What is an Application Layer in OSI Model?
Application Layer is the topmost layer in the Open System Interconnection
(OSI) model. This layer provides several ways for manipulating the data
(information) which actually enables any type of user to access network with
ease. This layer also makes a request to its bottom layer, which is presentation
layer for receiving various types of information from it.
Application Layer plays a key role in managing applications such as email,
file transfer, and remote login. To fully master the functionalities of the OSI
model, especially in relation to the application layer, exploring advanced
study materials can be beneficial. The Application Layer interface directly
interacts with application and provides common web application services.
This layer is basically highest level of open system, which provides services
directly for application process.
Functions of Application Layer
The Application Layer, as discussed above, being topmost layer in OSI
model, performs several kinds of functions which are requirement in any kind
of application or communication process. Following are list of functions
which are performed by Application Layer of OSI Model:
Data from User <=> Application layer <=> Data from Presentation Layer
Application Layer provides a facility by which users can forward several
emails and it also provides a storage facility.
This layer allows users to access, retrieve and manage files in a remote
computer.
It allows users to log on as a remote host.
This layer provides access to global information about various services.
This layer provides services which include: e-mail, transferring files,
distributing results to the user, directory services, network resources and
so on.
It provides protocols that allow software to send and receive information
and present meaningful data to users.
It handles issues such as network transparency, resource allocation and so
on.
This layer serves as a window for users and application processes to
access network services.
Application Layer is basically not a function, but it performs application
layer functions.
The application layer is actually an abstraction layer that specifies the
shared protocols and interface methods used by hosts in a communication
network.
Application Layer helps us to identify communication partners, and
synchronizing communication.
This layer allows users to interact with other software applications.
In this layer, data is in visual form, which makes users truly understand
data rather than remembering or visualize the data in the binary format
(0’s or 1’s).
This application layer basically interacts with Operating System (OS) and
thus further preserves the data in a suitable manner.
This layer also receives and preserves data from it’s previous layer,
which is Presentation Layer (which carries in itself the syntax and
semantics of the information transmitted).
The protocols which are used in this application layer depend upon what
information users wish to send or receive.
This application layer, in general, performs host initialization followed by
remote login to hosts.
Working of Application Layer in OSI Model
The application layer in the OSI model generally acts only like the interface
which is responsible for communicating with host-based and user
applications. This is in contrast with TCP/IP protocol, wherein the layers
below the application layer, which is Session Layer and Presentation layer,
are clubbed together and form a simple single layer which is responsible for
performing the functions, which includes controlling the dialogues between
computers, establishing as well as maintaining as well as ending a particular
session, providing data compression and data encryption and so on.
Working of Application Layer
At first, client sends a command to server and when server receives that
command, it allocates port number to client.
Thereafter, the client sends an initiation connection request to server and
when server receives request, it gives acknowledgement (ACK) to client
through client has successfully established a connection with the server.
Therefore, now client has access to server through which it may either
ask server to send any types of files or other documents or it may upload
some files or documents on server itself.
Features Provided by Application Layer Protocols
To ensure smooth communication, application layer protocols are
implemented the same on source host and destination host.
The following are some of the features which are provided by Application
layer protocols-
The Application Layer protocol defines process for both parties which are
involved in communication.
These protocols define the type of message being sent or received from
any side (either source host or destination host).
These protocols also define basic syntax of the message being forwarded
or retrieved.
These protocols define the way to send a message and the expected
response.
These protocols also define interaction with the next level.
Services Provided by Application Layer
The services provided by application layer in OSI model are:
It provides interface between user and application.
It is used for remote login.
It is used for file transfer.
It is used for mail services and transfers.
It is also used to transfer multimedia files.
It is used for resource sharing.
It is used for data synchronization.
It is used for authentication services.
Application Layer Protocols
The application layer provides several protocols which allow any software to
easily send and receive information and present meaningful data to its users.
The following are some of the application layer protocols .
TELNET: Telnet stands for Telecommunications Network. This protocol
is used for managing files over the Internet. It allows the Telnet clients to
access the resources of Telnet server. Telnet uses port number 23.
DNS: DNS stands for Domain Name System. The DNS service translates
the domain name (selected by user) into the corresponding IP address.
For example- If you choose the domain name as www.abcd.com, then
DNS must translate it as 192.36.20.8 (random IP address written just for
understanding purposes). DNS protocol uses the port number 53.
DHCP: DHCP stands for Dynamic Host Configuration Protocol. It
provides IP addresses to hosts. Whenever a host tries to register for an IP
address with the DHCP server, DHCP server provides lots of information
to the corresponding host. DHCP uses port numbers 67 and 68.
FTP: FTP stands for File Transfer Protocol. This protocol helps to
transfer different files from one device to another. FTP promotes sharing
of files via remote computer devices with reliable, efficient data transfer.
FTP uses port number 20 for data access and port number 21 for data
control.
SMTP: SMTP stands for Simple Mail Transfer Protocol. It is used to
transfer electronic mail from one user to another user. SMTP is used by
end users to send emails with ease. SMTP uses port numbers 25 and 587.
HTTP: HTTP stands for Hyper Text Transfer Protocol. It is the
foundation of the World Wide Web (WWW). HTTP works on the client
server model. This protocol is used for transmitting hypermedia
documents like HTML. This protocol was designed particularly for the
communications between the web browsers and web servers, but this
protocol can also be used for several other purposes. HTTP is a stateless
protocol (network protocol in which a client sends requests to server and
server responses back as per the given state), which means the server is
not responsible for maintaining the previous client’s requests. HTTP uses
port number 80.
NFS: NFS stands for Network File System. This protocol allows remote
hosts to mount files over a network and interact with those file systems as
though they are mounted locally. NFS uses the port number 2049.
SNMP: SNMP stands for Simple Network Management Protocol. This
protocol gathers data by polling the devices from the network to the
management station at fixed or random intervals, requiring them to
disclose certain information. SNMP uses port numbers 161 (TCP) and
162 (UDP).
