OSI Model – The Application Layer
The Application layer (Layer-7) provides the interface between the user application and the network. A web browser and an email client are examples of user applications. The OSI Model user application itself does not reside at the Application layer – the protocol does. The user interacts with the application, which in turn interacts with the application protocol. Examples of Application layer of OSI Model protocols include:
• FTP, via an FTP client
• HTTP, via a web browser
• POP3 and SMTP, via an email client
The Application layer provides a variety of functions:
• Identifies communication partners
• Determines resource availability
• Synchronizes communication
The Application layer interacts with the Presentation layer below it. As it is the top-most layer, it does not interact with any layers above it.
OSI Model – The Presentation Layer
The Presentation layer (Layer-6) controls the formatting and syntax of user data for the application layer. This ensures that data from the sending application can be understood by the receiving application. Standards have been developed for the formatting of data types, such as text, images, audio, and video. Examples of Presentation layer formats include:
• Text – RTF, ASCII, EBCDIC
• Images – GIF, JPG, TIF
• Audio – MIDI, MP3, WAV
• Movies – MPEG, AVI, MOV
If two devices do not support the same format or syntax, the Presentation layer can provide conversion or translation services to facilitate communication. Additionally OSI Model’s the Presentation layer can perform encryption and compression of data, as required. However, these functions can also be performed at lower layers as well. For example, the Network layer can perform encryption, using IPsec OSI Model .
OSI Model – The Session Layer
The Session layer (Layer-5) is responsible for establishing, maintaining, and ultimately terminating sessions between devices. If a session is broken, this layer can attempt to recover the session. Sessions communication falls under one of three categories:
• Full-Duplex – simultaneous two-way communication
• Half-Duplex – two-way communication, but not simultaneous
• Simplex – one-way communication
Many modern protocol suites, such as TCP/IP, do not implement Session layer protocols. Connection management is often controlled by lower layers, such as the Transport layer. The lack of true Session layer protocols can present challenges for high availability and failover. Reliance on lower-layer protocols for session management offers less flexibility than a strict adherence to the OSI model.