OSI 7 Layer Model Tutorial

The OSI Reference model is clearly and simply explained in this free tutorial from PC Network Advisor.

This is a plain text version of the original article. Click OSI Reference Model PDF Version for the original version complete with pictures.

If you spend much time in the company of network technicians you will eventually hear them say something like "That's Layer 2 only" or "That's our new Layer 4 switch". The technicians are referring to the OSI 7 Layer Reference Model. "OSI" stands for Open System Interconnection.

The OSI Reference model defines seven layers that describe how applications running upon network-aware devices may communicate with each other. The model is generic and applies to all network types, not just TCP/ IP, and all media types, not just Ethernet. It is for this reason that any network technician will glibly throw around the term" Layer 4" and expect to be understood.

It should be noted, however, that most protocols in day-to-day use work on a slightly modified layer system. TCP/ IP, for example, uses a 6-rather than a 7-layer model. Nevertheless, in order to ease the exchange of ideas, even those who only ever use TCP/ IP will refer to the 7-layer model when discussing networking principles with peers from a different networking background.

Confusingly, the OSI was a working group within the ISO (International Standards Organisation) and, therefore, many people refer to the OSI Reference model as the ISO Reference model. They are referring to the same thing.

Traditionally, layer diagrams are drawn with Layer 1 at the bottom and Layer 7 at the top. The remainder of this article describes each layer, starting from the bottom, and explains some of the devices and protocols you might expect to find in your data centre operating at this layer.

Layer 1 of the 7 layer Model is the Physical Layer and defines the physical and electrical characteristics of the network. The NIC cards in your PC and the interfaces on your routers all run at this level since, eventually, they have to pass strings of ones and zeros down the wire.

Layer 2 is known as the Data Link Layer. It defines the access strategy for sharing the physical medium, including data link and media access issues. Protocols such as PPP, SLIP and HDLC live here.

On an Ethernet, of course, access is governed by a device's MAC address, the six byte number that is unique to each NIC. Devices which depend on this level include bridges and switches, which learn which segment's devices are on by learning the MAC addresses of devices attached to various ports.

This is how bridges are eventually able to segment off a large network, only forwarding packets between ports if two devices on separate segments need to communicate. Switches quickly learn a topology map of the network, and can thus switch packets between communicating devices very quickly. It is for this reason that migrating a device between different switch ports can cause the device to lose network connectivity for a while, until the switch, or bridge, re-ARPs.

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