Computer Networks - Topic 1 Activity

·        Explain the OSI Model.

·        Identify and explain the 7 layers of the OSI Model.

-         Description

-         Purpose

-         Protocols

The Open Systems Interconnection model (OSI model) is a conceptual model that characterizes and standardizes the communication functions of a telecommunication or computing system without regard to their underlying internal structure and technology. Its goal is the interoperability of diverse communication systems with standard protocols. The model partitions a communication system into abstraction layers. The original version of the model defined seven layers.

Layer 1:  Physical Layer

Layer 2: Data link layer

Layer 3 : network layer

Layer 4: transport layer

Layer 5; Session layer

Layer 6: Presentation layer

Layer 7: Application layer

Layer 1:  Physical Layer

Physical layer is the electrical and physical specification of data connection. it define the relationship between a device and a physical transmission medium such as copper or a fiber optical cable. it is responsible for a transmission and reception of unstructured raw data in a physical medium.

Layer 2: Data link layer

the data link layer is a link between two directly connected nodes. it detect and possibly correct errors that may occur in the physical layer. it define the protocol to establish and terminate a connection between who physically connected device. the data link layer is divided into two sub layer which is Media access control (MAC) and Logical link control (LLC).

Media access control is responsible for controlling how a device in a network gain to a medium and permission to transmit data.

Logical link control (LLC) is responsible for identifying network layer protocols and then encapsulating them and controls error checking and frame synchronization.

Layer 3 : network layer

Network layer provides the functional and procedural means of transferring variable length data sequences called datagrams from one node to another connected to the same "network". 

Layer 4: transport layer

the transport layer provide the functional and procedural means of transferring varaible-length data sequences to a destination host via one or more networks, while maintaining the quality of service function.

an example of a transport-layer protocol in the standard internet stack is Transmission Control Protocol (TCP), usually built on top of the internet protocol (IP).

The transfer layer control the reliability of a given link through flow control, segmentation/desegmentation and error control.

Layer 5; Session layer

the session layer controls the dialogues (connections) between computers. it establishes, manages and terminates the connections between the local and remote application. The OSI model made this layer responsible for graceful close of session, which is a property of the TCP and also for session check pointing and recovery.

Layer 6: Presentation layer

Presentation layer establishes context between application-layer entities, in which the application-layer entities may use different syntax and semantics if the presentation service provides a mapping between them.

this layer provides independence from data representation for example encryption, by translating between application and network formats. this layer formats and encrypts data into the form that the application accepts.

Layer 7: Application layer

the application layer is the OSI layer closest to the end user, which means both the OSI application and the user interact directly with the software application. This layer interact with software applications that implement a communicating component.

application-layer functions typically include identifying communication partner, determining resources availability communication partners, and synchronizing communication. when identifying communication partners, the applications layer determine the identity and availability of communication partners for an application with data to transmit. when determining resource availability, the application layer must decide whether sufficient network resources for the requested communication are available.