OSI Layer Model


OSI Layers

The Open System Interconnect model (OSI) is made up of seven layers which are ranked in a hierarchical structure. This model usually depicts the procedure in which protocols and computer services can be standardized. The process allows sharing of information by computers. This model was developed to execute the different interconnection kinds of networks.

The interconnection is performed regardless of the differences in the programming language, hardware, application software, media and operating system. It is composed of seven layers that are subdivided into other layers. It is always important to understand the open System Interconnection model as it fosters the knowledge, building computer networks and designing. The people who developed the model analyzed the processes of transmitting information across a system. They lowered them to its logical elements.


The Open System Interconnection Model layers

The items that executed similar functions were connected and encompassed within one layer. The seven layers include;

The Physical Layer

It is the first layer which is effective in transmitting and receiving wire level data. The physical circuits are developed on this layer of the model. It outlines the optical or electrical signals used for transmission. The physical layer correctly utilizes the optical and electrical signaling approaches. The techniques involve impedance properties, synchronization, and connector’s physical shape, the voltage used in transmitting signals and media type.

The kind of media can be the optical fiber, twisted pair or coaxial cable. This layer is always restricted to the procedures that are required in placing the transmission signals above the media. It is also confined to acquire the signals that originate from the media. The physical layer has a lower partition known as the physical connector. It is also connected to the communication media. The transmission media is referred to as the Layer 0 of the model and is always situated outside the physical layer. 

Data link layer

Data link layer is the second in the ranking of the Open System Interconnection model. The second layer is always situated under the network layer but over the physical layer. Data link delivers an all-around validity of the transmitted information which runs from end to end. This layer is separated into two other layers known as the Logic Link Control sub layer and The Media Access Control sub layer.

The logical link layer is subjected to checking errors, controlling the flow and synchronizing of frames. Media Access Control sublayer (MAC), dictates the hosts’ physical addressing. It allows for communication with other devices that are present on the network by maintaining the physical device addresses.

Media Access Control sublayer constitutes the low-level address and is burned to the network cards. The low-level address is useful in determining the origin and terminal of the network traffic. The MAC addresses are also known as layer two addresses, hardware addresses, and physical addresses. Their names are also derived from the work they perform and are always efficient in their operation. This layer is responsible for activating, maintaining, and deactivating the data link connections.

Network layer

The network layer is the third in the ranking of the Open System Interconnection layers. This layer is active in managing local addressing information. The information is always contained in the packets which are delivered by the network layer to their correct terminal. The routers are utilized in directing the information that is conveyed by the Network layer.

This data is usually contained in the routing table. The routers are known to be efficient in building the system thus provide the intended actions in the network layer. The routing table is a list that describes the available destinations which are often kept in the router’s memory.

 The network layer works in close cooperation with the logical addresses in delivering the best performance. The logical addresses locate the computers on the network. It also determines the system in which the computer operates. Most layer protocols utilize the logical addresses in conveying the packets to the appropriate network.

The Internet protocol address is the logical addressing system that is used in the Network layer. The internet protocol addresses are also called the layer three addresses or the logical addresses. The network layer also activates, maintains and ends the network layer connections as well as transferring data between users.

Transport layer

This layer is ranked the fourth in the Open System Interconnection model. It is responsible for handling transport functions which involve reliable and unreliable delivery of the information to the terminal. It is also useful in dividing the information into smaller packets on the sending computer. The breaking down of data allows the missing packets to be sent for the second time in case they get lost during the transmission.

The ACKs (Acknowledgements) determines the missing packets from remote devices when they receive the packages. The transport system unravels all the packets at the receiving system and refining the initial information. The transport layer also helps in TCP segment sequencing. Sequencing is a service that carries the TCP segments that are disorganized and ensures they are in the right order.

This layer allows for the option of describing the service address for the application on the origin and terminal computer. This option allows the computer to outline the application where the request originated and application in which the request will follow. Most of the network applications operate on s system concurrently.

Other mechanisms will locate the applications that should obtain the incoming information. The User Datagram Protocol and Transmission Control Protocol use a technique known as port numbers that allow for multiplexing and de-multiplexing

Session layer

The session layer is the fifth layer of the Open System Interconnection model. It is located between the presentation layer and the transport layer. It is a layer which is responsible for activating, managing and ending connections of applications at each end of the transmission.  The service and rules are proposed in the connection establishment phase for communication between the devices.

The two devices must comply with the rules for the process to be efficient. The data phase commences immediately the rules are put in place. Termination of the communication always happens once the session is completed. This process usually becomes successful once it is terminated. The session layer is practically joined with the transport layer to improve the transmission process.

Presentation layer

It is the sixth layer of the model and is situated below the Application layer. This layer receives data from the application layer and ensures a correct format of the data to be conveyed across the network. In case the data is not in a proper format, the presentation layer will convert it to the appropriate size. It also happens in a similar way once if it receives information from the session layer. It does this by converting the data when it is not in the right format.

There is formatting function that is executed at the presentation layer. They include encryption, compression and making sure the character code set can be interpreted the other side.  These figures include Unicode, ASCII and the Extended Binary Coded Decimal Interchange Code utilized by the servers of IBM. In case there is a need for compressing the information from a network application in use, the presentation layer receives the request from the application layer, although the presentation layer does the compression.

Application Layer

The application layer is the seventh layer of the Open System Interconnection model. It is usually at the top of the hierarchy where the real traffic information is derived from the Application layer. The transmission may be generated by a command in the telnet protocol, HTTP protocol, and FTP protocol. This layer is often concerned with the application requirements. The application layer provides the service elements to all the application processes.

These items often execute the interprocess communication. They also outline the procedures that are used in the construction of application protocols. The protocols are used to access the services made available by the servers within the network

The benefits of Open System Interconnection model

  • The can be substituted by new protocols such as the dynamic technology
  • The rules in the model are made invisible as they are well hidden
  • It reinforces both the connection-oriented services and those that are not related to the connection
  • The model is useful in differentiating between the protocols, interfaces, and services


The downsides of Open System Interconnection model

  • It is usually tiresome to fit the rules in the model
  • It is only utilized as a reference model
  • It was formulated before the protocols were invented


The engineer of communications is often situated at the protocols that run at the bottom four layers of the OSI model. The Data Link, Network Layer, physical layer and transport layer are the ones that offer the basic communication service.

The above layers assist computer scientists in coming up with distributed applications programs. They utilize the service made available by the system. The OSI model is adjustable and can change with the technology. This model has acted as a perfect reference because of the useful functions in each layer.