There are seven layers in the OSI Reference Model, shown in Figure 2-1: application, presentation, session, transport, network, data link, and physical. The functions of the application, presentation, and session layers are typically part of the user’s application. The transport, network, data link, and physical layers are
responsible for moving information back and forth between these higher layers.
Each layer is responsible for a specific process or role. Remember that the seven layers are there to help you understand the transformation process that data will undergo as it is transported to a remote networking device. Not every networking protocol will fit exactly into this model. For example, TCP/IP has four layers. Some layers are combined into a single layer; for instance, TCP/IP’s application layer contains the functionality of the OSI Reference Model’s application, presentation, and session layers. The following sections go into more detail concerning the seven layers of the OSI Reference Model.
Application Layer
The seventh layer, or topmost layer, of the OSI Reference Model is the application layer. It provides the interface that a person uses to interact with the application. This interface can be command-line-based or graphics-based. Cisco IOS routers and switches have a command-line interface (CLI), whereas a web browser uses a graphical interface.
Note that in the OSI Reference Model, the application layer refers to applications that are network-aware. There are thousands of computer applications, but not all of these can transmit information across a network. This situation is changing rapidly, however. Five years ago, there was a distinct line between applications that could and couldn’t perform network functions. A good example of this was word processing programs, like Microsoft Word—they were built to perform one process: word processing. Today, however, many applications—MicrosoftWord, for instance—have embedded objects that don’t necessarily have to be on the same computer. There are many, many examples of application layer programs. The most common are telnet, FTP, web browsers, and e-mail.
Presentation Layer
The sixth layer of the OSI Reference Model is the presentation layer. The presentation layer is responsible for defining how information is presented to the user in the interface that they are using. This layer defines how various forms of text, graphics, video, and/or audio information are presented to the user. For example, text is represented in two different forms: ASCII and EBCDIC. ASCII (the American Standard Code for Information Interchange, used by most devices today) uses seven bits to represent characters. EBCDIC (Extended Binary-Coded Decimal Interchange Code, developed by IBM) is still used in mainframe environments to represent characters. Text can also be shaped by different elements, such as font, underline, italic, and bold.
There are different standards for representing graphical information—BMP, GIF, JPEG, TIFF, and others. This variety of standards is also true of audio (WAV and MIDI) and video (WMV, AVI, and MPEG). There are literally hundreds of standards for representing information that a user sees in their application. Probably one of the best examples of applications that have a very clear presentation function is a web browser, since it has many special marking codes that define how data should be represented to the user.
The presentation layer can also provide encryption to secure data from the application layer; however, this it not common with today’s methods of security, since this type of encryption is performed in software and requires a lot of CPU cycles to perform.
Session Layer
The fifth layer of the OSI Reference Model is the session layer. The session layer is responsible for initiating the setup and teardown of connections. In order to perform these functions, the session layer must determine whether data stays local to a computer or must be obtained or sent to a remote networking device. In the latter case, the session layer initiates the connection. The session layer is also responsible for differentiating among multiple network connections, ensuring that data is sent across the correct connection as well as taking data from a connection and forwarding it to the correct application.
The actual mechanics of this process, however, are implemented at the transport layer. To set up connections or tear down connections, the session layer communicates with the transport layer. Remote Procedure Call (RPC) is an example of an IP session protocol; the Network File System (NFS), which uses RPC, is an example application at this layer.
Transport Layer
The fourth layer of the OSI Reference Model is the transport layer. The transport layer is responsible for the actual mechanics of a connection, where it can provide both reliable and unreliable delivery of data. For reliable connections, the transport layer is responsible for error detection and correction: when an error is detected, the transport layer will resend the data, thus providing the correction. For unreliable connections, the transport layer provides only error detection—error correction is left up to one of the higher layers (typically the application layer). In this sense, unreliable connections attempt to provide a best-effort delivery—if the data makes it there, that’s great, and if it doesn’t, oh well!
Examples of a reliable transport protocol are TCP/IP’s Transmission Control Protocol (TCP) and IPX’s SPX (Sequenced Packet Exchange) protocol. TCP/IP’s User Datagram Protocol (UDP) is an example of a protocol that uses unreliable connections. Actually, IPX and IP themselves are examples of protocols that provide unreliable connections, even though they operate at the network, and not transport, layer. In IPX’s case, if a reliable connection is needed, SPX is used. For IP, if a reliable connection is needed, TCP is used at the transport layer.
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