3. Network layer
The network layer is responsible to deliver the packets of
data from the source to the destination. The major duties of network
layer are following:
The network layer adds a header to the packet coming
from the upper layer plus the logical addresses of the sender
Routing: To create a large network, the connecting devices are used
to route the packets to their destination.
4. Transport layer
The transport is responsible to deliver a message from one
process to another. The major duties of transport layer are
On computer several processes run at the same time. The
transport layer, therefore, must add a header to plus a port
address, to transfer the packet to the correct process on the
Segmentation and reassembly:
A message is
divided into segments and each segment has a sequence number.
This number enables the transport layer to reassembles the
message correctly and replace the packet that was lost in
The transport layer can be connectionless or
connection oriented. A connection oriented transport layer first
makes a connection with the transport layer at the destination
and then deliver packets while a connectionless network layer
deliver packets without making any connection.
5. Session layer:
The session layer is responsible to establish, maintain and
synchronies the interaction between communicating systems. The major
duties of session layer are following:
This layer is the network-dialed controller. It allows the
communication between two processes either in half duplex or
full duplex mode.
This layer adds synchronization points into a stream
of bits. These points are checked after every transmission for
efficient and error free transmission.
6. Presentation layer:
The presentation layer is responsible for the syntax and
semantics of the information exchanged between two systems. The
major duties of presentation layer are following:
This layer translates the encoded information at both ends
i.e. at sender and receiver.
This layer provides privacy to data. Encryption means that
sender transform the original information into another form and
send it over the network. Decryption at the receiver reverses
The number of bits can be reduced by the data compression.
This is important in the transmission of multimedia such as
text, audio and video.
7. Application layer:
The application layer is responsible to provide services to
the user. The major duties of application layer are following:
For email forwarding and storage this application is the
File transfer and access and management:
application allows a user to access and to retrieve files from a
remote computer and to manage or control files in a remote
information about various objects and services this application
provides distributed databases.