OSI Model Handout

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OSI Model

Layer 7: Application Layer

The application layer is the layer that is closest to the user. It provides network services to the user’s applications. It differs from other layers in that it does not provide services to any other OSI layer; instead, it provides services only to applications outside the OSI model. Examples of such applications are spreadsheet programs and word-processing programs. The application layer establishes the availability of intended communication partners and also synchronizes and establishes agreement on procedures for error recovery and control of data integrity. Examples of Layer 7 applications included Telnet and HTTP.

Layer 6: The Presentation Layer

The presentation layer ensures that the information that the application layer of one system sends out can be read by the application layer of another system. If necessary, the presentation layer translates among multiple data formats by using a common format. One of the more important tasks of this layer is encryption and decryption. The common Layer 6 graphic standards are PICT, TIFF, and JPEG. Examples of Layer 6 standards that guide the presentation of sound and movies are MIDI and MPEG.

Layer 5: The Session Layer

As its name implies, the session layer establishes, manages, and terminates sessions between two communicating hosts. The session layer provides its services to the presentation layer. It also synchronizes dialogue between the two hosts’ presentation layers and manages their data exchange. In addition to handling session regulation, the session layer offers provisions for efficient data transfer, class of service, and exception reporting of session layer, presentation layer, and application layer problems. Examples of Layer 5 protocols are the Network File System (NFS), X-Window Systems, and AppleTalk Session Protocol (ASP).

Layer 4: The Transport Layer

The transport layer segments data from the sending host’s system and reassembles it into a data stream on the receiving host’s system. The boundary between the transport layer and the session layer can be thought of as the boundary between application protocols and data-flow protocols. Whereas the application, presentation, and session layers are concerned with application issues, the lowest four layers are concerned with data-transport issues.

The transport layer attempts to provide data-transport service that shields the upper layers from transport-implementation details. Specifically, issues such as reliability of transport between two hosts are the concern of the transport layer. In providing communication service, the transport layer establishes, maintains, and properly terminates virtual circuits. Transport error detection and recovery and information flow control are used to provide reliable service. Examples of Layer 4 protocols are Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Sequenced Packet Exchange (SPX).

Layer 3: The Network Layer

The network layer is a complex layer that provides connectivity and path selection between two host systems that might be located on geographically separated networks. Additionally, the network layer is concerned with logical addressing. Examples of Layer 3 protocols are Internet Protocol (IP), Internetwork Packet Exchange (IPX), and AppleTalk.

Layer 2: The Data Link Layer

The data link layer provides reliable transit of data across a physical link. In so doing, the data link layer is concerned with physical (as opposed to logical) addressing, network topology, network access, error notification, ordered delivery of frames, and flow control. Examples of Layer 2 protocols included Ethernet, Token Ring, ISDN, PPP and Frame Relay.

Layer 1: 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. Such characteristics 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.

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