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7 Layers of OSI Reference Model
Posted Date: 23 May 2008 Resource Type: Articles/Knowledge Sharing Category: Computer & Technology
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Posted By: Ramkumar Member Level: Diamond
Rating:  Points: 1
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OSI-RM, OSI Reference Model, seven layer model- A model of network architecture and a suite of protocols (a protocol stack) to implement it, developed by ISO in 1978 as a framework for international standards in heterogeneous computer network architecture.
The OSI architecture is split between seven layers, from lowest to highest: 1 physical layer, 2 data link layer, 3 network layer, 4 transport layer, 5 session layer, 6 presentation layer, 7 application layer.
Each layer uses the layer immediately below it and provides a service to the layer above. In some implementations a layer may itself be composed of sub-layers.
OSI is the umbrella name for a series of non-proprietary protocols and specifications, comprising, among others, the OSI Reference Model, ASN.1 (Abstract Syntax Notation 1), BER (Basic Encoding Rules), CMIP and CMIS (Common Management Information Protocol and Services), X.400 (Message Handling System, or MHS), X.500 (Directory Service), Z39.50 (search and retrieval protocol used by WAIS), and many others. Apart from its actual application to real protocols, it also serves as a useful teaching model.
physical layer:
Layer one, the lowest layer in the OSI seven layer model. The physical layer encompasses details such as electrical and mechanical connections to the network, transmission of binary data as changing voltage levels on wires or similar concepts on other connectors, and data rates.
The physical layer is used by the data link layer.
Example physical layer protocols are CSMA/CD, token ring and bus.
data link layer:
Layer two, the second lowest layer in the OSI seven layer model. The data link layer splits data into frames (see fragmentation) for sending on the physical layer and receives acknowledgement frames. It performs error checking and re-transmits frames not received correctly. It provides an error-free virtual channel to the network layer. The data link layer is split into an upper sublayer, Logical Link Control (LLC), and a lower sublayer, Media Access Control (MAC).
Example protocols at this layer are ABP, Go Back N, SRP.
network layer:
The third lowest layer in the OSI seven layer model. The network layer determines routing of packets of data from sender to receiver via the data link layer and is used by the transport layer. The most common network layer protocol is IP.
transport layer:
The middle layer in the OSI seven layer model. The transport layer determines how to use the network layer to provide a virtual error-free, point to point connection so that host A can send messages to host B and they will arrive un-corrupted and in the correct order. It establishes and dissolves connections between hosts. It is used by the session layer.
An example transport layer protocol is Transmission Control Protocol (TCP).
session layer:
The third highest protocol layer (layer 5) in the OSI seven layer model. The session layer uses the transport layer to establish a connection between processes on different hosts. It handles security and creation of the session. It is used by the presentation layer.
presentation layer:
The second highest layer (layer 6) in the OSI seven layer model. Performs functions such as text compression, code or format conversion to try to smooth out differences between hosts. Allows incompatible processes in the application layer to communicate via the session layer.
application layer:
The top layer of the OSI seven layer model. This layer handles issues like network transparency, resource allocation and problem partitioning. The application layer is concerned with the user's view of the network (e.g. formatting electronic mail messages). The presentation layer provides the application layer with a familiar local representation of data independent of the format used on the network.
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