7 Layers of OSI Model and Their PDUs

 What is OSI Model:-

Open System Interconnection or OSI Model layers were first proposed by International Standard Organizations (ISO) to solve the multiple problems faced in telecommunication.
It is a conceptual model of telecommunication networking where the whole process is divided into seven effective layers. It synchronizes the whole process and helps one understand the interoperation of the diverse communications system under the standard protocol. The seven layers of the OSI model we are talking about are abstraction layers and each layer is connected to either one or two layers depending on its position. The interlinked layers serve one another to forward the communication process.

Understanding the Seven Layers of OSI Model :

Brief description of each of the OSI Model Layers:

OSI Model Layer 1 : Physical Layer :

This layer is the primary OSI Model layer and consists of connecting devices such as Ethernet cable, Token ring, Hubs, repeaters and electrical specification for data connection. This layer is responsible for connecting the device to physical transmission medium such as copper or optical cable or radio frequency or pulses of infrared. 

Layer 2 : Data Link Layer :

Data Link layer encodes and decodes data packets into bits and using its transmission protocol it handles the errors in the physical layer. It also controls flow and synchronization of the frames and at the same time it manages controlling access of the physical layer network devices.

There are two sub-layers in the 2nd layer of OSI Model layers,

  • The Media Access Control (MAC) layer
  • The Logical Link Control (LLC) Layer

While MAC determines how the computer in the network will have access to the LLC layer, the LLC layer works in frame synchronization.

Layer 3 : Network Layer :

This is the 3rd layer of the layers of OSI model. The network layer adds the concept of routing above the Data Link layer. When data arrives at the Network layer, the source and destination addresses contained inside each frame are examined to determine if the data has reached its final destination. If the data has reached the final destination, this Layer 3 formats the data into packets delivered up to the transport layer.

In layer 3 logical path is created for data transmission through network devices such as switch and routing devices. This works for transmitting data from one node to another.

Addressing, routing, and forwarding are the three major functions at this layer. To support routing, the Network layer maintains logical addresses such as IP addresses for devices on the network.  The Network layer also manages the mapping between these logical addresses and physical addresses. In IP Networking, this mapping is accomplished through the Address Resolution Protocol (ARP).

Layer 4: Transport Layer: 

The transport layer is the fourth in the layers of OSI model. Variable-length data sequence has to be transferred from source to destination host through one or more network. This transport Layer delivers data across network connections.

The transport layer is to be delivered the entire message from source to destination, and receiving data without any errors. Different transport protocols may support a range of optional capabilities including error recovery, flow control and support for re-transmission.

The supported protocols at this layer are TCP (Transmission Control Protocol) UDP ( User Datagram Protocol) and SPX (Sequential Packet Exchange – Novell Netware Operating System).

Layer 5: Session Layer :

In the session layer, Communication or connection between computers is controlled in the managed environment. It has the sole capacity of managing and terminating connection and helping in establishing connections between local and remote applications.

At layer 5 , it is built to support multiple types of connections that can be created dynamically and run over individual networks. Dynamically created connections that can run over individual networks can be controlled through the layer.

Layer 6 : Presentation Layer :

The Presentation layer is simplest in function of any piece of OSI Model layers. At layer 6, it handles syntax processing of message data such as format conversations and encryption/decryption needed to support the application layer above it.

This Presentation layer is the most simplified layer in the OSI model where data segregation is done based on the file type. Data can be classified in 4 types like ASCII – text format, audio format, picture format, and video format. Encryption to formats such as GIF, ASCII, PICT, JPEG, and much more are done to make it suitable for the application layer. The major job of this layer is to translate between application and network format.

Layer 7 : Application layer :

This is the last layer of the 7 layers of OSI Model and this layer represents the data that is understood by the end-user. The application layer supplies network services to end-user applications. Network services are typical protocols that work with users’ data. For Example, in a web browser application, the Application Layer protocol “HTTP” (Hypertext transfer protocol) “

In this layer the user interacts with the software application and communicating component is implemented. In this layer communication partners are identified along with quality services.

This Layer 7 provides data to (and obtains data from) the presentation layer.

PDU in the OSI Model: 

In telecommunication, the term protocol data unit (PDU) has the following meaning,

In an OSI Model, a unit of data is specified in a protocol of a given layer and consists of protocol-control information and possibly user data of that layer.

The PDU for each layers of OSI model is listed below-

Physical Layer – Raw Bits (1s or 0s) transmitted physically via the hardware

Data Link Layer – A-Frame (or series of bits)

Network Layer –  A packet that contains the source and destination address

Transport Layer – A segment that includes a TCP header and Data

Session Layer – The data passed to the network connection.

Presentation Layer – The data formatted for presentation.

Application Layer – The data received or transmitted by a software application.

As you can see, the protocol data unit changes between the seven different layers. The resulting information that is transferred from the application layer to the physical layer (and vice versa) is not altered, but the data undergoes a transformation in the process. The PDU defines the state of the data as it moves from one layer to the next.


  • OSI is a conceptual model of telecommunication networking where the whole process is divided into seven effective layers.
  • OSI stands for Open System Interconnection.
  • The seven layers of OSI model are, Physical layer, Data Link layer, Network layer, Transport layer, Session layer, Presentation layer and Application layer.
  • Each layer is connected to either one or two layers and data transmission is done between those layers.
  • Layer 1 or the Physical layer consists of connecting devices and the layer 7 or the application layer is the layer where user operates.
  • PDU stands for Protocol Data Unit. Every layer has a unique PDU. it is important for every network engineer to use PDU terms in their technical communication within IT environments.

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