OSI Network Model

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Open Systems Interconnection(OSI) Model Overview

The Open Systems Interconnection (OSI) model began as a reference model, but has since been implemented. It was created by the International Organization for Standardization (ISO) to provide a logical framework for how data communication processes should interact across networks. Standards were created for the computer industry allowing different networks to work together efficiently.

OSI Model Layers

There are 7 layers in the OSI model. Each layer is responsible for a particular aspect of data communication. For example, one layer may be responsible for establishing connections between devices, while another layer may be responsible for error checking during transfer. The layers of the OSI model are divided into two groups: the upper layer and lower layer. The upper layers focus on user applications and how files are represented on the computers prior to transport. For the most part, network engineers are more concerned with the lower layers. It's the lower layers that concentrate on how the communication across a network actually occurs.

The Term "Open" in OSI

The term “open” in OSI was used because the intent of the model was to provide a way for networks to be interconnected without regard to the underlying hardware, as long as the communications software adhered to the standards. The term has since come to imply modularity as well.

Modularity

Modularity refers to the exchange of protocols at a particular level without interfering with or damaging the relationship or tasks of another level. Within a layer, protocols can be exchanged, allowing communications to still take place seamlessly. These exchanges may need to occur based on hardware from different vendors or for a variety of different reasons.

Illustration with Images Text Consider the courier example. Modularity at the transportation level implies that it really doesn't matter how the package gets to the airplane. It could travel from the sender by truck to the plane or by boat. Either way, the sender still sends the package and expects it to get to Toronto. The plane still flies the package to Toronto regardless of how it was delivered to the pane.

The term "open" in OSI was used because the intent of the model was to provide a way for networks to be interconnected without regard to the underlying hardware, as long as the communications software adhered to the standards.

				True
				False

Correct Answer: 1

The OSI model organizes communication protocols into how many layers?

				4
				16
				7
				12

Correct Answer: 3

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The 7 OSI Layers

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The 7 OSI Layers

The OSI Model consists of the following seven layers: Application Presentation Session Transport Network Data Link Physical

What do the 7 layers really do?

When data is being transferred and shared over a network, it must pass through each of the 7 layers in one machine, from the application layer down, before zipping across a network and working its way up the layers in the receiving machine.

The OSI Model

The main use of the model is to help network designers understand the functionality involved with the development and flow of data communications. Included within this model are the protocols for network properties and transmission methods. The model is divided into 7 layers, with individual characteristics and tasks within each layer. Each layer must communicate with the layer directly above and below through a series of standards and protocols.

Illustration with Images Text Application Layer: Provides network services to user applications. It is responsible for exchanging information between programs running on the machine, such as an e-mail program, and other services running on a network, such as a print server or another computers' application. Presentation Layer: Concerned with how data is converted and formatted for data transfer. Examples of format conversions include ASCII text for documents and .gif and JPG for images. This layer performs code conversion, data translation, compression and encryption. Session Layer: Determines how two devices establish, maintain and manage a connection - how they talk to each other. These connections are called sessions. Transport Layer: Responsible for breaking the data into segments, establishing an end-to-end logical connection between machines, and providing for error handling. Network Layer:  Responsible for determining addressing on the network, determining the routes that information will take on its journey, and managing network traffic congestion. Data at this level is packaged into packets. Data Link Layer: Provides the link for how data, packaged into frames is communicated through hardware to be transported across a medium. It communicates with network cards, manages physical layer communications between connecting systems and handles error notification. Physical Layer: Specifies how data is processed into bits and physically transferred over medium, such as cables. It's responsible for activating and maintaining the physical link between systems.

Which of the OSI layers provides segmentation and re-assembly

				Applications
				Presenation
				Session
				Transport
				Network
				Data Link
				Physical

Correct Answer: 4

What is the information structure produced at OSI level 3 called?

				Segment
				Data
				Packet
				Frame

Correct Answer: 3

Which of layers provides code formatting?

				Applications
				Presentation
				Session
				Transport
				Network
				Data Link
				Physical

Correct Answer: 2

Which of layers provides the electrical and mechanical connections between systems?

				Applications
				Presenation
				Session
				Transport
				Network
				Data Link
				Physical

Correct Answer: 7