e one component or a collection of components that carry out a task. A protocol stack, or protocol suite, is made up of multiple protocols used to exchange information between computers. One protocol in the stack might specify how network inter- face cards (NICs) communicate, and another might specify how a computer reads information from the NIC. Chapter 5 Data Communications Basics For More Protocol Information The Web site www.protocols.com provides easy-to-understand information about protocols. A layer is a section of a protocol stack that is responsible for performing one particular aspect of information transfer. Because some protocols are capable of performing more than one function, one layer in a protocol stack may not neces- sarily correspond to one layer in the OSI model. Tunneling describes the process of using a protocol to transfer information through a network, using a different type of protocol. The OSI Seven-Layer Reference Model In the early 1970s, a problem was brewing. There were many different computer manufacturers, and there were many incompatibilities among them. Furthermore, each manufacturer created different product lines, and even within one company, there were often incompatibilities between product lines. So the International Organization for Standardization (ISO; www.iso.org) got involved and created the Open Systems Interconnection (OSI) reference model, which is a blueprint for device manufacturers and software developers to use when creating products. The OSI model, shown in Figure 5.8, has seven layers that describe the tasks that must be performed to transfer information on a network. When data is being transferred over a network, it must pass through each layer of the OSI model. As the data passes through each layer, information is added to that data. At the destination, the additional information is removed. Layers 4 through 7 occur at the end node, and Layers 1 through 3 are the most important to telecommunications networks. It is important to understand that the OSI model 