1

UGC NET

DAILY
CLASS NOTES
Computer

Data Communication and Computer Networks

Lecture – 17
Application layer
 2

Content
1. Session Layer
2. Presentation Layer
3. Application Layer
4. Protocols Of Application Layer

SESSION LAYER
1. The session layer is the 5th layer in the OSI model as shown in below figure. It provides services to
presentation layer which is above it and takes services of the transport layer.
2. This layer is called as the network dialog controller.
3. The session layer is expected to carry out the following duties:
a. Establish interaction between systems
b. Maintain the connection
c. Provide the synchronization

DIALOG CONTROL
1. The session layer has a responsibility to allow two willing systems to enter into a dialog.
2. It helps to establish, maintain and synchronize the interaction between the communicating systems.
3. The communication can be either in the half duplex or in the full duplex mode.
SYNCHRONIZATION
1. The session layer will allow a system to include the synchronization points (also called as check points)
in between the data stream.
 3

2. The inclusion of sync point is helpful when large files of data are being transferred from one system to
the other.
3. For example, if a file of 1000 pages is being sent by a system, then a sync point can be inserted after
every 50 pages.
4. This will ensure that each unit of 50 pages is received independently and a separate acknowledgement for
the same is sent back.
5. If a page is lost during communication, then only that unit is required to be retransmitted.

RELATION WITH PRESENTATION LAYER

1. The relation of session layer with the presentation layer is illustrated in below figure.
2. When the data from presentation layer is to be transmitted, it is handed over to the session layer.
3. The session layer will add the check points (sync) in between the data bits. It also adds the header H 5 to
the presentation layer data.

4. The communication between the presentation and session layer is bi-directional as shown in above figure.
PRESENTATION LAYER
1. The presentation layer is the 6th layer the OSI model as shown in below figure.
2. Above it there is the application layer and below it there is the sessions layer.
 4

3. The presentation layer is related to the syntax and semantics of the information being exchanged
between the interested systems.
4. Some of the important responsibilities of the presentation layer are:
1. Translation
a. Encoding b. decoding
2. Encryption
3. Compression.

1. Translation:
a. The communication systems usually exchange the information in the form of strings of characters,
numbers etc.
b. This information needs to be changed into bit stream before transmission.
c. This is essential because different systems use different systems use different encoding techniques. The
presentation layer does the job of translation.
d. The presentation layer at the sending end converts the information into a common format and the
presentation layer at the receiving end will convert this common format into the one which is compatible
to the receiver.
2. Encryption:
a. For ensuring the security and privacy of the information that is being communicated, a process called
data encryption is essential.
b. Encryption is carried out at the sending end. In this the sender transforms the original information to
another form, and sends the transformed information.
c. At the receiving end, an exactly opposite process called Decryption is carried out in which the received
information is transformed back to its original form.
d. Encryption and Decryption are carried out by the presentation layer.
 5

3. Compression:
a. The data compression technique is used for reducing the number of bits required to send an information.
b. Data compression is essential for transmission of multimedia such as text, audio and video.

RELATION WITH APPLICATION AND SESSION LAYERS

1. The relation of presentation layer with the application layer and session layer is illustrated in below
figure.

2. The data from the application layer (L7 data) is encrypted, encoded and compressed at the
presentation layer. A presentation layer header H-6 is also added as shown in below figure.
3. This is then sent to the session layer as L-6 data.
4. While receiving the data from session layer, the operations carried out by the presentation layer are
exactly opposite to those carried out while transmitting.
5. The received data from the session layer undergoes decryption, decompression and decoding at the
presentation layer.
6. The header H-6 is detached from the data and then the L-7 data is sent to the application layer.
Application Layer:-
It is present at the top of the OSI model. It is the layer through which the users interact. It provides
various services to the user.

Protocols of Application Layer :-

 6

1. Telnet :-
It stands for the TELecomunications NETwork. Helps in terminal emulation. It allows the Telnet
client to access the resources of the Telnet server. This is used for managing the files on the internet
and also used for initial set up of devices like switches. It is a command that uses the Telnet protocol
to communicate with a remote device or system. Port number is 23.
2. FTP:
It stands for file transfer protocol. The protocol that actually lets us transfer files.It can facilitate this
between any two machines using it. It is not just a protocol but it is also a program.It promotes
sharing of files via remote computers with reliable and efficient data transfer. Port number is 20 for
data and 21 for control.
3. TFTP(Trivial File Transfer Protocol):
It is the stripped-down, stock version of FTP, but it’s the protocol of choice if you know exactly what
you want and where to find it. This technology used for transferring files between network devices
and is a simplified version of FTP

4. NFS(Network File System):-

It allows the remote hosts to mount file systems over a network and interact with those file systems
as though they are mounted locally. NFS enables system administrators to consolidate resources onto
centralized servers on the network.
5. SMTP:
SMTP stands for Simple Mail Transfer Protocol. SMTP is a part of the TCP/IP protocol. It uses a
process called “store and forward,” .It moves your email on and across networks. Port number is 25.
6. DNS:-
DNS stands for Domain Name System. Every time you use a domain name, therefore, a DNS service
must translate the name into the corresponding IP address. For example, the domain name
www.xyz.com might translate to 198.106.233.4.
Port number is 53.
7. Hyper Text Transfer Protocol (HTTP): The Hyper Text Transfer Protocol (HTTP) is the
foundation of World Wide Web. Hypertext is well organized documentation system which uses
hyperlinks to link the pages in the text documents. HTTP works on client server model. When a
user wants to access any HTTP page on the internet, the client machine at user end initiates a
TCP connection to server on port 80. When the server accepts the client request, the client is
authorized to access web pages.
8. Post Office Protocol (POP): The Post Office Protocol version 3 (POP 3) is a simple mail
retrieval protocol used by User Agents (client email software) to retrieve mails from mail
server. When a client needs to retrieve mails from server, it opens a connection with the server
on TCP port 110. User can then access his mails and download them to the local computer.
POP3 works in two modes. The most common mode the delete mode, is to delete the emails
from remote server after they are downloaded to local machines. The second mode, the keep
 7

mode, does not delete the email from mail server and gives the user an option to access mails
later on mail server.
9. DHCP: It stands for Dynamic Host Configuration Protocol (DHCP). It gives IP addresses to
hosts. There is a lot of information a DHCP server can provide to a host when the host is
registering for an IP address with the DHCP server. Port number for DHCP is 67, 68.
10. SNMP: It stands for Simple Network Management Protocol. It gathers data by polling the
devices on the network from a management station at fixed or random intervals, requiring
them to disclose certain information. It is a way that servers can share information about their
current state, and also a channel through which an administrate can modify pre-defined values.
The Port number of SNMP is 161(TCP) and 162(UDP).

PW Web/App : https://smart.link/7wwosivoicgd4
Library - https://smart.link/sdfez8ejd80if