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
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
- Connectionless communication
- Host addressing (IP address)
- Message forwarding
4 Transport Layer¶
- TCP: segment
- UDP: datagram
How to transport data between 2 hosts reliably and effectively
- connection-oriented communication
- Same order delivery
- flow control
- Congestion avoidance
- 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¶
Also called “syntax layer”.
- Data conversion
- Character code translation
- Encryption and Decryption
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
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.
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.