What is an Application Layer?

A computer network is typically designed based on a network diagram. This topology is defined based on the open systems interconnection model (OSI) and consists of seven layers of network functionality. The application layer is the highest level within OSI, which is known as layer seven. It is the network layer that manages communication programs on the network, including file transfer protocol (FTP), hypertext transfer protocol(HTTP), and email. These programs enable Internet browsers and email applications to a share data across a network.

A network begins with a physical cable and ends with a computer application. Data is forced through several levels of transformations during a typically communication transfer. This is a necessary process to ensure proper data quality and security rules are followed. The OSI model was created to enable flexibility in software protocols.

The OSI model is made up of seven layers. These are the physical, data link, network, transport, session, presentation, and application layers. The layers work within a hierarchal structure to send messages across a network.

Most computer networks today use standard transmission control protocol (TCP/IP) as the communication language for the Internet. This network protocol uses the OSI model as a blueprint on how network devices and software should interact. Each layer in the model has a specific responsibility within the network topology.

The OSI model is designed based on a hierarchal chaining of responsibility. These rules force each layer to follow specific handoff processes when sending a message through the topology. The application layer is required to follow the handoff rules of the presentation layer, which is the next level within the hierarchy chain. These rules typically include format, encryption, and specific size requirements before the message can be accepted by the next level in the chain.

Instant messaging (IM) is a poplar communication protocol that is used on cell phones and computer systems. IM is an example of software that uses the application layer protocol. The IM standard protocol makes it possible for companies to build instant messaging software. Some examples of these products include AOL® Instant Messenger, Yahoo® mail and MSN® messenger. Each program uses the OSI standards to send and receive messages across the network at this layer.

Microsoft Outlook® is an example of an email application that uses the application layer. This is the lowest level of the OSI model, which is responsible for packaging messages from a software program into an understandable computer message. The application layer sends a message to the presentation layer, which is responsible for the encryption and format rules of the message.