The OSI Reference model

The OSI Reference model is divided in to 7 layers. The layers are the Application, Presentation, Session, Transport, Network, Data Link and Physical layers. Each layer has its own job to perform to make sure data makes if from one end of the communication to the other.

Each layer adds information to the previous layer. This adding of information at each level is called encapsulation. Once the data reaches that other side of the communication, the striping of information set by each layer at the destination host is called decapsulation.

Data added at each level of the OSI model is referred to as a Protocol Data Unit or PDU. Each PDU has different names at each layer of the OSI model. For the application layer a PDU is known as Data. At the transport layer it is known as a Segment. The Internetwork Layer refers to a PDU as a Packet. The Network Access layers a PDU is a Frame. While a PDU at the Physical Layer is at a bit level.

Once it is received at the destination host, the data encapsulated by the source host is then stripped and decapsulated with each layer reading the information previously set from the source host. The first step in this process of layering information starts at layer seven or the Application Layer.

OSI 7 Layer Reference Model
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|  application  | (7)
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|  presentation | (6) 
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|    session    | (5)
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|    transport  | (4) 
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|    network    | (3) 
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|    data link  | (2) 
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|    physical   | (1) 
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Application Layer 7

The Application Layer of the OSI model provides an interface between the software you use and the network which it is sent over. Software like web browsers, FTP and DNS are at the application.

Presentation Layer 6

After the data from the Application Layer is passed down, it reaches the Presentation Layer. The Presentation Layer is responsible for formatting the data in a way readable for the destination host. This can include compression and encryption of the data being sent and also formats for images such as JPEG and PNG.

Session Layer 5

The Session Layer is responsible for the management of sessions between users. This includes things like session establishment and termination of connections between applications.

Transport Layer 4

The data transfer between end systems and error correction. Protocols like TCP operate at this layer of the OSI model.

Network Layer 3

Routing and Switching of the data packet to the destination are done through the Network Layer. It creates a path between two hosts across a network.

Data Link Layer 2

At the Data Link layer information is encoded and decoded in to bits. The Data Link layer is further divided in to two sub-layers called the Media Access Control (MAC) and Logical Link Control layer (LLC). The Media access control layer provides access control to channels and addressing. The MAC layer makes it possible for multiple hosts on a network to communicate on a shared medium like Ethernet. The Logical Link Control layer acts as a interface between the Media Access layer and the Network Layer.

Physical Layer 1

The Physical Layer deals with the actual transmission of encoding and decoding of bits in to electrical signals which are then sent over a transmission medium. Physical Layer acts as an interface between transmission devices and transmission medium. It also controls data transmission rates measured in bits per second.