The OSI, Open System Interconnection, networking model is divided into 7 layers. They are physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and application layer. With aach layer having a different responsibility, all the layers work together to provide network data communication.
Layer 1: Physical - The Physical layer is relating to the hardware connection, the electronics, logic circuitry, and wiring that send the actual signal. The only function of this layer is to move bits of data on and off the network medium. Most network problems are often found at this layer.
Layer 2: Data Link - The Data Link layer is the interface between the upper "software" layers and the lower "hardware" Physical layer. One of its main functions is to create and interpret different frame types based on the network type in use. It is divided into two sub-layers: the Media Access Control (MAC) sub-layer and the Logical Link Control (LLC) sub-layer, which is described in the following:
1. LLC sub-layer is used to maintain connections between devices, that is, server and workstation.
2. MAC sub-layer makes multiple devices to share the same medium and it is used to maintain physical device (MAC) addresses for the local communications.
Layer 3: Network - The Network layer is about addressing messages and translating logical addresses and names into physical addresses. It manages data traffic and congestion involved in packet switching and routing and also enables the feature of specifying a service address (sockets, ports) to point the data to the correct program on the destination computer.
Layer 4: Transport - The tasks of the Transport layer are controling data flow, handling errors, and correcting transmission or reception problems. It also breaks up large data files into smaller packets, combines small packets into larger ones for transmission, and puts together the incoming packets into the original sequence.
Layer 5: Session - The Session layer deals with security and name recognition to enable two applications on different computers to talk to each other over the network and manages communications between computers by using simplex, half-duplex or full-duplex. The phases involved in a session communication are establishment, data-transfer, and termination.
Layer 6: Presentation - The Presentation layer manages data exchange formats and translates specific files from the Application layer format into a commonly recognized data format. It provides protocol conversion, data translation, encryption, character-set conversion, and graphics-command expansion.
Layer 7: Application - The Application layer deals with user applications, such as software for file transfers, database access, and email. It manages general network access, flow control, and error recovery and provides a consistent neutral interface for software to access the network and advertises the computers resources to the network.
To remember these 7 layers, you can just memorize the following sentence:
All People Seem To Need Data Processing
Note: The first letter of each word relates to the first letter of the layers starting with Application and ending with the physical layer.
