Defining the Layers of OSI Model

The Application Layer

• The OSI layer that is closest to the user;
• It provides network services to the user's applications
• It differs from the other layers in that it does not provide services to any other OSI layer.
• Examples of such application processes are spreadsheet programs, word processing programs, and banking terminal programs.
• If you want to remember Layer 7 in as few words as possible, think of browsers.

The Transport Layer

• The transport layer segments and reassembles data into a data stream.
• Issuses such as how reliable transport over an internetwork is accomplished are the concern of the transport layer.
• It provides mechanisms for the establishment, maintenance, and termination of virtual circuits, transport fault detection and recovery, and information flow control.
• If you want to remember Layer 4 in as few words as possible, think of quality of service, and reliability.

The Networking Layer

• The network layer is a complex Layer that provides connectivity and path selection between two end systems that may be located on geographically diverse networks.
• The network Layer is concerned with logical addressing (IP).
• If you want to remember Layer 3 in as few words as possible, think of path selection, swithshing, addressing, and routing.
• A router is a Layer 3 device.

The Data Link Layer

• The data link layer provides reliable transit of data across a physical link.
• The data link layer is concerned with physical addressing (MAC), network topology, media access, error notification, ordered delivery of frames, and flow control.
• If you want to remember Layer 2 in as few words as possible, think of naming, framing, and of media access control.

The Physical Layer

• The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems (data transmission across the network media).
• Charateristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other similar attributes are defined by physical layer specifications (various types of networking media.
• If you want to remember layer 1 in as few words as possible, think of signals and media.