The Osi Reference Model

The OSI Reference Model        

The OSI Reference Model

In the early 1980s the Open System Interconnection (OSI) reference model was initially introduced by the International Organization for Standardization (IOS) the models intentions are to promote interoperability of network devices. It is a reference tool for understanding data communication between any two networked systems. The model has become the standard in the rules of networking. The OSI Reference Model is a seven layer hierarchy that defines the separate platforms that data travels through to get from one device to another device over a network. The remainder of this presentation will provide a detailed description of each of the seven layers functions. Then will proceed to go in depth on how packet-filtering routers and firewalls protect a network and what layer they reside on.

Application Layer (Layer 7)

Layer seven the Application Layer is designed to support all applications and end-user processes, it is the highest layer of the hierarchy. A few services this layer provides are resource allocation, file transfer, and the identification and verification of computer availability.  Sending an email is an example of an application that works at this level (Tuma & Fajfar, 2000).

Presentation Layer (Layer 6)

Layer six the Presentation Layer translates and then proceeds to present data to the application layer for the use of the end-user. At this layer, the implementation of data decryption and encryption can take place along with data translation. After the translation of the data, the presentation layer will send it across the network providing relief of compatibility issues. All interactions with photographs, movies, and music take place at this level.

Session Layer (Layer 5)

Layer five is the Session Layer where the establishment of sessions between computers are maintained and managed. When users make requests for specific information over the web such as checking a bank account balances the session layer will make the first contact with the host computer. The functions performed at this layer are formatting the data, establishing a communication link, and handling the recovery and restart processes.

Transport Layer (Layer 4)

Layer four the Transport Layer also known as the data end-to-end layer. Here is where data between end hosts or nodes is transferred, and delivery is controlled. The responsibility of this layer is to ensure data is delivered error-free with no duplications or losses. It works as a transition point between the lower layers and higher layers. An example would be Transmission Control Protocol/Internet Protocol (TCP/IP) this protocol provides universal connectivity across the internet using a reliable delivery mechanism. It handles data in bundles called packets, keeping them from getting lost, damaged, or disordered.

Network Layer (Layer 3)

Layer three the Network Layer controls the flow of packets. Packets are bundles of organized data that include the internet address of the destination. Based on the priority of service, network conditions, and other influences this layer decides what physical route the data should take. It establishes, maintains, and terminates connections throughout the network. However, unlike the Transport Layer, the Network Layer does not know where the destination to deliver your data originates. It only knows how to address the data and drop the packet onto the network for the Transport Layer to route.

Data Link Layer (Layer 2)

Layer two the Data Link Layer is responsible for establishing and terminating the links between two systems, frame error checking, and frame traffic control. It handles errors from the physical layer to deliver error-free data frames from one node to another, allowing layers above it to retrieve error-free data. This layer has two sections the first is Media Access Control (MAC) layer that controls how a computer obtains access to the data and permission to transmit it. The second portion is the Logical Link Control (LLC) layer that handlings error checking, frame synchronization, and flow control.