OSI Model

Open Systems Interconnection (OSI) is a set of internationally recognized, non proprietary standards for networking and for operating system involved in networking functions.

THE OSI REFERENCE MODEL The model was developed by the International Organisation for Standardisation (ISO) in It is now considered the primary Architectural model for inter-computer communications. The Open Systems Interconnection (OSI) reference model is a descriptive network scheme. It ensures greater compatibility and interoperability between various types of network technologies.

The OSI model describes how information or data makes its way from application programmes (such as spreadsheets) through a network medium (such as wire) to another application programme located on another network. The OSI reference model divides the problem of moving information between computers over a network medium into SEVEN smaller and more manageable problems. This separation into smaller more manageable functions is known as layering.

A LAYERED NETWORK MODEL The OSI Reference Model is composed of seven layers, each specifying particular network functions. The process of breaking up the functions or tasks of networking into layers reduces complexity. Each layer provides a service to the layer above it in the protocol specification. Each layer communicates with the same layer’s software or hardware on other computers.

The lower 4 layers (transport, network, data link and physical —Layers 4, 3, 2, and 1) are concerned with the flow of data from end to end through the network. The upper four layers of the OSI model (application, presentation and session—Layers 7, 6 and 5) are orientated more toward services to the applications. Data is Encapsulated with the necessary protocol information as it moves down the layers before network transit.

Tasks involved in sending letter

Computer AComputer B

LAYER 7 – The APPLICATION Layer Network virtual terminal File transfer, access and management Mail services Directory services

The application layer is responsible for providing services to the user.

LAYER 6 – The PRESENTATION Layer Translation Encryption Compression

The presentation layer is responsible for translation, compression, and encryption.

LAYER 5 – The SESSION Layer Dialog Control Synchronization

The session layer is responsible for dialog control and synchronization.

LAYER 4 – The TRANSPORT Layer Service point addressing Segmentation and reassembly Connection control Flow control Error control

The transport layer is responsible for the delivery of a message from one process to another.

LAYER 3 – The NETWORK Layer LOGICAL ADRESSING ROUTING

The network layer is responsible for the delivery of individual packets from the source host to the destination host.

LAYER 2 – The DATA LINK Layer FRAMING PHYSICAL ADDRESSING FLOW CONTROL ERROR CONTROL ACCESS CONTROL

The data link layer is responsible for moving frames from one hop (node) to the next.

LAYER 1 – The PHYSICAL Layer Physical characteristics of interfaces and medium Representation of bits Data rate Synchronization of bits Line configuration Physical topology Transmission mode

The physical layer is responsible for movements of individual bits from one hop (node) to the next.

Layering & Headers Each layer needs to add some control information to the data in order to do it’s job. This information is typically prepended to the data before being given to the lower layer. Once the lower layers deliver the the data and control information - the peer layer uses the control information.

Headers Process Transport Networ k Data Link Process Transport Network Data Link DATA H H H H HH