Introduction: The Open Systems Interconnection (OSI) model describes how information from a software application in one computer moves through a network medium to a software application in another Computer. It divides the communications processes into seven layers. It is a theoretical representation of what happens between two nodes communicating on a network. 4/18/2011 2 Manohar Prasad.
7 Layers: The Seven Layers of the OSI Model can be divided into two categories:
The Upper Layers of the OSI Model deal with application issues and are generally implemented only in Software. The Lower Layers of the OSI Model handle data transport issues and are implemented in hardware and Software. 4/18/2011 3 Manohar Prasad.
1. Physical Layer: The Physical Layer of the OSI Model defines connecter & interface specifications, as well as medium requirements. Protocols at the Physical Layer accept frames from the Data Link Layer & transmit signals. When receiving data, physical layer protocols detect & accept signals and pass on to the Data Link Layer. Connectivity devices such as Hubs and Repeaters operate at the Physical Layer. Physical Layer protocols also set the data transmission rate. 4/18/2011 4 Manohar Prasad.
2. Data Link Layer: The primary function of the Data Link Layer is to divide data received from the Network Layer into distinct frames that can be transmitted by the Physical Layer. The Data Link Layer is divided into two sublayers:
The Upper sublayer of the Data Link Layer, called the Logical Link Control, provides an interface to the Network Layer protocols , manages flow control & issues requests for transmission of data.
The Lower sublayer of the Data Link Layer, called the Media Access Control manages access to the Physical medium.
Ex. A Network Interface Card having MAC address. 4/18/2011 5 Manohar Prasad.
3. Network Layer: The primary function of the network layer is to translate network address into their physical counter parts & decide how to route data from the sender to the receiver. Each node has two types of addresses: (a) Network address (b) Physical address Network layer protocols determine the path from point A on one network to point B on another network by: (a)Delivery priorities (b)Network congestion. 4/18/2011 6 Manohar Prasad.
4. Transport Layer The transport layer accepts data from session layer & manages the end-to-end delivery of data. It breaks large data units into multiple smaller units, called segments. Generally, it is responsible for making sure that the data is delivered error-free & in proper sequence. Some transport layer protocols known as connection-oriented take steps to ensure that the data arrives exactly as it was sent. 4/18/2011 Manohar Prasad. 7
5. Session Layer The session layer, provides various services, including tracking the number of bytes that each end of the session has acknowledged receiving from the other end of the session. It allows applications functioning on devices to establish, manage, and terminate a dialog through a network. Session layer functionality includes: Virtual connection between application entities Synchronization of data flow Creation of dialog units Connection parameter negotiations Partitioning of services into functional groups Acknowledgements of data received during a session Retransmission of data if it is not received by a device 4/18/2011 Manohar Prasad. 8
6. Presentation Layer The presentation layer, is responsible for how an application formats the data to be sent out onto the network. The presentation layer allows an application to read (or understand) the message. Examples of presentation layer functionality include: • Encryption(such as the scrambling of passwords) and decryption of a data for security. • Compression and expansion of a data so that it travels efficiently. • Graphics formatting. • Content translation. • System-specific translation. 4/18/2011 Manohar Prasad. 9
7.Application Layer The application layer, provides an interface for the end user operating a device connected to a network. This layer is what the user sees, in terms of loading an application (such as Web browser or e-mail); that is, this application layer is the data the user views while using these applications. Examples of application layer functionality include: • Support for file transfers. • Ability to print on a network. • Electronic mail. • Electronic messaging. • Browsing the World Wide Web. 4/18/2011 Manohar Prasad. 10