OSI Reference Model (OSI)
The International Organization for Standardization (ISO) developed the Open System Interconnection (OSI) Reference Model to describe how information is transferred from one machine to another machine from the point when a user enters information using a keyboard and mouse to when that information is converted to electrical or light signals transferred along a piece of wire or radio waves transferred through the air. It is important to understand that the OSI Reference Model describes concepts and terms in a general manner and that many network protocols such as IP and IPX fail to fit nicely into scheme explained in OSI model. Therefore the OSI Reference Model is most often used as a teaching and troubleshooting tool. By understanding the basics of the OSI Reference Model you can apply these to real protocols to gain a better understanding of them as well as to more easily troubleshoot problems.
OSI Reference Model (OSI)
Layer 6 Presentation
Layer 5 Session
Layer 4 Transport
Layer 3 Network
Layer 2 Data link
Layer 1 Physical
(1)Application Layer
The top (seventh) layer of the OSI Reference Model is the application layer. It provides the user interface. Example of TCP/IP applications include telnet, FTP, HTTP, and SMTP. The seventh layer or topmost layer of the OSI Reference Model is the application layer. It provides the interface that a person user to interact with the application. The interface can be command-line interface (CLI) whereas a web browser uses a graphical interface. In the OSI Reference Model the application layer refers to applications that are network-aware. There are thousands of computer application across a network. This situation is changing rapidly however . Five years ago there was a distinct line between application that could and could not 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 Microsoft word for instance
Have embedded objects that do not necessarily have to be on the same computer are telnet FTP web browsers and e-mail.
(2)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 or audio information are presented to the user. For example text is represented in two different forms: ASCI and EBCDIC. ASCII (the American Standard Code for on formation 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 b 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 is not common with today methods of security since this type of encryption is performed in software and required a lot of CPU cycles to perform.
(3)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 later 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 (PRC) 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 attempted to provide a best effort delivery if the data makes it there that great and if it dose not oh well! Example of a reliable transport protocol are TCP/IP Transmission Control Protocol (TCP) and IPX SPX (Sequenced Packet Exchange) protocol. TCP/IP User Datagram Protocol (UDP) is an example of a protocol that uses unreliable connections. Actually IPX and IP themselves are example of protocols that provide unreliable connections even though they operate at the network and not transport layer. In IPX case if a reliable connection is needed SPX is used. For IP if a reliable connection is mechanics is discussed in more depth in the section Transport Layer later in this chapter.
Network Layer
The third layer of the OSI Reference Model is network layer. The network layer provides quite a few functions. First it provides for a logical topology of your network using logical or layer 3 addresses. These addresses are used to group machines together. As you will see in chapter 3 these addresses have two components a network component and a host component. The network component is used to group devices together. Layer 3 addresses allow devices that are on the same or different media types to communicate with each other. Media types define types of connections such as Ethernet Token Ring or serial. These are discussed in the section Data Link Layer later in this chapter. To move information between devices that have different network numbers a router is used. Routers use information in the logical address to make intelligent decisions about how to reach a destination. Routing is discussed in more depth in chapter 9, 10 and 11. Examples of network layer protocols include Apply Talk DEC net IPX, TCP/IP (or IP for short), Vines and XNS. The network layer is discussed in much more depth in the section Network Layer later in this chapter.
Data Link Layer
The second layer in the OSI Reference Model is the data link layer. Whereas the network layer provides for logical addresses for devices the data link layer provides for physical or hardware addresses. These hardware addresses are commonly called Media Access Control (MAC) addresses. The data link layer also defines how a networking device access the media that it is connected as well as defining the media frame type. This include the fields and components of the data link layer or layer 2 frame. This communication is only for devices on the same data link layer media type (or same piece of wire). To traverse media types Ethernet to Token Ring for instance typically a router is used. The data link layer is also responsible for taking bits (binary 1 and o) from the physically layer and reassembling them into the original data link layer frame. The data link layer dose error detection and will discard bad frames. It typically does not perform error correction as TCP/IP TCP protocol does however some data link layer protocols do support error correction functions. Examples of data link layer protocols and standards for local area network (LAN) connections include IEEE 802.3 and 802.5 Ethernet II and ANSI FDDI. Example of WAN connections include ATM Frame Relay HDLC (High Level Data Link Control) SLIP (Serial Line Internet Protocol ) and X.25. Bridges switches and network interface controllers or cards (NIC) are the primary networking devices functioning the data link layer which is discussed in more depth in the section Data Link Layer later in this chapter.
Physical Layer
The first or bottommost layer of the OSI Reference Model is the physical layer. The physical layer is responsible for the physical mechanics of a network connection which include the following:
- The type of interface used on the networking device.
- The type of cable used for connecting devices.
- The connectors used on each end of the cable.
- The pin-outs used for each of the connections on the cable.
The type of interface is commonly called a NIC A NIC can be a physical card that you put into a computer like a 100BaseT Ethernet card or a fixed interface on a switch like a 100BaseTX port on a Cisco Catalyst 1900 series switch. The physical layer is also responsible for how binary information is converted to a physical layer signal. For example if the cable uses copper as a transport medium the physical defines how binary 1 s and 0 s are converted into an electrical signal by using different voltage
