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The (Open Systems Interconnection) OSI has been developed by International Organization for Standardization (ISO). The OSI model provides a framework for creating and implementing networking standards and devices and describes how network applications on different computers can communicate through the network media. In this post, each of the seven layers of the OSI model will be explained in simple terms.

 

 

Layer 1 – Physical layer

Physical layer defines the physical medium itself. It details how cables, connectors and network interface cards are supposed to work and how to send and receive bits. When a networking problem occurs, many networking pros go right to the physical layer to check that all of the cables are properly connected and that the power plug hasn’t been pulled from the router, switch or computer, for example.

 

Layer 2 – Data Link

Data Link layer defines the format of data on the network. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. MAC layer is used for hardware addressing and for controlling the access method. The LLC layer is used for used for flow control and error detection.

 

Layer 3 – Network

Network Layer defines device addressing, routing, and path determination. Network layer acts as a boundary between the host and the subnet. It deals with routing issues, deadlock and conjestion issues caused by increased number of packet data transfer and decreasing the performance etc.

NFS uses Internetwork Protocol (IP) as its network layer interface. IP is responsible for routing, directing datagrams from one network to another. Even though IP packets are addressed using IP addresses, hardware addresses must be used to actually transport data from one host to another. The Address Resolution Protocol (ARP) is used to map the IP address to the hardware address.

 

 

Layer 4 – Transport

Transport layer is responsible for getting the entire message across, establishes and terminates connections between two computers and keeping track of fragmentation and out-of-order packets. Used for flow control and data recovery.

 

Two transport protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), sits at the transport layer.

TCP establishes connections between two hosts on the network through ‘sockets’ which are determined by the IP address and port number.

UDP on the other hand provides a low overhead transmission service, but with less error checking.

 

Layer 5 – Session

This layer defines how to establish, manage and terminate connections between applications.

 

Layer 6 – Presentation

The Presentation layer defines the data formats. The compression and encryption are also defined at this layer.

 

Layer 7 – Application

This layer provides network services to the end-users. In general: Layer 7 is the layer that users interact with directly, for example, users browse applications like web browsers (Google Chrome, Firefox, Safari, etc.), Email Applications (Outlook, Thunderbird, etc.), Chat Applications ( WhatsApp, Skype, Viber, etc.) directly are the examples of layer 7 applications.

 

 

 

References: Network World

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