OSI is a 7-layer architecture and each layer has specific functionality. This article describes the 7 layers of the OSI model.
The users of computer networks are located all over the world (a wide physical range). So, to ensure that data communication whether nationwide or worldwide can be developed and are compatible with each other, an international standard has been developed. These standards fit into the framework designed by the “International organization of standardization” (ISO). This framework is called the “Model for Open System Interconnection (OSI)”. This is normally referred to as OSI.
The OSI model has seven different layers. They are:
- Layer 1: Physical Layer
- Layer 2: Data Link Layer
- Layer 3: Network Layer
- Layer 4: Transport Layer
- Layer 5: Session Layer
- Layer 6: Presentation Layer
- Layer 7: Application Layer
The lowest layer of the OSI model is the physical layer. And the highest layer in the OSI model is the application layer. All applications do not require using all seven layers of the OSI model. Generally, the lower three layers of the OSI model are enough for most applications. Each layer of the OSI model is made from electronic circuits and/or software and also has a separate existence from the remaining layers.
Each layer is supposed to handle data or messages from the layers which are immediately above or below it. This task is done by following some protocols. So, each layer takes data from the adjacent layer, handles it according to the protocol rules, and then passes the processed data to the next layer on the other side.
Functionality of each layer
Layer 1: The Physical Layer
The physical layer of the OSI model performs the following functions:
- The physical layer of the OSI model will activate, maintain and deactivate the physical connection.
- The physical layer of the OSI model will define the voltages and data rates needed for transmission.
- The physical layer of the OSI model converts the data bits into electrical signals.
- The physical layer of the OSI model also decides whether the transmission is simplex, half duplex, or full duplex.
A physical layer does not perform the following functions:
- The physical layer of the OSI model does not detect or correct errors.
- The physical layer of the OSI model does not decide the medium or modulation.
Examples of the physical layer protocols are RS-232 and RS-449 standards.
Layer 2: The Data Link Layer
The data link layer of the OSI model performs the following functions:
- The data link layer of the OSI model synchronizes the data and does error control for the information that is to be transmitted over the physical link.
- To enable error detection, the data link layer of the OSI model adds error detection bits to the data which needs to be transmitted.
- The encoded data from the data link layer of the OSI model is then passed to the physical layer.
- These error detection bits are used by the data link layer on the side to detect and correct the errors.
- At this level, the outgoing messages are assembled into frames, and the system waits for the acknowledgment to be received after every frame is transmitted.
- Correct operation of the data link layer ensures reliable transmission of every message transmitted. Examples of the data link layer protocols are HDLC, SDLC, and X.25 protocol.
Layer 3: The Network Layer
The network layer of the OSI model performs the following functions:
- The network layer of the OSI model routes the signals through various channels to the other end.
- The network layer of the OSI model acts as the network controller by deciding which route data should be taken.
- The network layer of the OSI model also divides the outgoing messages into packets and to assemble incoming packets into messages for the higher levels.
Hence, the network layer acts as a network controller for routing data.
Layer 4: The Transport Layer
The transport layer of the OSI model has the following functions:
- The transport layer of the OSI model decides if the data transmission should take place on parallel paths or a single path.
- The transport layer of the OSI model does functions such as multiplexing, splitting, or segmenting the data.
- The transport layer of the OSI model guarantees the transmission of data from one end to the other.
- The transport layer of the OSI model breaks the data groups into smaller units so that they are handled more efficiently by the network layer.
Layer 5: The Session Layer
The session layer of the OSI model has the following functions:
- The session layer of the OSI model manages and synchronizes conversations between two different applications. The session layer of the OSI model is the level at which the user will establish the connection between the two systems to systems.
- The session layer of the OSI model controls logging on and off, user identification, billing, and session management.
- In the transmission of data from one system to the other, at the session layer of the OSI model streams of data are marked and resynchronized properly so that the ends of messages are not cut prematurely. It also ensures that data loss is avoided.
Layer 6: The Presentation Layer
The presentation layer of the OSI model has the following functions:
- The presentation layer of the OSI model ensures that the information is delivered in such a form that the receiving system will understand and use it.
- The form and syntax (language) of the two communicating systems can be the same and they may be different also. For example, one system is using ASCII code for file transfer and the other one uses IBM’s BCDIC.
- In such cases, the presentation layer of the OSI model will provide the “translation of data” from ASCII to EBCDIC and vice versa.
Layer 7: The Application Layer
The application layer of the OSI model has the following functions:
- The application layer of the OSI model is at the top of all the layers. The application layer of the OSI model provides different services such as manipulation of information in various ways, retransferring the files of information, distributing the results etc. to the user who is sitting above this layer.
- The functions such as login or password checking are also performed by the application layer of the OSI model.
Advantages of OSI Model
The followings are the advantage of the OSI model.
- It helps in determining the required software and hardware for building the network.
- With the use of the OSI model, it is easy to understand and communicate the process across a network.
- It becomes easy to troubleshoot as it can be pinpointed which layer is creating the problem.
