Those who start towards the path of CCENT understand the OSI model as a core component that continues throughout the rest of the Cisco certification tracks: CCNA, CCNP, and CCIE.
The Open Systems Interconnection model (OSI model) is a seven layer conceptual model that characterizes and standardizes the communication functions of a telecommunication or computing system.
The physical layer, which is the bottom layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and it carries the signals for all of the higher layers.
The data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link.
The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors.
The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications.
The session layer allows session establishment between processes running on different stations.
The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known by the application layer at the receiving station.
The application layer serves as the window for users and application processes to access network services.
The Cisco Learning Network has developed an OSI model chart to help aid in your learning. This chart organizes and describes the seven layers, providing examples in the form of protocols and devices, as well as in relation to the DOD TCP/IP Model.