OSI Model

The Open System Interconnection (OSI) Model

A Brief History

During the 1970s two organizations set about with the goal of creating a standard for the architecture of networks; one being the International Organization for Standardization (ISO) and the other being the International Telegraph and Telephone Consultative Committee (CCITT). In the mid-1980s (1983-84) both concepts, being very simular in design, were brought together to create what is known today as the OSI Model.

The OSI Model

The OSI Model is broken into 7 layers, each performing its own task in order to allow for communication over a network.

Layer 7: Application.

This is the layer at which the user interacts with the network via software.

Operations at this layer: Telnet, Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), and Simple Mail Transfer (SMTP).

Layer 6: Presentation.

This layer provides translation from application to network formats (or network to application) via various commonly used encrpytion schemes.

Operations at this layer: Motion Pictures Experts Group (MPEG), Grapics Interchange Format (GIF), and Joint Photographic Experts Group (JPEG).

Layer 5: Session.

This layer provides for the establishment, management, and termination of communication between applications. It deals with session and connection coordination.

Operations at this layer: Zone Information Protocol (ZIP) and Apple Talk.

Layer 4: Transport.

This layer provides for the transfer of data between hosts. It is responsible for error recovery, segmentation/desegmentation, and flow control.

Operations at this layer: Transmission Control Prorocol (TCP) and User Datagram Protocol (UDP).

Layer 3: Network.

This layer performs routing and switching functions. It also handles addressing, packet sequencing, and quality of service.

Operations at this layer: Internet Protocol (IP).

Layer 2: Data Link.

This layer encodes data into bits (or decodes them) and provides a bridge for the upper software layers, and the physical media of the network.

This layer has 2 sub layers, the Media Access Control (MAC) which manages access to the physical medium, and the Logical Link Control (LLC) which is the link between the MAC and the Network layer.

Layer 1: Physical.

This layer provides the means for which bits are sent and recieved.

Copper cables, fiber cables, radio signals (Wi-Fi), network cards, switches, or any actual hardware on a network act as part of this layer.

For anyone having a difficult time remembering the layer/order of the OSI model, here are 2 acronyms that may help.

All People Seem To Need Data Processing (layers 7-1)

or

Please Do Not Throw Sausage Pizza Away (layers 1-7)

References: 1. http://en.wikipedia.org/wiki/OSI_model 2. http://www.tcpipguide.com/free/t_HistoryoftheOSIReferenceModel.htm 3. http://www.webopedia.com/quick_ref/OSI_Layers.asp 4. http://www.cisco.com/en/US/docs/internetworking/technology/handbook/Intro-to-Internet.html#wp1020560