Network Basics: OSI Application Layer
The highest layer of the Open Systems Interconnection (OSI) model, the Application layer, deals with the techniques that application programs use to communicate with the network.
In addition to sharing files, networks allow users to communicate with each other in various ways. For example, messaging applications let network users exchange messages with each other using an e-mail application such as Microsoft Outlook. Users can also hold online meetings over the network. In fact, with inexpensive video cameras and the right software, users can hold videoconferences over the network.
The name of this layer is a little confusing. Application programs such as Microsoft Office or QuickBooks aren’t a part of the Application layer. Rather, the Application layer represents the programming interfaces that application programs such as Microsoft Office or QuickBooks use to request network services.
Some of the better-known Application layer protocols are
DNS (Domain Name System) for resolving Internet domain names.
FTP (File Transfer Protocol) for file transfers.
SMTP (Simple Mail Transfer Protocol) for e-mail.
SMB (Server Message Block) for file sharing in Windows networks.
NFS (Network File System) for file sharing in Unix networks.
Telnet for terminal emulation.