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The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one device, proceeding to the bottom layer, over the channel to the next device and back up the hierarchy.

Application

Presentation

Session

Transport

Network

Data Link

Physical

Real physical medium

Layer 1: Physical Layer

At the lowest level, the physical layer models the interface of a computer system to the physical medium for the provision of bit transmission. It includes such aspects as:

• When one side sends a 1, the other side receives a 1
• Determining the voltages used to represent a 1 and a 0
• The shape and type of the physical connections to be used

RS232, RS485, Fast Ethernet are examples of protocols with physical layer components.

Layer 2: Data Link Layer

The data link layer provides a framework around data for transmission by the physical layer; detection and correction of errors may be performed in this layer.
The data link layer:

• Recognizes the boundaries of each frame
• Determines the correct sequence of frames
• Regulates how many frames arrive over a specific period
• Provides error detection and correction mechanisms upon these frames

Layer 3: Network Layer

The network layer makes use of the underlying data link services to provide data transmission services across sub-networks. It provides the transfer of data between two hosts without the transport layers on the machines knowing about the mechanism.

The network layer must therefore be responsible for the establishment, maintenance and termination of the connection between two hosts across any intervening communications facility. It must deal with problems of addressing, routing and prevention of bottlenecks.

Layer 4: Transport Layer

This layers primary task is to hide all the network dependent characteristics from the layers above it. It provides transparent data transfer so all the protocols defined for the transport layer will only need to be implemented on the host computers and not on any intermediate computers in the network. The transport layer establishes and maintains a logical connection with the corresponding transport layer on a remote host and uses this connection to ensure complete data transfer.

Layer 5: Session Layer

The session layer provides services for the management and control of data flow between two computer systems. The period of time for which two users remain logically connected (even though not transmitting data continuously) is known as a session.

The purpose of the session layer is to provide a user-oriented connection service. A session protocol may provide a user interface by adding to the basic connection service, possibly by imposing a structure on the dialogue between the users.

Layer 6: Presentation Layer

The presentation layer is concerned with the format of the data being exchanged. It provides a set of data transformation services, typically including formatting and data translation.

For example, one user might use ASCII codes for character representation whereas another user might use EBCDIC. The presentation layer provides the code conversion.
Additionally, the presentation layer may provide sophisticated text compression techniques or may perform a data encryption operation on data to be transmitted.

Layer 7: Application Layer

At the highest layer in the OSI model, it is in this environment in which a users programs operate and communicate. All lower layers of the model exist to support, and make possible, the activities that take place at the application layer. This layer therefore contains management functions and generally useful mechanisms to support distributed applications.

The layer embodies a wide range of system-independent application features, some of which are well recognised and are therefore standardised by ISO. These include:

• file transfer and file directory operations (delete, rename, etc.)
• message handling services (e.g. email)

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