The OSI Model is a conceptual model, or framework, describing how two computing devices establish and maintain communications in order to transfer information between them. It is the international standard for inter-computer communication.
The Application Layer
This layer is at the top of the OSI stack, and because it is, it does not provide a service, or services, to any other layer. Instead, it uses the services of the presentation layer, which is immediately below it.
The application layer interacts with software applications, which the user in turn interacts with, that implement a communicating component. Application-layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. (HSdataline) It determines the identity and availability of communication partners, decides whether sufficient network resources for the requested communication exist, and then manages all communication between the communicating applications.
The Presentation Layer
This layer is responsible for data conversion and compression. It can be considered to be the translator of the OSI model. This layer takes data from the Application layer and converts it to a format that can be read by the Application layer of the receiving computer. The Presentation layer is also responsible for data encryption (if required by the application used in the Application layer) and data compression, which will reduce the size of the data. (Habraken, page 93)
The Presentation layer provides application layer applications with a choice of data types for transmission over the network media, which include: ASCII and HTML for text, JPEG and GIF for graphics, and MPEG for video.
The Session Layer
This layer is responsible for establishing, managing and terminating communications between two computing devices. It offers three modes of communication: simplex, half-duplex and full duplex.
The Transport Layer
This layer is responsible for data transport issues such as: flow control, multiplexing, and error checking and recovery. It has two main means of transporting data: connection oriented and connectionless.
Flow control manages data transmission between devices so that the transmitting device does not send more data than the receiving device can process, while multiplexing enables data from several applications to be transmitted onto a single physical link. Error checking involves creating various mechanisms for detecting transmission errors, while error recovery involves taking an action, such as requesting that data be retransmitted, to resolve any errors that occur. (HSdataline)
Connection oriented protocols use some form of acknowledgment system between the sender and recipient, whereas connectionless protocols do not use any form of acknowledgment from the recipient that it has received the packet.
The Network Layer
This layer is responsible for addressing the data packets that are to be sent from the source device to the destination device, and for determining the route that the packets will take in reaching the destination device.
The Network layer has two primary functions: logical addressing – providing logical addresses for the devices, and routing – determining the routes that the packets will take over the network.
The Data Link Layer
This layer is responsible for getting data reliably transferred across the actual physical network link. Its responsibilities include rendering the physical addresses of network devices, the network topology, and the sequencing of packets or frames.
The IEEE has sub-divided the Data Link layer into two sub-layers: the Logical Link Control layer, which establishes and maintains the link between the two computing devices as data moves across the physical network, and the Media Access Control layer, which uniquely identifies each computing device on the network based on their hardware addresses.
The Physical Layer
This layer is at the bottom of the OSI stack, and because it is, it does not provide utilise the service, or services, of any other layer. It only provides a service, or services, to the layer immediately above it. 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. (HSdataline)
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