v  Transport layer provides the logical connection to two applications
processes running on different hosts within a network layers.

v  Transport layer is responsible to send the entire message from
sender to the receiver also ensure that the message reached to the destination
in same sequence as it was sent from the sender.

v  Transport layer makes the data transfer reliable as it divides the large
data into small chunks so data can be handling or transferred efficiently
through the network.

v  Transport layer provides different services

Ø  Reliable transfer of data

Ø  Control  flow of data

Ø  Transmission of data in same order as it was sent from sender

Ø  Multiplex ports






1.1.1.      Functions of transport layer

Ø  Addressing

Transport layer include service point addresses which is port
address.  This layer
gets the message to approve the process on the computer unrelated to the
Network Layer, which gets each packet to correct the computer.

Ø  Segmentation & end to end connection

A single message is divided  into chunks and sent the segments one by one
and at destination point it again reassemble the segments and chunks of that
message to decrypt it into original from ,each segment will receive in same
sequence as it was sent from sander.

Ø  Quality of services




Ø  Realization

Software and hardware controls.

layers involve different protocols TCP transfer control protocol is a connection oriented protocol TCP/IP
  application which sends the
data receives acknowledgement or verification that the data was actually
received and UDP is a connectionless protocol that does not provide the
reliable transfer of protocol. Transport layer is responsible of transparent
transfer of data.

1.1.2.      Issues with transport

Ø  Insecurity

I transport layer when data is being transferred through wireless
networks or other mediums, we are not sure about the secure transmission of
data because wireless devices are not always secure.

Ø  Lack of central authority

In wired networks it would be possible to monitor or check the data
traffic by applying some central security checks but in wireless networks these
security checks are not available so lack of central check authority is the
issue with transport layer.

Ø  Physically vulnerable

Nodes used for the transmission purpose are easily breakable which
can cause vulnerable threat to the loss of data being transmitted. 



above diagram the data which is first divided into segments is sent from
application layer to network layer and these segments passes through network
layer and receives  to application layer
and when data is received to the recipient it again merged into original form
as it was before.


1.2.   Session layer

v  Session layer or port layer controls the connection between two
computers or communication devices.

v  It performs the full duplex, half duplex and simplex duplication operation.

Ø  Full duplex

Full duplex communication refers to the transmission of data in two
directions at same time means data can be sent and received at the same time
through a single communication medium. Telephone is the example of full duplex

Ø  Half duplex

Half duplex mode was used in old networks it refers the
transmission of data only in one direction over a medium at same time, walkie
talkie is the example of the half duplex mode

Ø  Simplex

Simplex mode only allows the transfer of data in on direction it
does not the switching of direction while transmission of data. For example in
radio broadcast transmissions information only transfer in one direction to
multiple receivers.


v  Synchronization of the information from different sources is also handling
by session layer, for example sessions are used in live television programs
where video and audio streams coming from two different sources merged together,
sessions avoid overlapping and in case of connection loss it tries to recover
or manage the failed connection.

v  Session layer deals with the sessions and coordination of the
applications and networks.

v  Role of session layer is important in keeping the track of the
multiple telnet connections, download of multiple files requested by the FTP
applications and retrieval of the multiple web pages from a server at a time.

v  Dialogue control and synchronization are the functions of session

v  Check points in session layer are considered as recovery points in
case of system failure these check points are considered to be recovery configurations
and action.

v  Different protocols like NFS, NetBios names, RPC, SQL work at this


1.2.1.      Dialogue control

Dialogue control allows two systems to get start communication in
half duplex or full duplex mode.

1.2.2.      Synchronization

Synchronization performs checkpoints into streams of data for example
if a system is sending a file of 1600 pages it applies checkpoint after every
100 pages to ensure the quality and acknowledgement of data transmitted
successfully or not .

v  Web browsers and OSI protocol known as X.225 or ISo8327 are the
excellent examples of session layer protocols.


1.3.    Presentation layer

v  Presentation sometimes known as syntax layer is the 6th
layer of OSI model work to present data to application layer which is above to
presentation layer.

v  Presentation layer defines the syntax which is used by the host
that communicates through a network.

v  Functions of presentation layer involve characterization, data
translation, encryption and decryption of data and data conversion as well.

v  It works as a translator when different language is used between
sender and the recipient.

v  Http (Hyper text transfer protocol), ASCII, EBCDIC, TIFF, GIF are the examples of the
presentation layer.




1.4.   Application layer

v  Application layer provide functions on for different application
programs. It provides and manages the connection of all lower layers.

v  It is also responsible to manage the both application and users end
tasks and overall system management also dependent on the application layer.

v  Application layer provides application services for the file
transfer this layer is closed to end users which mean both the OSI application
layer and the end udders can interact directly with the software application.

v  It identifies the communication partners determine the resource
availability through internet protocol network and provide synchronized
communication to end users.

v  Examples of application layers are Telnet, FTP (File Transfer
Protocol), FTFP (Trivial File Transfer Protocol), NNTP (Network news transfer
protocol) and Electronic or simple mail etc.




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