The Seven Layers of the OSI Network Model
The OSI Reference model is comprised of seven layers. This network model is used to describe the different layers, or "parts" of a network. Ranging from software related to hardware related, they are: Application, Presentation, Session, Transport, Network, Data-link, and Physical. The Application layer refers to the software used on the network, such as a web browser, and ftp client, or the client's operating system, all of which are used to access resources from the host. Even though all these applications are considered part of the Application level, they may develop their own methods of communicating between each other, just as long as they are compatible with the level below them. This means following the standard protocol for tasks such as file transfer and email. FTP would be the most common example of a file transfer protocol, and the X.400 message protocol and the X.500 directory protocol are worldwide accepted protocols for their two functions. The second layer from the top is the presentation layer. This layer follows a common protocol in communicating with the Application layer, and of course the layer below it, the Session layer. The presentation layer, though, is concerned with taking the message contents and transforming them appropriately for transmission across the network. This means changing the Application contents into digital, or binary, data. ASCII (American Standard Code for Information Interchange) is an example of a component of the presentation layer. This takes displayed characters and converts them into binary values, so they can be transmitted over the network.. In addition to external data representation transformation, which has just been covered, the presentation layer is also used for compression and/or encryption of the data. However, the presentation layer on the other end must also know how to decrypt or decompress this data. The third layer from the top is the Session layer. This layer includes functions that establish a connection with the host. After the presentation layer transforms the data for transmission, the session layer makes it easy for the data connection to be maintained between processes on different systems. In other words, when a process requests a connection with another process on another system, the session layer helps establish the link. Therefore, this layer is responsible for the following tasks: 1. Starting and activating the session 2. Stopping and releasing the session 3. Synchronizing the two ends 4. Dialog control 5. Transfer of data The session layer is the last "upper" or software-related layer, and provides the connection to the "lower", or hardware-related layers. The first of the two middle layers is the Transport layer. This layer is for decomposing packets that are sent from the session layer, so they may be transmitted over the network. On the other end, the transport layer puts these back together again. When Transport breaks up these layers, it assigns a sequence number to each one, so if they are received out of order on the other side, the remote Transport layer can recompose them as if they had never been broken up. Afterward, the transport layer puts the correct destination address on the packets and does simple routing of them between networks if it is in an internetwork. The next layer down is called the Network layer. This layer accomplishes the task of telling the packet exactly which hosts to go through to reach the final destination host. It does detailed routing of the message so the message knows how to get there. A simple analogy is comparing what Network does to looking carefully at a roadmap and getting explicit directions, rather than saying merely that you want to go from location x to location y. A widely accepted Network Layer protocol is X.25. Now that the message is properly encoded, broken down, and routed, the only thing left to with it is physically transport it. The Data-link layer, the first of the two physical layers, does data transport. It sends data from one end of a line to the other, it activates and deactivates the link, it detects errors in the data link, it allows the link to be shared, it provides transparent data flow, and allows for error recovery and modification. Commonly accepted standards for the Data-link layer are ANSI's ADCCP (Advanced Data Communications Control Procedure), HDLC (High-level Data Link Control), SDLC (SNA's Data Link Control), and IEEE's 802.2. The final layer accomplishes the task of actually sending the message over the physical link. This link is called the Physical Link. There are different ways to transmit this binary data, such as analog signals, amplitude or frequency modulation (AM or FM), light signals, or radio signals. Standards for transmission of binary data in this layer are: CCITT's x.21 for digital transmission, and v.35 & v.24 for analog transmissions. The whole object of the OSI Reference Model is in its name: Open Systems Interconnect. This means that it has to be open for different types of protocols or methods for each layer, while still providing compatibility between the other layers. This will allow flexibility for programmers while still maintaining a strict standard for how these layers must interact with the ones above and below it.