The upper layers of the OSI model deal with less esoteric concepts.
The Session Layer
Protocols that operate at the Session layer of the OSI model are responsible for establishing, maintaining, and breaking sessions, or dialogs. This is different from the connection services provided at the Transport layer because the Session layer operates at a higher level and looks at the bigger picture— the entire conversation, not just one sentence. Many gateways operate at the Session layer. Novell’s Service Advertisement Protocol (SAP) is a Session layer protocol, as well as NetBIOS.
The Presentation Layer
The Presentation layer does what you might think it does: It changes the look, or presentation, of the data from the lower layers into a format that the upper-layer processes can work with. Among other services, the Presentation layer deals with encryption, data compression, and network redirectors.
In addition, the Presentation layer deals with character-set translation. Not all computer systems use the same table to convert binary numbers into text. Most standard computer systems use the American Standard Code for Information Interchange (ASCII). Mainframe computers (and some IBM networking systems) use the Extended Binary Coded Decimal Interchange Code (EBCDIC) and Unicode, which is popular on the Internet as one character set that assigns a unique number to every character regardless of the language or the operating system used to display the character. The three are totally different. Protocols at the Presentation layer can translate between the three.
The Application Layer
Now, you might be thinking, “This layer is for my programs, right?” Wrong. The Application layer defines several standard network services that fall into categories such as file transfer, print access, and e-mail relay. The applications that access these network services are located above the Application layer and are not even part of the layered model.
There are only a few upper-layer devices, none of which operate at any specific layer. Because they perform a range of functions for the network, they fall into the class of devices known as gateways. A gateway translates one type of network data into another. Gateways can be either hardware or software, but the most popular way to run a gateway is as a software program on a dedicated computer.
There are many, many types of gateways, but the one most people think of is an e-mail gateway. E-mail gateways translate e-mail messages from one type of e-mail system so that they can be transmitted on another (for example, from GroupWise e-mail to SMTP mail for the Internet).
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The Osi Model
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