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Conceptually, data flows down into the "stack" from the operating system in one computer, moves downwards (being acted on by each layer in turn) and then goes across (on the Physical Layer connection) to another computer. The data then moves "up" the stack in the recipient computer where the data is, again, acted on, in turn, by each corresponding layer. Because the functionality is divided into layers it's possible for software and hardware developers to create separate, modular components and then fit them together in interesting ways. For example, a data can move between an 802.11 wireless LAN and an 802.3 Ethernet LAN because only the lowest layers are being changed. The upper layer aspects of the conversation (the "TCP/IP" and application data part) remains unchanged. There are entire textbooks written on the significance of this modular categorization of layered architecture but, for the present discussion, it will be sufficient to explain briefly what the role of each layer is.

Physical Layer: This refers to the way the communicators are physically connected. Copper cable and the Ethernet electrical rules that represent bits on the cable are Physical Layer specifications as are the rules for representing bits with light on a fiber-optic cable. In 802.11 the Physical Layer medium is the air and the Physical Layer rules differentiate 802.11b from 802.11g and 802.11a. These standards use different ways of representing bits with alterations in the electromagnetic RF signal.

Data Link Layer: This layer takes care of moving frames of data from a sender's hardware address to an immediately reachable receiver's hardware address. In the realm of 802.11 there are actually two separate addressing issues at the Data Link Layer. Consider frames originating with a wireless client destined for a web server on the Internet. The frame must go from the client to the access point and then on to the Ethernet interface on the inside of the Internet gateway router. There is a Data Link source address that represents the client. There is an 802.11 receiver address for the access point and an "Ethernet-like" destination address for the Ethernet interface on the router. The Data Link Layer is also responsible for taking turns accessing the medium. It's the Data Link Layer that performs the "listen before you talk" function. In summary, the Data Link Layer is concerned with moving frames from one device to another directly reachable device (even if "directly reachable" means through an access point). It also is responsible for medium access control (the MAC function). Switches make forwarding decisions based on Data Link Layer address.

Network Layer: This is where Internet Protocol (IP) comes in to give an identify to the ultimate destination device in a conversation (a web server on the Internet, for example). If the Data Link Layer thinks in terms of computer "boxes" here and there then the Network Layer thinks in terms of a logical device identity, the IP address. Routers make forwarding decisions based on the Network Layer address.

Transport Layer: TCP (Transport Control Protocol) operates at the Transport Layer. This layer is responsible for the delivery of packets from one software program running in a computer to a software program running in a different computer. For example, web servers use TCP Port 80 and mail servers use TCP Port 25. The port number, at the Transport Layer, identifies a program running in a computer. The Transport Layer offers guaranteed delivery of packets by implementing a sequence number and retransmission mechanism.

The Session, Presentation, and Application Layers are not always as clearly delineated as the lower layers. Technically (and briefly) speaking, the Session Layer manages an entire communication session. In practice it typically the Transport Layer (specifically TCP) that does all the work and there is very little behavior for the Session Layer protocols to do. The Presentation Layer is responsible for formatting the data (ie: ASCII versus UNICODE Dual-Byte characters) and the Application Layer implements a set of commands that the operating system can use to do work across the network ("Open File", "Read File", "Close File"). It's common to find that software commands in an application program are mapped directly to Application Layer commands so a program can operate on remote resources as if they were local.