Open Source Interconnections(OSI) Model
Open Systems Interconnection Basic Reference Model (OSI
Reference Model or OSI Model) is an abstract description for layered
communications and computer network protocol design. It was developed as part
of the Open Systems Interconnection (OSI ) initiative. 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
Network Complexities
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Different types of hardware and software
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Different operating systems
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Different types of data to be transferred –
text, images, music, video, etc
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Data must be transferred without errors
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Many different paths may have to be taken
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Yet computers must communicate with each
other in a network
This layer supports application and
end-user processes. Communication partners are identified, quality of service
is identified, user authentication and privacy are considered, and any
constraints on data syntax are identified. Everything at this layer is
application- Application specific.
This layer provides
application services for file transfers.
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e- (Layer 7) mail, and other network software
services. Telnet and FTP are applications that exist entirely in the
application level. Tiered application architectures are part of this layer.
This layer 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 Presentation transform data into the form that
the application layer can accept.
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(Layer 6) This layer formats and encrypts
data to be sent across a network, providing freedom from compatibility
problems. It is sometimes called the syntax layer. This layer establishes,
manages and terminates connections between applications. The session layer sets
up, coordinates, and Session terminates conversations, exchanges
·
(Layer 5) applications at each end. It deals
with session and connection coordination. This layer provides transparent
transfer of data between end Transport systems, or hosts, and is responsible
for end-to-end error recovery
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(Layer 4) and flow control. It ensures
complete data transfer. This layer provides switching and routing technologies,
creating logical paths, known as virtual circuits, for transmitting data from
Network node to node. Routing and forwarding are functions of this layer,
·
(Layer 3) well as addressing,
internetworking, error handling, congestion control and packet sequencing. At
this layer, data packets are encoded and decoded into bits. It furnishes
transmission protocol knowledge and management and handles errors in the
physical layer, flow control and frame synchronization. The data link layer is
divided into two sub layers: Data Link The Media Access Control (MAC) layer and
the Logical Link Control
·
(Layer 2) (LLC) layer. The MAC sub layer
controls how a computer on the network gains access to the data and permission
to transmit it. Electrical impulse, light or radio signal -- through the
network at the electrical and mechanical level.
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