In this section, we'll introduce the network technologies related to IEEE 802.11, including the Open System Interconnection Reference Model (OSI/RM) and physical components of IEEE 802.11.
In OSI/RM, the computer network system is conceived as a seven-layer framework. Their names and relationships are shown in Figure 7.1.
IEEE 802.11 architecture consists of four major physical components, as shown in Figure 7.2.
Wireless Medium (WM)
WMs refer to the physical layer where wireless MAC frame data is transmitted. Initially, two radio frequency (RF) physical layers and one infrared physical layer were introduced, but the RF layers turned out to be more popular.
Stations comprise all devices and equipments that are connected to the wireless LAN. Battery-operated laptops and handheld computers are typical STAs, but "portable" is not a must. In some cases, desktops are also connected to wireless LANs to avoid pulling new cables.
Access Points (AP)
As a branch of STA, AP provides access to distribution services for associated STAs.
Distribution System (DS)
When several APs are connected to cover a larger area, they have to communicate with each other to track the movements of mobile stations. This is conducted through the distribution system, an external data network. It is responsible for transmitting data frames to their destinations.
The physical components above constitute a wireless network. The basic building block of an IEEE 802.11 network is the Basic Service Set (BSS). It comes in two categories, the Independent BSS and the Infrastructure BSS, as shown in Figure 7.3.
Stations in an independent BSS communicate with each other directly without AP.
In an infrastructure BSS, STAs must associate with an AP to obtain network services. APs function as the control centre of the set. This is the most common network architecture. Every STA needs to go through association and authorisation before joining a certain BSS. Infrastructure BSS is the most common network architecture.
Network architectures are accompanied with identifications:
BSS Identification (BSSID)
Each BSS has a physical address for identification, called BSSID. For an Infrastructure BSS, the BSSID is the MAC address of the AP. It comes with a factory default value and can be changed according a fixed naming format.
Service Set Identification (SSID)
Each AP has an identifier for user identification. In most cases, one BSSID is associated with one SSID. It is usually a readable string, which is what we call the Wi-Fi name.