Feature of OSI Model :
1. Big picture of
communication over network is understandable through this OSI model.
2. We see how hardware and
software work together.
3. We can understand new
technologies as they are developed.
4. Troubleshooting is easier
by separate networks.
5. Can be used to compare
basic functional relationships on different networks.
Principles of OSI
Reference Model
The
OSI reference model has 7 layers. The principles that were applied to arrive at
the seven layers can be briefly summarized as follows:
1. A layer should be created
where a different abstraction is needed.
2. Each layer should perform a
well-defined function.
3. The function of each layer
should be chosen with an eye toward defining internationally standardized
protocols.
4. The layer boundaries should
be chosen to minimize the information flow across the interfaces.
5. The number of layers should
be large enough that distinct functions need not be thrown together in the same
layer out of necessity and small enough that architecture does not become
unwieldly.
Functions of Different
Layers :
Layer 1: The Physical
Layer :
1. It is the lowest layer of
the OSI Model.
2. It activates, maintains and
deactivates the physical connection.
3. It is responsible for
transmission and reception of the unstructured raw data over network.
4. Voltages and data rates
needed for transmission is defined in the physical layer.
5. It converts the digital/analog
bits into electrical signal or optical signals.
6. Data encoding is also done
in this layer.
Layer 2: Data Link Layer
:
1. Data link layer
synchronizes the information which is to be transmitted over the physical
layer.
2. The main function of this layer
is to make sure data transfer is error free from one node to another, over the
physical layer.
3. Transmitting and receiving
data frames sequentially is managed by this layer.
4. This layer sends and
expects acknowledgements for frames received and sent respectively. Resending
of non-acknowledgement received frames is also handled by this layer.
5. This layer establishes a
logical layer between two nodes and also manages the Frame traffic control over
the network. It signals the transmitting node to stop, when the frame buffers
are full.
Layer 3: The Network
Layer :
1. It routes the signal
through different channels from one node to other.
2. It acts as a network
controller. It manages the Subnet traffic.
3. It decides by which route
data should take.
4. It divides the outgoing
messages into packets and assembles the incoming packets into messages for
higher levels.
Layer 4: Transport Layer
:
1. It decides if data
transmission should be on parallel path or single path.
2. Functions such as
Multiplexing, Segmenting or Splitting on the data are done by this layer
3. It receives messages from
the Session layer above it, convert the message into smaller units and passes
it on to the Network layer.
4. Transport layer can be very
complex, depending upon the network requirements.
Transport
layer breaks the message (data) into small units so that they are handled more
efficiently by the network layer.
Layer 5: The Session
Layer :
1. Session layer manages and
synchronize the conversation between two different applications.
2. Transfer of data from
source to destination session layer streams of data are marked and are
resynchronized properly, so that the ends of the messages are not cut
prematurely and data loss is avoided.
Layer 6: The Presentation
Layer :
1. Presentation layer takes
care that the data is sent in such a way that the receiver will understand the
information (data) and will be able to use the data.
2. While receiving the data,
presentation layer transforms the data to be ready for the application layer.
3. Languages(syntax) can be
different of the two communicating systems. Under this condition presentation
layer plays a role of translator.
4. It perfroms Data
compression, Data encryption, Data conversion etc.
Layer 7: Application
Layer :
1. It is the topmost layer.
2. Transferring of files
disturbing the results to the user is also done in this layer. Mail services,
directory services, network resource etc are services provided by application
layer.
3. This layer mainly holds
application programs to act upon the received and to be sent data.
Merits of OSI reference
model:
1. OSI model distinguishes
well between the services, interfaces and protocols.
2. Protocols of OSI model are
very well hidden.
3. Protocols can be replaced
by new protocols as technology changes.
4. Supports connection
oriented services as well as connectionless service.
Demerits of OSI reference
model:
1. Model was devised before
the invention of protocols.
2. Fitting of protocols is
tedious task.
3. It is just used as a reference
model.
