Friday, September 7, 2018

OSI & TCP/IP models

OSI & TCP/IP Models

The Open Systems Interconnection Model (OSI) was created by the International Organization for Standardization (ISO), the international standardization body. It was designed as a reference model for describing the functions of the communication system. The OSI model provides the basis for creating and implementing network standards and devices and describes how network applications on different computers can interact through networked media. It has seven levels, each layer describes another function of the data passing through the network.

Here is a graphical representation of these layers:

OSI model
The layers are usually numbered from the last, which means that the physical layer is considered the first layer. It is useful to remember these layers, because in the CCNA exam there will certainly be some questions. Most people study the mnemonics "Please do not throw pizza at sausage":

OSM Mnemonic
So, what is the purpose of these layers?
They are most often used by suppliers. They allow them to implement certain functions in the network device, which facilitates interaction with devices from other providers.

Below is a brief description of each level of the OSI model.

Physical: defines how to move bits from one device to another. It describes how cables, connectors and network adapters should work, and how to send and receive bits.
Data transmission channel: encapsulates the packet into a frame. The frame contains a header and a preliminary transition for device interaction. The header typically contains the source and destination MAC address. The trailer contains the frame control sequence field, which is used to detect transmission errors. The data link layer has two sub-layers:
1. Logical link management: used for flow control and error detection.
2. Media Access Control: used for hardware routing and access control.

Network: Specifies the routing, routing, and routing of the device. The device address (logical) is used to identify the host on the network (for example, by its IP address).
Transportation: segments large parts of the data received from the top-level protocols. Install and terminate the connections between the two computers. Used to control the flow and restore data.
Session: Determines how to establish and terminate a session between two systems.
Representation: definition of data formats. Compression and encryption are defined in this layer.
Application: this layer is closer to the user. Allows network applications to communicate with other network applications.

The following table shows which protocols are at the level of the OSI model:

OSI Protocols

TCP / IP model
The TCP / IP model was created in the 1970s by the Defense Research Projects Agency (DARPA). Like the OSI model, it describes general recommendations for the development and implementation of computer protocols. It consists of four layers: access to the network, the Internet, transport and the application:

IP-TCP model

The following figure shows a comparison between the TCP / IP model and the OSI model:

Comparison of TCP IP and OSI Models

As you can see in the image above, the TCP / IP model has fewer layers than the OSI model. The application, presentation, and OSI session layers are combined in one layer in the TCP / IP model. In addition, physical and data links are known as the network access layer in the TCP / IP model.

Differences between OSI and TCP / IP Models

There are other differences between these two models, in addition to the obvious difference in the number of layers. The OSI model prescribes the necessary steps for data transmission through the network and is very specific, determining which protocol is used for each level and how. The TCP / IP model is not specific. We can say that the prescribed OSI model and the TCP / IP model are described.