Introduction To OSI Reference Model

What Is OSI Reference Model?

The International Standards Organization (ISO) created the OSI or Open System Interconnection, model in 1984. It provides a layered networking framework that describes how communications between heterogeneous systems should be done. It consists of seven connected layers. The physical layer, data link layer, network layer, transport layer, session layer, presentation layer and application layer are the seven layers of the OSI Model.

The network support layers are the physical layer, data link layer and network layer. The layers control the actual data transfer from one device to another. The user support layers are the session layer, presentation layer and application layer. These layers enable interaction between unrelated software running in various environments. Linking the two groups is the transport layer.

The main functions of each of the layers are as follows −

• Physical Layer: Its function is to transmit individual bits from one node to another over a physical medium.
• Data Link Layer: It is responsible for the reliable transfer of data frames from one node to another connected by the physical layer.
• Network Layer: It manages the delivery of individual data packets from source to destination through appropriate addressing and routing.
• Transport Layer: It is responsible for the delivery of the entire message from the source host to the destination host.
• Session Layer: It establishes sessions between users and offers services like dialogue control and synchronization.
• Presentation Layer: It monitors the syntax and semantics of transmitted information through translation, compression and encryption.
• Application Layer: It provides high-level APIs (application program interfaces) to the users.

Characteristics Of OSI Model

• Upper layers and lower layers make up the OSI Model’s two layers.
• The top layer handles problems with the software’s applications.
• Application layers are located closest to users.
• The upper layer is referred to as the layer above another.
• Data transportation using hardware and software is handled by the lower layer.
• The physical layer, which deals with information stored on physical media, is the lowest layer.
• Helps users comprehend how networks communicate.
• Different network layers make troubleshooting simpler.
• Through the use of this model, new technological development is simpler to comprehend.
• It is possible to compare primary functional relationships on different layers.

1. Physical Layer

It is the final layer of the model that is in charge of setting up physical network devices for data acceptance. It can also terminate a network node connection with another node.

This layer receives raw data, which is then sent to higher layers. Additionally, it changes digital bits into different signals.

Functions of Physical Layer of OSI Model

1. Bit synchronization: The physical layer provides the synchronization of the bits by providing a clock. This clock controls both sender and receiver thus providing synchronization at the bit level.
2. Physical topologies: Physical layer specifies the way in which the different, devices/nodes are arranged in a network i.e. bus, star, or mesh topology.
3. Transmission mode: Physical layer also defines the way in which the data flows between the two connected devices. The various transmission modes possible are Simplex, half-duplex and full-duplex.
4. Bit rate control: The Physical layer also defines the transmission rate i.e. the number of bits sent per second.

* Hub, Repeater, Modem, Cables are Physical Layer devices.
** Network Layer, Data Link Layer, and Physical Layer are also known asLower Layers orHardware Layers.

2. Data Link Layer

By dividing the data into frames, this layer makes it possible to access the data for easier analysis. This makes sure that data is accurate and timely as it moves up the stack. It transmits data in packet form. Logical Link Control and Media Access Control are its two sublayers.

By identifying the protocol address from the header, logical link control is used to move the packets to the following layer. The network’s physical layer is connected by the media access control layer. This is accomplished by using Address Resolution Protocol to discover the receiver’s address.

Functions of Data Link Layer of OSI Model

1. Framing: Data link layer functionality includes framing. It offers a method for a sender to send a group of bits that are significant to the receiver. By including unique bit patterns at the start and end of the frame, this can be achieved.
2. Physical addressing: The Data link layer adds the physical addresses (MAC addresses) of the sender and/or receiver to each frame’s header after creating it.
3. Error control: The error control mechanism provided by the data link layer allows for the detection and retransmission of broken or lost frames.
4. Flow Control: The amount of data that can be sent before receiving acknowledgement is coordinated by flow control because the data rate must be constant on both sides in order to prevent data corruption.
5. Access control: The MAC sub-layer of the data link layer aids in identifying which device is in control of a communication channel when it is shared by several devices.

* Packet in Data Link layer is referred to asFrame.
** Data Link layer is handled by the NIC (Network Interface Card) and device drivers of host machines.
*** Switch & Bridge are Data Link Layer devices.

3. Network Layer

Data transmission between nodes in a network is handled by the network controller layer. Each node has a distinct address, which this layer reads to send data to the right place. Data is sent in packet form and after processing, a connection is made.

Functions of Network Layer of OSI Model

• Internetworking: Internetworking is the main responsibility of the network layer. It offers a logical link between various devices.
• Addressing: The source and destination addresses are added to the frame header by a network layer. On the internet, addressing is used to identify the device.
• Routing: The main element of the network layer, routing selects the most efficient route from the source to the destination from among all possible routes.
• Packetizing: The packets from the upper layer are received by and converted into packets by a network layer. The action in question is called packetizing. Internet Protocol is used to accomplish it (IP).

** Segment* in Network layer is referred to as **Packet**.
** Network layer is implemented by networking devices such as routers.

4. Transport Layer

At the core of the OSI model, the transport layer controls packet delivery. It keeps track of the data, segmentation/desegmentation and manages errors. This layer, which is used to fragment or reassemble data, runs from TP0 to TP4. It follows two protocols.

Transmission Control Protocol

a common protocol for host-to-host internet communication. Data is divided into smaller chunks that are transported via various internet routes to their destination. Additionally, it modifies the packets’ order at the receiving end.

User Datagram Protocol

It is a very unreliable transport layer protocol. The sender does not even attempt to send it, and the recipient does not receive any acknowledgement.

Functions of Transport Layer of OSI Model

• Service-point addressing: Due to the fact that computers run multiple programmes at once, data is transmitted from the source to the destination not only from one computer to another, but also from one process to another. The header containing the address known as a service-point address or port address is added by the transport layer. Data transmission between computers is the responsibility of the network layer, while message transmission to the right process is the responsibility of the transport layer.
• Segmentation and reassembly: In order to uniquely identify each segment, the transport layer divides the message into several pieces when it receives it from the upper layer. Each piece is then given a sequence number. The transport layer then reassembles the message based on their sequence numbers once the message has reached its destination.
• Connection control: Two services are offered by the transport layer. both connection-oriented and connectionless services are available. Each segment is treated as a separate packet by a connectionless service and they all take different paths to the same place. Before sending the packets, a connection-oriented service establishes a connection with the destination machine’s transport layer. All packets in a connection-oriented service take the same route.
• Flow control: The transport layer is also in responsible of controlling flow, but it does so end-to-end rather than over a single link…
• Error control: Error control is another duty of the transport layer. End-to-end error control is used instead of error control across a single link. The sender transport layer guarantees error-free the message delivery to the target location.

5. Session Layer

A connection in this layer is established by the number of computers in the network. Additionally, it handles the creation, administration and termination of local and remote applications. They can also choose to terminate a complete session/transmission.

Functions of Session Layer of OSI Model

1. Session establishment, maintenance and termination: The layer enables connection establishment, use, and termination between the two processes.
2. Synchronization: This layer enables the addition of checkpoints, or synchronisation points, in the data by a process. These synchronisation points assist in locating the error so that the data can be correctly resynchronized and ends of the messages are not cut prematurely and data loss is avoided.
3. Dialog Controller: The session layer enables the beginning of half-duplex or full-duplex communication between two systems.

**In the TCP/IP model, the “Application Layer” is made up of the three layers listed below (including the session layer).

**These three layers are implemented by the network application itself. These are also referred to as software layers or upper layers.

6. Presentation Layer

This layer is in charge of transforming data into semantics that applications can understand. It then sends the data to other layers after formatting it with things like compression and encryption. It is the networking model’s translator.

Functions of the Presentation layer of OSI Model

• Translation: Information is exchanged between two systems’ processes in the form of character strings, numbers and other data. The presentation layer manages the interoperability between the various encoding methods used by various computers. The data is transformed from a sender-dependent format into a common format, which is then transformed at the receiving end into a receiver-dependent format.
• Encryption: To maintain privacy, encryption is required. Encryption is a process that transforms the data sent by the sender into a different format before sending the resulting message over the network.
• Compression: Data compression reduces the amount of bits that must be transmitted by compressing the data. Data compression is crucial for multimedia content like text, audio, and video.

7. Application Layer

Users interact with the data on this layer. The layer identifies the communication channels through which an application’s data can be transmitted. Some of the applications are – Browsers, Messengers, and more. Application Layer is also called Desktop Layer.

Functions of the Application layer of OSI Model

• File transfer, access and management (FTAM): A user can access files on a remote computer, get files from a computer, and manage files on a remote computer using an application layer.
• Mail services: An application layer provides the facility for email forwarding and storage.
• Directory services: An application provides distributed database sources and is used to provide global information about various objects.

Interaction Between OSI Model Layers

Each layer of the OSI is traversed during data transfer between devices. Each of them is connected to the others and depends on communication to operate. Each layer communicates with its peer layer and subordinate layer. For instance, to obtain the information, the network layer and data link layer will work together. The information will then be transmitted to the physical layer.

Advantages Of The OSI Model

• Boosts standardization of hardware devices.
• The protocols are replaceable with technological changes.
• The standardization decreases complexity in interfaces.
• Ensure interoperable technology and evolution acceleration.
• It is useful for facilitating modular engineering.
• Supports connection-oriented and connectionless services.

Disadvantages Of The OSI Model

• The protocol fitting is very long and dull.
• It doesn’t follow a particular protocol.
• Many layers have similar or the same services.
• It is only useful as a reference model.
• These layers can’t function parallelly and need to wait for details from the previous layer.