The upper three layers are
application-level layers and the lower four are known as data-level layers.
Each layer used the information from the below layer and provide the services
to above layer as requested. OSI model may have some duplication at various
layers and to find and fit the new protocol is difficult in OSI model (Bora et al. 2014). Each layer
is dependent on other layer and that’s why they have to wait for each other to
receive and process the data.
Layers are differentiated based on
the services, interfaces and protocols of each layer. These seven layers of OSI
model defines the different standards like:
Ø The way of communication between devices
Ø Tell the devices when to transmit the data and when to not transmit
Ø To ensure that the data flow in a correct method from sender to
Ø Make sure the physical transmission media is arranged in right
manner and the sent data is received by the right receiver to whom sender sends
Figure 1: OSI 7 Layers Model
2.2.1. Physical Layer
A physical layer is the extreme lower
layer of OSI model. The main function of physical layer is concerned with the
raw bits transmission and reception over a physical medium. As it is the bottom
most layer it carries the signals of all high level layers which are above it.
In physical layer all the data is converted into code words and cipher and then
converted to physical signals to transmit.
2.2.2. Data link layer
Data link layer resides above the
physical layer and below the network layer; it is responsible for the end to
end validity of the data being transmitted. Data transmitted in data link layer
is in the form of frames. Data link layer is responsible for the delivery of
frames between different devices but on the same network.
2.2.3. Network layer
In network layer, data is being
transmitted in the form of packets through logical paths. The order format of
the packets is handled by the network layer. It maintains the quality of the
services while the transmission of the data from source to destination.
2.2.4. Transport layer
2.2.5. Session layer
2.2.6. Presentation layer
Presentation layer sometimes known
as syntax layer is the 6th layer of OSI model. The layer defines the
syntax which is used by the host that communicates through a network. Functions
of presentation layer involve characterization, data translation, encryption
and decryption of data and data conversion as well.
2.2.7. Application layer
Application layer provide functions
for different application programs. It provides and manages the connection of
all lower layers. It is also responsible to manage the both application and
users end tasks and overall system management also dependent on the application
layer. Application layer provides application services for the file transfer.
This layer is close to end users which mean both the OSI application layer and
the end users can interact directly with the software application.
3. Transport layer and Session layer
Transport layer provides the logical
connection of two application processes running on different hosts within a
network layers. 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. 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 (Dierks, T. 2008). Transport
layer defines the five different classes of protocols with different levels of
reliability. 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
3.1.1. Functions of transport layer
Transport layers involve different
protocols. TCP transfer control protocol is a connection oriented
protocol, TCP/IP application which sends
the data and 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 (Dixit, V. 2014). Transport layer provides the maximum data transfer reliability
and make that the packets of data reaches in the same order at destination.
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.
& 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 form, each segment will be received in same
sequence as it was sent from sender.
Ø Quality of
· Error Recovery
· Overcrowding control
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.
3.1.2. Issues with transport
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.
Nodes used for the transmission purpose are easily breakable which
can cause vulnerable threat to the loss of data being transmitted.
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.
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.
3.2.1. 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.
3.2.2. 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 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
OSI model is seven layer based model that
describes the networking protocol and establishes a communication. Every layer
has its own duties and responsibilities and the report was mainly based upon
session and transport layer. Transport layer is layer 4 and session is layer 5 both
have the responsibility of communication and session management respectively. Services
of every layer are important for the layer above and every layer receives
services from the layer below. Session layer and Transport layer serve each
other in which Transport provide the services to session and Session layer
establishes a session to initiate communication.
Li, Y., Li, D.,
Cui, W., & Zhang, R. (2011, May). Research based on osi model. In Communication
Software and Networks (ICCSN), 2011 IEEE 3rd International Conference on (pp.
Bora, G., Bora,
S., Singh, S., & Arsalan, S. M. (2014). OSI reference model: An
overview. International Journal of Computer Trends and Technology
(IJCTT), 7(4), 214-218.
Dalal, S., & Dixit, V. (2014). The OSI model: Overview on the seven layers
of computer networks. International Journal of Computer Science and
Information Technology Research, 2(3), 461-466.
(2008). The transport layer security (TLS) protocol version 1.2.
Zimmermann, H. (1980).
OSI reference model–The ISO model of architecture for open systems
interconnection. IEEE Transactions on communications, 28(4), 425-432.