What is OSI Model ?

In this article we will discuss about OSI (Open System Interconnection) Model. It is a reference model which is design to understand large and complex network architecture in a simple layered approach. OSI model itself does not have any direct relationship with network. Actually, it defines a networking framework in which various networking protocols are works in different layers.

OSI Model in Networking
OSI Model

History of Open System Interconnection (OSI) Model

In 1970, we had several network communication protocols but they designed for same types of machines.For example, IBM machines could communicate only with IBM machines.

In 1978, ISO (International Standard Organisation) starts working to develop a network reference module. In 1984, finally a reference model has been developed and this is known as Open System Interconnection.

OSI model have 7 different layer. Each layer have different functions. Different types of protocols are works on different layers.

7 Different layers of Open System Interconnection (OSI) Model

As we already discussed, OSI Model have 7 different layers. In this session we will cover all these layers.

Application Layer

This is the 7th protocol of OSI Model. In this layer, different protocols have direct interact with user application. On this layer, various protocols works. They encapsulate their header on this layer. After the encapsulation, they handover this data to presentation layer.

Examples : HTTP, HTTPS, FTP, RIP, DNS, DHCP, BGP etc.

Presentation Layer

This layer is 6th layer of OSI Model. In this layer various operations are performed on original data from application layer. Here, mainly Data Encryption, Data Decryption, Data conversion, Compression and Serialization operations are performed on original data. After performing various operation, it handovers this data to Session Layer.

Session Layer

Session Layer is 5th layer of OSI model.  This layer maintain sessions between different computers. Session layer is responsible for session creation, session managing and session termination between different computers. This layer is also important for session checkpointing and recovery. After creation or termination of data, it handover the data to transport layer (described below).

Transport Layer

Transport layer is 4th layer of OSI model. In this layer, two major protocol TCP and UDP is works. This layer, chucks the actual data into segments and then encapsulate their head on this. After the encapsulation process, it handover this segment to network layer on which Internet Protocol (IP) protocol is working.

Ports : Port is a virtual path from source to destination. Both protocols TCP and UDP, having two 16 bit fields which is known as Source Port and Destination Port. Protocols at application layer having their per-defined port numbers. These port number is also known as “Well define ports“. During transmission of data, a client puts a random port number in Source Port filed and an well defined port in Destination field.

Protocols : TCP, UDP.

Network Layer

Network layer is a 3rd layer of OSI model. In this layer, Internet Protocol (IP) works. This layer receive data from Transport layer. Here, IP protocol, encapsulate their header on received data and thus form an data packet. IP protocol, here mainly adds source and destination IP address. This data packet further sends to Data Link Layer of OSI model.

Protocols : IPv4, IPv6, EIGRP, OSPF.

Data Link Layer

Data Link layer is 2nd layer of OSI model. This layer is responsible for node to node data transfer. This layer have two sub layer.

Medium Access Control (MAC) layer: Responsible how devices gain access to transmit data over a medium.

Logical Link Control (LLC) layer: It is responsible of identifying and encapsulating Network Layer protocols. It is also used for error checking and error correction.

Example: On data link layer, IEEE standards 802.3 (Ethernet) and 802.11 (Wi-Fi) works.

Physical Layer

This is first protocol of OSI model. This layer is responsible for sending and receiving data bits from one end to another end.

Example: Here, Transmission media like Wire, RJ45 etc works.

Summary

OSI (Open System Interconnection), is a reference model developed by ISO (International Standard Organisation) in 1984. This model have 7 different layers. Each layer have different protocols. It is very helpful for understanding data flow between computers.

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