OSI, TCP/IP and Hybrid Models

OSI Model is developed by International Organization for Standardization or ISO, the Open Systems Interconnection model or OSI model is a critical building block in networking. OSI Model helps in troubleshooting and understanding networks because of the layered approach that it follows. It consists of seven layers in which each layer has its functionality to perform.

Layers of OSI Model

• Physical layer
• Data Link layer
• Network layer
• Transport layer
• Session layer
• Presentation layer
• Application layer 

These layers are given from bottom to up as below. 

1. Physical Layer: Physical layer is the lowest layer in the OSI Model. This layer works on establishing actual physical connections among devices.

2. Data Link Layer: DLL works for the node-to-node delivery of the message.

3. Network Layer: The Network Layer is responsible for data transmission from one host to another located in multiple networks.

4. Transport Layer: The transport layer is responsible for message delivery across the points covered in the path to the destination.

5. Session Layer: The Session Layer works in the establishment, termination, and management sessions.

6. Presentation Layer: The presentation Layer has the work of encrypting and decryption data, and converting it to such form that can be accessed by Application Layer.

7. Application Layer: The application Layer is the topmost layer of the OSI Model, where the user interacts with the system.

OSI Model

Advantages of OSI Model

• Provides standards and interoperability.
• Split development(a person working in layer 3 need not be concerned with layer 7).
• Quicker development (as each layer is independent of the other, development in an OSI model is faster as compared to the old proprietary models). 

TCP/IP Model

OSI model was used for connectionless protocols like CLNS and CLMNP; but with the advent of TCP (connection-oriented protocol) a new model; i.e., TCP/IP Model came into play. In this model, the Application, Presentation, and Session layers of the OSI model were combined to form the Application layer in the TCP/IP model, and the Datalink and Physical layers in the OSI model were combined to form the Network access layer in the TCP/IP model and the Internet layer in the TCP/IP model was the equivalent of the Network layer in OSI model. 

Layers of TCP/IP Model

• Application Layer
• Transport Layer
• Internet Layer
• Network Access Layer

1. Network Access Layer: It is the lowest layer of the TCP/IP Model which comprises of Physical and Data Link Layer of the OSI Model. It helps in data transmission among the same network.

2. Internet Layer: This layer comprises of Network Layer of the OSI Model. It has the work of moving data packets from destination to source.

3. Transport Layer: This layer is similar to the Transport layer of the OSI Model. It has the work of Error-Free Message Delivery.

4. Application Layer: It is the topmost layer of the TCP/IP Model. This Layer is the combination of the Application, Presentation, and Session Layer of the OSI Model. 

OSI vs TCP/IP vs Hybrid

Advantages of TCP/IP Model

• TCP/IP supports various Network Routing Protocols.
• It is scalable and based on client-server architecture.
• It is an open protocol suite i.e., it’s not proprietary, so anyone can use it.
• TCP/IP works independently of the OS.

Difference between OSI Model and TCP/IP Model

OSI Model	TCP/IP Model
OSI refers to Open Systems Interconnection.	TCP refers to Transmission Control Protocol.
It consists of 7 layers: Physical layer Data Link layer Network layer Transport layer Session layer Presentation layer Application layer	It consists of 4 layers: Application Layer Transport Layer Internet Layer Network Access Layer
OSI Model deals with the functioning of the network.	TCP/IP Model is a standard protocol that allows connection with hosts over a network.
OSI Model follows a vertical approach.	TCP/IP Model follows a horizontal approach.
OSI Model is the reference model.	TCP/IP is the implementation of the OSI Model.
OSI Model provides both Connection-Less and Connection-Oriented Services.	TCP/IP Model provides only Connection-Less Services.

Hybrid Model

In the real world, we use a mix of both the OSI model and the TCP/IP model, called the Hybrid model. In the Hybrid model, the Application layer is a combination of the Application, Presentation, and Session layers of the OSI model (similar to the TCP/IP model). The remaining layers. The transport Layer, Network Layer, Data Link Layer, and Physical Layer are the same as the OSI model.