The Seven Layers Although shown as separate layers in the OSI model in practice the line between Application and Presentation it often blurred. Application layer does not include application software, rather this layer is the interface to the applications and programs that use the network services provided by OSI. It contains many complete services like file transfer, network management, privacy mechanisms, Virtual Terminal Protocol and authentication. It is this layer that pulls all the functionality of the other layers together and provides a method for programmers to more easily interface their programs with the network.
Similar to the application layer, the Presentation layer prepares data to be passed to the applications using the network. Some of the transformations it performs are text compression, encryption / decryption and reformatting.
The session layer is responsible for establishing, controlling, managing and terminating connections between actual computers. Settings like full or half duplex are decided here. Services such as HyperText Transfer Protocol (HTTP) and Simple Network Management Protocol (SNMP) are provided in this layer. Contrary to RFC that defines HTTP as being an application layer protocol.
This layer creates a barrier between the ones above it and the ones below. It is designed to ensure that the link between computers remains reliable, separating out the application-related functions from the connection functions. One of the features in this layer is error recovery, A system that detects errors or corruption and requests retransmissions of the damaged data.
The Network layer receives data from the transport layer takes charge of routeing it through the network. It establishes the actual connections used and finds the most appropriate pathway for each piece of data. Relaying it through each node that makes up the network. Then receiving the packets that arrive in return. It is this system the enables multiple data links to exist at once on the same network.
The Data Link layer defines how devices are physically connected, such as bus or a ring topology. It provides several functions to help simplify the flow management of the upper layers:
![]() The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems. Physical layer specifications define characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors.
The layers working together On a correctly implemented OSI system a network-enabled program is able to access the application layer and pass it an e-mail message for example. As the message is passed down through each layer it is broken down into smaller "packets" and header information is attached at each layer. When each Packet reaches the bottom of the OSI stack it is sent along the physical network. When the packet reaches the receiving computer the process is reversed, it travels back up the layers; error checking takes place and the reassembled data it presented to the program. |
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