Open Systems Interconnection ( OSI ) is a standard reference model for communication between two end users in a network. The model is used in developing products and understanding networks.OSI divides telecommunication into seven layers. The layers are in two groups. The upper four layers are used whenever a message passes from or to a user. The lower three layers are used when any message passes through the host computer. Messages intended for this computer pass to the upper layers. Messages destined for some other host are not passed up to the upper layers but are forwarded to another host.
Layer 1: The physical layer ...
This is the level of the actual hardware. It defines the physical characteristics of the network such as connections, voltage levels and timing. This layer conveys the bit stream through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier.
Layer 2: The data-link layer ...In this layer, the appropriate physical protocol is assigned to the data. Also, the type of network and the packet sequencing is defined.This layer provides synchronization for the physical level and does bit-stuffing for strings of 1's in excess of 5. It furnishes transmission protocol knowledge and management.
Layer 3: The network layer ...he way that the data will be sent to the recipient device is determined in this layer. Logical protocols, routing and addressing are handled here. This layer handles the routing of the data (sending it in the right direction to the right destination on outgoing transmissions and receiving incoming transmissions at the packet level). The network layer does routing and forwarding.
Layer 4: The transport layer ...This layer maintains flow control of data and provides for error checking and recovery of data between the devices. Flow control means that the Transport layer looks to see if data is coming from more than one application and integrates each application's data into a single stream for the physical network. This layer manages the end-to-end control (for example, determining whether all packets have arrived) and error-checking. It ensures complete data transfer.
Layer 5: The session layer ... It establishes, maintains and ends communication with the receiving device. This layer sets up, coordinates, and terminates conversations, exchanges, and dialogs between the applications at each end. It deals with session and connection coordination.
Layer 6: The presentation layer ...It takes the data provided by the Application layer and converts it into a standard format that the other layers can understand. This is a layer, usually part of an operating system, that converts incoming and outgoing data from one presentation format to another (for example, from a text stream into a popup window with the newly arrived text). Sometimes called the syntax layer.
Layer 7: The application layer ...This is the layer that actually interacts with the Operating System or application whenever the user chooses to transfer files, read messages or perform other network-related activities. This is the layer at which communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. (This layer is not the application itself, although some applications may perform application layer functions.)
Layers 1 - 4 are called as "Transport Set" and Layers 5 to 7 are called as "Application set".
To know which protocols are being used in different layers we can see in the following link.
Wonderful chart describing relation between all protocols
Protocols in different Layers
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