About the Application Layer – OSI Model
– Application layer is just name given to an abstraction layer for the computer network programming in the OSI model. – This is the seventh layer of this model.
– It is meant for the communication protocols as well as methods that have been designed exclusively for the process – to – process communications across the network based on internet protocol.
– The protocols operating at this level make use of the underlying protocols at the transport layer level for establishing the host to host connections.
– The OSI model does not defines this layer in a broad sense or scope.
– It has simply defined as the user interface.
– The application layer holds the responsibility for the display of images as well as data in a user recognizable format to the user.
– The application layer is also responsible for interfacing with this layer.
– Another major purpose of the application layer is to divide the functionality above the 4th OSI layer (i.e., transport layer) in two levels namely:
a) The session layer and the
b) The presentation layer.
– A strict modular separation has been specified by the OSI model especially for these two layers.
– It also focuses on providing the protocol implementation to each of them.
– This layer is the last and the outermost layer in the OSI model and therefore closest to the user.
– With this, it becomes obvious that there is a direct interaction between the layer and the user and the software application.
– This software application has a component via which it can communicate.
– But there’s one problem with such applications that they are not counted inside the OSI model’s scope.
Following are some functions of the application layer functions:
a) Identification of the communication partners
b) Determination of the resource availability
c) Synchronization of the communication
– During the identification of the communication partners, the identity as well as availability is determined by the application layer.
– During the determination of the availability of the resources, it is important for the application layer to decide regarding the existence of the requested communication or the sufficient network.
– In the third function i.e., the synchronization, the required cooperation between the applications is managed by this layer.
– Few examples of application layer implementations are:
On TCP/ IP stack: It includes:
a) HTTP or hypertext transfer protocol
b) FTP or file transfer protocol
c) SMTP or simple mail transfer protocol
d) SNMP or simple network management protocol
On OSI stack: It includes:
a) CMIP or common management information protocol
b) X>400 mail
c) FTAM or file transfer and access management protocol
Some other issues that are handled by this layer exclusively are:
a) Network transparency
b) Resource allocation and
c) Problem partitioning
– This layer is more concerned with the user’s view rather than technical view of the network.
– To this layer, a local representation of data is provided by the presentation layer that does not depends up on the format that is used by the network.
– The application layer provides an interfacing between the communications environment used by various application process.
– This layer is categorized under the host layers category of the OSI topologies.
– The accurate delivery of the data in the network is ensured by this layer.
– Communications between the applications are managed by the application layer.
– Structure to the data grams is added by the preceding layer i.e., the presentation layer.
– Control mechanism to the data exchange is added by the session layer.
– These three layers represent the internet architecture’s application layer.
– This layer then serves as window for the various applications operating on the open environment through which they can exchange information.
– Applications using this layer have been termed as the application processes.
– The protocols operating at the application layer level are classified in to two categories namely:
a) CASE or common application specific elements and
b) SASE or specific application specific elements