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23 Οκτ 2013 (πριν από 5 χρόνια και 3 μήνες)

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The international organization for standardization (ISO) developed the Open Systems Interconnection
(OSI) reference model to describe how information is transferred from one system to another system. When
information is converted into electric
al or light signals, it is transferred along a piece of wire or radio
waves. It is very important to define a set of rules for their transmission. Various types of rules and
regulations come under the ISO’s reference model. And it is very important to unde
rstand that why many
network protocols, such as IP and IPX, fail to fit properly into the scheme explained in ISO’s model.

This model covers all aspects of network communicatio
n and is known as OSI model.


Physical layer is required for transmitting a bit stream over a physical medium. It deals with mechanical
and electrical specification of the interface and transmission medium. It also defines the processor
and the
functions that the physical devices and interface have to perform for the transmission to occur, so in general
physical layer is concerned with the following.

1. Physical characteristics of interface

2. It specifies data transmission rate.

3. It al
so defines the physical topologies of the network.

4. It deals with sender and receiver synchronization at bit level.

5. Representation of bits , that is, to convert digital bit into electrical signal and vice


6. It also defines the direct
ion of the transmission of two devices whether it is simplex,

duplex or
full duplex.


The data link layer transforms the raw bit stream recei
ved from the physical layer into reliable information
and is responsible for node to node delivery. It makes the physical layer appear error free to the upper

Some of the responsibilities of data layer are as follows:


It deals with the physical

address of both the source and the destination.


When two or more devices are connected to the same link, Data link layer protocols determine which
device will have control over the link at any given time.


This layer also deals with error control
mechanism, that is, it detects and re
transmits damaged or loss
FRAME. It uses a mechanism to prevent duplication of frames.


It also deals with the rate at which the sender should send the data to the receiver so that a fast sender
does not overwhelm a

slow receiver, this mechanism is called FLOW CONTROL.


It divides the stream of bits received form the network layer into manageable data units called FRAME.


At the receiver end, again it performs the function of assembling the raw bits received

from the physical
layer into frames before passing them on to the network layer.


The network layer is responsible for source to destination delivery o
f packets across multiple networks.
Whereas data link layer's delivery is hop to hop delivery. Network layer assures that each packet gets from
its point of origin to its destination.

The specific responsibilities are as follows:


It deals with the logi
cal addressing of sender and receiver.


It deals with the routing of data between different links and networks.


This layer also deals with the overall congestion control mechanism of the network.


It divides the outgoing message into packets and assembles the incoming packets into message.


The transport layer is respon
sible for process to process delivery of the entire message, that is, this layer
ensures that the whole message arrives intact and in order over error control and flow control process to
process level.

Some responsibilities are as follows:


It deals wit
h segmentation and re
assembly of the message with proper sequence number in case the
message is very large.


It deals with connection control, that is, whether an end to end connection should be connectionless or
connection oriented.


This layer
also deals with flow control of the data but here the flow control is performed process to
process rather than across a single link.


It deals with service point addressing, that is, the network layer gets each packet to correct computer and
the transp
ort layer gets the entire message to the correct process on the computer.


This layer also deals with error control so that the receiving transport layer receives the message without
any error or duplication; error correction is usually achieved by re


The job of session layer is to establish, maintain and synchronize the interaction between the
communicating systems.

Specific responsib
ilities of session layer are as follows:


It allows two systems to start a communication (dialog control) with each other. The communication
between two systems is either in HALF or FULL DUPLEX mode.


This layer allows addition of check points, that
is, synchronization points into a stream of data, so that in
case of crash during the transmission of data, data can be re
transmitted from the check point last inserted,
of re
transmitting from

the beginning.


The presentation

layer is concerned with the syntax and semantics of information exchange between two
communicating systems.

Some responsibilities are as follows:


It deals with protocol conversion.


It carries out data compression to reduce the bandwidth of the dat
a to be transmitted.


It translates data between the format that the network requires and the format that the computer expects.


For security and privacy purpose, it carries out encryption of data at the sender's end and decryption at
the receiver's



This layer enables the user, whether human or software, to access the network. It provides user interface
and support for services such as,

Data base access


Remote file access and transfer.

Some specific func
tions are as follows:


This layer allows a user to access file in a remote computer.


This layer provides the bases of Email forwarding and storage.


This layer provides distributed database sources and access for global information about various
and objects.


This layer creates software emulation of a terminal at remote host. This application is known as network
virtual terminal exchange.