Network Layer
The
network layer manages device addressing. It defines protocols for
opening and maintaining network path between systems. It is also manages
data transmission and switching procedures. Routers
operate at the network layer. The network layer looks at packet
addresses to determine routing methods. If a packet is for the system on
the local network, it is sent directly there. If it is addressed to the
system on another segment, the packet is sent to the router, which
forwards it on the desired network.
Data-Link Layer
The
data link layer provides the rules for sending and receiving
information across the physical connection between two systems. This
layer provides error detection and control. Because this layer provides
error control, higher layers do not need to handle such services. Switches and Bridges operate at this layer.
Physical Layer
Physical
layer sends and receives the bits. This layer defines the physical
characteristics of the medium such as connectors, electrical
characteristics such as voltage levels, and functional aspects such as
setting up and maintaining the physical link. Well-known physical layer
interfaces for local area networks (LANs) include Ethernet, Token-Ring,
and Fiber Distributed Data Interface (FDDI). Hubs and Repeaters work at this layer.
Presentation Layer
The
presentation layer protocols are part of the user’s operating system
and applications. In this layer information is formatted for display or
printing. Tasks like interpretation of codes within the data (such as
tabs or special graphics sequences), data compression, decompression,
encryption and the translation of other character sets are performed
here.
Session Layer
Session
layer sets up manages, and then tears the sessions between Presentation
layer entities. This layer coordinates communication between nodes, and
offers three different modes of communications: Simplex, Half Duplex
and Full Duplex.
Transport Layer
This
layer breaks and reassembles the large data into data stream. It
provides a high level of control for moving the information between
systems, including prioritization, more sophisticated error handling,
and security features. It controls packet sequence, regulates traffic,
and finds duplicate packets. If data is missing from the packet, the
receiving end transport layer protocol asks the sending end transport
layer protocol to retransmit packets. This layer ensures that all data
is in the proper order and received completely.
The OSI
model is divided in seven layers. These layers have been arranged in
two groups. Top three layers define how the applications in the
computers will communicate in with each other or with users. The bottom
four layers define how the data is transmitted from one end to another.
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Application
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Presentation
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Session
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Transport
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Network
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Data Link
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Physical
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Application
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Presentation
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Session
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Transport
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Network
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Data Link
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Physical
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The OSI reference model = Top 3 Layers + Bottom 4 Layers
Application Layer
It
is the layer where users actually communicate to the computer system.
Applications access the network services using defined procedures in
this layer. The application layer is used to define the applications
that handle file transfers, network management, terminal sessions, and
message exchange etc.