Welcome. This video looks at the application layer of the OSAI model and how it relates to the TCP IP model. We will see how people use the application layer to communicate across a network and explain how protocols ensures services running on one kind of device can send data to and receive data from many different network devices. The OSI reference model is a layered abstract representation created as a guideline for network protocol design. In the model, information is passed from one layer to the next, starting at the top with the application layer on the transmitting host, proceeding down the hierarchy to the physical layer. Then the information passes over the communication channel to the destination host where the information proceeds back up the hierarchy ending at the application layer. Unlike the OSI model, the TCPIP application layer protocols were developed before the emergence of personal computers, graphical user interfaces, and multimedia objects. This is the reason for additional layers in the OSI model. The functionality of the TCP IP application layer protocols fit roughly into the framework of the top three layers of the OSI model. The other layers in the process of passing information up and down the hierarchy work similarly in both models. The two forms of software programs or processes that provide access to the network within the application layer include applications and services. When we open a web browser or an instant message window, an application is started and the program is put into the devices memory where it is executed and referred to as a process. Application layer protocols establish consistent rules for exchanging data between applications and services loaded on the participating devices. Within the application layer, protocols specify what messages are exchanged between the source and destination hosts, the syntax of control commands, the type and format of the data being transmitted, and the appropriate methods for error notification and recovery, the application layer protocols implemented on the source and destination host must match in order for communication to be successful. When people attempt to access information that is not stored on their local device, then the client server model is initiated. In the client server model, the device requesting information is a client and the device responding to the request is a server. Both of which are considered to be in the application layer. In addition to the client server model, there is also a peer-to-peer model. Peer-to-peer networking involves two distinct forms. The first is peer-to-peer network design and the second is peer-to-peer applications. In a peer-to-peer network, two or more computers are connected via network and can share resources having a dedicated server. Each of these end devices can function as either a server or a client on a per request basis. The most widely known TCP IP application layer protocols are those that provide for the exchange of user information. These protocols specify the format and control information necessary for many of the common internet communication functions. They include protocols for email such as simple mail transfer protocol or SMTP and protocols for the web such as hypertext transfer protocol HTTP. Numerous other protocols are also used in the application layer. As a network professional, it is important to know how an application is able to format, transmit, and interpret messages that are sent across a data network. This knowledge will help you implement services and troubleshoot problems on your network.