· network layer and these segments passes through

·   Overcrowding control 

Ø  Realization

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Software and hardware controls.

Ø  Flow Control

It manages the
rate of data transfer between two nodes and increases the speed of data


Figure 2: Transport layer

In 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 by application layer
and when data is received to the recipient. It again merged into original form as
it was before.

1.1.1.     Issues with transport

Ø  Insecurity

In 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

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

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

Session layer

Session layer or port layer controls
the connection between two computers or communication devices. Session layer
deals with the sessions and coordination of the applications and networks. The
major role of session layer is authentication, authorization and session restoration
(Zimmermann, H. 1980).

Figure 3: OSI model session layer

In above diagram, header and data is
separated and passed to the application layer. 
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. Other side of medium cannot
transfer data until the data send by the first side is not received. When
message from the first side is completely transferred and received by the other
side, then half duplex allows other side to transmit their data.

Ø  Simplex

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

1.2.1.     Protocols of session layer

There are two protocols RPC and Netbios
and commonly found in use of LAN environments. These protocols normally suffer
from any issue as they are in LAN environment.

1.2.2.     Functions of Session transport layer

Session checkpoints and recovery in
the session layer is established through session layer. Different streams
information is allowed through this which may be originative from different
resource for proper synchronization. Synchronization of the information from
different sources is also handled 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. Role of
session layer is important in keeping the track of the multiple telnet
connections, downloading of multiple files requested by the FTP applications
and retrieval of the multiple web pages from a server at a time. Dialogue
control and synchronization are the functions of session layer. Check points in
session layer are considered as recovery points in case of system failure;
these check points are considered as recovery configurations and action and different
protocols like NFS, NetBIOS names, RPC, SQL work at this layer.          
Dialogue control

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

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

Issues with transport

Normally this layer is confused with
session of TCP/IP model session. It upholds network traffic, make connection,
and torn down connection. There are no such logical issues with the session


2.    Conclusion



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