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Networking
Basics:
What is the OSI model?
Explains
the Open Systems Interconnection framework for networking
standards
The
OSI model:
- Allows various “open” systems
to communicate.
- The Open Systems Interconnection
model was created by the International
Standards Organization in the late 1970's.
- Serve as a blueprint for
all network communication technologies.
- Dividing up all the processes
of networking activity into seven layers.
- Each layer has its own distinct
functions and services.
The OSI model
consists of seven layers which are:
- The Physical Layer: transmits
raw data bits over a communication channel (mostly
mechanical and electrical issues)
- The Data
Link Layer: guarantees to the network layer that there
are no transmission errors by breaking the input data
stream up into frames and sending back acknowledgement
frames
- The Network
Layer: controls the operation of the involved subnet;
main issues are routing (determine a way from source
to destination) and dealing with problems of heterogeneous
networks, e. g. different size requirements of transmitted
data blocks
- The Transport
Layer: splits up data from the session layer if necessary
(segmentation) and ensures that the pieces arrive
correctly
- The Session
Layer: allows users on different computer systems
to establish a session between them, i. e. they are
able to transfer files or log into a remote system;
the conditions of communication are laid down, for
example full-duplex or half-duplex
- The Presentation
Layer: unlike the layers before it is concerned with
the syntax and semantics of the transmitted information;
it is concerned with all aspects of information representation
such as data encoding, data compression and encryption
- The Application
Layer: contains a variety of commonly needed protocols
like handling with different terminal types and file
systems; a label to identify the communication process,
its origin and destination application is added to
the transmitted information
Layers 4 to 7 are true end-to-end
layers, i. e. the layer on the source system carries on
a communication process with the same layer on the destination
system. In the lower layers the protocols are between
a system and its immediate neighbour, for example the
source system and a system "on the way" to the
destination.
Some of the
functions of the physical and the data link layer are
combined in the Medium Access Control (MAC) sublayer,
which in particular is important to Local Area Networks
(LANs). It determines how devices attached to the network
gain access to the transmission medium.
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A communication
between two systems via a relay system (another computer
or an interconnection device), causes the following
flow of information between the OSI architectures of
the involved systems:
Click on image to
enlarge
Note
that the OSI model does not lay down the specific protocols
used to communicate between two computers on a specific
layer. Although ISO recommends which protocols to use
with the OSI model, the model itself is in proper speaking
no standard of computer networking. Which protocol in
a single layer is actually used, depends on several
factors like the physical network, the needed reliability,
etc.
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