Open Systems Interconnection (OSI) Reference Model

Open Systems Interconnection (OSI) Reference Model

The Open Systems Interconnection (OSI) Reference Model is a modular framework for developing standards that are based on a division of network operations into seven, sets of network services.

At one time, most vendors agreed to support OSI in one form or another, but the OSI was too loosely defined and proprietary standards were too entrenched. Except for the OSI-compliant X.400 and X.500 e-mail and directory standards, which are still widely used, what was once thought to become the universal communications standard now serves as the teaching model for all other protocols.

Most of the functionality in the OSI model exists in all communications systems, although two or three OSI layers may be incorporated into one.

Example of how the OSI Layers work using an e-mail sent from the computer on the left.

Data travels from the sending computer down through all the layers to the physical layer where the data is put onto the network cabling, and then sent to the physical layer of the receiving computer where the process reverses and the data travels up through the layers to the application layer of the receiving computer.

A look at each of the OSI layers , and the role it plays.

Some common network devices and protocols and where they are implemented in the OSI model.

The unofficial other OSI Layer 2.5

While not a part of the official OSI model, the term “Layer 2.5” has been used to categorize some protocols that operate between the data link layer 2 and the network layer 3.

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For example, Multiprotocol Label Switching (MPLS) operates on packets (layer 2) while working with IP addresses (layer 3) and uses labels to route packets differently.

Interfaces

In addition to standards for individual protocols in transmission, there are also interface standards for different layers to talk to the ones above or below, usually operating system specific. For example, Microsoft Winsock, and Unix’s sockets and System V Transport Layer Interface, are interfaces between applications (layers 5 and above) and the transport (layer 4). NDIS and ODI are interfaces between the media (layer 2) and the network protocol (layer 3).

notes:

X.400 An ISO and ITU standard for addressing and transporting e-mail messages. It conforms to layer 7 of the OSI model and supports several types of transport mechanisms, including Ethernet, X.25, TCP/IP, and dial-up lines.

X.500 An ISO and ITU standard that defines how global directories should be structured. X.500 directories are hierarchical with different levels for each category of information, such as country, state, and city. X.500 supports X.400 systems.

Media Access Control Layer is one of two sublayers that make up the Data Link Layer of the OSI model. The MAC layer is responsible for moving data packets to and from one Network Interface Card (NIC) to another across a shared channel.

The MAC sublayer uses MAC protocols to ensure that signals sent from different stations across the same channel don’t collide.

Different protocols are used for different shared networks, such as Ethernets, Token Rings, Token Buses, and WANs.