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OSI Layering Model

Definition

The Open System Interconnection (OSI) is a network framework that implemented through network protocol in 7 layers. Open System Interconnection (OSI) was first developed by The International Organization for Standardization (ISO) in 1084. The 7 OSI Layer is a concept and theory for mapping a system in the network. For the function it self, each layer has a different function. All these 7 layers work collaboratively to transmit the data from one to another across the globe.

The 7 Layer in OSI Model

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Physical Layer (Layer 1)

The lowest layer of the OSI reference model is the physical layer. Physical layer is responsible for the actual physical connection between the devices. Bits are the form of information that stored inside physical layer. Physical layer is responsible to transmit individual bits from one node to another. The function of the physical layer are:

  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 bit level.

  2. Bit rate control : The physical layer also defines the transmission rate.

  3. Physical topologies : Physical layer specifies the way in which the different, devices/nodes are arranged in a network.

  4. 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.

The data link layer is responsible for the node to node delivery of the message. The main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer.

Data Link Layer is divided into two sub layers, such as Logical Link Control (LLC) and Media Access Control (MAC).

The functions of the data Link layer are :

  1. Framing : The packet received from Network layer is divided into frames depending on the frame size of NIC(Network Interface Card).

  2. Physical addressing : This frame is then added to the MAC address of the sender and the receiver in the header.

  3. Error control : From the addition of MAC addresses to this data, the data link layer provides the mechanism of error control in which it detects and retransmits damaged or lost frames.

  4. Flow Control : The data link layer also takes care of data transfer error-free frames by implementing flow control to maintain the data rate constant on the transmitter side and receiver side.

  5. Access control : When a single communication channel is shared by multiple devices, MAC sub-layer of data link layer helps to determine which device has control over the channel at a given time.

Network Layer (Layer 3)

Network layer works for the transmission of data from one host to the other located in different networks. This layer manages the route of the packets by select the shortest path to transmit the packet from the number of routes available. The sender & receiver’s IP address are placed in the header by the network layer to help answer the device uniquely. This layer is implemented using network devices such as routers.

The functions of the Network layer are :

  1. Routing : The network layer protocols determine which route is suitable from source to destination. This function of network layer is known as routing.

  2. Logical Addressing : In order to identify each device on internetwork uniquely, network layer defines an addressing scheme. The sender & receiver’s IP address are placed in the header by network layer. Such an address distinguishes each device uniquely and universally.

Transport Layer (Layer 4)

Transport layer provides services to application layer and takes services from network layer. The data in the transport layer is referred to as Segments. It is responsible for the End to End Delivery of the complete message. At sender's side, Transport layer receives the formatted data from the upper layers, performs Segmentation and also implements Flow & Error control to ensure proper data transmission. It also adds Source and Destination port number in its header and forwards the segmented data to the Network Layer. At receiver's side, ransport Layer reads the port number from its header and forwards the Data which it has received to the respective application. It also performs sequencing and reassembling of the segmented data.

The function of the transport layer are :

  1. Segmentation and Reassembly : This layer accepts the message from the (session) layer , breaks the message into smaller units . Each of the segment produced has a header associated with it. The transport layer at the destination station reassembles the message.

  2. Service Point Addressing : This layer has the function to deliver the message to correct process, transport layer header includes a type of address called service point address or port address. The transport layer makes sure that the message is delivered to the correct process.

The services provided by the transpor layer :

  1. Connection Oriented Service : In this transmission, the receiving device sends an acknowledgement, back to the source after a packet or group of packet is received thus making this transmission is reliable and secure. This transmission has a three-phase process which include Connection Establishment, Data Transfer, Termination / Disconnection.

  2. Connection Less Service : In this transmission, the receiver does not acknowledge receipt of a packet, so it allows for much faster communication between devices. Connection-oriented service is more reliable than connectionless Service. This transmission has a one-phase process and includes Data Transfer.

Session Layer (Layer 5)

This layer is responsible for establishment of connection, maintenance of sessions, authentication and also ensures security. The functions of the session layer are :

  1. Session establishment, maintenance, and termination : The layer allows the two processes to establish, use and terminate a connection.

  2. Synchronization : This layer allows a process to add checkpoints which are considered as synchronization points into the data. These synchronization point help to identify the error so that the data is re-synchronized properly, and ends of the messages are not cut prematurely and data loss is avoided.

  3. Dialog Controller : The session layer allows two systems to start communication with each other in half-duplex or full-duplex.

Presentation Layer (Layer 6)

Presentation layer is also called the Translation layer. The data from the application layer is extracted here and manipulated as per the required format to transmit over the network.

The presentation layer ensures the information that the application layer of one system sends out is readable by the application layer of another system. On the sending system it is responsible for conversion to standard, transmittable formats. On the receiving system it is responsible for the translation, formatting, and delivery of information for processing or display. In theory, it relieves application layer protocols of concern regarding syntactical differences in data representation within the end-user systems. An example of a presentation service would be the conversion of an extended binary coded decimal interchange code (EBCDIC-coded) text computer file to an ASCII-coded file. If necessary, the presentation layer might be able to translate between multiple data formats using a common format.

As previously explained, presentation layer has 3 functions:

  1. Translation: from ASCII to EBCDIC.

  2. Encryption/Decryption: Data encryption translates the data into another form or code. The encrypted data is known as the cipher text and the decrypted data is known as plain text. A key value is used for encrypting as well as decrypting data.

  3. Compression: Reduces the number of bits that need to be transmitted on the network.

Application Layer (Layer 7)

These applications produce the data, which has to be transferred over the network. This layer also serves as a window for the application services to access the network and for displaying the received information to the user. Ex: Application – Browsers, Skype Messenger etc.

An application layer is an abstraction layer that specifies the shared communications protocols and interface methods used by hosts in a communications network. Application layer is the top layer of OSI reference Model. The application layer abstraction is used in both of the standard models of computer networking: the Internet Protocol Suite (TCP / IP) and the OSI model. Application layer produced the data, which has to be transferred over the network, also serves as a window for the application services to access the network and for displaying the received information to the user.

The function of the Application layer are :

  1. Network Virtual Terminal, is a communications concept describing a variety of data terminal equipment (DTE), with different data rates, protocols, codes and formats, accommodated in the same network.

  2. FTAM-File transfer access and management, is an OSI application layer (layer 7) protocol that specifies a standard mechanism for access and management of a distributed network file system.

  3. Mail Services.

  4. Directory Services.

References

Microdata Indonesia. (2019, July 15). Pengertian Dan Fungsi 7 Model Osi Layer. http://microdataindonesia.co.id/news/read/169/pengertian-dan-fungsi-7-model-osi-layer#

GeeksforGeeks. (2020, February 4). Layers of OSI Model. https://www.geeksforgeeks.org/layers-of-osi-model/