Layered Approaches:

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In networks, the study of a network and working on a live network are two different things. Studying a network needs more detail whereas working on a network is relatively simple. Working is simpler than study because we work after we have studied that network.

Layered approach is a methodology to explain or see the working of a network. When we differentiate this approach, we find one that is best to study the network while another that is best to work on a network.

These two approaches are:

1. OSI Layers
2. TCP / IP Layers

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OSI Layers:

OSI stands for Open System Interconnection. It is an abstract model comprising of seven discreet layers that collectively show how a network works. The entire storyline that starts from composing the data on source machine and ends at arrival of that data on destination machine is differentiated among these layers.

The layers in OSI are:

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

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Application Layer

This layer is responsible for composing or constructing data that has to be sent. On receiver’s end it works for the proper arrival of data and the appropriate usage for the user.

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Presentation Layer

On source, this layer implements protocols for compressing and encrypting the data. Similarly on destination, it implements protocols for decompression and decryption.

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Session Layer

On source this layer designs a session that must be followed for timely delivery of the data. On destination, it checks whether the data arrives within the allowed session or not.

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Transport Layer

Transport layer performs three important functions which are as follows:

1. On source it packetizes the data whereas on destination it assembles all packets.
2. On source it decides whether to implement circuit-switching or to implement packet-switching. For first, it runs TCP and for the other it runs UDP protocol. On destination it all is verified.
3. On source a process ID (Port address) is assigned to every packet. On destination, it is checked.

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Network Layer

On source, it defines route for every packet whereas on destination, it checks whether the packet has followed the assigned route or not. This route definition is in consequence to the protocol running on transport layer (TCP or UDP).

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Data Link Layer

This layer is the busiest layer in OSI model. It implements the following functionalities:

1. Framing. (Packets are encapsulated in frames)
2. Flow Control. (To ensure collision-free transmission)
3. Error Control. (To ensure error-free transmission)

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Physical Layer

This layer works for the most fundamental workout. On source, it modulates the signal whereas on destination, it demodulates it. Modulation refers to the boarding of data on to the signal and demodulation means to landing the data from signal.

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TCP/IP Layers:

TCP/IP layers are also known as DoD (Department of Defense) layers. Since these layers were proposed by the Dept. of Defense of US Govt., these layers are standardized by the same name.

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