ISO & OSI model – layers of the Network

In network world, there is one major standard of network layers by ISO (International Standardization Organization) institution and one of the most used models for networking IP/TCP model (uses also the layers described in standardization model from ISO). One is known as standard which is suitable for any protocol standard, and the other is IP/TCP model used in networking for packet transmission between devices in the IP network. Both models are made out of layers – ISO has 7 layers, IP/TCP has 4 layers, that will be described further on.

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OSI model and TCP/IP layers

ISO model stands for Open System Interconnection; I will begin with the “lowest” in the hierarchy¬† (the physical) and proceeding to the “highest” (application layer).

Physical > Data Link > Network > Transport > Session > Presentation > Application

1. PHYSICAL LAYER
Carries a signals for all the higher layers (electrical/optical power). Data encoding: modifies digital signal (1s and 0s) used by PC.

2. DATA LINK LAYER
Provides error-free transfer of data frames from one node to another over the physical layer. Link establishment and termination; logical link between to nodes. Frame traffic control and other frame quality/transmission controls.

3. NETWORK LAYER
Controls the operation of the subnet, deciding which physical path of the data should take based on network conditions, priority of service and other factors, example: Logical – physical address mapping: translates logical addresses or names, into physical addresses.

4. TRANSPORT LAYER
This layer ensures that messages are delivered error-free; with no losses or duplications. The size and complexity of a transport protocol depends on the type of service it can get from the network layer.

5. SESSION LAYER
The session layer allows session establishment between processes running on different stations, example: Session support; performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on.

6. PRESENTATION LAYER
The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known to the application layer at the receiving station.

Example: Character code translation: for example, ASCII to EBCDIC and Data encryption: encrypt data for security purposes. For example, password encryption.

7. APPLICATION LAYER
The application layer serves as the window for users and application processes to access network services, such as electronic massaging like email or network terminals.

Layer explanations and also recommendation for further reading by: Microsoft: The OSI Model’s Seven Layers Defined and Functions Explained

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OSI model compared to TCP/IP and other known network protocols

 

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Difference between TCP/IP model and OSI model

Next time: Major differences between OSI Reference Model and TCP/IP Reference Model

I recommend you also read the IETF (Internet Enginnering Task Force) document about internet protocols: https://tools.ietf.org/id/draft-baker-ietf-core-04.html

And RFC1122 is also interesting because it has details how hosts operate with protocols and describes different relations between layers of the network: https://www.ietf.org/rfc/rfc1122.txt

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