Network Essentials Notes – Section 1

How a Network Functions

The OSI Models

• International Standard Organization ISO Specifications for network architecture.
• Called the Open Systems Interconnect or OSI Model.
• Seven layered model, higher layers have more complex tasks.
• Each layer provides services for the next higher layer.
• Each layer communicates logically with its associated layer on the other computer.
• Packets are sent from one layer to another in the order of the layers, from top to bottom on the sending computer and then in reverse order on the receiving computer.

OSI Layers

• Application
• Presentation
• Session
• Transport
• Network
• Data Link
• Physical

Application Layer

• Serves as a window for applications to access network services.
• Handles general network access, flow control and error recovery.

Presentation Layer

• Determines the format used to exchange data among the networked computers.
• Translates data from a format from the Application layer into an intermediate format.
• Responsible for protocol conversion, data translation, data encryption, data compression, character conversion, and graphics expansion.
• Redirector operates at this level.

Session Layer

• Allows two applications running on different computers to establish use and end a connection called a Session.
• Performs name recognition and security.
• Provides synchronization by placing checkpoints in the data stream.
• Implements dialog control between communicating processes.

Transport Layer

• Responsible for packet creation.
• Provides an additional connection level beneath the Session layer.
• Ensures that packets are delivered error free, in sequence with no losses or duplication’s.
• Unpacks, reassembles and sends receipt of messages at the receiving end.
• Provides flow control, error handling, and solves transmission problems.

Network Layer

• Responsible for addressing messages and translating logical addresses and names into physical addresses.
• Determines the route from the source to the destination computer.
• Manages traffic such as packet switching, routing and controlling the congestion of data.

Data Link Layer

• Sends data frames from the Network layer to the Physical layer.
• Packages raw bits into frames for the Network layer at the receiving end.
• Responsible for providing error free transmission of frames through the Physical layer.

Physical Layer

• Transmits the unstructured raw bit stream over a physical medium.
• Relates the electrical, optical mechanical and functional interfaces to the cable.
• Defines how the cable is attached to the network adapter card.
• Defines data encoding and bit synchronization.

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