The Application Layer is launched when an application operation starts. It starts the communication with another computer when an application starts. It is in software packages that use client-server software. Examples of services within this layer are DNS, web browser, and NFS. The Presentation Layer decides how to format the data and the encryption if any is used it formulates the data so that it is acceptable to the layer above. This layer gives functionality between the host OS and software layers. Basically it formats the data that is being sent into a presentable format for the application layer.
MIDI, HTML, GIF, and JPEG are examples of the data formats used. The Session Layer manages the sequencing of the transaction and sometimes the authorization. This layer creates notifications if messages fail it also manages conversations going both ways and can handle many different conversations. The session layer will not pass the data along until the conversation of data is complete. RPC, SQL, NetBIOS, are some examples of these protocols. The Transport Layer reassembles the packets which are broken up and placing the data in the proper order to be transmitted across certain media. At this layer error recovery is also performed.
The following are examples TCP, UDP, and SPX. The Network Layer is accountable for implementing a logical addressing scheme and for transporting the packets from one end to the other end. The network layer can be connectionless or it can be connection oriented. This layer is also responsible for breaking larger packets up for delivery depending on the media this is known as packet switching. These protocols include Apple talk DDP, IP, and IPX. The Data Link Layer provides the connections between entities on the network it handles getting data over a specific medium. This layer handles Flow Control and layer sequencing if required.
The correct sequence is preserved within the frames it also handles transit delay. This layer also checks the frame for errors using the FCS if it detects errors it discards the frame. Examples are IEEE 802. 2, 802. 3, HDLC, and Frame Relay. The Physical Layer handles the material aspects of the media being used to send out data. It sets up the binary pattern, buffers the data and ensures synchronization. It decodes the bit stream then notifies the data link layer the frame was received then passes it up. Ethernet, RJ45, V. 24, V. 35, and NRZ are examples of specifications used in this layer.