This article explains the OSI Model along with a detailed explanation for its 7 layers. Explore the advantages and disadvantages of this networking model.
When we talk about a networking model, the two models which come to our mind are: TCP / IP model and the OSI model. A networking model describes how data / message / information can be transmitted from one networking component (or system or computer) to the other. A network model defines protocols that are required to build a network at the local or global level. In this article, let us discuss the OSI model, its seven layers and what are the advantages and disadvantages of using the OSI model.
What is OSI Model?
By definition, “The Open Systems Interconnection model (OSI model) is a conceptual model that characterizes and standardizes the communication functions of a telecommunication or computing system without regard to its underlying internal structure and technology.” The main purpose of the OSI Model is to make diverse communication systems interoperable with standard communication protocols. The OSI Model partitions the communication system into different abstraction layers. In layman terms, we can say that the OSI Model provides a standard to different types of computer systems which enables them to communicate with each other.
International Organization of Standardization launched the OSI Model as a product of the Open Systems Interconnection in 1984.
In the OSI Model, each layer has a specific job to handle. Each of these layers communicates with the layer above and below itself. The roles of these layers are so precise that the DDoS attacks a particular layer of the OSI Model to disrupt the communication.
Importance of the OSI Model
The modern internet does not follow the OSI Model strictly. Instead, it follows the TCP / IP suite, which is simple with four layers. But, the OSI Model is still very useful for troubleshooting different network problems. The OSI Model helps tp break down the problem into more unaffected parts, isolate the defective layer and then points out the source of trouble. By this, one can save a lot of time, energy and resources by doing unnecessary work.
Layers of the OSI Model
The server architecture system of the OSI Model has seven different layers. Each of these layers has specific functions and is defined accordingly. These layers work together to transmit data from one layer to another. These layers are categorized into upper Layer and lower Layer:
- The Upper layers: The upper layers mainly concern themselves with the end system and the application. The highest layer is closest to the end-user in the communication system. Here the communication from end-user to another commences with the interaction between end-user and the application.
- The lower layers: These layers deal with all the activities which are concerned with the transportation of data.
Basically, the seven layers are:
- Data link
- Physical layer
Physical layer (1st layer)
This is the lowest layer of the OSI model. The physical layer helps the user to define the physical and the electrical specification of the data connection in the communication. Here the information is contained in the form of This layer transmits individual bits from one node to the next. When the physical layer receives the data, it converts the signals into 0s and 1s and sends them to the data link layer, which puts the frame back together. The signals can be of the following types: 1100, 0101, 0011, 0001. Let’s have a look at the functions of the Physical Layer:
- Bit rate control: The Physical Layer defines the rate of transmission. That is, it defines the number of bits sent per second.
- Transmission mode: This Layer defines the way the given data should travel between two devices. The transmission modes available in the OSI model are half-duplex, duplex and simplex.
- Physical topologies: The physical Layer determines how different nodes or devices can arrange themselves in a network, for example, star, bus or mesh topology.
- Bit Synchronization: The bits can be synchronized in the OSI model with the help of the physical layer. Here, the physical layer provides a clock which offers the synchronization of bits. This particular clock controls both sender and receiver, and hence it can provide sync at the bit level.
The examples of the physical layers can be a modem, repeater, and cables.
Data Link Layer (DLL) (Layer 2)
The data link layer concerns itself with the node to node delivery. It ensures that the data transfer from one node to the other through the physical layer is completely errored free. When a data packet arrives in a network, the DLL transmits it to the host taking the help of its MAC address. The data link layer is further divided into two sub-layers, namely Logical Link Control (LLC) and Media Access Control (MAC). The main functions of the DLL are as follows:
- Access Control: Suppose multiple devices share a single communication channel. The MAC sub-layer of the DLL helps to identify which device should control the channel at a particular time.
- Flow control: To avoid data corruption, one should make sure that the rate of data flow is constant on both sides. DLL makes sure that this transfer rate remains constant.
- Error Control: DLL can provide error control where DLL detects and retransmits the lost or damaged frames.
- Framing: One of the functions of the data link layer is framing. Framing provides a way to a sender to transfer a set of bits which are meaningful to the receiver. This is accomplished by attaching some special patterns in the frame.
Network layer (Layer 3)
The network layer transmits data from one host to another, over several different networks. It also identifies different paths of data transfer, points out the shortest one. It is also responsible for taking care of packet routing. The network layer also places the sender’s and the receiver’s IP address in the header. Following are the functions of the network layer:
- Routing: The network layer is responsible for identifying the most suitable route for the transmission of data. This function of the network layer is called routing.
- Logical addressing: The network layer distinguishes different devices/systems/computers in a particular network or in an inter-network transmission by giving them a unique IP address. This becomes easy when one has to identify the sender and the receiver.
Transport Layer (Layer 4)
The role of the transport layer is to provide service to the application layer and in return, take services from the network layer. The data in the transport layer is termed as segments. It concerns itself with end to end delivery of the complete message. The message layer also gives confirmation of successful data transmission and in case of any error, re-transmits the data. The work of the transport layer can be categorized in two ways:
- At the sender’s side: After receiving data from the upper layers, the transport layer then performs segmentation on the data. The transport layer also executes the flow and error control to ensure proper transmission of data from the sender to the receiver. The layer also adds source and destination port number in its header and then forwards the segmented data to the network layer.
- At the receiver’s end: The transport layer first reads the port number from its header and then forwards the data to the respective applications. It sequences and reassembles the segmented data.
The functions of the transport layer are as follows:
- Segmentation and reassembly: The transport layer first receives data from the upper layer and then breaks the message/data into several smaller units called segments. Each of these segments has a header attached to it. The transport layer then reassembles the message in the destination system.
- Service Point Addressing: The main objective of the transport layer is to deliver the correct message to the correct process. Here, to make sure the correct delivery happens, the transport layer includes a type of address viz service point address or port address. Here, by specifying the address, transport layer ensures correct delivery of the message.
Session Layer (Layer 5)
The main role of this layer is to establish the proper connection, maintenance of the session and also to ensure proper security during transmission of a message. The functions of this layer are as follows:
- Synchronization: This Layer allows the process to add synchronization points into the data during the transmission. These synchronization points help to recognize the error and cut them. This helps reduce the chances of data loss significantly.
- Dialogue controller: Here, the session layer allows the two systems which are involved in communication to start communicating with each other. This communication is done in the form of half-duplex or full-duplex.
- Session establishment, maintenance and termination: This Layer enables the two processes involved in communication to establish, use or terminate the connection.
Presentation Layer (Layer 6)
The presentation layer is also known as the Translation layer. After receiving data from the Application layer, this layer extracts and manipulates the data as per the format requirement and then transmits them over the other network. The main functions of the presentation layers are as follows:
- Translation: Example, ASCII to EBCDIC
- Compression: This Layer, reduces the number of bits that is needed ot be transmitted on the network by compressing the data.
- Encryption/ Decryption: This Layer translates the data into another form or code. The encrypted data is called ciphertext, and the decrypted data is called plain text. The layer uses a key-value in both encrypting and decrypting data.
Application Layer (Layer 7)
The topmost layer in the OSI Model stack is the application layer. Network applications work on this layer only. The data required to be transmitted produced in these applications. This layer also enables the application services to access the network and for displaying received information to the user by serving as a window. The common examples of application layers are browsers, Skype Messenger, etc. We can refer Application layer as Desktop layer The main functions of the application layer are as follows:
- Mail services
- Directory Services
- FTAM- File transfer access and management
- Network virtual terminal
Advantages of the OSI Model
- This model, even though outshone by IP / TCP model in terms of usage, still serves a guidance tool to any network model.
- One of the advantages of being a layered model is that if one has to make changes in any layer, this does not affect the other layers in the model. But the condition is that there is no drastic change in the interface of the layer.
- The OSI model is separated services, interfaces and protocols distinctly. Because of this, it is flexible.
- The OSI model supports both connectionless services and connection-oriented services.
Disadvantages of the OSI Model
- The OSI model is purely a theoretical one, and hence there is no availability of appropriate technology. Hence its practical implementation is widely restricted.
- The OSI model was launched in 1984, and till then the TCP / IP model was already implemented. Hence companies were highly reluctant to use this model.
- Because of the complexity of the OSI model, the implementation was highly costly and cumbersome.
- Even though there are many layers, some layers like the presentation layer and the session layer have very less use when the OSI model is practically deployed.
- In the OSI model, the duplication of services is evident. Functions like addressing, flow control and error control is offered by multiple layers.
- Since the standards set by the OSI model are primarily theoretical, they do not offer a satisfactory practical solution during network implementation.
- Since its launch, the OSI model has never met its practical needs and was never able to compete with the TCP / IP suite model. Hence people used to term this model as of ‘inferior quality’.
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