OSI Model

OSI stands for Open System Interconnection and is a conceptual framework for the stack of protocols or sets of rules that govern network communication. OSI is called a model because it describes the collection of services that are required to allow information on a computer to be organized and prepared for transmission and then how that information is treated when it is received. OSI provides one possible model for network communications and while OSI is not the only model available, it is the predominant model. The OSI Model is comprised of seven layers with the beginning of a network transmission (such as sending an email) at the top or seventh layer and the actual sending of a message as a set of electrical signals onto an Ethernet cable at the bottom or first layer. The TCP protocol resides at the fourth or Transport layer and the IP protocol is at the third or Network layer. The OSI layers top to bottom are called: Application, Presentation, Session, Transport, Network, Data-Link, and Physical. See IP and TCP in this Glossary for more information.