Which is not a layer of the OSI model?

The OSI (Open Systems Interconnection) model consists of seven distinct layers that describe the different functions involved in network communications. These layers, from the lowest to the highest, are: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

The correct answer to the question is Security, as it does not correspond to any of the OSI model's defined layers. While security is an essential aspect of network communications, it is not treated as a standalone layer within the OSI model. Instead, security measures and protocols can be implemented across multiple layers of the OSI model, depending on the required level of security for the data being transmitted.

In contrast, the Data Link layer is responsible for node-to-node data transfer and error correction, the Physical layer deals with the physical connections and transmission of raw bitstreams over a medium, and the Presentation layer functions to translate, encrypt, or compress data being sent over the network to ensure proper format and interpretation by the receiving system. Thus, the presence of these components in the OSI model underscores that Security does not represent an individual layer within this framework.