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The NetBEUI Implementation

Because it is based on the NetBIOS protocol, NetBEUI (NetBIOS Enhanced User Interface) has datagram support and, thus, has support for connectionless transmission. It doesn’t, however, have support for connection-oriented services. NetBIOS does allow hosts to have logical names (using WINS), but the naming service, as with NDS, functions at the upper layers of the OSI model.

The OSI Model’s Upper Layers

he upper layers of the OSI model deal with less esoteric concepts. Even though we’re still discussing computer networking, the top three layers (Session, Presentation, and Application) seem easier to understand. Because the Network+ exam doesn’t cover the upper layers (and many times these top three layers are grouped together), the following sections will give only a brief overview.

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.

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). The two are totally different. Protocols at the Presentation layer can translate between the two.

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