Basic OSI reference model

The communication entities perform a variety of different functions during the communication process. These functions range from creating a message, formatting the message, adding information that can help detect errors during transmission, sending the data on the physical medium, and so on.

The OSI reference model defines a layered model for interconnecting systems, with seven layers. The layered approach allows the model to group similar functions within a single layer, and provides standard interfaces allowing the various layers to talk to each other.

Figure 1 shows the seven layers of the OSI model. It is important to note that the reference model defines only the functions of each layer, and the interfaces with the adjoining layers. The OSI model neither standardizes the interfaces between the various layers within the system (subsequently standardized by other protocol standards) nor delves into the internals of the layer, as to how the functions are implemented in each layer.

The OSI model describes the communication flow between two entities as follows:

• The layers have a strict peering relationship, which means that layers at a particular level would communicate with its peer layers on the other nodes through a peering protocol, for example, data generated at layer 3 of one node would be received by the layer 3 at the other node, with which it has a peering relationship.
• The peering relationship can be between two adjacent devices, or across multiple hops. As an example, the intermediate node in figure 1, that has only layers 1 through 3, the peering relationship at layer 7 will be between the layer 7 at the transmitting and receiving nodes, which are not directly connected but are multiple hops away.
• The data to be transmitted is composed at the application layer of the transmitting node and will be received at the application layer of the receiving node.
• The data will flow down the OSI-layered hierarchy from layer 7 to layer 1 at the transmitting node, traverse the intermediate network, and flow up the layered hierarchy from layer 1 to layer 7 at the receiving node. This implies that within a node, the data can be handed over by a layer to its adjacent layer only. Each layer will perform its designated functions and then pass on the processed data to the next layer:

The high-level functions of each layer are described as follows: