The 802.11 protocol represents a family of wireless radio communications based on different modulation techniques in the 2.4 GHz and 5 GHz ISM bands of the unlicensed spectrum. 802.11b and 802.11g reside in the 2.4 GHz band, while 802.11n and 802.11ac open up the 5 GHz band. The last chapter detailed the 2.4 GHz band and the different protocols that reside in that space. Wi-Fi is susceptible to the same noise and interference as Bluetooth and Zigbee and deploys a variety of techniques to ensure robustness and resiliency.
From a stack perspective, the 802.11 protocols reside in the link layer (one and two) of the OSI model, as shown in the following figure:
The stack includes various PHYs from older 802.11 specifications such as the 802.11 original PHYs (including infrared), a, b, g, and n. This is to ensure backward compatibility across networks. Most chipsets include the entire PHY collection and it is difficult to find a part with an older PHY alone.
802.11 systems support three basic topologies:
- Infrastructure: In this form, a Station (STA) refers to an 802.11 endpoint device (like a Smartphone) that communicates with a central access point (AP). An AP can be a gateway to other networks (WAN), a router, or a true access point in a larger network. This is also known as Infrastructure Basic Set Service (BSS). This topology is a star topology.
- Ad hoc: 802.11 nodes can form what is called an Independent Basic Set Service (IBSS) where each station communicates and manages the interface to other stations. No access point or a star topology is used in this configuration. This is a peer-to-peer type of topology.
- Distribution system (DS): The DS combines two or more independent BSS networks through access point interconnects.
Below are examples of the three basic topologies of an IEEE 802.11 architecture:
In total, the 802.11 protocol allows for up to 2007 STAs to be associated with a single access point. This is relevant when we explore other protocols, such as the IEEE 802.11ah for IoT, later in this chapter.