The OSI (Open Systems Interconnect) 7 Layer Model

Application
(Layer 7)

Applications access the underlying network services using defined procedures in this layer. The application layer is used to define a range of applications that handle file transfers, terminal sessions, network management, and message exchange.

Presentation
(Layer 6)

Protocols at the presentation layer are part of the operating system and application the user runs in a workstation. Information is formatted for display or printing in this layer. Codes within the data, such as tabs or special graphics sequences, are interpreted. Data encryption and the translation of other character sets are also handled in this layer.

Session
(Layer 5)

The session layer coordinates the exchange of information between systems by using conversational techniques, or dialogues. Dialogues are not always required, but some applications may require a way of knowing where to restart the transmission of data if a connection is temporarily lost, or may require a periodic dialog to indicate the end of one data set and the start of a new one.

Transport
(Layer 4)
PDU - Segment

The transport layer provides a high level of control for moving information between systems, including more sophisticated error handling, prioritization, and security features. The transport layer provides quality service and accurate delivery by providing connection oriented services between two end systems. It controls the sequence of packets, regulates traffic flow, and recognizes duplicate packets.

Network
(Layer 3)
PDU - Packet

The network layer defines protocols for opening and maintaining a path on the network between systems. It is concerned with data transmission and switching procedures, and hides such procedures from upper layers. Routers operate at the network layer. The network layer can look at packet addresses to determine routing methods.

Data Link
(Layer 2)
PDU - Frame

The data link layer defines the rules for sending and receiving information across the physical connection between two systems. This layer encodes and frames data for transmission, in addition to providing error detection and control.

Physical
(Layer 1)
PDU - Bit

The physical layer defines the physical characteristics of the interface, such as mechanical components and connectors, electrical aspects such as voltalge levels representing binary values, and functional aspects such as setting up, maintaining, and taking down the physical link.