OSI Model - Application Layer

OSI model consists of two layers : upper layer and lower layer. The upper layer are oriented more towards services to the application and lower layer are more oriented towards the data flow from end to end through the application. The upper layer describes the network function with respect to the application and operating system. The four lower layers of OSI reference model are Physical, Data Link, Network and Transport Layer. It describes how the data is transferred across physical devices. The lower layer is implemented with hardware and software.

OSI Model

Layer 1 - Application Layer

The Application Layer supports user and application tasks and overall system management, including resource sharing, file sharing, remote file servers, database and network management.


The Application Layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network or the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application layer implementations include:

• On OSI Model:
  • FTAM File Transfer and Access Management Protocol
  • X.400 Mail
  • Common management information protocol (CMIP)
• On TCP/IP Model:
  • Hypertext Transfer Protocol (HTTP),
  • File Transfer Protocol (FTP),
  • Simple Mail Transfer Protocol (SMTP)
  • Simple Network Management Protocol (SNMP) 

Important functions of Application Layer :

1. Identifying intended communication partners
2. Determining the resource availability of intended communication
3. Synchronizing communication
4. Authentication of intended communication partners