Here are the seven layersof the OSI Reference Model with their respective roles and responsibilities:

•Ø Layer 7: Application. This is the top layer of the OSI Model. It is responsible for allowing applications to gain network access. User applications and system services generally gain network access by interacting with a process running at this layer.

•Ø Layer 6: Presentation. This layer works closely with the Application layer. Its primary responsibility is to ensure that data being passed up to the Application layer is either converted to, or is already in, a format that will be understood by the Application layer's processes.

•Ø Layer 5: Session. The Session layer is responsible for establishing, maintaining, and terminating communications among applications or processes running across a network.

•Ø Layer 4: Transport. The Transport layer is responsible for the transmission of messages from the sending host to the final receiving destination. It is responsible for creating a virtual circuit between two points on the network as well as ensuring the integrity of the data (if the lower levels or protocols do not provide this service).

•Ø Layer 3: Network. This layer is responsible for routing packets through multiple networks. The Network layer operates without regard to the underlying protocols in use, and because of this, devices, such as routers, operating at this level can be used to connect networks that use different Data Link and Physical layer technologies.

•Ø Layer 2: Data Link. This layer was originally created as a single functional layer. However, the need became apparent to divide the Data Link layer into two sublayers-the Logical Link Control (LLC) sublayer and the Media Access control (MAC) sublayer. Together, these two sublayers are responsible for moving packets onto and off of the network. The MAC sublayer is specifically concerned with obtaining access to the network at an appropriate time, such as when no other machines are communicating or when permission to access the network has been specifically given. At this layer, bits and bytes are assembled into frames, or vice versa. The LLC sublayer packages the bytes it has received from the MAC sublayer below it into a format readable by the Network layer above it.

•Ø Layer 1: Physical. The Physical layer is concerned with the physical method by which the actual bits and bytes are being sent and received. This is the layer where hardware, connectors, cable length, and signaling specifications are defined.

To simplify how the OSI Model works, consider the following example: When the Application layer receives a package of electronic data from an application or service; it processes that information in some way, adds a header, and passes it down to the next layer of the OSI Model, which is the Presentation layer. The Presentation layer then performs an operation on the data, adds its own header, and then passes it down to the next OSI layer. The data is eventually passed all the way down the model to the physical media, with each layer performing an operation and adding header information. The data is then transmitted across the network to another machine. This machine receives the data and passes it back up through all of the layers. Each layer processes and removes its respective header information and passes the data up to the next layer.

Each layer of the OSI is only aware of the layers directly above and below it. Also, each layer maintains logical communication with a corresponding layer on another machine and is unaware of the underlying processes being performed by other layers.

Communication that occurs near the bottom of the OSI Model occurs as simple bits and bytes or as a stream of bytes. The data near the bottom of the model is generally handled as raw data or as frames of data. However, as the data moves up the model, the payload of numerous frames of data is recombined in a meaningful order. Bits become bytes, bytes become words and complex data, and words and data eventually become whole ideas, sentences, or even documents before they are handed up to the application or user.

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