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The Open System Interconnection Reference Model (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer network protocol design. In its most basic form, it divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model.
A layer is a collection of conceptually similar functions that provide services to the layer above it and receives service from the layer below it.
Description of OSI layers:
Layer 1: Physical Layer
·         Defines the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium.
·         Establishment and termination of a connection to a communications medium.
·         Participation in the process whereby the communication resources are effectively shared among multiple users.
·         Modulation or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel.
Layer 2: Data Link Layer
·         Provides the functional and procedural means to transfer data between network entities.
·         Detect and possibly correct errors that may occur in the Physical Layer. The error check is performed using Frame Check Sequence (FCS).
·         Addresses is then sought to see if it needs to process the rest of the frame itself or whether to pass it on to another host.
·         The Layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer.
·         MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it.
·         LLC layer controls frame synchronization, flow control and error checking.
 
Layer 3: Network Layer
·         Provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks.
·         Performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors.
·         Network Layer Routers operate at this layer—sending data throughout the extended network and making the Internet possible.
 
Layer 4: Transport Layer
·         Provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers.
·         Controls the reliability of a given link through flow control, segmentation/de-segmentation, and error control.
·         Transport Layer can keep track of the segments and retransmit those that fail.
Layer 5: Session Layer
·         Controls the dialogues (connections) between computers.
·         Establishes, manages and terminates the connections between the local and remote application.
·         Provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures.
·         Implemented explicitly in application environments that use remote procedure calls.
Layer 6: Presentation Layer
·         Establishes a context between Application Layer entities, in which the higher-layer entities can use different syntax and semantics, as long as the presentation service understands both and the mapping between them. The presentation service data units are then encapsulated into Session Protocol data units, and moved down the stack.
·         Provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.
Layer 7: Application Layer
·         This layer interacts with software applications that implement a communicating component.
·         Identifies communication partners, determines resource availability, and synchronizes communication.
o       When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit.
o       When determining resource availability, the application layer must decide whether sufficient network or the requested communication exists.
o       In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer.
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Posted on Tuesday, March 30, 2010 2:46 AM | Back to top


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