OSI Model Michael Hines NTC 360 Richard Bohn 29 November 2010 During the late 1970’s, the need for computers of all types to be able to communicate with each became more prevalent. In order to rectify this growing situation, the Internation Organization for Standardization (ISO) began to formulate the Open Systems Interconnection (OSI) model. The OSI model was created to standardize the rules of networking in order for all systems to be able to communicate. In order for communication to occur on a networking using different device drivers and protocol stacks, the rules for communication must be explicitly defined.
The OSI model deals with the following issues; • How a device on a network sends its data, and how it knows where to send it. • How a device on a network receives its data, and how it knows where to look for it. • How devices using different languages communicate with each other. • How devices on a network are physically connected to each other. • How protocols work with devices on a network to arrange data. By 1983, the OSI model became the standard reference model (Tomsho, Tittel & Johnson, 2004). The OSI model utilizes layers and is broken down into seven different layers.
Each layer has its own unique properties, tasks and activities. The seven layers (in order from “lowest” in the hierarchy to “highest”) are physical, data link, network, transport, session, presentation and application. The lowest layer on the OSI model is the physical layer. This layer deals with the physical, electrical, and cable issues involved with making a network connection. It associates with any part of the network structure that doesn’t process information in any way. The physical layer is responsible for sending the bits across the network media. It does not define what a bit is or how it is used merely how it’s sent.
The physical layer is responsible for transmitting and receiving the data. It defines pin assignments for serial connections, determines data synchronization, and defines the entire network’s timing base (Microsoft Support, 2002). The data link layer is responsible for the flow of data over the network from one device to another. It accepts data from the Network Layer, packages that data into frames, and sends them to the Physical Layer for distribution. In the same way, it receives frames from the physical layer of a receiving computer, and changes them into packets before sending them to the Network Layer (Microsoft Support, 2002).
The Data link Layer is also involved in error detection and avoidance using a Cyclic Redundancy Check (CRC) added to the frame that the receiving computer analyses (Tomsho, Tittel ; Johnson, 2004). This second also checks for lost frames and sends requests for re-transmissions of frames that are missing or corrupted at this level. The third layer to the OSI model is the network layer. This layer is responsible for making routing decisions and forwards packets that are more than one link away. The network layer is also responsible for determining routing and message priority.
By having this single layer responsible for prioritization, the other layers of the OSI model remain separated from routing decisions. This layer is also responsible for breaking large packets into smaller chucks when the original packet is bigger then the Data Link is set. Similarly, it re-assembles the packet on the receiving computer into the original-sized packet (Tomsho, Tittel ; Johnson, 2004). The transport layer’s main duty is to ensure that packets are sent error-free to the receiving computer in proper sequence with no loss of data or duplication.
The transport layer is also responsible for breaking large messages into smaller packets for the network layer, and for re-assembling the packets when they are received from the network layer for processing by the session layer (Microsoft Support, 2002). The session layer is the section of the OSI model that performs the setup functions to create the communication sessions between computers. It is responsible for much of the security and name look-up features of the protocol stack, and maintains the communications between the sending and receiving computers through the entire transfer process.
Using the services provided by the transport layer, the session layer ensures only lost or damaged data packets are re-sent, using methods referred to as data synchronization and checkpointing. This ensures that excess traffic is not created on the network in the event of a failure in the communications. The session layer also determines who can send data and who can receive data at every point in the communication. Without the dialogue between the two session layers, neither computer would know when to start sending data and when to look for it in the network traffic (Tomsho, Tittel & Johnson, 2004).
The presentation layer is responsible for protocol conversation, data translation, compression, encryption, character set conversion, and graphical command interpretation between the computer and the network (Microsoft Support, 2002). Lastly, the application layer provides services that support user applications, such as database access, e-mail services, and file transfers. The application layer also allows Remote Access Servers to work, so that applications appear local on remotely hosted servers (Microsoft Support, 2002). In order for each layer of the model to communicate with the levels above and below it, certain rules were developed.
These rules are called Protocols, and each protocol provides a specific layer of the model with a specific set of tasks or services (Tomsho, Tittel & Johnson, 2004). Each layer of the model has its own set of protocols associated with it. When you have a set of protocols that create a complete OSI model, it is called a Protocol Stack. An example of a protocol stack is TCP/IP, the standard for communication over the internet, or Appletalk for Macintosh computers. As stated before, protocols define how layers communicate with each other. Protocols specifically work with ONLY the layer above and below them.
They receive services from the protocol below, and provide services for the protocol above them. This order maintains a standard that is common to ALL forms of networking. When a message is sent from one machine to another, it travels down the protocol stack or layers of the model, and then up the layers of the stack on the other machine. As the data travels down the stack, it picks up headers from each layer through a process called encapsulation (Achiever, 2010). Headers contain information that is read by the peer layer on the stack of the other computer.
As the data travels up the levels of the peer computer, each header is removed by its equivalent protocol. This process is referred to as decapsulation (Achiever, 2010). These headers contain different information depending on the layer they receive the header from, but tell the peer layer important information, including packet size, frames, and datagrams. Each layer’s header and data are called data packages, or service data units (Achiever, 2010). Within the explanation of the OSI model, many advantages have been shown as a result. Firstly, the OSI model is a standardized model.
It can therefore be able to communicate with any other hardware or software that also utilizes the standard regardless of the operating system that is used. The use of layers is also a big benefit to the OSI model. Each layer works independently on their own tasks and then communicated within the protocol stacks. This is helpful because each layer is able to handle its own processes and issues without the necessity of the other layers (Tomsho, Tittel ; Johnson). Lastly, because the OSI Model has a layered architecture it gives the ability for new designs, protocols and network services to be added without difficulty (Mitchell, 2010).
Overall, the OSI model gave networking a great standardization to use as a reference guide. As networking continues to grow, the OSI model gives an inherent ability to progress under a singular view that allows seamless communication between those systems that share the same standardization.
Achiever, Freedom (2010), “Encapsulation and Decapsulation”. Retrieved from http://learncomputernetwork. blogspot. com/2010/02/encapsulation-and-decapsulation. html on November 29, 2010. Microsoft Support (2002),
“The OSI Model’s Seven Layers Defined and Functions Explained”. Retrieved from http://support. microsoft. com/kb/103884 on November 29, 2010. Mitchell, Bradley (2010),
“OSI Model Reference Guide”. Retrieved from http://compnetworking. about. com/cs/designosimodel/a/osimodel. htm on November 29, 2010. Tomsho, G. , Tittel, E. , ; Johnson, G. (2004).