The network is only as useful as the applications that are on it. As shown in the figure, within the application layer, there are two forms of software programs or processes that provide access to the network: network applications and application layer services.
Network Applications
Applications are the software programs used to communicate over the network. Some end-user applications are network-aware, meaning that they implement application layer protocols and are able to communicate directly with the lower layers of the protocol stack. Email clients and web browsers are examples of this type of application.
Application Layer Services
Other programs may need the assistance of application layer services to use network resources, like file transfer or network print spooling. Though transparent to an employee, these services are the programs that interface with the network and prepare the data for transfer. Different types of data, whether text, graphics, or video, require different network services to ensure that they are properly prepared for processing by the functions occurring at the lower layers of the OSI model.
Each application or network service uses protocols, which define the standards and data formats to be used. Without protocols, the data network would not have a common way to format and direct data. In order to understand the function of various network services, it is necessary to become familiar with the underlying protocols that govern their operation.