What is Application Layer.

Application layer is the top layer of the OSI (Open System Interconnectivity) server layer model. This layer handles issues like network transparency, resource allocation and problem partitioning. The application layer is concerned with the user's view of network (e.g. formatting electronic mail messages). The presentation layer provides the application layer with a familiar local representation of data independent of the format used on the network. burks.bton.ac.uk/burks/foldoc/59/6.htm

The application layer provides the interface to the communications environment which is used by the application process. It is responsible for communicating application process parameters. www.scit.w/v.ac.uk/cm1901/spos/comms/std.osim7.html

Application layer falls under the "host layers" in the OSI topologies used in network communication and it is also the top layer in the Internet model. The host layer also provide for accurate delivery of data between computers in the network.

Diagram of Application Layer of OSI.

  • Application
  • Presentation
  • Session
  • Transport
  • Network
  • Data Link
  • Physical

  Open System Interconnection Structure.

                 Application layer manages communications between applications.

          Presentation Layer adds structures to units of data being exchanged.

Session layer adds control Mechanism to data exchange.


Pointedly, the above three layers of the OSI model is represents the Application layer in the Internet architecture.

The application layer being the highest layer of OSI architecture, has the purpose to serve as a window between correspondent application processes so that they may exchange information on the open environment. The programs which use the application layer are known as Application Processes.

A user programme may interface directly with Presentation Layer in which case it must include protocol modules to initialise communication with peer application processes, establish the appropriate Presentation context and transfer files or messages. Alternatively the user process may include available modules which support commonly required application related services. Such modules are referred  to as Application Specific Elements and are defined as an integrated set of functions which together provide one or more application related communication capabilities. These capabilities are defined by a protocol in a very similar manner to that used to specify a service provided by the layer bellow. The protocol may specify direct use of presentation layer services and or those provided by other ASEs. The grouping of user process ( or application entity), relevant ASEs, and interfaces between them is known as an application process.


Classification of Application Layer Protocols.

Application layer protocols are classified into Common Application Specific Elements (CASE) and Specific Application Specific Elements (SASE).

CASE elements are commonly required by user elements and SASEs, whereas SASEs are only included in the application process when their particular service is specifically required.



Defined CASEs:

There are four CASEs currently defined. These are -

1. Association Control ACSE, which enables users to establish or terminate an association between application processes.

2. Reliable Transfer RTSE,- This enables reliable transfer of information between peers.

3. Remote Operation ROSE:  this enables users to initiate operations at a remote site.

4. Recovery ,CCR; - this enable users to recover from failure during execution of a task.

( Donncha Redmond: http://ganges.cs.tcd.ie/undergrad/4ba2/application


                                         Fig. 1 Datagram Application Process

  The above diagram shows the classified application protocols, interface with the application program.


 Application Services.

What is Application service.

This is the actual internet service or access that we follow to get work or services done through the internet.  Millions of people across the world access the internet everyday. The root of the internet lie in the academia and much research I think is still being carried out. Since the internet was opened up to  commerce in the early 1990's, many new facilities have arisen. Also with the advent of the world wide web, business has seized the opportunity to use the internet for communication, marketing, advertising and selling of different products.

Services Available on the Internet.

These are some of the services available on the internet which includes: The world wide web - is the fastest growing service on the internet          (not to be confused with the internet  itself). It describes the vast collection of hypertext-based files available on the internet and provides quick and easy way to retrieve these files so they can be viewed using a browser. It's popularity comes from its ability to handle files containing many different elements, such as graphics, sound, video and animation.

Note: As well as text and graphics, hypertext based files can also contain links to other files and resources. The hypertext based files on the internet effectively form an interconnected web of files spanning the globe ( hence the term world wide web). 

The next chapter will delve into the actual mechanics of the service providers at the operational levels and how this is achieved.


The Services Provided by the Application Layer.

These include FTP, FTAM, SMTP and TELNET.

FTP: File (Transfer Protocol) is the standard way to transfer files between different machines. In order to transfer files from the FTP server on the Unix/Telnet, you will need an FTP client program on your computer.

Note: Before transferring files, make sure there is enough disk space on your machine. The FTP program uses TCP/IP protocol to transfer files to and from remote nodes. Sometimes it will read the host file to determine the IP address. The TCP/IP and the OSI protocol have coexisted for sometime and will continue to coexist in the internet community because as more and more OSI hosts are fielded on the internet, the requirement for gateways between the protocol suites become more pressing.

This report will describe an application layer gateway providing interoperability between the TCP/IP  file Transfer Protocol (FTP) and the OSI File Transfer, Access and Management (FTAM) protocol.  The proposed application layer is based on a bi-directional set of mappings between the FTP and FTAM protocols. Since the protocols have quite different command structures, the mappings between them are not one-to-one. This report assumes the knowledge of File Transfer Protocol (FTP) [RFC959] and the File Transfer, Access and Management Protocol (FTAM) [ISO8571-1,2,3,4,5].


 The Goals of Mappings.

The two important goals of mappings are to:

Though it is anticipated that application layer gateway will be implemented on full protocol suites of both TCP/IP and OSI, at least one implementation of such a gateway ( included in the ISO Development Environment) can be configured to operate FTAM over, either OSI or TCP/IP lower layer services.

FTP Commands:

The FTP commands most frequently used are;-

ftp- starts FTP . Here if a machine is specified, a connection to that machine is made immediately. e.g. telgate.uel.ac.uk, otherwise the FTP command is displayed.

open - opens a connection to the specified machine.

cd- change directory on the remote machine.

lcd - change directory on the local machine.

pwd - print working directory on the remote machine.

ls - short directory listing on the remote machine.

lls - short directory listing on the remote machine.

dir- long directory listing on the local machine .

ascii - set ascii transfer type ( for text files)

binary - (or image) set binary transfer type ( for binary dates, executable files, gzipped files etc.

get - copy files from the remote machine to the local machine, or append "-" after the file name to display the contents of a remote file on your screen (e.g. the command get 00README - will display 00README file of current directory to your local screen.

put - copy file from the local machine to the remote machine.

mget - as for get but copies multiple files 

mput - as for put but copies multiple files .

close - disconnects from the remote machine.

quit - terminates FTP session and exit.