Things You Must Know About the OSI model
Overview
Open System Interconnection (OSI) is a conceptual model created by International Organization for Standardization to provide a standard to different computers to communicate with each other. The modern internet is not based on OSI however this is still used to troubleshoot network problems, to help visualize and communicate how networks operate, etc. This was introduced in 1983 but this was adopted by ISO as an international standard in 1984. This is a composition of 7 layers. Refer to figure 1.
Each layer of the OSI model has a specific job and each layer communicates with above and below layers. The following describes how each layer work from top to bottom.
Application Layer
This is the only layer that interacts with the client. The application layer uses protocols to send and receive messages, and present meaningful data to client. Some of the examples for the application layer protocols are Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Post Office Protocol (POP), Simple Mail Transfer Protocol (SMTP), and Domain Name System (DNS).
Presentation Layer
This is responsible for preparing data for the application layer. This implies the job of the presentation layer is to translate the incoming data into a syntax that the application layer of the receiving device can understand. Apart from that, the presentation layer is responsible for compressing the data it receives from the application layer before delivering it to the session layer.
Session Layer
The job of the session layer is to open and close the communication between two devices. This is responsible for transferring the data when the session is open and closing them when communication terminates. Here can set checkpoints during a transfer if the session is interrupted, devices can resume data transfer from the last checkpoint. But without the checkpoints, the entire transfer would have to begin again from scratch.
Transport Layer
This layer takes the data from the session layer and breaks it up into chunks called segments before sending it to the network layer. The transport layer on the receiving device reassembles the segments again and sends to the session layer. This layer handles both flow control and error control.
- Flow control: Determines the optimal speed of transmission that matches the connection speed of the receiving device.
- Error control: Checks whether the received data are complete and correct if not requests for retransmission.
Network Layer
This facilitates to transfer of the data between two networks. This will be helpful when the two devices are communicating on different networks otherwise the network layer is not necessary. Again, the network layer breaks up segments received from the transport layer into packets on the sender’s side and reassembles them on the receiving device. This layer uses Internet Protocol (IP) addresses to route the packets to the destination node.
Data Link Layer
This establishes the connection between 2 physically connected nodes on a network. Also, this breaks the packets received from the network layer into frames and sends them from the source to the destination. The data link layer is a composition of 2 parts including;
- Logical Link Control (LLC): Conducts error checking and synchronizes frames.
- Media Access Control (MAC): MAC addresses are used to connect devices.
Physical Layer
This layer includes the physical equipment such as cables, switches, wireless connections, etc. This layer transfers the data as bitstreams (a string of 0s and 1s).
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