Protocols that operate at this level establish, maintain and break sessions (also referred to as dialogs). It's different then what goes on at the Transport Layer because it looks at the whole exchange of information, not just single transmissions. Many of the types of gateways that exist operate at the session layer.
This layer changes the presentation of the data from the lower layers into a format the upper layer can use. Encryption, data compression and network redirectors happen here. Character set translation is also dealt with at this layer. Most, but not all, computers use the American Standard Code for Information Interchange (ASCII); some mainframe computers and some IBM networking systems use the Extended Binary Coded Decimal Interchange Code (EBCDIC).
The application layer is where standard network services are defined. FTP (File Transfer Protocol), printer access and e-mail relay are some examples of these services. The actual applications that access these services are located above the application layer.
There are very few upper layer devices and none of them operate at any specific layer. They fall into a class of devices known as gateways. A gateway translates one type of network data into another. Gateways can be hardware, software or, as is usually the case, a combination of both; with a software program running on a dedicated computer.
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