OSI Model

Open Systems Interconnection (OSI) Model


Use 7 layers to define basic functions of Internet.

Published by ISO in 1984.

The basic model for today’s Internet.

OSI is just a model, not any implementation.

The most popular TCP/IP protocol is designed based on OSI model, or, an implementation of several layers of OSI.

(Why include links? Repeating is meaningless. Here is just a summary)


  • Reduces complexity
  • Standardizes interfaces
  • Facilitates modular engineering
  • Ensures interoperable technology
  • Accelerates evolution
  • Simplifies teaching and learning

It defines standardized interfaces between different layers. So that all layers can be independent and can have different implementations.


From bottom to top.

7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical

1 Physical Layer


The most basic physical transmission.

“Is the wifi working?”

Binary raw bit transmission.

  • electrical, mechanical, procedural, and functional specifications of the physical link

From wikipedia:

The physical layer is responsible for the transmission and reception of unstructured raw data between a device and a physical transmission medium. It converts the digital bits into electrical, radio, or optical signals.


3 Network Layer


Addressing and Routing

  1. Connectionless communication
  2. Host addressing (IP address)
  3. Message forwarding

Data Delivery


4 Transport Layer

  • TCP: segment
  • UDP: datagram

host-to-host communication


How to transport data between 2 hosts reliably and effectively

  • connection-oriented communication
  • Same order delivery
  • reliability
  • flow control
  • Congestion avoidance
  • multiplexing
  • transportation issues handling
  • virtual circuits


5 Session Layer

Basically, the top 3 layers, application presentation and session layers, can be regarded as the “Application” since they are all controlled by applications.

Session layers manages different sessions between application processes between end-users

Establishes, manages, and terminates sessions between applications


6 Presentation Layer

data translation

Also called “syntax layer”.

  • Data conversion
  • Character code translation
  • Compression
  • Encryption and Decryption


7 Application

Different definitions for OSI and TCP/IP (actually same name, different layers).

The applications are responsible for 5,6,7 three layers. Some applications might be simple, will only implement functionality of 7th layer.

Network Processes to Application

defines user interface responsible for displaying received information to the user


Data Encapsulation

applications generate data

Data begin with 7th layer, the application layer. They will be transmitted from top to bottom. Each layer will include a HDR (header) to the data. Each layer stores its information in HDR.

The process of “data going throughout 7 layers from top to lowermost one” in which each layer concatenates its header (or footer or both) is called data encapsulation.


Upon receiving, data will be “de-encapsulated”.

Each layer will check the information it is responsible for.

From lowermost level to topmost layer.

At the end only the original data will be transmitted to application.

TCP/IP Stack

Now everyone uses TCP/IP. It is an implementation of OSI model.

It has 4 layers.


The most “important” ones are 3rd and 4th layer.


For networking, it usually refers to the 4 low layers.

For software engineering, people care the top 3 layers more.