application layer
  2. Presentation layer
  3. Session layer
  4. Transport layer
  5. Network layer
  6. Data-link layer
  7. Physical layer

How to Remember

All                    – Application

People            – Presentation

Seems            – Session

To                   –  Transport

Need of          – Network

Data                – Data Link

Processing    – Physical

Layer 7: Application layer

The topmost layer of the OSI model is the application layer. This layer defines services that are directly used by a user application. It allows the creation of a virtual terminal and gives File Transfer Access and Management that enables a user to access, retrieve, manage or control files in a remote computer.

Protocol Used at Application Layer: HTTP, FTP, DNS, SNMP, Telnet

Layer 6: Presentation layer

The presentation layer prepares data for the application layer, based on syntax and makes sure that data is delivered to the receiving system will be understandable and usable. Some important responsibilities of this layer are translation, encryption, and compression.

Protocol Used at Presentation Layer: SSL, TLS, IMAP, SSH

Layer 5: Session layer

The session layer helps to establish, maintain, and synchronize conversations between two different applications. This layer allows two systems to enter in dialog, which can be half-duplex or full-duplex. This layer allows a process to add checkpoints that are considered as synchronization points into the stream of data.

Protocol Used at Session Layer: NetBIOS, PPTP

Layer 4: Transport layer

This layer is responsible for transportation services. This layer decides whether the data transmission should occur on parallel paths or single paths. This layer is responsible for multiplexing, splitting, or segmenting on the data.

This layer is capable of both connectionless and connection-oriented transfer of data. Flow control is a fundamental function of this layer. The Transport layer is a true end-to-end layer, all the way from the source to the destination.

Protocol Used at Transport Layer: TCP, UDP, ECN, SCTP

Layer 3: Network layer

The main function of the network layer is to transfer packets from source to destination across multiple networks. It translates logical network addresses into physical addresses. There is no need for this layer if two systems are connected on the same network. The network layer is responsible for routing, i.e., it selects the shortest path to transmit the packet.

Protocol Used at Network Layer: IP, ARP, ICMP, IPSec, IGMP

Layer 2: Data-link layer

The main function of a data-link layer is to handle the errors which might occur at the physical layer. The Data Link layer adds a header to the frame in order to define the physical address of the sender or receiver of the frame if the frames are to be distributed to different systems on the network. This layer consists of two sub-layers:

  1. Logical Link Control (LLC): The primary function of LLC is to multiplex protocols over the MAC layer while transmitting and likewise to de-multiplex the protocols while receiving.
  2. Media Access Control (MAC): This sub-layer uses MAC addresses for connecting devices and permits to transmit and receive data.

Protocol Used at Data Link Layer: PPP, ATM, Ethernet, SLIP, FDDI

Layer 1: Physical layer

The lowest layer of the OSI reference model is the physical layer. The physical layer describes the electrical and physical specifications of the data connection. It contains information in the form of bits, and the digital data bits are converted into electrical signals.

It defines the transmission mode, whether it is simplex, half-duplex, or full-duplex between two devices on the network. The physical layer defines voltages and data rates needed for transmission. The physical layer isn’t concerned with protocols or other higher-layer items.

Protocol Used at Physical Layer: Ethernet, USB, Bluetooth

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