The Open Systems Interconnection (OSI) is a network model it is a framework of standards containing three subsections with a total of seven inner layers. The OSI is a network model and provides a set way for computers and devices to ‘talk’ to each other as a means of avoiding compatibility issues. During the 1970s, computers started to communicate with each other without regulation. It was not until the late 1970s that the speed of this transmission started to increase and standards were created. These standards consist of the group of three application layers the two internetwork layers and the two hardware layers.
When a message is transmitted from one computer to another through these seven layers protocols are wrapped around the data, the layers in the network use a formal language or protocol that is a set of instructions of what the layer will do to the message, these protocols are labelled or encapsulated onto the data. You could think of the protocols as layers of paper with a message that only the individual layer understands. Each layer handles other aspect of the connection these will be discussed below.
The first layer is the application layer it controls what data is submitted and deals with communication links such as establishing authority, identifying communication partners and the level of privacy. It is not the interface of what the user sees, the client program creates this. When a user clicks a web link the software on the computer which understands HTTP (such as internet explorer and Netscape communicator) transfers it into a HTTP request message.
The presentation layer may perform encryption and decryption of data, data compression and translation between different data formats. This layer is also concerned about displaying formatting and editing user inputs and outputs. A lot of requests such as website requests do not use the presentation layer also there is no software installed at the presentation layer and is therefore rarely used.
The session layer as the name suggests deals with organisation of the session. The layer creates the connection between the applicants, enforces the rules for carrying session and if the session does fail the layer will try to reinstate the connection. When computers communicate they need to be in synchronisation so that if either party fails to send information the session layer provides a synchronisation point so the communication can continue.
The transport layer ensures that a reliable channel exists between the communicating computers. The layer creates smaller easy to handle packets of data ready for transmission in the data link layer, it will also translate the address into a numeric address ready for better handling on the lower levels. The protocols that the transport layer uses must be kept the same in all computers. It is in this layer that protocols such as Transport Control Protocol (TCP) is used this protocol allows computers running different applications and environments to communicate effectively.
The website request now has been encapsulated with two different protocols, HTTP and TCP and almost ready to move around in the network. The network layer routes the data from node to node around the network as multiple nodes in the network exist and will avoid a computer if it not passing packets on. Any computer connected to the Internet must be able to understand TCP/IP, as it is the internetwork layers that enable the computer to find other computers and deliver messages to them.
The IP packet, containing the TCP and HTTP protocols all inside one another is now ready for the data link layer. The data link layer manages the physical transmission in next layer. The data link layer decides when to transmit messages over the devices and cabling. The data link layer also allocates stop and start markers onto the message and detects and eliminates any errors that occur during transmission. This is because the next layer sends data without understanding its meaning. A protocol called and Ethernet frame is wrapped around the message and passed onto the physical layer.
It is in the physical layer that the Ethernet frame (and the other protocols inside Ethernet frame) is transferred into a digital signal consisting of a series of ones and zeros (Binary) and through cabling the website request message is sent to the website server. When the server receives the web request message the whole process is reversed. The Ethernet frame is “unpacked” going back through each layer until it reaches the application layer and the message is read. The process is then started again as the web page requested is sent back in another message, to the person requesting it.
Carr, H. H. & Synder, C. A. (2007) Data Communications & network security. United States of America: McGraw-Hill/Irwin pg 124-129
Dennis, A. (2002). Networking In The Internet Age Application Architectures. United States of America: John Wiley and Sons, Inc
Dostálek, L., & Kabelová. A. (2006). Understanding TCP/IP. Retrieved August 6, 2006 from http://www.windowsnetworking.com/articles_tutorials/Understanding-TCPIP-Chapter1-Introduction-Network-Protocols.html