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OSI Model


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This model was developed in 1983 by the International Standards Organization (ISO). This was the first step taken to standardized the international protocols used in various layers. As it deals with connecting open systems, that is, systems that are open for communication with other systems, the model is called the ISO-OSI (Open System Interconnection) Reference Model.

OSI is a layered framework designed for network systems. Due to this model connection between all types of a computer system is possible. It has seven separate layers but each layer is related to each other. Each layer describes a part of the process of transferring data across a network.

This is the OSI model.

OSI Model

The OSI model has seven layers. The function of each layer is given below:-

Physical Layer

It is the lowest layer of the OSI model. The physical connection between the devices is possible due to this layer. Information stored in the physical layer is in the form of bits. Individual bits from one node to next are transmitted by with the help of this layer. The data received by this layer is converted into 0s and 1s and it is transferred to the Data Link Layer.

Function in the physical layer of the OSI Model

Physical layer performs the following functions :

  1. Synchronization of Bits: At the bit level, the transmitter and receiver should be synchronized. In another way transmitter and receiver, clocks should be synchronized.
  2. Data Rate: The number of bits sent per second which is also known as transmission rate is defined by this layer.
  3. Physical Topologies: To create a network the connection of devices is defined by physical topology. There are various topologies through which devices can be connected i.e, star topology (devices are connected through a central device), a ring topology (each device is attached to the next, making a ring), a mesh topology (each device connected to every other device), or a bus topology (each device on a common link)
  4. Mode of Transmission: The way of transmission between two devices i.e., simplex, half-duplex, or full-duplex is also defined by this layer.
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Data Link Layer

The node to node delivery of a message is possible due to the data link layer. This layer makes sure that the transfer of data takes place error-free from one node to another, over the physical layer. In a network when a data packet arrives, the data link layer is responsible to transmit it to the host, using its MAC address.

This layer is sub-divided into two layers:

  1. Logical Link Control (LLC)
  2. Media Access Control (MAC)

Depending on the frame size of NIC(Network Interface Card) the packets received from the network layer are further divided into frames. To get the receiver’s MAC address, a request is placed by the sender by sending the IP address whose MAC address is required and in returns gets the destination MAC address.

Function in the data link layer of the OSI Model

Data Link Layer performs the following functions:

  1. Framing: The Data link layer performs a function known as framing. This creates a path by which significant information can be transferred to the receiver by the sender. From the network layer data packets are received by this layer which is encapsulated into frames. The hardware receives each frame bit-by-bit sent by the data link layer. A special bit pattern at the beginning and end of each frame is attached so that it can be assembled again by the receiver.
  2. Physical Addressing: The MAC address or physical address is added to the header of each frame of the sender and/or receiver by this layer.
  3. Flow Control:  The data rate should be constant on both side i.e., the rate of generating the data must be equal to the rate of absorbing the data by the receiver. This flow of data is controlled by this layer.
  4. Error Control: By adding mechanisms to detect and retransmit damaged or lost frames, the data link layers adds reliability to the physical layer. It also deploys a method to restrict duplication of frames. Generally, error control is obtained via a trailer added to the frame end.
  5. Access Control: At a channel when there are more than one device connected this layer helps it to determine which device has command over it.

Network Layer

The delivery of packets across multiple networks(links) from source to destination is the responsibility of the network layer. The network layer is not needed when both the systems are connected to the same link. Although, when two systems are connected to the different networks, this layer is required as it achieves the transmission of data from one host to another. Networking devices such as routers are used to implement this layer.

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Function in the network layer of the OSI Model

Following functions are performed by the network layer:

  1. Routing: The suitable route from source to destination is determined by protocols of this layer and this function of the network layer is called routing.
  2. Logical Addressing: An addressing scheme is defined by protocols of the network layer, which is used to identified different devices on internetwork (a network of a network) or a large network.

Transport Layer

The delivery of the entire message from source-to-destination (end-to-end) is the responsibility of this layer. Data is known as Segments in this layer. While the network layer manages end-to-end delivery of each packet, it does not recognize any connection among these packets. The intact arrival of the entire message is ensured by this layer.

Function in the transport layer of the OSI Model

Functions of the transport layer are as follows:

  1. Segmentation and Reassembly: Multiple transmittable segments are formed by splitting a message, each having a sequence number. These numbers allow the transport layer to reassemble the message accurately and for identifying and replacing the packets which were lost during transmission.
  2. Service-point Addressing: Many times a computer runs multiple programs simultaneously. Due to this reason, the delivery of packets only from source-to-destination is not sufficient but they should also be delivered to the specific process on one computer to a specific process to another computer.  To assure the transmission of those packets, a kind of address is added known as service -point address (or port address) by the transport layer. Thus each packet is received by the network layer to the correct computer.

Session Layer

The establishment of connection, maintenance, synchronization, and termination of connection is the responsibility of this layer. This layer also ensures the security of connection and authentication.

Function in the session layer of the OSI Model

The functions of the session layers are:

  1. Synchronization: In a data stream, multiple synchronization points or simply checkpoints are added by this layer. These checkpoints check whether the data is properly synced or not and this helps in avoiding any kind of data loss.
  2. Dialog Control: he communication between two systems are permitted by this layer which can be in any mode i.e., half-duplex (one away at a time) or full-duplex (both ways data at a time).
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Presentation Layer

The presentation layer ensures that communicating machines can interoperate properly.  The data received by the receiver should be understandable and it is the responsibility of this layer. If syntax between the two communicating users is different then this layer operates as a translator.

Function in the presentation layer of the OSI Model

The following functions are performed by this layer:

  1. Encryption/Decryption: A system must be capable of assuring privacy for carrying sensitive information. The data is translated into another form using data encryption. Cypher text is the term assigned to the encrypted data and the decrypted data is referred to as plain text.  Both encryption and decryption are carried out using a key value.
  2. Translation: Since different computers use different encoding method, the interoperability is the responsibility of the presentation layer.  The information before transmission must be converted into bitstreams. Information is converted into a common format from sender dependent format by this layer at the transmitter. At the receiving machine, this layer again converts the common format into its receiver-dependent format.
  3. Compression: The number of bits sent through a network can be reduced by data compression. It is especially essential in the transmission of multimedia such as audio, video, etc.

Application Layer

At the topmost layer of the OSI model, this layer is implemented by the network application. This layer allows the user to access the network. The data that has to be transferred over the network is produced by these applications. The UI (user interface) and maintenance for services such as e-mail and remote file access control are implemented through this layer.

Function in the application layer of the OSI Model

The functions of the application layer are:

  1. FTAM (File Transfer Access and Management): This function allows the user to read, write, and perform various operations on a file on a remote computer.
  2. Network Virtual Terminal: This function allows the user to log on to a remote host.
  3. Directory Services: The access to database sources for global information is provided by this application.
  4. Mail Services: The e-mail forwarding and storage functionalities are also provided by this application.

The late invention of the OSI Model is the reason it is not implemented on the internet and is used just as a reference.

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