OSI Model
Table of Contents
What is OSI Model? #
The ISO OSI (Open Systems Interconnection) Model is a set of rules that explains how different computer systems communicate over the TCP / IP network.
The OSI Model was developed by International Organization for Standardization (ISO) and it consists of 7 layers.
What is OSI Model | Real World Examples
Layers of the OSI Model #
Each OSI layer has it’s own functions and responsibilities. Communication process entails both directions (encapsulation and decapsulation).
Layers 5,6 and 7 can be grouped together. They are all responsible to produce the data (Protocol Data Unit - PDU) for the Transport layer.
7. Application Layer #
Applications produce the data for the Transport layer and often provide a user interface to interact with.
Example protocols used in the Application layer: HTTP(S), SMTP, FTP,DNS, SSH…
The application layer is concerned with the specific type of application.
6. Presentation Layer #
The presentation layer is primarily responsible for syntax of the data for the applications to send and consume (e.g. HTML, CSV, JSON).
- Encryption / Decryption
- Compression
5. Session Layer #
Session Layer is responsible for establishing, managing, terminating sessions between two devices.
Network File System (NFS) or Server Message Block (SMB) are commonly used protocols at the session layer.
OSI and TCP IP Models - Best Explanation
4. Transport Layer #
The primary role of the Transport Layer is to ensure that data packets arrive in the right order.
- TCP - data must be intact (e.g. file sharing)
- acknowledgment (ACK) of the successful data transmission and re-transmission of the data in error
- slower than UDP but more reliable
- UDP - retaining packets is less critical (e.g. video streaming)
- faster than TCP but less reliable (no ACK)
The data in the transport layer is referred to as segments. In this layer source and destination port is added in the header. Segmentation is essential for multiplexing.
3. Network Layer #
The Network Layer is responsible for the transmission of data (in this layer referred to as packets) from one host to another that is located in different networks.
Network Layer also takes care of routing - finding the shortest path to transmit the packet, from a number of routes available.
Network Layer is utilizing IP protocol in order to route. Network devices (routers and switches) are appending the source and destination IP address to the packet’s header.
2. Data Link Layer #
The Data Link Layer refers to technologies used to connect two machines across a network.
Data in this layer is referred to as frame.
- Ethernet is an example of a standard at this layer.
- Switches and Bridges are common Data Link Layer devices.
The Data Link Layer is divided into two sublayers:
- Logical Link Control (LLC)
- Media Access Control (MAC)
1. Physical Layer #
The Physical Layer refers to the physical communication medium and technologies to transmit data across that medium.
The physical layer contains information in the form of bits. Physical Layer is responsible for transmitting individual bits from one node to the next.
When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will put the frame back together. Common physical layer devices are Hub, Repeater, Modem, and Cables.
» Sources « #
What is OSI Model:
- https://www.geeksforgeeks.org/computer-networks/open-systems-interconnection-model-osi/
- https://aws.amazon.com/what-is/osi-model/